Je Souhaiterais Remercier

Catherine Faur

Pierre Bérubé

Murielle Rabiller-Baudry

Christophe Dagot

Emma Roubaud

Matthieu Dorel

Sylvain Pageot

Keywords: membrane bioreactor, ultrafiltration, hollow-fiber, wastewater treatment, ageing, full-scale Bioréacteur à membrane, Ultrafiltration, Fibre creuse, Traitement des eaux résiduaires urbaines, Vieillissement, Echelle industrielle

à rendre cette expérience extraordinairement enrichissante.

GENERAL INTRODUCTION

Membrane bioreactors (MBR) have experienced an exponential growth during the past decade.

The cumulative treatment capacity of large-scale facilities (>100,000 m 3 /d), increased from 500,000 m 3 /d in 2010 and is expected to reach over 14,000,000 m 3 /d by 2025 [START_REF]The MBR site[END_REF]. The main drivers of this MBR expansion across the globe are stricter legislation concerning the reuse of the treated effluent, discharge to sensitive water bodies, space limitations, and retrofitting/upgrading wastewater treatment plants (WWTP) [START_REF] Krzeminski | Membrane bioreactors -A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects[END_REF]. Membranes act as a reliable physical barrier, and thus MBR perform significantly better than classic processes. To illustrate, Bertanza et al. ( 2017) compared a full-scale MBR with a conventional activated sludge system (CAS) from a WWTP in Italy. Treating the same influent, MBR was able to remove 99.5% of chemical oxygen demand (COD) versus 90.6% for the CAS [START_REF] Bertanza | A comparison between two full-scale MBR and CAS municipal wastewater treatment plants: techno-economic-environmental assessment[END_REF].

Despite the increasing interest in the application of MBR, its operating costs are still higher than CAS. [START_REF] Xiao | Current state and challenges of full-scale membrane bioreactor applications: A critical review[END_REF] reported operating costs of MBRs in China ranging from 0.13 -0.25 USD/m 3 compared to 0.11 USD/m 3 for the average operating costs of municipal wastewater treatment plants [START_REF] Xiao | Current state and challenges of full-scale membrane bioreactor applications: A critical review[END_REF]. These costs are mainly associate to membrane fouling and the operations to overcome it. Several strategies are applied to reduce fouling such as aeration cycles, physical cleaning, i.e., relaxation or backwash, and chemical cleaning either by chemically enhanced backwash or cleaning-in-place (CIP) protocols [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF]. These strategies directly affect operating costs, but may also cause unexpected changes in membrane properties, which, simultaneously to accumulation of residual fouling, decrease filtration performance.

The combined effect of residual fouling and changes in membrane properties in the long term results in membrane ageing, which ultimately leads to membrane replacement [START_REF] Fenu | Elaborating the membrane life concept in a full scale hollow-fibers MBR[END_REF]. Membrane lifespan is also a decisive factor in the economic efficiency of an MBR. For instance, a 19% reduction in the total costs of an installation was reported if membranes were replaced after 10 years instead of after 5 [START_REF] Verrecht | The cost of a large-scale hollow fibre MBR[END_REF].

As a result, interest in understanding the ageing process of MBR has increased. Traditionally, studies focused on comprehending the changes in membrane properties due to exposure to chemicals often used in cleaning protocols, such as oxidants, bases, and acids. Among them, sodium hypochlorite (NaOCl) is the most commonly studied and widely used reagent in ultrafiltration processes thanks to its cleaning efficiency and associated low cost; unfortunately, NaOCl is also involved in the degradation of polymeric membranes. Polyvinyl difluoride (PVDF), a chlorine-resistant polymer, has emerged as one of the most commonly used membrane materials for drinking water and wastewater treatment applications [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF]. It has a high hydrophobic character and is usually mixed with hydrophilic and pore-forming additives, i.e., polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), in the manufacturing of ultrafiltration membranes.

Since 2010, at least 22 studies have been published on PVDF membrane ageing. The main approach used in these studies was to accelerate ageing by single soaking in a hypochlorite solution and to observe the changes in membrane properties [START_REF] Abdullah | Filtration and cleaning performances of PVDF membranes aged with exposure to sodium hypochlorite[END_REF][START_REF] Abdullah | Assessing the effects of sodium hypochlorite exposure on the characteristics of PVDF based membranes[END_REF][START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF][START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF][START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF][START_REF] Puspitasari | Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane[END_REF][START_REF] Rabuni | A systematic assessment method for the investigation of the PVDF membrane stability[END_REF][START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF][START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF][START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF][START_REF] Vanysacker | Effect of chemical cleaning and membrane aging on membrane biofouling using model organisms with increasing complexity[END_REF][START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF][START_REF] Wu | Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods[END_REF][START_REF] Zhang | A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes[END_REF]. Generally, the chlorine exposure dose, which is the product (C × t) between chlorine concentration (C) and soaking time (t), is used to monitor the ageing process. This parameter allows us to compare the ageing effects between bench scale and full scale. Other approaches included filtration of model fouling solutions and backwash hypochlorite cycles to include the impact of fouling [START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF][START_REF] Gao | Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes[END_REF][START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF].

Finally, one study investigated PVDF membrane ageing in full-scale direct filtration facilities [START_REF] Robinson | Membrane ageing in full-scale water treatment plants[END_REF].

The main conclusions drawn from these studies are: (i) the current understanding of PVDF membrane ageing is mainly based on chemical exposure at bench scale and (ii) contradictions can be found regarding permeability, pore size, and fouling reversibility changes with ageing, probably because of the differences in ageing conditions, in analytical methods, and in membrane type. Regarding these two points, the changes in PVDF membrane properties over time by direct filtration in drinking water production facilities were elucidated in an initial study and the findings were later compared with changes observed in ageing of the same membranes at bench scale by single soaking and by cycles of model fouling solution filtration/NaOCl backwash [START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF][START_REF] Robinson | Membrane ageing in full-scale water treatment plants[END_REF]. By applying the same analytical methodology, the authors were able to conclude that chemical changes were similar for both scales, but performance differed, which means that bench-scale approach can be used only when chemical changes is a concern.

However, operating conditions in direct filtration and in MBR are substantially different and a comprehensive study of changes in membrane properties over time under MBR on-site conditions is still lacking. For direct filtration units, filtration cycles may take several hours at higher fluxes, i.e., over 30 L.m -2 .h -1 (LMH) before backwash, due to a low-charged influent in terms of dissolved organic carbon (DOC), i.e., approximately 3.5 mgC/L [START_REF] Yu | Performance of hollow fiber ultrafiltration membrane in a full-scale drinking water treatment plant in China: A systematic evaluation during 7-year operation[END_REF]. For MBR, membranes are expected to handle much more charged water (DOC over 100 mgC/L) with higher amounts of fouling agents and suspended solids, leading to smaller filtration fluxes (~25 LMH) and more frequent relaxation/backwash (every 10-90 min) [START_REF] Krzeminski | Membrane bioreactors -A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects[END_REF]. In addition, there are major differences between these applications regarding: (i) membrane structure, since for MBR membranes are reinforced by other polymers, (ii) filtration is mostly performed from the outside towards inside membranes and (iii) aeration cycles are frequently performed which enables oscillation of fibers and effective control of cake formation.

The dynamics of filtration and fouling in MBR are expected to play a major role in membrane ageing compared with direct filtration facilities. [START_REF] Cote | Hollow fiber membrane life in membrane bioreactors (MBR)[END_REF] analyzed the data set of ZeeWeed® MBRs shipped to North America over 15 years and listed five end-of-life triggers for membranes, namely, hollow-fiber breakage, mechanical module or cassette failure, upgrade to a higher performance product, weakening of the potting resin-membrane fiber bond and increasing in cleaning flow throughput. They stated that a slow increase in operating pressure and the need for more frequent cleaning are the dominant reasons for membrane replacement.

Regarding the membrane structure itself, replacement can be caused by: (i) a loss of integrity due to either pore enlargement, skin tearing or hollow-fiber breakage, which would lead to a permeate of poorer quality or (ii) an increasing transmembrane pressure (TMP) approaching to the maximum TMP that the membrane can withstand. The later would lead to more frequent cleaning and a decline in hydraulic efficiency.

Only two studies were published about membrane ageing in full-scale MBR installations. on the permeability index decline. Authors reported similar lifespans based on the permeability index decline and on the loss of membrane integrity [START_REF] Mailler | Longterm performances and membrane lifespan of full-scale MBR treating filtrate from sludge ultra-dewatering[END_REF]. They went further and correlated the decline on permeability index to the cumulated load of supernatant COD rather than the chlorine exposure, which is a new indicator to be integrated in membrane ageing studies. It is worth noting that the MBR in this study treated filtrate from sludge ultra-dewatering, which is a much more concentrated effluent (in terms of carbon and nitrogen sources) than common MBRs applied to urban wastewater treatment.

Therefore, a clear understanding of supported PVDF membrane ageing in full-scale MBR conditions is still lacking. Because of that, the representability of traditional approach of accelerated ageing by chlorine exposure towards ageing under operating conditions is questionable. Therefore, this thesis aims to:

1) Understand membrane ageing due to NaOCl exposure: pristine ZeeWeed® 500D membranes will be aged by soaking in NaOCl solutions at pH and concentration similar to real operation and membrane properties will be characterized;

2) Compare ageing due to chlorine exposure to full-scale MBR ageing to demonstrate the representability of accelerated ageing protocol at bench-scale:

ZeeWeed® 500D membranes will be harvested from a full-scale MBR in order to monitor changes in properties and compare with those samples aged by NaOCl soaking at bench-scale; supply and demand is projected to reach 40% by 2030, according to the Water Resources Group [START_REF] Martins | Annual Report: Scaling Up for Impact -2030 Water Resources Group -World Bank Group[END_REF]. The role of sanitation and wastewater management in public health is widely known.

280,000 deaths from diarrhea are assigned to the lack of these two factors every year, making them priority intervention areas in World Health Organization (WHO) strategic plan for 2018-2025 [START_REF]WHO | WHO water, sanitation and hygiene strategy 2018-2025[END_REF]. These facts urge for improvements in wastewater treatment (WWT), which can provide a safe disposal of the gathered effluent into the environment [START_REF] Salgot | Wastewater treatment and water reuse[END_REF].

Wastewater treatment plants are usually composed of a preliminary treatment (physical), i.e.

coarse screening, grit chambers and aeration, to get rid of heavy and larger entrained solids and protect pumping equipment. After that, a primary treatment, such as decantation, is used to remove suspended solids, phosphor based compounds and colloids in a physicochemical sedimentation, and the addition of coagulants enhances the decantation performance in this step.

Then, organic matters from effluents (carbon, phosphor and nitrogen based substances) are biochemically oxidized by microorganisms in an activated sludge matrix as a secondary treatment. There are few technologies to be highlighted in this step, namely, conventional activated sludge process (CAS) followed by decantation as a clarification step, which has been optimized over more than 100 years and membrane bioreactor (MBR), which is the combination of a AS biological treatment and a membrane-assisted separation for clarification [START_REF] Muralikrishna | Chapter Twelve -Wastewater Treatment Technologies[END_REF]. Besides these two, biofiltration is another process often used in secondary treatment, which makes possible the treatment of organic matter by passing through biologically activated filters, but this process will not be part of this report [START_REF] Reungoat | Biofiltration of wastewater treatment plant effluent: Effective removal of pharmaceuticals and personal care products and reduction of toxicity[END_REF].

The solid fractions from the above-mentioned methods are separated in a clarification step and undergo other treatments, i.e. digestion or incineration, in order to reduce putrescibility and pathogenic contents before their ultimate disposal, which is usually fertilization. Finally, in some wastewater treatment plants, there is also a tertiary treatment step to increase water quality by reducing micropollutants contents. These are advanced physicochemical processes, such as, adsorption on activated carbon and ozonation [START_REF] Muralikrishna | Chapter Twelve -Wastewater Treatment Technologies[END_REF] or filtration processes, such as nanofiltration.

An overview of a simplified wastewater treatment plant is shown in Figure 1.1.

This chapter presents a bibliographic synthesis of the biological treatment in wastewater treatment plants (section 2), including MBR, followed by a focus on the membrane technology involved in this process. Afterwards, the major drawback of this process, i.e. fouling, is presented in section 3 and the main strategies to control this phenomenon are discussed in section 4. Finally, since these operating conditions cause changes in membrane properties, membrane ageing is discussed in the following section (section 5), with a further look into PVDF membranes ageing.

WASTEWATER TREATMENT

CONVENTIONAL ACTIVATED SLUDGE PROCESS (CAS)

The CAS was first developed into a full-scale plant for Manchester's sewage treatment (UK) in the beginning of the 20 th century. Since then, this technology is the most widely used in wastewater treatment, because of its low energetic and capital costs, besides high efficacy in removing carbon-based pollutants [START_REF] Krzeminski | Membrane bioreactors -A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects[END_REF][START_REF] Van Bentem | MBR Varsseveld: 5 years of operational experience[END_REF]. This process consists of an aerated biological basin, where oxygen gas is transferred into mixed liquor (composed of raw or primary treated wastewater and microbial organisms) and metabolic processes assure the stabilization of organic matter by microorganisms. Then, a secondary clarifier, where solids (biomass) are separated out from treated water and recirculated to the biological reactor in order to keep biomass concentration at levels that ensure treatment.

In the biological basin, aeration is the key parameter, because it provides oxygen to microorganisms, but also induces mixing which allows maintaining solids in suspension and improves the contact between organic matter and microorganisms. Moreover, the sludge's flocculation properties are also a key point, because it ensures the separation of the sludge and treated water. However, this property can be easily shifted by the variability in influent's content and operating parameters [START_REF] Seyhi | État actuel des connaissances des procédés de bioréacteur à membrane pour le traitement et la réutilisation des eaux usées industrielles et urbaines[END_REF]. Some of the main parameters in CAS process are listed in the Table 1.1 along with their characteristic values.

MEMBRANE BIOREACTORS (MBR)

MBR technology dates from late 1960s, when microfiltration membranes became commercially available. This process was introduced by the Japanese enterprise Dorr-Oliver Inc. and it was composed by a biological basin with a crossflow filtration unit operating in tangential mode with flat sheet membranes [START_REF] Le-Clech | Fouling in membrane bioreactors used in wastewater treatment[END_REF]. At this stage, the technology struggled to find viable applications due to energetic costs related to high crossflow velocities, aeration and loss of performance as membranes fouled. The first breakthrough came in 1989, when Yamamoto and colleagues immerged membranes into the biological basin, where the aeration had the double role of transfer oxygen to microorganisms and control fouling by increasing shear stress nearby membranes, thus decreasing costs [START_REF] Stricot | Bioréacteurs à membranes à configuration externe : influence de la configuration du procédé sur la structuration des matrices biologiques et le colmatage des membranes[END_REF]. Although important advances were achieved, it was only in 1998 that the first full-scale MBR was installed for domestic wastewater treatment in Portland, UK, capable to treat up to 1,900 m 3 /day [START_REF] Le-Clech | Membrane bioreactors and their uses in wastewater treatments[END_REF].

In the recent years, as a result of improvements in fouling control strategies, optimization in operating conditions and new membrane materials, treatment capacity of super-large MBRs (>100,000 m 3 /day) has reached 6.0 million m 3 /day and its market value has been exponentially increasing as can be seen in Figure 1.2 [START_REF] Krzeminski | Membrane bioreactors -A review on recent developments in energy reduction, fouling control, novel configurations, LCA and market prospects[END_REF][START_REF] Xiao | Current state and challenges of full-scale membrane bioreactor applications: A critical review[END_REF]. In most of the current MBRs, membranes are immerged in a dedicated tank, after the biological basin. This strategy enables to separate aerations and optimize it for purposes of the aerobic tank and for membrane fouling in each tank. Currently, this technology is preferred over CAS process in many cases. There are several advantages associated to MBRs, including better quality of effluents, limited sludge production and easy automation. As membranes replace clarifiers from CAS, the overall treatment footprint is considerably decreased. Indeed, with the increasing tendency in land prices, MBR technology is favored as it is a more compact technology. The use of membranes also enables better treatment performances as a result of the physical separation of the suspended solids from the effluent and the enhanced biological process due to the higher concentration of microorganisms and related sludge retention time (SRT), increasing metabolic activity [START_REF] Le-Clech | Membrane bioreactors and their uses in wastewater treatments[END_REF]. Although MBR presents better performance, capital expenses, membrane replacement and energy consumption, mainly associated to the efforts in fouling control and oxygen transport, are still the major drawback of these systems. Comparing to CAS, energy costs remains up to 30% higher in MBR applied to municipal wastewater treatment plants, but studies have presented promising results in reducing this gap [START_REF] Van Bentem | MBR Varsseveld: 5 years of operational experience[END_REF].

BIOLOGICAL TREATMENT STAGE

MBR application is finding its way mainly on urban wastewater treatment as it efficiently treats ordinary pollutants, such as carbon, nitrogen and phosphor based compounds, it has an effect on less biodegradable compounds, such as endocrine disruptor chemicals, and it allows complete disinfection from bacteria and partially from viruses. The biological stages applied in large scale installations are generally composed by anoxic, anaerobic and aerobic zones [START_REF] Xiao | Current state and challenges of full-scale membrane bioreactor applications: A critical review[END_REF].

ORGANIC MATTER OXIDATION

Under aerated conditions, microorganisms and pollution compose the activated sludge. These microorganisms oxidize carbon-based compounds in wastewater decreasing its biochemical oxygen demand (BOD), this reaction is generically represented in Eq. 1.

𝐶 𝑥 𝐻 𝑦 𝑂 𝑧 + 𝑂 2 → 𝐶𝑂 2 + 𝐻 2 𝑂 Eq. 1
Air is usually injected by diffusers. They are provided at the bottom of the tank, which also enable mixing of activated sludge. Air supply may be tapered along the length of the reactor in the case of a plug flow tank, reducing oxygen supply to match the quantity of oxygen demand.

For this process, oxygen transfer from air to mixed liquor should be optimized for better treatment, in this case fine bubbles are preferred. Usually air is supplied in the order of 6 m 3 air/m 3 of sewage [START_REF] Scholz | Chapter 15 -Activated Sludge Processes[END_REF].

TREATMENT OF NITROGEN-BASED COMPOUND

Nitrification also takes place in the aerated tank, which is the oxidation of ammonia to nitrite (Eq. 2) by proteobacteria, such as Nitrosomonas, and the further oxidation of nitrite to nitrate (Eq. 3) by genera Nitrobacter and Nitrococcus, for example. The later reaction yields much less energy than the first, but both rely on slow growth bacteria which is suitable in MBRs due to the possibility of operating in longer solid retention time (SRT > 12 days).

2 𝑁𝐻 4 + + 3 𝑂 2 → 2 𝑁𝑂 2 -+ 4𝐻 + + 𝐻 2 𝑂 Eq. 2 2 𝑁𝑂 2 -+ 𝑂 2 → 𝑁𝑂 3 - Eq. 3
The nitrates produced in the aerobic zone will be consumed under anoxic conditions. When sludge is under anoxic conditions (absence of molecular oxygen -O2), microorganisms use nitrate as electron acceptor by the action of enzymes such as, nitrate reductase, forming nitrogen gas or nitrous oxides (NO and N2O). As a result, the denitrification process, described in Eq. 4, reduces concentrations of nitrogen based compounds in wastewater [START_REF] Fenchel | Chapter 1 -Bacterial Metabolism[END_REF].

4 𝑁𝑂 3 -+ 5 𝐶 𝑥 𝐻 𝑦 𝑂 𝑧 + 4 𝐻 + → 2 𝑁 2 + 5 𝐶𝑂 2 + 7 𝐻 2 𝑂 Eq. 4

TREATMENT OF PHOSPHORUS-BASED COMPOUNDS

Phosphorus-based compound are mostly reduced in the primary treatment step, by coagulation with addition of metallic salts, such as ferric chloride (FeCl3) or aluminum sulfate (Al2(SO4)3).

However, in the secondary treatment, biological phosphorus reduction can be achieved in the anaerobic and aerated zones.

In anaerobic conditions, low weight fatty acids and sulfate are transformed into more simple substances, such as sulfite-amide and carbon dioxide, by sulfate-reducing bacteria. In low weight fatty acid metabolism, bacteria produce poly-β-hydroxybutyrate (PHB) using polyphosphate reserves within the cells, this releases then orthophosphates in the media, initially increasing phosphate concentration in solution to be further consumed in aerated zones [START_REF] Fenchel | Chapter 1 -Bacterial Metabolism[END_REF].

In the aerated zone, PHB is used for bacteria growth and carbon dioxide and water formation.

These reactions release excess energy and as a result bacteria consume great amounts of orthophosphate reducing its concentration in wastewater [START_REF] Le-Clech | Membrane bioreactors and their uses in wastewater treatments[END_REF][START_REF] Fenchel | Chapter 1 -Bacterial Metabolism[END_REF]. Anaerobic and aerated zones are dimensioned to yield higher consumption of phosphorus in aerated zone than its formation in anaerobic zone. Phosphorus elimination may be completed by the addition of metallic salts.

MBR X CAS: OPERATING CONDITIONS OF BIOLOGICAL TANKS

There are important operating conditions that ensure the efficient performance of these reactions. The HRT for MBRs range from 4.0 -8.0 h, and differently from CAS, this parameter is not dependent on SRT due to the membranes, which are able to keep sludge within bioreactor for more important time (10 -80 days). This enables operation at high MLSS (4.0 -10.0 g/L), which reduce reactor volume making installations more compact and less sludge waste is produced. At high MLSS concentration, the ratio food to microorganism (F/M) may be as low as 0.05 -0.15 kgBOD5.kgMVLSS -1 .d -1 , presenting efficient treatment of diluted municipal wastewater. Dissolved oxygen concentration in MBRs is also affected by the high MLSS concentration. This parameter is fundamental for carbon and nitrogen based compounds removal and it should be maintained at the minimum value of 2.0 mg/L, in order to insure aerobic conditions. However, to keep this concentration of dissolved oxygen, oxygen transfer from gas to liquid must be induced. When MLSS concentration increases, this transfer is limited by viscous and steric effect. To quantify this effect, the ratio between the rate of oxygen transfer in clean water and in mixed liquor (α-factor) is usually adopted. It is conventionally assumed that α-factor of 0.3-0.7 is acceptable for MBRs, depending on MLSS concentration and SRT [START_REF] Le-Clech | Membrane bioreactors and their uses in wastewater treatments[END_REF].

CHAPTER 1: BIBLIOGRAPHIC STUDY

35

Regarding physicochemical properties of pollutants, temperature and pH present strong effects on viscosity, hydrophobicity and solubility, which can affect the performance of biological treatment, thus they must be controlled. Although temperature usually varies with weather conditions, pH remains suitable for management and it ranges from 7.0 -8.0 in full-scale wastewater treatment plants [START_REF] Seyhi | État actuel des connaissances des procédés de bioréacteur à membrane pour le traitement et la réutilisation des eaux usées industrielles et urbaines[END_REF].

MEMBRANE FILTRATION STAGE

After biochemical treatment, treated water is separated from sludge by filtration.

The membranes used in this specific application are placed mainly according to two configurations, namely side-stream and submerged. In side-stream MBRs (Figure the reactor, as a result there is no need for cross-flow pumps, however, conditions around membranes cannot be separately controlled from the biological basin, thus aeration has a double role of transferring oxygen to biomass and inducing shear stress along membranes to prevent from fouling [START_REF] Sadr | 14 -Membrane technologies for municipal wastewater treatment[END_REF]. The configuration presented in Figure 1.3.C enables to separate the membranes in a dedicated tank. Mixed liquor is pumped in a membrane tank and usually permeate is produced by a suction pump. This configuration enables to separate aeration in the biological tank with fine bubbles for optimal oxygen transfer and coarse bubbles in the membrane tank for fouling control, in addition to air flowrate. with either microfiltration or ultrafiltration membranes, so their pore size ranges from 0.01 to 0.5 µm and they are capable to reject sludge flocs, protozoa, bacteria and some viruses [START_REF] Sadr | 14 -Membrane technologies for municipal wastewater treatment[END_REF][START_REF] Deowan | 5 -Membrane bioreactors for water treatment[END_REF].

MEMBRANE MATERIAL FOR MBR APPLICATION

There are several materials that are suitable to compose membranes in MBRs, i.e. ceramic and polymers; however polymers are mostly applied as they are cheaper. Cellulose acetate, polysulfone (PSU) and polyethersulfone (PES) are the main hydrophilic polymers found in these membranes and polypropylene (PP) and polyvinylidene fluoride (PVDF) are the hydrophobic ones. The main drawbacks of these polymeric membranes include sensitivity to pH, oxidants and high temperature ranges and some of them present low mechanical resistance.

To overcome the later, more resistant materials are often used as a matrix to support the selective part of membranes. In WWT, polyethylene terephthalate (PET) and polyamides (PA) are examples of those materials. Besides, hydrophilic agents are blended to membranes to decrease interactions between mixed liquor and membranes, avoiding severe fouling [START_REF] Deowan | 5 -Membrane bioreactors for water treatment[END_REF][START_REF] Benavente | Low fouling membranes for water and bio tech applications[END_REF].

The membrane material for a given application is chosen based on several factors, such as feed stream properties, cleaning protocols and scale of process. PVDF is usually the polymer that matches the majority of these parameters and as a result it is the market leader in MBRs applications [START_REF] Puspitasari | Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane[END_REF]. PVDF presents high thermal stability, chemical resistance, especially against halogens, oxidants, inorganic acids and aliphatic, aromatic and chlorinated solvents. Its 35% to 70% and presents four semi crystalline phases, namely, α, β, γ and δ [START_REF] Benavente | Low fouling membranes for water and bio tech applications[END_REF]. PVDF-based membranes usually undergo hydrophilic treatments to enhance their performance in WWT by blending, co-polymerization, surface-grafting and surface coating, for example. Blending is one of the most applied method and polyethylene glycol (PEG), polyethyleneimine, polyvinylpyrrolidone (PVP) and tannin are widely used as hydrophilic agents in these type of membranes [START_REF] Liu | Surface hydrophilic modification of PVDF membranes by trace amounts of tannin and polyethyleneimine[END_REF][START_REF] Lv | Improvement of antifouling performances for modified PVDF ultrafiltration membrane with hydrophilic cellulose nanocrystal[END_REF]. In that case, even if the hydrophilic agent is soluble in water, which is the case for PVP, the additive will be entrapped in the membrane structure and only a small part present on the extreme surface will be washed away from the membrane structure by single water contact.

FOULING

THEORETICAL ASPECTS OF FILTRATION

In filtration process of pure solvents, permeate flux density (J0, expressed in m.s -1 ) is defined by Darcy's law (Eq. 5), which describes the flow of a fluid through a porous medium.

𝐽 0 = 𝑄 𝑃 𝑆 = 𝐿 𝑃 µ 𝑇 ∆𝑃 = ∆𝑃 µ 𝑇 𝑅 𝑚 Eq. 5
Where, 𝑄 𝑃 is the permeate flux (m 3 .s -1 ), S is the membrane surface (m 2 ), 𝐿 𝑃 is the hydraulic permeability of the membrane (m), ∆𝑃 is the transmembrane pressure (TMP, Pa), which is the difference between the average of feed and retentate pressures and permeate pressure, 𝜇 𝑇 is the dynamic viscosity of permeate (in that case pure solvent, Pa.s) at temperature T and 𝑅 𝑚 is the hydraulic resistance of the membrane (m -1 ).

When the parameters in Eq.5 are used outside the international system of the units, the permeability of a membrane is expressed in L.h -1 .m -2 .bar -1 and corresponds to the ratio LP/µT.

When fouling takes place, the fouling resistance (𝑅 𝑓 ) is added to the membrane resistance. As a result, an increase in TMP is observed in MBRs operating at constant flux or a decrease in flux for a constant TMP operation. In that last case the permeate flux (J) can be estimated by Eq. 6.

𝐽 =

∆𝑃 𝜇 𝑇 (𝑅 𝑚 +𝑅 𝑓 )

Eq. 6

Indeed, when filtering different species, selectivity is another important parameter to characterize the process. The retention rate of a determined specie defines membrane selectivity and can be calculated by Eq. 7.

𝑅𝑒𝑡𝑒𝑛𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 = 1 -𝐶 𝑃 𝐶 0 Eq. 7

Where:

FOULING MECHANISMS

The fouling resistance results from interactions of mixed liquor with membrane surface, following four different mechanisms, i.e. organic adsorption, mineral precipitation, pore blocking and biofouling.

Fouling process is initiated by the adsorption of organic foulants on membrane surface and inside its pores due to physical-chemical interactions. These interactions may be Van der Waals-based and electrostatic-based, but are mainly hydrogen bonds-based in MBRs. The attractive and repulsive effect of these forces create an energy barrier that foulants must overcome to adsorb on the membranes. According to [START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF], this energy barrier is higher than the one associated to floc-to-floc interaction, which explains the first slow conditioning period of membrane surface, followed by the rapid fouling phenomena in MBRs.

These interactions are affected by several parameters, including floc size, surface charge, and surface roughness [START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF].

Besides adsorption, inorganic fouling can occur by crystallization, when salts, such as CaSO4 and CaCO3, are rejected by the membrane, increasing their concentration on the surface. This accumulation in the vicinity of the membrane leads to a decrease in flux due to higher osmotic pressure, allowing the nucleation and subsequently precipitation of minerals on membrane surface [START_REF] Zhang | A Review on Flux Decline Control Strategies in Pressure-Driven Membrane Processes[END_REF].

Four different models , including complete, standard, intermediate blocking and cake filtration (Figure 1.6), can explain pore blocking. For complete blocking, each particle arriving on membrane surface blocks a pore without overlapping others. The standard blocking considers that particles will be deposit within pores decreasing the pores volume. For cake filtration, it is assumed that particles do not block pores, they form a compressible layer on membrane surface which has its own porosity, acting as a second membrane. Lastly, intermediate blocking considers that particles either deposit over others or completely block a pore, integrating principles from cake filtration and complete blocking mechanisms [START_REF] Guo | A mini-review on membrane fouling[END_REF][START_REF] Wang | Pore blocking mechanisms during early stages of membrane fouling by colloids[END_REF]. Finally, biofouling occurs when membrane surface is conditioned by macromolecules, microorganisms easily attach onto it by bioadhesion and bioadsorption to form microcolonies and ultimately develop mature biofilms. Biofilms consist in layers of substrate, living and dead microorganisms and extracellular products. They strongly affect productivity of MBRs in WWT [START_REF] Guo | A mini-review on membrane fouling[END_REF]. The formation of biofilms is coordinated through a signaling system known as quorum sensing (QS). Microorganisms produce signal molecules, e.g. N-acyl homoserine lactones (AHLs), autoinducer peptides (AIPS) and autoinducer-2 (AI-2), which freely diffuse through cell membranes and dictate production of specific compounds, formation of biofilms, surface motility and interspecies competition. On one hand, AHLs are mostly produced for the communication of Gram-negative bacteria. On the other hand, AIPS are used by Gram-positive bacteria. AI-2 is considered an inter-species signal molecule used by both [START_REF] Lee | Quorum sensing and quenching in membrane bioreactors: Opportunities and challenges for biofouling control[END_REF].

Complete Standard Intermediate Cake filtration

Moreover, the higher solute/particulate concentration nearby membrane creates a gradient with the bulk solution, leading to a back diffusion into the bulk, which may trigger the formation of a gel layer and precipitation on membrane surface. This phenomenon, known as concentration polarization, is inherent in membrane process and can be described as the film theory [START_REF] Zhang | A Review on Flux Decline Control Strategies in Pressure-Driven Membrane Processes[END_REF][START_REF] Guo | A mini-review on membrane fouling[END_REF][START_REF] Hernandez Rojas | Bioréacteur à membranes immergées pour le traitement d'eaux usées domestiques : influence des conditions biologiques sur les performances du procédé[END_REF].

FOULANT SPECIES IN MBR

The different components of mixed liquor are responsible for these four fouling mechanisms.

The main structures that play a major role in fouling include extracellular polymeric substances (EPS), soluble microbial products (SMP), colloids and sludge flocs. These components can greatly affect permeability in MBRs in different ways that will be further discussed.

Most microorganisms on Earth live in aggregated forms within a matrix of biosynthetic polymers of biological origin, namely EPS. For a long time, they were called extracellular polysaccharides, as this is their main component, however a broader definition was established to consider the different classes of organic macromolecules, i.e. proteins, lipids, nucleic acids, humic acids, exocellular enzymes, structural components of cells and metabolites. Besides adhesion of cells in flocs and biofilms, EPS protect the cell against toxins and accumulate substrates [START_REF] Kunacheva | Analytical methods for soluble microbial products (SMP) and extracellular polymers (ECP) in wastewater treatment systems: A review[END_REF]. These biopolymers exhibit complex linkage, hydrogen bonds and cation bridging interactions, and several functional groups, such as carboxyl, hydroxyl, phenolic, sulfhydryl and phosphoric groups. These groups assign a negative charge density at neutral pH, approximately, and contribute to aggregate formation, affecting therefore, hydrophobicity, adhesion, flocculation and dewatering properties of sludge. As a result EPS are highly involved in the membrane fouling process [START_REF] Stricot | Bioréacteurs à membranes à configuration externe : influence de la configuration du procédé sur la structuration des matrices biologiques et le colmatage des membranes[END_REF][START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF][START_REF] Guo | A mini-review on membrane fouling[END_REF].

As mentioned before, EPS are mainly composed by polysaccharides and ribose, mannose, glucose and uronic acids are their predominant monomeric units [START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF]. Due to their high molecular weight (>100 kDa) and low biodegradability, polysaccharides are more likely rejected in filtration and their gelling properties enable the formation of impermeable gel layer on membranes, that can increase filtration resistance and act as a support and substrate for microorganisms developing a biofilm [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF].

Although proteins appear in lower content than sugars, it is also considered that they affect membrane fouling. Among the amino acids most abundant in the matrix, aspartic acid, glutamic acid, tyrosine acid and cysteine acid present negatively charged side chain responsible for major changes in EPS's hydrophobicity, surface charge and adhesion forces, thus affecting fouling as well [START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF].

Sludge flocs and EPS compose the solid fraction in mixed liquor. SMP are soluble products from cell metabolism, mainly soluble colloidal carbohydrates, humic acids and proteins, which freely diffuse in supernatant. They can be determined as a function of the COD in the soluble fraction of the mixed liquor [START_REF] Mailler | Longterm performances and membrane lifespan of full-scale MBR treating filtrate from sludge ultra-dewatering[END_REF]. [START_REF] Shen | Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China[END_REF] collected data of SMP from 10 full-scale MBRs and found concentrations of polysaccharides from 3-18 mg/L, humic substances from 2-10 mg/L and proteins at less than 5 mg/L with a broad molecular weight distribution from less than 1 kDa to greater than 100 kDa [START_REF] Shen | Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China[END_REF]. As SMP and EPS consist roughly of similar macromolecules, some authors found relations between them. Laspidou et al. [START_REF] Laspidou | A unified theory for extracellular polymeric substances, soluble microbial products, and active and inert biomass[END_REF] inferred that EPS can be synthesized using SMP as electron-donor or simply by adsorption on sludge flocs, but can also be hydrolyzed to form SMP. These interrelations are represented in Regarding operating conditions affecting these fouling species, although it is believed that SMP is one of the main source of membrane fouling, only the SMP concentration does not seem to correlate with fouling propensity in MBRs [START_REF] Drews | Membrane fouling in membrane bioreactors-Characterisation, contradictions, cause and cures[END_REF]. However, some indirect relations have been found and were assembled in Drew's review [START_REF] Drews | Membrane fouling in membrane bioreactors-Characterisation, contradictions, cause and cures[END_REF]. The main parameter influencing SMP concentration is SRT, these concentrations tend to drop with higher SRT, and as a result, fouling propensity decreases. Dissolved oxygen is another important parameter for SMP concentration.

At limiting oxygen conditions, SMP elimination is not properly performed, hence cell hydrophobicity decreases, thereby promoting deflocculation and fouling. [START_REF] Shen | Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China[END_REF] also

found important relationships between fouling potential and some fractions of SMP, such as TOC, polysaccharides and humic acids contents [START_REF] Shen | Characterization of soluble microbial products in 10 large-scale membrane bioreactors for municipal wastewater treatment in China[END_REF]. TOC showed high impact on initial fouling, which consists of both adsorption and pore blocking. Indeed, studies suggest that SMP can increase the fouling propensity of EPS by conditioning membrane surface to the subsequent adhesion of EPS. When supernatant presented 0.03 g/L of SMP, cake resistance increased 8 times in the filtration of 13 g/L sludge floc solution [START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF].

The formation of the cake layer in MBRs operated at constant flux is a dynamic process decomposed in three stages according to the TMP behavior as presented in increase, TMP builds steadily at a lower rate in the second stage with further adsorption of soluble species and biomass deposition. With the blocking of some regions on the membrane surface, flux rises in specific areas aside with fouling rate promoting a "jump" in TMP. After this stage, EPS were found to be the main foulants in the cake [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF][START_REF] Gkotsis | Fouling Issues in Membrane Bioreactors (MBRs) for Wastewater Treatment: Major Mechanisms[END_REF].

Figure 1.8: Cake formation on membrane surface and its effects on TMP over time. Figure from [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF].

Therefore, it seems that firstly, SMP species interact with membrane surface and pore walls, and then the insoluble species, such as EPS and sludge flocs, build up the cake layer and increase pressure as a result. Currently, efforts are still needed into identifying the main causes of fouling and on developing strategies to better control it in order to reduce energetic costs of MBRs. Regarding these strategies, fouling can be classified as reversible, when the fouling layer can be removed by pressure release or specific physical cleaning, and as irreversible, otherwise [START_REF] Zhang | A Review on Flux Decline Control Strategies in Pressure-Driven Membrane Processes[END_REF].

FOULING CONTROL

Fouling is the major challenge in MBRs because it leads to a decline on productivity and a rise on energy consumption, thus on operating costs. That is the reason why researches have focused on antifouling strategies in the recent years [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF]. As a result, the progress achieved with these strategies have gradually reduced energy consumption. In China, MBRs commissioned from 2006 to 2014 presented energy consumption of about 0.5-0.7 kWh/m 3 , against 0.35-0.65 kWh/m 3 for MBRs built thereafter [START_REF] Xiao | Current state and challenges of full-scale membrane bioreactor applications: A critical review[END_REF].

There are several approaches to prevent extensive clogging and fouling of membranes. Table 1.2 summarizes these techniques and principle involved in each one. Some of them are widely applied on full-scale operation and others are still been developed, this is indicated in Table 1.2.

These strategies are further discussed on the following sections.

MECHANICAL APPROACH

Among the mechanical approaches, aeration, relaxation and backwashing are the most applied on full-scale MBRs. They contribute to the turbulence on membrane surface, which limits cake formation. Aeration can be performed in continuous at a fixed air flowrate, but it was observed that same results to limit cake formation could be achieved with cyclic aeration with 10s/10s intervals of aeration on and off achieving up to 50% of saving related to aeration consumption [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF]. Besides air flowrate is beneficial for permeability of membranes up to an optimal value, higher values may damage sludge flocs leading to EPS and colloids release, being detrimental for permeability [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF].

Along with aeration, operating MBRs in intermittent cycles of filtration and relaxation, i.e.

when permeate flowrate is set to zero, have shown great results to overcome fouling. Relaxation could decrease by half the fouling rate [START_REF] Habib | Influence of relaxation modes on membrane fouling in submerged membrane bioreactor for domestic wastewater treatment[END_REF]. The same study showed that relaxation time is a key parameter: cycles of 10 min of filtration and 1.5 min of relaxation presented the lowest fouling rates, whereas applying 2 min of relaxation increased fouling rate as high as operating with no relaxation.

Since aeration and relaxation cannot deal with internal fouling, backwashing is also employed in the majority of wastewater treatment plants with the purpose of reducing internal fouling. It can be achieved by pushing clean water or permeate on the other sense of filtration and it is efficient on removing deposited layer, reducing gel polarization layer and preventing compaction of gel layer. In a pilot-scale study, using cycles of 250 s filtration/50 s relaxation and 30 s of backwashing each ten cycles, membranes operated for 2 years kept permeability above 100.0 L.h -1 .m -2 .bar -1 filtering up to 25 g/L of suspended solids at fluxes between 9 and 13.3 L.h -1 .m -2 and air scouring at 0.3 Nm 3 /(h.m 2 ) [START_REF] Robles | Factors that affect the permeability of commercial hollow-fibre membranes in a submerged anaerobic MBR (HF-SAnMBR) system[END_REF]. Indeed, the flowrate (10-25 L.h -1 .m -2 )

and duration (10 -30 s) of backwash was verified and authors concluded that higher intensities were more effective than long duration [START_REF] Zsirai | Efficacy of relaxation, backflushing, chemical cleaning and clogging removal for an immersed hollow fibre membrane bioreactor[END_REF].

CHEMICAL CLEANING

The use of chemical reagents is also a widely spread practice on MBRs. Chemical cleaning of fouled membranes follows six main steps. When reagents are injected, they can (i) react in the bulk solution, followed by their (ii) transportation to the interface between bulk solution and fouling layer. Cleaning reagents (iii) penetrate into fouling layer, (iv) the oxidization of foulants and/or (v) their solubilization and detachment take place. After that, oxidized/detached foulants and the waste cleaning agents are (vi) transported back to the interface and (vii) finally to the bulk solution [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF].

Several species are employed as chemical cleaning agents of fouled membranes including acids, bases, oxidants and surfactants. In traditional cleaning protocols, each of those solutions can be Among the acids, oxalic, citric, nitric, hydrochloric and phosphoric acids are the most used and they are effective in removal of mineral fouling by neutralization and double decomposition reactions with the precipitated salts on membrane surface. On one hand, mineral acids are low cost and effective on foulants removal but they can damage membranes and easily change pH of medium, affecting microbial activity in the activated sludge. On the other hand, oxalic and citric acids have buffering properties decreasing pH shock and membrane damage and they are able to remove metal cations by forming complexes with them [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF].

Bases, such as sodium and potassium hydroxides, are preferred to remove organic fouling. They are applied at pH ranging from 11.0 to 12.0 to hydrolyze and solubilize proteins and carbohydrates. They also react with fats and oils through saponification forming water-soluble micelles. These reagents present high permeability recovery in a short period of cleaning [START_REF] He | Chemical Cleaning of Membranes in a Long-Term Operated Full-Scale MBR for Restaurant Wastewater Treatment[END_REF][START_REF] Levitsky | Cleaning UF membranes with simple and formulated solutions[END_REF].

Organic fouling can also be removed by oxidants. These agents oxidize functional groups of foulants to ketone, aldehyde and carboxylic groups, making them more hydrophilic, thus less attracted to membrane surface. Besides oxidation, they also disinfect membranes from biological fouling [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF]. Sodium hypochlorite and hydrogen peroxide are examples of oxidants used in cleaning protocols. At full-scale, sodium hypochlorite is preferred due to its efficiency and low cost. The study of [START_REF] Brepols | Strategies for chemical cleaning in large scale membrane bioreactors[END_REF] showed that at lab-scale, both oxidants presented same cleaning efficiency, but when applied to full-scale, hydrogen peroxide was not able to maintain permeability over a period of several weeks even at concentrations as high as 2000 ppm [START_REF] Brepols | Strategies for chemical cleaning in large scale membrane bioreactors[END_REF]. Moreover, when compared to NaOH, NaOCl was more efficient in cleaning a membrane fouled by a solution of 0.3 g/L bovine serum albumin (BSA) [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF]. Surfactants are other class of chemicals often used in MBRs to clean organic foulants, mainly from specific effluents rich in fats, oils and proteins. These amphiphilic compounds reduce surface tension between liquids and solids and are able to solubilize macromolecules. Their hydrophobic tail adsorbs organic foulants, while their hydrophilic moiety interferes with the ion bridges [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF]. [START_REF] Levitsky | Cleaning UF membranes with simple and formulated solutions[END_REF] found that Tween-20 presented the highest cleaning efficiency on polyether sulfone (PES) membranes and when filtering a solution of BSA, membranes previously cleaned by the surfactant presented less protein adsorbed than the ones cleaned by NaOCl [START_REF] Levitsky | Cleaning UF membranes with simple and formulated solutions[END_REF]. However, a cocktail of both was the most effective cleaning solution.

Electrostatic repulsions and surfactant hydrophobicity help to loosen bonds between proteins and membrane surface. As a result, mass transfer of oxidants through the fouling layer is facilitated and the detached foulants are enclosed in surfactant micelles, avoiding repeatable attachment.

Recently, laboratory studies have demonstrated the efficiency of some emerging chemicals as cleaning reagents of fouled membranes. Rhamnolipids are a class of low cost and low toxicity surfactants that have been successfully applied for fouling prevention and cleaning with high degree of biofilm detachment. They enhance surface hydrophobicity of foulants by release of lipopolysaccharides (LPS) and EPS, mitigating fouling on hydrophilic membranes. However, CHAPTER 1: BIBLIOGRAPHIC STUDY 53 in long-term operation, membranes are expected to become more hydrophobic and the performance of these reagents has never been accessed [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF].

Another promising chemical is free nitrous acid (FNA), which is formed at low pH nitrite solutions, and is able to inactivate microorganisms responsible for biofouling by damaging lipids, proteins and deoxyribonucleic acid (DNA). It has been studied as cleaning reagent since 2015 and has shown to be more effective than NaOH on cleaning of heavily and moderately fouled membranes and could also descale membrane surfaces. Some of the drawbacks of this compound that must be overcame include possible membrane damage at very low pH (~2.0) and possible increase of NO2 concentration in effluent impairing treatment performance [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF].

Next to their performance in cleaning protocols, most of these chemical agents have been reported to change membrane properties, such as, mechanical strength, hydraulic resistance, pore size and hydrophilicity. Moreover, they negatively affect microbial process in MBRs by inactivating important enzymes for organic matter biodegradation, nitrification and denitrification, increasing EPS production or producing toxic intermediates. With these changes, interactions between foulants and aged membranes may differ, resulting in different cleaning efficiency for the same protocols over time. For this reason, researchers are still developing other methods to avoid consume of harmful chemicals.

ALTERNATIVE METHODS

The development of antifouling membranes has emerged in literature as a promising way to avoid all those down times from physical and chemical cleaning. Several hydrophilic agents, such as PEG and polyvinyl alcohol based polymers compose commercial membranes used in ultrafiltration. These agents decrease interactions between foulants and membrane surface and are very effective in the early stages of full-scale operations. Nanomaterials including silver nanoparticles, titanium and zinc oxides and all carbon-based nanomaterials have been tested as CHAPTER 1: BIBLIOGRAPHIC STUDY 54 components of ultrafiltration membranes. These species present photocatalytic activity, antimicrobial ability and hydrophilic characteristics that can act in synergy to alleviate membrane fouling. These membranes can be classified as active or passive approaches for fouling control. For example, silver nanoparticles and graphene-based nanomaterials can actively affect microbial cells undermining cell growth on membrane surface, whereas titanium dioxide nanoparticles passively retard bacterial adhesion and growth by the development of a thin layer of water on membrane surface, due to hydrophilic interactions [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF][START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF].

Besides changing membrane properties to reduce interaction with foulants, researchers have proposed to reduce the amount of foulants that reaches membranes by cell immobilization. On one hand, biofilm carriers, previously discussed, are a passive way to immobilize cells onto a surface to avoid contact between foulants and membrane surface. On the other hand, cell entrapment, i.e. immobilized inside a porous polymer matrix, enables to prevent fouling and protects cells from toxins, prolongs SRT and has a simple design for MBR application. MBRs relying on entrapped cells are similar to a hybrid system of biofilters with membranes. The membranes enable overcoming the removal of pathogens and large particles, one of the major drawbacks of process based only on entrapped cells. Agar, alginate, Carrageenan as well as polyvinyl alcohol and xanthan gum have been introduced as matrix. The successful application of these materials depends on their durability and size, cell to matrix ratio and cell viability [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF].

Quorum quenching (QQ) is another strategy to mitigate fouling. It consists on avoiding the intercellular communication among bacteria (Quorum sensing). Bacteria produce autoinducers in proportion to the cell density that are identified by receptor proteins. When a threshold level is reached, the transcription of specific genes is activated and actions such as secretion of EPS and biofilm formation are promoted. AI-2 and AHL are two important autoinducers targeted by QQ. AI-2 is considered a universal autoinducer secreted by both Gram-positive and Gramnegative bacteria, but, as Gram-negative bacteria are predominant in MBRs, controlling AHL is also effective to prevent fouling [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF][START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF]. QQ is often based on either avoiding autoinducers synthesis, deactivating signal molecules or interfering transport and reception.

Among the novel approaches, electrical methods, such as electrocoagulation, electrophoresis and microbial fuel cell (MFC), have been proposed to control fouling in recent years. In electrocoagulation process, two electrodes in the MBR are powered with direct current, where the metal composing the anode is electrically dissolved forming a positively charged coagulant that neutralizes the negative organic matter, reducing its accumulation on membrane surface.

As barriers, this process exhibits high-energy costs due to electricity consumption. The electrophoresis consists in the application of a uniform electrical field to generate an electric repulsive force between foulants and membrane surface. This process share the same drawbacks as electrocoagulation, however, some membranes can be used as cathodes, such as polypyrrolebased membranes, decreasing energetic costs of this process [START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF]. Some studies have coupled MBRs with MFCs in order to generate the electrical repulsion between foulants and membranes, by microorganisms' electrochemical reactions. Besides the electrical repulsion, generation of hydrogen peroxide on the cathode can enhance the degradation of biopolymers and the detachment of foulants from membrane surface [START_REF] Meng | Fouling in membrane bioreactors: An updated review[END_REF].

All these methods to mitigate fouling coupled with the operating conditions promote changes on membrane properties over time. These changes alter the treatment productivity, increasing process costs. Therefore, the study of changes on membrane properties over time, i.e. membrane ageing, is a subject of great interest in the wastewater treatment field and will be further discussed in the next section.

MEMBRANE AGEING

The mechanical stress and chemical exposure of the strategies to control fouling may alter physical/chemical properties of the materials composing membranes with time. These changes CHAPTER 1: BIBLIOGRAPHIC STUDY 56 define the process of ageing. Membrane ageing leads to a loss of productivity, an increase of operating costs and, ultimately, the membrane replacement, which accounts for 25-40% of the total membrane plant costs. That is the reason why researchers have been focusing on this particular subject lately and publications related to ageing have been multiplied by a factor of 10 from 2000 to 2010 [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF].

It is worth noting that ageing is a comparative study between used and pristine membranes, consequently, it is impossible to quantify it without a reference. Ageing generally leads to membrane failure, but they must not be confused. Membrane failure is linked to the loss of removal efficiency and can be determined by integrity tests, such as pressure decay tests or particle monitoring [START_REF] Tng | 11 -Membrane ageing during water treatment: Mechanisms, monitoring, and control[END_REF]. It is a concept often applied to the production of potable water, which uses non-supported membranes. In wastewater treatment, productivity is the key important, therefore, it is usually prioritized over loss of permeate quality.

Since the process efficiency is closely related to ageing and failure, anticipating and scheduling activities for the membrane replacement is vital for operators of full-scale plants. Currently, the membrane lifespan is estimated based on membrane manufacturers' warranty, which can range from 3 to 10 years. However, there are cases reporting membrane lifespan of less than 6 months and others showing that membranes could withstand their operating conditions and filtration efficiency for up to 13 years [START_REF] Akhondi | The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review[END_REF]. Therefore, there is a clear need for membrane ageing assessment tools based on reliable properties that describe ageing correctly.

PROPERTIES AFFECTED BY AGEING

Soaking membranes into cleaning agent solutions is the most used method to age membranes in laboratory and to predict what could happen in full-scale operation. This way membrane ageing can be described by monitoring the behavior of physical/chemical properties and filtration performance characteristics, such as pore size distribution, tensile strength,

PHYSICAL PROPERTIES

Among the existing techniques to assess mechanical characteristics of membranes, traction tests are the most frequently applied and parameters such as ultimate elongation and ultimate tensile strength can determine the ageing process. The first quantifies the deformation of membranes at break and provides information about the elasticity of membranes, whereas the latter is the uniaxial stress that can be applied prior to fracture. Aged membranes tend to become stiffer and more brittle, however, contrastive results were already reported for PVDF membranes soaked in hypochlorite [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF][START_REF] Tng | 11 -Membrane ageing during water treatment: Mechanisms, monitoring, and control[END_REF].

The morphology of membranes are often evaluated by visualization techniques, such as atomic force microscopy (AFM), scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). AFM measures surface morphology and roughness by the force to drag a precision tip along the membrane surface [START_REF] Tng | 11 -Membrane ageing during water treatment: Mechanisms, monitoring, and control[END_REF]. In membrane ageing, this assay mainly provides comparative studies of topography, which trends to increase over time. When comparing different materials of aged membranes, this increase was more pronounced on PVDF-based than on PES-based membranes [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF]. The other two techniques (SEM and FESEM) can provide further information on pore size, thickness, symmetry and physical damage via high-resolution images [START_REF] Tng | 11 -Membrane ageing during water treatment: Mechanisms, monitoring, and control[END_REF]. For example, enlargement of the pore size of PVC hollow fibers aged with NaOCl was already found by FESEM [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF]. However, the control of imaging variables is primordial to avoid bias when applying these techniques, indeed pore distribution can be overestimated as they cannot differentiate surface pits and true pores [START_REF] Abdullah | SEM imaging of membranes: Importance of sample preparation and imaging parameters[END_REF].

Compared to these imaging techniques, bubble pressure measurement is a more reliable tool to determine pore size. It provides the size of the largest pore in a sample. This method correlates the pore size with the pressure needed for a wetting fluid (mainly water) to be displaced by another or by air. Narrow pores exhibit higher capillary forces, thus higher pressures are expected for these pores. This behavior is described in the equation of Young-Laplace (Eq. 8), which correlates the size of the largest pore (rmax) and the capillary pressure in a tube (Pb).

𝑟 𝑚𝑎𝑥 = 2 𝛾 𝑐𝑜𝑠 (𝜃) 𝑃 𝑏

Eq. 8

With:

𝛾: Water superficial tension, 𝜃: contact angle of water on membrane surface.

CHEMICAL PROPERTIES

To assess chemical properties of aged membranes, there are a wide range of techniques available that can be as simple as contact angle measurement, or highly sophisticated, such as mass spectroscopy (MS). Contact angle is widely used to determine hydrophobicity of membrane surfaces as they age. It is based on the angle between the membrane surface and the droplet of a probe liquid, generally hydrophilic. Similarly, the captive bubble method can be applied by immersing the sample in a liquid and place an air bubble on the membrane surface, this way porosity plays a less important role in the measurement. Thus, a high angle indicates a more hydrophobic surface, which is generally attributed for aged membrane to reactions between cleaning agents and membrane polymers or accumulation of irreversible foulants, as example [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF][START_REF] Causserand | Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation[END_REF]. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) is another widely used technique that evaluates chemistry surface of membranes by identification of functional groups. By this technique, many studies have concluded the PVP leach out in hydrophilic membranes or the degradation mechanism of PES membranes in contact with hypochlorite [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF].

To evaluate macromolecular changes in the main polymer of membranes during ageing, thermogravimetric analysis (TGA) and size exclusion chromatography (SEC) were already reported. TGA is based on the thermal degradation of polymers: as the temperature increases, components are removed from the membrane matrix and the material is continuously weighted.

In polymers, shorter chains are volatilized at lower temperatures, so the TGA profile of an aged membrane can highlight the predominance of cross-linking or chain scission among the polymers molecules with ageing and the degradation of co-polymers. Whereas, SEC provides detailed information about the molecular mass distribution of polymers. It consists of dissolving the membrane material into a solvent and passing through a column that separate larger molecules from smaller ones, giving a quantitative measure of polymeric chain scission and crosslinking of aged samples [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF].

Less frequently, Nuclear magnetic resonance (NMR) spectroscopy, MS, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) are employed in membrane ageing studies.

NMR and MS are techniques that are more sophisticated. They, respectively, identify chemicals and determine the molecular masses of substances present in samples. NMR is also able to quantify the species in the membrane, which gives an advantage over ATR-FTIR. DSC and XRD assess the crystallinity of polymers. The first is based on measurements of melting enthalpy by the melting temperature, and the latter allows the examination of the crystal structure by measuring angles and intensities of diffracted X-ray beam of a sample [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF].

Assessing the elemental composition of a material surface can be useful in ageing, as reactions of elimination or irreversible fouling can be evidenced. X-ray Photoelectron Spectroscopy (XPS) is an interesting tool as changes in common atoms in polymer membranes, such as C, F, O, N and S, can be observed. This method also gives information about the oxidation state of these elements, whereas, a simpler way to assess just the elemental composition of samples is the combination of SEM with energy dispersive X-ray spectroscopy (EDX) [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF][START_REF] Tng | 11 -Membrane ageing during water treatment: Mechanisms, monitoring, and control[END_REF].

FILTRATION PERFORMANCE

Membrane filtration properties, such as intrinsic permeability, fouling rate and selectivity are useful macro-scale properties to monitor membrane ageing, since they are impacted by most of the other physical-chemical properties.

Intrinsic permeability is measured by filtration of clean water, either distilled, ultra-pure or potable, at different flowrates. For instance, changes in intrinsic permeability of membranes during ageing are often related to changes in pore size and hydrophobicity. Intrinsic permeability may be measured in-situ for application such as evaluating cleaning efficiency with a standard liquid (clean water), but also to monitor membrane ageing.

Fouling rate may be determined by filtration of fouling agents and tracking the changes of permeate flux, if TMP is constant, or changes in TMP, if permeate flux constant. These changes are also related to hydrophobicity: as membranes aged, they usually become more hydrophobic, thus more susceptible for interactions with organic matter. Analysis of fouling rate can also be performed in-situ, to determine whether fouling potential of the matrix has changed in the shortterm, but also to give information about membrane ageing in the long-term.

Challenge tests are also useful tools to estimate pore size evolution by the measurement of the retention of polymeric probes, such as dextran, or of the rejection capability by filtering bacteria, such as Escherichia coli, or viruses [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF][START_REF] Pellegrin | Filtration performance and pore size distribution of hypochlorite aged PES/PVP ultrafiltration membranes[END_REF]. These are also interesting indicators of integrity loss for in-situ measurements of permeate, if the presence of these species increase overtime in permeate, it may be a sign of membrane degradation or fiber breakage.

Membrane ageing studies consist in the comparison of a number of properties between aged and pristine membranes. Changes in these properties affect each other during ageing. Thus, ageing can be demonstrated by correlating changes in elemental, molecular, microscopic and macroscopic properties. According to the literature, the direct correlation among these properties for PVDF membranes, either supported or not, is illustrated in the Figure 1.10 below.

CHAPTER 1: BIBLIOGRAPHIC STUDY 63

Several studies related filtration performance to pore size and hydrophobicity of membranes.

The changes in hydrophobicity were linked to additive or PVDF degradation, which also affected pore size. In addition, membrane mechanical resistance was mostly influenced by PVDF chain reactions. The relationship between these modifications will be further discussed in section 5.3. Membrane properties evolve differently depending on its composition, operating conditions and cleaning agents. As the most used cleaning agent is sodium hypochlorite, its impacts on membranes are the most explored in the literature. Thus, the chemistry of sodium hypochlorite in solution and its effects on membranes properties will be hereby summarized.

CHEMICAL AGEING BY SODIUM HYPOCHLORITE SOLUTION

CHEMISTRY OF SODIUM HYPOCHLORITE SOLUTION

Sodium hypochlorite can dissociate into different oxidant species depending on pH, which will determine its action on membrane properties. These species present different oxidationreduction potentials that make NaOCl generally more effective on disinfecting membranes at around pH 9.0 when it can act as a detergent at pH 11.5, for example. Typically, chlorine (Cl2), hypochlorous acid (HClO) and hypochlorite ion (ClO -), which are predominant at acid (pH<3.0), neutral (4.0<pH<7.0) and basic (pH>9.0) solutions, respectively, are the species that contribute to total chlorine concentration. However, these are not the only species involved on the ageing of polymeric membranes. As can be seen in equations 9 -11, ClO• and OH• radicals are two reactional intermediates likely to be present in solution and believed to have an effect on polymeric membranes degradation [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF].

𝐻𝑂𝐶𝑙 + 𝐶𝑙𝑂 -→ 𝐶𝑙𝑂 • + 𝐶𝑙 -+ 𝑂𝐻 •

Eq. 9

𝑂𝐻 • +𝐶𝑙𝑂 -→ 𝐶𝑙𝑂 • + 𝑂𝐻 - Eq. 10 𝐶𝑙𝑂 • +𝐶𝑙𝑂 -→ 2𝐶𝑙 -+ 𝑂 2 + 𝑂𝐻 • Eq. 11

EFFECTS OF NAOCL EXPOSURE ON DIFFERENT MEMBRANES

Some of the most used polymers in water and wastewater treatment plants, i.e. cellulosederived, polyvinylchloride (PVC), PSU, PES and PVDF based membranes, were reviewed regarding the ageing effects of sodium hypochlorite by Regula et al. (2014) [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF]. In this review, the authors observed that cellulose-based membranes, i.e. cellulose acetate (CA) and cellulose ester (CE), and PVC membranes are less used membranes for water treatments and one of the reasons is that they are highly sensitive to hypochlorite. CA and CE membranes presented PSU membranes are the most used membranes in drinking water production along with PES and PVDF membranes, but they are also applied in hemodialyzers. In both applications, they must be cleaned and bleach is the most used agent. Although these are more NaOCl resistant membranes, Regula et al. ( 2014) mentioned several studies reporting increasing permeability after NaOCl exposure for these membranes [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF]. These changes were attributed to PSU degradation, which led to chain scission and contributed for loss in mechanical properties [START_REF] Rouaix | Experimental study of the effects of hypochlorite on polysulfone membrane properties[END_REF].

In another study, [START_REF] Regula | Ageing of ultrafiltration membranes in contact with sodium hypochlorite and commercial oxidant: Experimental designs as a new ageing protocol[END_REF] showed that this increase in permeability and the decline in elongation at break were underlined as temperature, contact time and concentration increased [START_REF] Regula | Ageing of ultrafiltration membranes in contact with sodium hypochlorite and commercial oxidant: Experimental designs as a new ageing protocol[END_REF]. Decline in elongation at break reached 80% at 40 °C and pH 11.7. Moreover, they observed that the breaking force also decreased continuously, making membranes rigid and fragile, which increased chances of early breakage.

The impact of NaOCl on PES membranes is one of the most developed subjects in membrane ageing recently. Chlorine tends to increase permeability because of an increase in pore size at pH 6.0 -9.0. These changes in pore size were related to changes in PES chemical structure observed on ATR-FTIR spectra. The proposed ageing mechanism of PES membranes evolved over time [START_REF] Tsehaye | Stability of polyethersulfone membranes to oxidative agents: A review[END_REF] and actually the most recent results consider two mechanisms occurring at the

INTERACTIONS BETWEEN HYDROPHILIC AGENT AND NAOCL

Most of the selectivity skins of membranes used in water treatment processes are composed of a secondary material (additive) that is responsible to increase hydrophilicity of membrane surface in order to decrease fouling and increase permeability. In the case of hydrophilic treatment by membrane coating, this additive is present only on the membrane surface. In the case of hydrophilic agent addition by blending in membrane material, even if the additive is supposed to be present all over the membrane thickness, it is significantly detected only within the first 30 µm of the membrane thickness [START_REF] Pellegrin | Filtration performance and pore size distribution of hypochlorite aged PES/PVP ultrafiltration membranes[END_REF]. However, most of these additives are highly reactive with some of hypochlorite dissolution products and these reactions can greatly affect overall membrane properties. single soaking and showed that membrane surface was more negatively charged with ageing as a result of a slight increase in oxygen atoms due to ring opening of PVP and formation of carboxylic acid and carboxylate groups (COOH/COO -) [START_REF] Hanafi | Electrokinetic analysis of PES/PVP membranes aged by sodium hypochlorite solutions at different pH[END_REF]. With the PVP dislodgement, they expected a decline in hydrophilicity, however contact angle measurements remained constant after ageing, possibly due to the partial ionization of the membrane surface previously explained that counterbalances the loss in hydrophilic agent in the overall hydrophilicity of the surface.

These modifications contributed to the loss of permeability after filtration of a dextran solution, which evolved from 12% to 45% for pristine and 144 h aged membranes (200 ppm chlorine), respectively. Authors attributed this decline to the loss of membrane hydrophilicity, which favored its fouling, and to structural modifications of membrane.

In [START_REF] Causserand | Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation[END_REF], the authors investigated the impact of different ageing conditions and procedures for PES/PVP membranes applied to the drinking water industry in contact with NaOCl solutions. The parameters investigated were NaOCl concentration, soaking duration and the exposure mode, i.e. static by soaking, under stirring and dynamic by filtration/backwashing cycles of NaOCl solution [START_REF] Causserand | Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation[END_REF]. Similarly to others studies, the authors observed an increase in permeability, which correlated well with the PVP content decline as membranes are exposed to NaOCl. Regarding concentration and exposure duration, authors reported that NaOCl concentration contributed the most for the membrane degradation, monitored via the elongation at break and PVP degradation. This implies that, for a same exposure dose, results may vary if concentrations or exposure time (t) are different, therefore, a correction factor (n) was proposed in the calculation of the exposure dose, resulting in Eq. 12. Since exposure time contributed the least, "n" was found to be usually below 1.0. Regarding exposure mode, static or dynamic, aged membranes. This study compared PVDF membranes with and without PVP exposed to hypochlorite at several pH (6 -11.5) [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]. The authors observed PVP degradation regardless the pH of exposure. Concerning the decline in pore size, they hypothesized that this structure modification is linked to the presence of additive but the mechanism is yet to be established.

Besides, in the same study, the authors found that the presence of PVP changed the way that PVDF reacted with hypochlorite. These results will be further discussed in the next section along with ageing of PVDF-based membranes.

POLYVINYLIDENE DIFLUORIDE-BASED MEMBRANES

PVDF is the market leader for membranes applied on wastewater treatment [START_REF] Delattre | Ageing of Hollow Fiber Membranes in Polyvinylidene Fluoride (PVDF) used in Water Treatment[END_REF]. Its tolerance to chlorine is the primary reason that this material is often chosen for full-scale applications [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF]. Although this polymer is resistant to chlorine, several studies have demonstrated changes on physical/chemical structure and filtration performance for these types of membranes in contact with NaOCl [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF].

The results of 24 scientific papers about ageing of PVDF membranes will be discussed in this section regarding membrane characteristics and ageing methods. Among these studies, 16 of hypochlorite solutions [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF][START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF][START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF][START_REF] Rabuni | The contrastive study of chemical treatment on the properties of hydrophobic PVDF membrane[END_REF]. The other four studies evaluated different aspects of ageing in membranes from either pilot or full-scale operation [START_REF] Fenu | Elaborating the membrane life concept in a full scale hollow-fibers MBR[END_REF][START_REF] Robinson | Membrane ageing in full-scale water treatment plants[END_REF][START_REF] Van Bentem | MBR Varsseveld: 5 years of operational experience[END_REF][START_REF] Cote | Hollow fiber membrane life in membrane bioreactors (MBR)[END_REF]. Firstly, results from the 20 scientific papers that used artificially aged membranes (by soaking or filtration cycles at lab scale) will be discussed, interestingly only three of those papers aged supported membranes, which are often used in wastewater treatment plants.

PHYSICAL AND STRUCTURAL PROPERTIES

Regarding physical properties, PVDF membranes tend to decrease their elongation at break and Regarding MBR applications, hollow-fibers are reinforced with other polymers to withstand the high shear-stress conditions, i.e. supported membranes. Thus, the mechanical resistance of these membranes is less related to PVDF properties and is expected not to change under chlorine exposure. However, [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF] characterized NaOCl aged membranes reinforced with a PET nonwoven fabric and found a slight decline of 10% in ultimate tensile strength [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF]. With only one study dealing with reinforced fibers, there is a clear lack of investigation on these type of membranes.

Under the range of physical characteristics, roughness of membrane surface was evaluated by three studies [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF][START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF][START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF]. All of them remarked rougher membranes as they aged, which could facilitate adsorption of foulants onto surface.

The most controversial parameter among the physical/structural aspects of membranes is the evolution of pore size with ageing. This parameter was evaluated in 11 publications. In 6 researches, pore size tends to increase, most likely due to the degradation of additives, leading to the appearance of bigger pores. Contrastively, Li et al. ( 2021) obtained no changes in pore size in SEM images, although, a degradation of additives was observed [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. Only one study presented a decline of pore size [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]. Authors hypothesized that changes in porosity involve modifications at a dimension scale not directly linked to molar mass of PVDF or additive degradation. They speculated an indirect mechanism between change in porosity and degradation of the additive that is yet to be established. In the same study, no changes in pore size were observed for PVDF membranes without additive. [START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF][START_REF] Puspitasari | Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane[END_REF]. The first observed by capillary flow porometer smaller pores at first, followed by higher pores as exposure doses increased. The authors linked the initial decrease of pore size to the additive leach out, which produced smaller pores and decreased mean pore size on the membrane structure, but as exposure increased, more additive was degraded and bigger pores were formed [START_REF] Puspitasari | Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane[END_REF]. The later found an increase followed by decrease of mean pore size by challenge tests using PEG and Dextran solutions. They discussed that these tests do not exclude the contribution of membrane hydrophobicity, which could be misleading these conclusions [START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF]. Some differences might be influencing these contrastive conclusions:

CHAPTER 1: BIBLIOGRAPHIC STUDY 71 i)
Ageing conditions variability: the values of NaOCl concentration, pH and temperature was not always mentioned, and the range of dose studied varied (from 5,000 -120,000 ppm.h to 1.680,000 -3,528,000 ppm.h). Some studies exposed the membranes to NaOH in combination to NaOCl, or used acids to control pH.

ii) Analytical method: pore size was determined in some studies by SEM imagery, which is sensible to the analyzed zone and topography. Solute transport tests were also used, which are influenced by chemical interactions between solutes and membrane surface. Additionally, capillary flow porometry and nitrogen adsorption methods were employed, in which measure of small pores sizes can be challenging since these technics apply high pressures which can be detrimental to membrane structure;

iii) Membrane type: even though these studies used PVDF-based membranes, they are often from different manufacturers. This means that production method and compositions may vary, affecting the overall changes as membranes age.

CHEMICAL PROPERTIES

Membrane chemical properties are assessed by interpretation of ATR-FTIR spectra in most of the times. To follow the hydrophilic agent degradation, authors usually refer to peaks appearing at 1660 -1675 cm -1 attributed to amide bond of PVP, for example, and peaks appearing at 1730 -1750 cm -1 assigned to carbonyl bond originated from both PEG and PVP as examples [START_REF] Causserand | Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation[END_REF][START_REF] Regula | Ageing of ultrafiltration membranes in contact with sodium hypochlorite and commercial oxidant: Experimental designs as a new ageing protocol[END_REF].

By following the evolution of bands at the range of 760 -1403 cm -1 alterations in PVDF fingerprint can be identified. Moreover, dehydrofluorination can be indicated by the presence of peaks at a range of 1590 -1650 cm -1 ascribed to carbon double bond and 2100 cm -1 , which represents carbon triple bond [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF]. For all the studies that followed the evolution of hydrophilic agent content, a decline was to 96,000 ppm.h and obtained PVP degradation, but no changes in the PVDF fingerprint were observed [START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF]. At higher doses (240,000 -1,920,000 ppm.h), Gao et al. (2016) found the complete elimination of the hydrophilic agent after 480,000 ppm.h (1665 cm -1 ), whereas C-F peak (1000-1100 cm -1 ) at first increased, but after the total departure of PVP, this peak decreased [START_REF] Gao | Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes[END_REF]. The first increase was assigned to internal crosslinking among PVDF structure, which was possible as PVP leached out. After 480,000 ppm.h, authors hypothesized that defluorination and oxygenation reaction due to high pH could be the reason to C-F peak decline as already seen for sodium hydroxide exposure. Indeed, these results were supported by thermogravimetric analysis. Complementarily, Li et al. ( 2021) aged membranes for 500,000

ppm.h at different pH and only observed defluorination at pH 11 [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. Thus, it seems that for chlorine exposure lower than around 500,000 ppm.h PVDF molecules are stable, but these membranes undergo defluorination at higher doses and pH> 9.0.

Another widely discussed chemical parameter is membrane hydrophobicity, usually determined by contact angle measurements. As already seen, with the hydrophilic agent degradation, membranes are expected to become more hydrophobic (increasing contact angle), this is true for seven out of eleven studies using hydrophilised membranes. Among the other four, decline [START_REF] Vanysacker | Effect of chemical cleaning and membrane aging on membrane biofouling using model organisms with increasing complexity[END_REF][START_REF] Wang | Membrane cleaning in membrane bioreactors: A review[END_REF]. The first attributed this decline to partial ionization of membrane surface analogously to PES aged membrane, whereas the later assigned it to the presence of carbonyl on membrane surface confirmed by XPS. These groups are originated from the defluorination process of PVDF that forms firstly carbon double bonds, then carbonyl groups as presented in (XDLVO) theory to quantify surface energy of PVDF membranes aged with NaOCl from 240,000 ppm.h up to 1,920,000 ppm.h [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF]. According to this theory, the total interaction energy in an interface is the sum of Van der Waals, Lewis acid-base and electrostatic double layer energies. These energies are determined by contact angle measurements with probe liquids of known surface tension. Authors found that aged membranes presented more apolar characteristics and high electron donor components. The sum of Van der Waals and acid-base interactions when a solid is immersed in water provides a quantitative information about the hydrophilicity/hydrophobicity of the given surface. In this case, membranes became more hydrophobic as they aged due to hydrophilic agent degradation. Nevertheless, after 576,000 ppm.h, membranes presented a decline in hydrophobicity, that authors ascribed to defluorination and oxygenation of PVDF.

Besides these reactions in the PVDF molecules, crosslinking and chain scission are often part of discussion in ageing of PVDF membranes, but they are hardly ever directly assessed.

Ravereau et al. ( 2016) dissolved aged membranes and analyzed obtained samples by SEC [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF].

CHAPTER 1: BIBLIOGRAPHIC STUDY 75
Authors observed the predominance of chain scissions over crosslinking for membranes with or without hydrophilic agent and aged at pH of 6.0 and 7.5. Furthermore, the presence of additive significantly favored chain scission. For higher pH (11.5), chain scissions were less extensively favored. The authors speculated that non-random reactions can be superposing the random ones, thus selective attack of short chains might be changing macromolecular structure of these membranes [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF].

A fewer studies evaluated the effects of NaOCl on the electrical charges of PVDF membranes by zeta potential [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF][START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. For membranes aged at pH 7.5, Gao et al. (2017) firstly found an increase in negative charge on aged membranes at 240,000 ppm.h due to the ionization of weak acids formed from the ring opening of PVP molecules. Then these charges are reduced at 576,000 ppm.h probably due to the elimination of these PVP degradation products. Finally, after 960,000 ppm.h negative charge increased again, which was probably related to dehydrofluorination and oxygenation with formation of carbonyl and carboxylate groups in PVDF molecules [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF]. [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF] exposed PVDF membranes to 500;000 ppm.h of NaOCl at pH ranging from 3 to 11 using HCl or NaOH for adjustment [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. Ageing increased the negatively charged character of these membranes for all tested pH. At higher pH (9 and 11), this increase was higher and authors attributed this result to the formation of carboxylic groups from the opening of PVP ring.

Therefore, ageing seemed to increase membrane hydrophobicity because of PVP degradation.

Additionally, membranes are likely to become more negatively charged, especially at higher pH and higher doses due to PVP ring opening and defluorination and oxygenation of PVDF.

These changes along with membrane structural changes will highly interfere in filtration properties. These changes are reviewed in the following section.

FILTRATION PERFORMANCES

Besides understanding physical/chemical characteristics of membranes in ageing studies, they are also tested for their filtration parameters, aiming to understand the way ageing could affect membranes performance on operating conditions.

The most frequent parameter assessed is pure water flux or permeability, i.e. fifteen studies presented results on this topic. Even if the pure water permeability of MF/UF membranes is expected to be impacted by membrane degradation, its monitoring in the frame of membrane ageing studies is quite tricky. Indeed, this parameter is simultaneously modified by (i) the decrease in hydrophilicity due to the additive degradation, (ii) the increase in hydrophilicity due to partial material ionization and (iii) the increase in porosity due to pore enlargement following hydrophilic agent dislodgment.

Water permeability tends to grow with ageing in most of the works as a result of pore enlargement or higher hydrophilicity for doses ranging from 7,000 -2,000,000 ppm.h at different pHs [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF][START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF][START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF][START_REF] Zhang | A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes[END_REF][START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF][START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF][START_REF] Levitsky | Cleaning UF membranes with simple and formulated solutions[END_REF][START_REF] Rabuni | The contrastive study of chemical treatment on the properties of hydrophobic PVDF membrane[END_REF]. In addition, [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF] observed a connection between the permeability increase and the pore enlargement due to PVDF chain breakage as a result of dehydrofluorination in alkali conditions [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF]. [START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF] found a greater correlation between water permeability and the maximum pore size for membranes aged up to 960,000 ppm.h at pH 11 [START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF], while the correlations are poorer with the mean pore size and hydrophilicity. This suggests that the formation of macro voids is the dominant factor on permeability increase. 2018) also stated that the decline in hydrophilicity was responsible to the decline in permeability, but the following increase was due to pore enlargement linked to hydrophilic agent degradation [START_REF] Wu | Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods[END_REF].

Filtration with pure water is unavoidable to characterize the effect of membrane structure evolution on filtration capability but this parameter is far from real conditions applied on these membranes, that is the reason some studies evaluate filtration of activated sludge or model solutions simulating foulants from full-scale process. To evaluate filtration performance of membranes under real conditions, the critical flux is one of the important parameters. Critical flux concept was firstly defined as the flux below which a decline of flux with time does not occur; above it fouling is observed [START_REF] Field | Critical flux concept for microfiltration fouling[END_REF]. With a methodology point of view, different protocols can be followed to determine this parameter. [START_REF] Robles | Sub-critical filtration conditions of commercial hollow-fibre membranes in a submerged anaerobic MBR (HF-SAnMBR) system: The effect of gas sparging intensity[END_REF] focused on MBR membranes [START_REF] Robles | Sub-critical filtration conditions of commercial hollow-fibre membranes in a submerged anaerobic MBR (HF-SAnMBR) system: The effect of gas sparging intensity[END_REF], the filtration is performed at a defined flux for a certain duration before physical cleaning and then increased by a constant flux-step size, TMP evolution is followed, the critical flux can be defined as the threshold flux which TMP build up exponentially. [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF] arbitrarily defined critical flux when TMP increased above 0.3 kPa within 15 min of filtration of activated sludge (HRT 4h; SRT 30 d; MLSS 6 g/L) [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF]. They observed that critical flux decreased slowly as membranes aged and they assigned this behavior to increasing hydrophobicity and roughness, and more electron acceptor components on membrane surface, which was defined by the XDLVO theory in their work. Unlike the concept of critical flux, Abdullah and Bérubé (2018) evaluated the filtration capacity of their membranes as being the filtered volume (HRT 10 h; SRT 12d; MLSS 4 g/L; at 25 LMH) before permeability decreased by 10% [START_REF] Abdullah | Filtration and cleaning performances of PVDF membranes aged with exposure to sodium hypochlorite[END_REF]. A slight decline in filtration capacity was observed for membranes exposed up to 1,000,000 ppm.h of sodium hypochlorite (≈ 18% of decrease) but for samples exposed to 2,000,000 ppm.h, filtration capacity sharply fell from 5,600 L.m -2 (pristine membranes) to values as low as 600 L.m -2 .

All these different parameters can give information about the fouling propensity of membranes over ageing. The majority of studies that evaluated fouling propensity found increasing trends for higher doses which means that as they age, membranes become more likely to interact with foulants, decreasing cleaning efficiency and increasing the down time of installations for maintenance [START_REF] Abdullah | Filtration and cleaning performances of PVDF membranes aged with exposure to sodium hypochlorite[END_REF][START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF][START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF][START_REF] Zhang | A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes[END_REF][START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF][START_REF] Gao | Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes[END_REF]. Moreover, Vanysacker et al. ( 2014) found that membrane ageing not only increased protein-fouling fraction in EPS, but also influenced EPS production in biofoulant in the three filtration cases, namely, monospecies feed (P. aeruginosa culture) or duospecies (P. aeruginosa + E. coli culture) or spiked activated sludge (P. aeruginosa + activated sludge) [START_REF] Vanysacker | Effect of chemical cleaning and membrane aging on membrane biofouling using model organisms with increasing complexity[END_REF]. Authors suggested the growth behavior or metabolism of these species on the surface of aged membranes were changed in comparison to pristine ones, since cell density was not affected by membrane aging.

Instead of filtering real activated sludge, a way to decrease variability to assess these interactions is to filter specific compounds that mimic components present in raw wastewater or activated sludge. Specific interactions between membrane surface and these compounds and retention of these last as membranes age are then evaluated. The most tested compounds in these situations are BSA, humic acids, sodium alginate, polysaccharides, bacteria, such as Escherichia coli, and viruses.

Eight publications tested filtration performance and retention of aged membranes to some of humic acid sodium salt, 10 mg.L -1 alginic acid sodium salt and 10 mg.L -1 BSA and observed a decrease in retention analyzing permeate by TOC [START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF]. They attributed this effect to possible pore enlargement or changes in membrane surface, accordingly to Li et al. (2021) [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF].

Interestingly, they observed that this decline in retention was less extensive when they aged membranes by filtration/cleaning cycles of model solution and sodium hypochlorite solution than when membranes were only soaked in the reactive agent at the same doses.

Gao et al. (2017) also filtered these compounds with aged PVDF membranes and decomposed interactions between these foulants and membranes [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF]. They observed that interactions between foulants and membranes were largely determined by hydrophilicity and electrical charges. New membranes were apolar and presented higher electron donor characteristics and negative charges resulting, globally, in a hydrophobic material. Because of this high hydrophobicity, membranes were repulsive to humic acid, the only hydrophilic foulant, causing low fouling potential. Regarding BSA and sodium alginate, they both presented hydrophobic interactions with membranes, but due to the high electron donor character, they were more repulsive to sodium alginate and more attractive for BSA. When these foulants were mixed, BSA acted as a bridge interacting directly with membrane surface and improving sodium alginate fouling. All aged membranes presented a hydrophobicity even higher than the pristine one, having higher interactions with foulants, but at 960,000 ppm.h hydrophobicity started to slightly decrease, decreasing also fouling propensity. Authors argued that these changes are due to a decline in the C-F peak and the appearance of a carbonyl peak. They associated these changes to the formation of sodium carboxylate between the PVDF and NaOCl in these aged membranes [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF]. Retention of polysaccharides is expected to follow a different path from that of proteins.

Polysaccharides, such as dextran, are often uncharged, thus a separation governed by size exclusion mechanism is expected [START_REF] Pellegrin | Multi-scale analysis of hypochlorite induced PES/PVP ultrafiltration membranes degradation[END_REF]. However, since Zhang et al. (2017) obtained higher retention and hydrophilicity as membranes aged, they proposed that a hydration layer would tightly bound to the membrane surface as its hydrophilicity increased, which could prevent dextran to pass through the membrane even if pore size increase during ageing [START_REF] Zhang | A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes[END_REF]. Contrarily, Ren et al. (2021) observed a decrease in hydrophilicity aging PVDF membranes. They tested aged membranes for polymer flooding, which are effluents rich in polymers that promote a much quicker fouling. In this environment, authors found that aged membranes presented greater flux decline during filtration and this loss was more related to irreversible fouling rather than physically reversible fouling. They stated that the increase in hydrophobicity increased interactions between membrane surface and polymers present in their effluent [START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF]. that they confirmed thanks to a thoroughly characterization based on nuclear magnetic resonance, size exclusion chromatography and differential scanning calorimetry [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF].

SYNTHESIS OF CHANGES IN PVDF MEMBRANE PROPERTIES DUE TO AGEING

Rejection and fouling propensity to specific molecules were also controversial among these researches. When facing foulants, aged membranes mostly increased their fouling propensity due to increasing roughness or hydrophobicity, with higher interactions towards proteins. These increasing interactions resulted in some cases increasing retentions due to adsorption on membrane surface and formation of a denser cake layer, but in some studies pore enlargement allowed more molecules to flow through membranes, decreasing retention.

These papers evaluated ageing of PVDF membranes by either i) soaking in NaOCl solutions or ii) cycles of filtration/cleaning with foulants and chemicals, respectively. These conditions are far from the real conditions that these membranes are operated. Four other publications directly assessed the concept of ageing in membranes operating at full or pilot scale and these results are presented in the following section.

+ (increase)

The last review about PVDF membrane ageing was performed in 2016 [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF]. Since then, one of the major findings was the effect of ageing on membrane surface electrical charges. Gao et al.

(2017) and Li et al. (2021) reported that membranes become more negatively charged with ageing [START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF][START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. Moreover, the first study reporting results of membrane ageing in full-scale installations was published, which confirmed the degradation of hydrophilic agent at the chemical level, but highlighted differences in permeability and fouling propensity behavior with ageing compared to accelerated ageing [START_REF] Robinson | Membrane ageing in full-scale water treatment plants[END_REF].

MEMBRANE AGEING UNDER OPERATING CONDITIONS

Ageing in pilot or full-scale operating conditions was investigated by a few studies, mainly for the drinking water industry. Regarding chlorine contact, they estimated a life span of 15 -16 years considering the maximum exposition recommended by the manufacturer of 500,000 ppm.h. As life-time estimated using the permeability decay was shorter than this one, authors considered more convenient to increase frequency of cleaning than excessively foul membranes from an investment cost perspective. With a financial point of view, authors concluded that energy costs could not be considered a trigger for membrane replacement as operations close to the end of life are still economically feasible, even though energy consumption doubled. Finally, a life span concept based on membrane mechanical resistance was not advised, since episodes of increasing turbidity of permeate were related to punctual breakage in potting resin. Reparation and partial substitution of these parts solved the problem. Replacing whole membrane lanes because of punctual breakages would not be economically viable. [START_REF] Cote | Hollow fiber membrane life in membrane bioreactors (MBR)[END_REF]. These data revealed that fiber breakage was never an issue thanks to a resistant support layer that composes these membranes. The frequency of failure of surrounding structures such as connections, pipes and resin decreases with time as these membrane cassettes/modules aged. The origins of these failures were virtually eliminated from the most recent cassettes. Weakening of the potting resin at the juncture was not a concern within the first 12 years of operation, when the advised maximum chlorine exposure (40,000 ppm.h/year) was respected. They concluded that the constant TMP rise and the necessity of more frequent chemical cleanings were the triggers for membrane end of life. Thus, planned replacements were proposed over 2 -4 years of the entire inventory in sub-sets representing 10 -25% of the plant membrane surface in a way to avoid operation of aged and new membranes by the same pumping system. By following this methodology, membranes could operate for an average of 10 years.

CONCLUSION

Although several studies have been published about ageing of PVDF membranes, many of them are focused in membranes applied in drinking water plants. Knowledge about membrane ageing in MBRs treating urban wastewater is still scarce. From the scientific literature, ageing is highly dependent on operating conditions besides chemicals and materials used. Moreover, the contradictory results from chemical ageing, especially pore size, hydrophobicity and permeability, show that this process is not completely understood and the strategies used to accelerate ageing in lab can mislead those results. The representability of these assays regarding real complex operation was only assessed in drinking water applications and the characterization of membrane ageing in full-scale MBRs with respect to (i) membrane

properties and (ii) in-situ filtration performance is still lacking. In MBRs, membranes are loosely attached, filtration cycles are usually shorter, biological activity is higher leading to higher concentrations of foulants and other metabolites that may interact with membranes and alter their ageing.

INTRODUCTION

This chapter aims to present the membranes studied in the present thesis. Furthermore, ageing procedure at bench-scale is presented. For full-scale ageing, a detailed description of each wastewater treatment plant and their respective membrane bioreactor is provided. Finally, the various methods used to characterize membranes are described.

MATERIALS AND AGEING PROCEDURE

ZEEWEED® MEMBRANES

This study was performed with hollow-fibers harvested from ZeeWeed® 500D modules (SUEZ Water Treatment Solutions, France). These are ultrafiltration membranes with a mean pore size of 0.04 µm. Table 2.1 presents detailed information about these membranes. Since their primary application is MBRs in which they encounter important mechanical stresses, these membranes present an internal support to ensure their mechanical resistance. The material of this support is not provided by the membrane manufacturer, but according to the comparison of its infrared spectrum to the ones of species in the HR Nicolet sampler library with OMNIC software (Thermo fischer, USA), the support material signature presented 64% similarity to polyethylene terephthalate (PET) spectrum (Figure 2.1). The chemical cleanings performed in-situ for these membranes include usually acids (citric acid, hydrochloric acids, etc) and bases/oxidants (sodium hydroxide, sodium hypochlorite, etc).

PVDF membranes are not sensible to acid exposure as shown elsewhere [START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. Since several researches confirmed changes in PVDF membrane properties after exposure to oxidants and After rinsing in a large volume of pure water, laboratory ageing was performed by single soaking the pristine membranes in NaOCl solution at a total free chlorine concentration of 1000 ppm at room temperature. The intensity of ageing is presented as C × t ranging from 0 to 750,000 ppm.h, which is higher than the maximum exposure dose advised by membrane manufacturers (500,000 ppm.h). The volume of fibers did not exceed 1% of the soaking solution volume in order to avoid significant consumption of chlorine. Total chlorine concentration was checked three times a week via silver nitrate titration (Hach, USA). When a deviation of more than 10% from the target concentration was measured, the concentration was corrected by adding NaOCl. Concentrated sulfuric acid was added to adjust the pH to 9.0 that was corrected whenever changes greater than 5% were observed over time. These chlorine concentration and pH are similar to the ones found in cleaning in place (CIP) protocols for MBRs applied to municipal wastewater treatment. These are precisely the conditions applied to the membranes in the MBR from Seine aval WWTP as described in section 2.3. The actual free chlorine concentrations and pH measured during the bench-scale ageing ranged from 950 to 1060 mg Cl/L (ppm) and from 8.98 to 9.09, respectively. Temperature for these expositions was kept at 21 ± 2 °C. Therefore, the conditions were considered very stable.

After harvesting from single soaking expositions, membrane samples were stored in 1 g/L sodium bisulfite at room temperature (21 ± 2 °C) prior characterization.

FULL-SCALE AGEING

Membranes in the full-scale operation were sampled from the Seine-Morée (SEM) and Seine aval (SAV) WWTP in France (SIAAP). In parallel, data from MBRs installed in both WWTP

were collected in order to analyze the evolution of membrane performances. The list of the collected parameters is presented in Table 2.2 and each MBR is presented in details in the following sections.

SEINE MORÉE WWTP

This facility was commissioned in 2014 and designed to treat up to 50,000 m 3 /d wastewater (300,000-population equivalent) from the northern Paris urban area. This WWTP is composed of screening, grit and grease removal in its pretreatment unit, followed by a primary lamellar settler. The biological treatment and post-clarification are performed by MBR (Ultrafor®, Suez, France). Pretreated wastewater is divided equally into three biological basins (total volume of 39,500 m 3 ) composed of anaerobic, anoxic, and aerated zones. Then, the mixed liquor is sent to eight membrane tanks of 300 m 3 each (SEM-UF1 to SEM-UF8). Each membrane tank contains 10 immerged cassettes composed of ZeeWeed® 500D membrane modules. The total filtering surface of this MBR is 106,000 m 2 (13,250 m 2 per tank). A schematic representation of this WWTP is presented in Figure 2.4. This WWTP was supposed to be coupled with a biogas unit for the sludge treatment, but the project was not approved by all institutions involved. To avoid the high costs and flow of truck evacuation of sludge produced, the WWTP is operated at 30% of its designed capacity. This choice implied a discontinuous way to operate membranes. Due to this choice, the average daily filtration time for each membrane tank was 5.4 ± 1.9 h.

SEINE AVAL'S WWTP

The 2.3) and similar specific aeration demand (SADm) (Table 2.4) over time. Thus, these parameters were not able to differentiate them in order to reduce the number of analyzed tanks.

COMPARISON OF CHEMICAL CLEANING

PARAMETERS

To illustrate the potential variations of the doses and pH that membranes are exposed for each CIP, Figure 2.9 presents the chlorine concentration (Figure 2.9.A) and pH (Figure 2.9.B) measured in laboratory for the soaking solutions harvested during CIP for SAV-MBR row A and B and SEM-MBR, except pH for SEM-MBR (data not available). In addition, plant operators from SAV-MBR only started tracking chlorine concentration and pH after the third CIP. Chlorine concentration during CIP for SEM-MBR varied from 610 to 1030 ppm, whereas for SAV-MBR, it presented a higher dispersion varying within the range of 532 -1136 ppm.

As a result pH during CIP for SAV-MBR varied between 8.3 and 9.7.

CHAPTER 2: MATERIAL AND EXPERIMENTAL METHODS

SAV-A1 SAV-A2 SAV-A3 SAV-A5 SAV-A8 SAV-A12 SAV-A13 SAV-B1 SAV-B5 SAV-B8 SAV-B10 SAV-B11 SAV-B13 SAV-B14 SEM-UF1 SEM-UF4 SEM-UF5 SEM-UF8

MEMBRANE HARVESTING AND PREPARATION

Fibers were harvested by cutting in both potted ends and stocked in a sodium bisulfite solution

HIGH FREQUENCY DATA TREATMENT

Besides, high frequency (15 min) data related to filtration (permeate flowrate, transmembrane pressure and temperature) were obtained for key periods between 15 days before and 15 days after a CIP protocol for these same membrane tanks. TMP is calculated thanks to pressure sensor in the permeate collector pipe after membranes and level sensors in the membrane tank.

This way, TMP is automatically determined in mbar according to Eq. 2.4. Additionally, permeate flux at 20 °C (J20) was calculated based on temperature (T) and permeate flowrate (Qp) by Eq. 2.5 [START_REF] Rosenberger | Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment[END_REF].

TMP = ℎ 𝑖 * 1000 10.13 -P 𝑝𝑒𝑟𝑚é𝑎𝑡 𝑖 × 1000 Eq. 2.4

J 20 = 𝑄 𝑝 𝑆 𝑒 -0.0239(𝑇-20)

Eq. 2.5

Where:

hi: mixed liquor level in the membrane tank i (m), measured by a pressure sensor Vegabar 82 (Vega Technique, France); 𝑃 𝑝𝑒𝑟𝑚é𝑎𝑡 𝑖 : permeate pressure (bar), measured by pressure sensor Cerabar PMP71 (Endress+Hauser, Switzerland); S: filtering membrane surface (m 2 ).

CHAPTER 2: MATERIAL AND EXPERIMENTAL METHODS

117

After determination of permeate flux at 20°C and transmembrane pressure, in order to monitor membrane ageing of these membrane tanks, the permeability index was determined. This is the ratio between permeate flux at 20°C and transmembrane pressure when membrane tanks are filtering mixed liquor. Since this indicator can be used to monitor long-term changes in membrane behavior when filtering mixed liquor, factors other than the membrane properties can affect this measure, i.e. filtration conditions, fouling state and mixed liquor characteristics.

To minimize such impacts, the method to calculate permeability index in the long-term is presented in the following section.

DETERMINATION METHOD OF PERMEABILITY INDEX FOR LONG-TERM ANALYSIS

Permeability index, similarly to membranes intrinsic permeability, represents the inverse of the hydraulic resistance of membranes in filtration. This indicator can be determined after a Cleaning-In-Place (CIP), when membranes are at their cleaned stage. On site, permeability index is currently used to monitor whether CIP are successful, but it can also be used to observe membrane ageing, since a continuous decline in this index is expected due to residual irreversible fouling accumulation [START_REF] Fenu | Elaborating the membrane life concept in a full scale hollow-fibers MBR[END_REF].

Before calculating these permeabilities for the studied MBRs, high frequency data (5 s) of Qp and TMP were collected to evaluate the dynamics of permeability before and after CIP. These data were recovered for the three initial cleaning in-place events for membrane tank A1 from SAV, since this membrane tank was representative of a regular operation, with only 8% of nonfiltration days since commissioning, an average permeate net flux of 26.6 ± 6.3 LMH and SADm of 0.22 ± 0.06 m 3 /m 2 /h. These data were firstly cleaned following two simple steps: (i)

All zeros (non-filtration periods) were deleted and (ii) values of flowrate below the first percentile (value below which 1% of values in its frequency distribution falls) were also deleted. Then, permeate flux (J) adjusted to 20°C is calculated according to Eq. 2.5 and divided by TMP, to obtain the permeability index (Eq. 2.11).

𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑖𝑛𝑑𝑒𝑥 = 𝐽 20 𝑇𝑀𝑃

Eq. 2.11

These values were smoothed by the median of values during 2 min to obtain the chronic presented in Figure 2.12 for this first period. From these data, large variations of J20 are observed between 20 and 50 LMH, which is due to the variable volume of influent sent to the station during the day and the availability of membrane tanks. These oscillations in J20 seemed to affect permeability index even in short filtration periods (~10 min). Permeability index is supposed to be independent of the J20 for short periods of filtration according to Darcy's law, assuming that fouling does not take place.

Therefore, these changes were investigated. Some of these periods are extracted from Figure 2.12 and highlighted in blue in Figure 2.13. From the blue zones in Figure 2.13 it is possible to validate the reversible characteristic of these evolutions. Thus, two phenomena may be involved in this behavior:

(i) At higher flowrates, the cake is compressed, overly increasing TMP (Figure 2.14.B), or reducing drastically permeability index (Figure 2.14.A).

Most points of TMP above 500 m 3 /h are placed above the linear regression in (ii) The geometry of pipes and hydraulic flow between membranes and pressure sensors cause friction and local losses in pressure resulting in an overestimation of TMP at higher fluxes [START_REF] Philippe | Analyse statistique et modélisation multivariée de l'évolution long terme de la perméabilité dans un bioréacteur à membranes à échelle réelle[END_REF]. Nicolas Philippe (2014) proposed a method to estimate a pressure drop coefficient to adjust these variations in the calculation of TMP and permeability index as a result. This method is based on determining the nature of flow by the Reynolds number, followed by the calculation of the pressure drop, based on the Moody's or Darcy's method, which involve specific information about material and geometry of permeate line.

In order to reduce variations on the permeability index not linked to long-term operation, two points will be taken into consideration in the following sections:

1. The effect of permeate flux: a simple approach is proposed to calculate permeability index as a function of the permeate flux;

2. The duration of filtration period: to calculate the permeability index after CIP to be representative.

EFFECT OF PERMEATE FLUX

To control the impact of flux variation, instead of fixing a given permeate flux and limiting the analysis to this only flux, 9 flux intervals were established between 18 and 36 L.h -1 .m -2 , covering Thus, a sensitivity study was carried out to determine the impact of the number of days after a CIP to be considered in the permeability index calculation. The tested length ranged from 1 to 7 days after CIP (CIP+1 up to CIP+7), as a complete cycle of filtration and chemical enhanced backwash takes 7 days. Besides, tests were performed considering the 7 following days of a CIP, but excluding the first (CIP +1 to +7) and second (CIP +2 to +7) days just after CIP. The results of average J/TMP calculated, standard deviations and number of observations (filtration cycles) are presented in Tables 2.8 and 2.9 for each condition of the CIPs of April/2018 and October/2019, respectively, for membrane tank SAV-A1 and Tables 2.10 and 2.11 for membranes SEM-UF4. From these tables, we seek the conditions with simultaneously, a relevant number of observations, so that values are representative, and the minimization of absolute standard deviations to limit variability. Firstly, J/TMP were only calculated when they presented 10 or more filtration cycles. As expected, the total number of cycles increased with increasing number of days considered between CIP+6 and CIP+7. Standard deviation was low in CIP+1, because of the low number of filtration cycles, then increased and fluctuated between CIP+2 and CIP+6. After CIP+7 standard deviation decreased over most of the range of permeate fluxes. Thus, CIP+7, CIP+1 to +7 and CIP+2 to +7 seemed to satisfy these conditions.

Therefore, to consider the whole filtration cycle of each tank and avoid strong dynamics in the beginning of a cycle due to sludge conditions, CIP+1 to +7 is chosen to the general determination of permeability indexes.

Table 2.8: Impact of period length on J/TMP (CIP: April/2018, SAV-A1). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10. 2.9: Impact of period length on J/TMP (CIP: October/2019, SAV-A1). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10.

Length

Length

CHARACTERIZATION OF MEMBRANE PERFORMANCE FACTORS

The performance factors of aged membranes were measured to identify the impact of NaOCl exposure at bench-scale and the impact of operating conditions at full-scale.

MECHANICAL PROPERTIES

To evaluate mechanical resistance of membrane samples, uniaxial tensile tests were performed on an INSTRON 3340 series apparatus (INSTRON®, USA). The initial distance between grips was fixed at 100 mm and samples of wet membranes were extended at a constant elongation rate of 110 mm.min -1 . Maximum tensile strength, elongation at break, and Young's modulus were calculated from the experimental stress-strain curves using Bluehill 3 software.

Experimental error of the maximum tensile strength indicators are under 5% and elongation at break and Young's modulus are both around 10%. For each sample, two measures were performed. An example of stress-strain curve of the pristine ZeeWeed® membrane is presented in Figure 2.21.

FILTRATION PROPERTIES

WATER PERMEABILITY

Intrinsic membrane permeability was measured by frontal filtration of pure water at different permeate flowrates (Qp) in KrosFlo Research apparatus (SpectrumLab, USA). Temperature (T)

is measured during the essay and permeate flux is corrected to 20°C according to Eq. 2.5 [START_REF] Rosenberger | Impact of colloidal and soluble organic material on membrane performance in membrane bioreactors for municipal wastewater treatment[END_REF].

Pressures of inlet and outlet (Pin and Pout, respectively) were measured for each J20 to determine TMP (Eq. 2.8) and permeability (Lp/µ20) was then calculated by the slope of the curve (J20 versus TMP) according to the Darcy's law (Eq. 2.9).

𝑇𝑀𝑃 = P

Deformation (%)

Pristine Zeeweed membrane -0 ppm.h

Maximum tensile strength

Elongation at break

The Lp/µ20 values obtained for each sample were then normalized with respect to the value Lp0/µ20 of a pristine membrane as the ratio Lp/Lp0. Error propagation was calculated based on each physical quantity and precision of Lp/µ20 was around 11%. Experiments for each samples were performed in duplicates.

FOULING REVERSIBILITY

Evaluating how aged membranes would act when facing real fouling matrices is a key indicator to understand the impact of ageing in a practical way. Some studies assessed these behaviors by using model solutions for filtration tests, usually composed of bovine serum albumin (BSA), sodium alginate, and/or polysaccharides to mimic real effluents [START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF][START_REF] Gao | Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes[END_REF]. Only one study performed tests with real mixed liquor, and although these matrices are not standardized, these protocols are expected to be more transposable to full-scale applications [START_REF] Abdullah | Filtration and cleaning performances of PVDF membranes aged with exposure to sodium hypochlorite[END_REF]. In the present study, both sets of samples (bench-scale-and full-scale-aged membranes) were submitted to hydraulic cleaning recovery tests against foulants (mainly colloidal) from a real MBR mixed liquor in order to assess fouling reversibility.

To this aim, 11-cm-long modules composed of three fibers were constructed and submitted to three successive cycles of filtration and backwash of mixed liquor sample. The mixed liquor was collected from SAV-MBR recirculation channel and filtered at 1.2 µm on the same day.

This colloidal suspension was stored at 4°C for a maximum of 2 weeks before use. The procedure was performed to avoid high variations in the mixed liquor characteristics over the storage time and to focus on colloidal fouling. The resulting matrix is mainly composed of the colloidal fraction typically present in an MBR mixed liquor (COD: 15.6 ± 0.8 mg O2/L; DOC:

7.3 ± 2.4 mg C/L; KTN: 1 ± 0.1 mg N/L; NO2: 0.07 ± 0.01 mg N/L; NO3: 9.2 ± 1.9 mg N/L; PO4 -: 0.17 ± 0.04 mg P/L). These experiments were conducted with a Convergence Inspector Hydra device (Convergence Industry, The Netherlands).

CHAPTER 2: MATERIAL AND EXPERIMENTAL METHODS

138

In order to determine the fouling reversibility, filtration was performed at 0.55 bar of TMP for 60 min, which is the maximal advised TMP for filtration by membrane manufacturer, until the permeate flux reached the pseudo-stable condition (change of flux less than 5% over a 5 min period of filtration). These conditions ensure effective fouling. Then, conditions of permeate backwash to ensure full physical cleaning recovery were tested to establish an adapted protocol.

For this, backwash was performed at 0.8 bar for 15, 30 and 60 min with permeate. Backwash TMP was chosen based on the ratio between backwash and filtration TMP at full-scale MBR, which is 1.7. This ratio for the assay only reached 1.5, due to limitations of the apparatus.

Permeate flowrate, pressure and temperature were continuously measured. Permeate flowrate was adjusted to flux at 20 °C. The standard filtration behavior for these tests are presented in To evaluate the efficiency of backwashes at different duration, the cleaning recovery was calculated as Eq. 2.10, in which J0 is the pure water permeate flux measured at a TMP of 0.55 bar. Results of cleaning recovery for each backwash duration are presented in Table 2.12. These results indicated that a 30 min backwash was the optimum duration to maximize physical recovery among the tested conditions. Thus, the protocol to determine cleaning recovery for membrane samples in this research was established with a backwashing duration of 30 min.

Cleaning recovery (%) =

(J/J0)c2 (J/J0)c1 × 100% Eq. 2.10 For the determination of fouling reversibility, membrane modules of each sample were submitted to two cycles of 60-min filtration with this matrix at a fixed TMP of 0.55 bar and 30 min of permeate backwash at TMP 0.8 bar. The third filtration cycle was performed for 10 min prior to a final backwash of 30 min for practical reasons. Flowrate, pressure and temperature were automatically monitored every 2 s. Prior to these measures, membrane samples aged at bench-scale, which did not filter before, were compressed by filtration of pure water at 0.55 bar of TMP for 2 h (sufficient duration to reach a stabilized permeate flowrate).

Membranes filtered pure water for 10 min prior to the mixed liquor filtration/backwash cycles at the same TMP, and pure water permeate flux (J0) was measured. The permeate fluxes in mixed liquor (J) during filtration were then normalized by J0, giving the curve J/J0 presented in Cleaning recovery after a backwash could then be expressed as a percentage of the initial normalized flux (J/J0)c1. Cleaning recovery presented in this study was obtained for third filtration cycle (J/J0)c3 (Eq. 2.11). The experimental error of these two measures were estimated at 9 and 13%, respectively.

Cleaning recovery c3 (%) = (J/J0)c3 (J/J0)c1 × 100% = 𝐽 𝑐3 𝐽 𝑐1 × 100% Eq. 2.11
The cleaning recovery results allow us to estimate reversible fouling in terms of recovered filtration flux after backwash. The remaining non-recovered flux is related to the physically

CHARACTERIZATION OF MEMBRANE PHYSICAL-CHEMICAL PROPERTIES

In the same way as for performance factors, physical-chemical properties of aged membranes were measured to identify the impact of NaOCl exposure at bench-scale and the impact of operating conditions at full-scale.

CHEMICAL COMPOSITION BY INFRARED SPECTROMETRY

Functional chemical groups present on the material at the membrane surface were analyzed via attenuated total reflection-Fourier transform infrared (ATR-FTIR) using a Nicolet iS5 spectrometer with a germanium cell (Thermo Fisher Scientific, USA). Infrared (IR) spectra were recorded at wavelengths from 600 cm -1 to 4000 cm -1 (128 scans at 4.0 cm -1 ). Spectral analyses were performed with OMNIC® software. For each sample, 5 spectra were performed.

These membranes are mainly composed of PVDF, which is relatively stable over chlorine exposure; besides hydrophilic agents and pore formers are added to their composition to enhance membrane filtration ability. The FTIR spectrum of a pristine ZeeWeed® membrane is presented in Figure 2.24. The PVDF fingerprint is observed by the bands at wavelengths of 1403, 1277, 1178, 1072, 880, and 841 cm -1 . In addition, the band around 1674 cm -1 is usually related to carbonyl stretches bonded to an amide group. This is a characteristic band associated with PVP used as hydrophilic agent and pore former by polymeric membrane manufacturers [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF]. It is known that PVP is reactive in alkali conditions and in the presence of chlorine radicals [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF]. Therefore, changes in the 1674 cm -1 band were studied among other changes. Since the over chlorine exposure, it is used to normalize changes on the absorbance of the peak at 1674 cm -1 to a common scale [START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF]. In the present study, the PVP content of each aged sample is then expressed as a percentage of the value obtained for pristine membranes. Peaks at 1590 cm -1 (Carboxylate), 1700/1770 cm -1 (succinimide) and 1720/1730 cm -1 (carboxylic acid) were also observed, since they are supposed to be either related to PVP degradation products or residual fouling [START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF].

In case of full-scale aged membranes, the residual fouling agents are different compounds in mixed liquor that may present peaks in the range between 1600 -1700 cm -1 . However, since membranes are thoroughly cleaned before analysis, the effects of these compounds are supposed to be minimized in the spectra. . This method is used instead of the sessile drop technique in the case of porous materials to avoid capillarity effects and it provides a more realistic measure as the membrane surface is kept in wet conditions [START_REF] Pellegrin | Multi-scale analysis of hypochlorite induced PES/PVP ultrafiltration membranes degradation[END_REF]. For these measurements, PVDF surface skin from samples was first peeled off from the support material and fixed on a microscope slide, then immersed in the measuring cell filled with water. 10 measures were performed by sample and the experimental error is around 13%. This method was only applied to bench-scale samples as peeling off did not allow us to recover a piece of intact skin from the full-scale samples, and measurements on the whole hollow fiber led to large errors due to their concave surface.

PORE SIZE AND VOLUME

Porous volume and pore size distributions were measured by nitrogen adsorption for both bench-scale-and full-scale-aged samples. Membrane samples of 0.2 g were oven dried at 40 °C for 72 h and placed in the measurement chamber. Measurements were carried out once for each sample at 77 K in a Belsorp apparatus (Microtrac MRN, Germany). The sorption isotherms can be used to determine porous volume and pore size distribution by the BJH model [START_REF] Barrett | The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms[END_REF].

SURFACE MORPHOLOGY BY SCANNING ELECTRON MICROSCOPY

Scanning electron microscopy (SEM) was performed using a Hitachi TM3030. Firstly, random pieces of membranes from each sample were dried for 48h at 40°C then coated with 5 nm of gold. Then, images with an optical magnification of 60× and 500× were acquired in order to qualitatively evaluate the topographic aspects of the membranes.

STATISTICAL ANALYSIS

CONCLUSIONS

This chapter detailed the experimental tools used to quantify the evolution of PVDF membrane properties over ageing process.

The following chapter applies most of these methods to characterize membrane samples aged at bench-scale by chemical action (NaOCl) and full-scale in MBRs to establish how ageing changes membrane properties in these two scales.

SCALE AND FULL-SCALE 1. INTRODUCTION

Studies usually simplify membrane ageing by accelerating it by single soaking in a hypochlorite solution and by observing the changes in membrane properties [START_REF] Abdullah | Filtration and cleaning performances of PVDF membranes aged with exposure to sodium hypochlorite[END_REF][START_REF] Abdullah | Assessing the effects of sodium hypochlorite exposure on the characteristics of PVDF based membranes[END_REF][START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF][START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF][START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF][START_REF] Puspitasari | Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane[END_REF][START_REF] Rabuni | A systematic assessment method for the investigation of the PVDF membrane stability[END_REF][START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF][START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF][START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF][START_REF] Vanysacker | Effect of chemical cleaning and membrane aging on membrane biofouling using model organisms with increasing complexity[END_REF][START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF][START_REF] Wu | Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods[END_REF][START_REF] Zhang | A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes[END_REF]. Recently, some studies quantified the representativeness of accelerated ageing at bench-scale towards ageing in drinking water facilities [START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF][START_REF] Robinson | Membrane ageing in full-scale water treatment plants[END_REF]. With regard to membranes used in MBR, the evaluation of the representability of single soaking accelerated ageing is still lacking. In MBR, parameters such as more frequent cycles of filtration/backwashing, aeration, mixed liquor more charged in foulants, could have a greater impact on membrane ageing than the solely chemical action of sodium hypochlorite. Therefore, this chapter aims to establish the representativeness of accelerated membrane ageing by single soaking of pristine membrane at bench-scale (1000 ppm; pH 9.0) with respect to full-scale MBR operation. In parallel, membrane samples were harvested from the membrane tank UF3 of SEM-MBR from 2016 to 2021, corresponding to 7 years of operation since the date of commissioning (2014). In terms of hypochlorite exposure dose, these samples ranged from 33,000 ppm.h to 100,000 ppm.h.

For that, the following approach was carried out:

-Quantifying and understanding membrane ageing, i.e. changes in membrane properties, by sole sodium hypochlorite contact and under operating conditions at fullscale MBR; -Highlighting the differences between these two processes and discussing explicative hypothesis.

Data presented in this chapter were published in the journal Separation and Purification

Technology [START_REF] Oliveira Filho | Comprehensive study of supported PVDF membrane ageing in MBR: A direct comparison between changes at bench scale and full scale[END_REF].

CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE

147

RESULTS

First, the changes in performance factors, namely, mechanical and filtration properties, are presented for both ageing at bench scale and at full scale (section 2.1). Then, the physicalchemical properties are evaluated in order to establish cause-effect relationships (section 2.2).

Finally, these changes due to sodium hypochlorite contact and MBR operating conditions are discussed further in sections 3.1 and 3.2, respectively.

IMPACT OF AGEING ON MEMBRANE PERFORMANCE FACTORS AT BENCH SCALE AND FULL SCALE

MECHANICAL PROPERTIES

Regarding mechanical resistance, pristine ZeeWeed® 500D membranes presented an ultimate tensile strength of 395.8 ± 13.7 MPa, ultimate elongation of 40.5 ± 4.2 % and Young's modulus of 1075.2 ± 114.5 MPa.

At bench scale, membranes were single soaked in sodium hypochlorite between 0 and 750,000 ppm.h of exposure dose. This exposition did not cause statistically significant changes in the ultimate tensile strength as seen in Figure 3.1.A (Kruskal test, p = 0.468). This stability was also observed in percent ultimate elongation (Figure 3.1.B). Moreover, Young's modulus slightly increased between 0 and 60,000 ppm.h, then stabilized around 1200 MPa (Figure

3.1.C).

Regarding membranes aged at full scale, the ultimate tensile strength and Young's modulus showed a 45% and a 33% decline, respectively, over the study time that corresponds to a cumulative exposure dose to hypochlorite of 100,000 ppm.h (Figure 3.1.A and C).

Nevertheless, operators did not report significant fiber breakage during the whole operation For ageing at bench scale, these results can be divided into three phases. The first phase, corresponding to membranes exposed to a dose C × t of up to 78,000 ppm.h, showed an increasing trend until reaching 1.9 times the initial permeability. The second phase (C × t = 78,000 -150,000 ppm.h) presented a decreasing Lp/Lp0 ratio; and in the third phase, after 150,000 ppm.h, samples' intrinsic permeability reached a plateau at a value comparable to the one of the pristine membrane.

Moreover, the Lp/Lp0 ratio of membranes at full scale showed an increasing trend with C × t (and the associated time of operation), although to a lesser intensity than bench-scale-aged samples, at least during the 7 years of data collection in the present research.

CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE

Cleaning recovery

Mixed liquor fouling reversibility was performed using a pre-filtered mixed liquor (at 1.2 µm)

to focus mainly on the colloidal fouling. However, salts can also be present. Three cycles of filtration and backwashing were performed and cleaning recovery was defined as the recovered flux in the beginning of the third cycle with respect to the first one (as detailed in Chapter 2).

Pristine membrane recovered 56% of its flux. Samples aged at bench-scale reached 65% at 24,000 ppm.h and remained stable up to 125,000 ppm.h; subsequently, samples exposed to 500,000 ppm.h experienced a decline to a value of 51% (Figure 3.3). When comparing these values with those obtained for full-scale aged samples (Figure 3.3), we found that cleaning recovery was always higher in samples aged at bench-scale, regardless of the exposure dose with characteristic peaks of alpha-phase PVDF (875, 1070 and 1181 cm -1 ) and beta-phase PVDF (841, 1274 and 1403 cm -1 ) [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF]. Moreover, it presented a band at around 1674 cm -1 , which may be related to carbonyl stretches bonded to the amide group in the PVP chain, which is used in these membranes to increase hydrophilicity. for bench-scale aged membranes, we can observe the growth of a band at 1650 cm -1 . PVDF is well-known for its chemical stability. However, in alkali media, an alteration in chemical composition of PVDF can be observed, associated with the dehydrofluorination reaction. The dehydrofluorination process, an ionic mechanism, is mainly indicated by the occurrence of the peaks at 1650 cm -1 assigned for carbon-carbon double bond and 2100 cm -1 for the carboncarbon triple bond [START_REF] Regula | Chemical cleaning/disinfection and ageing of organic UF membranes: A review[END_REF]. In the case under study the band at 1650 cm -1 can then be attributed to carbon-carbon double bonds formation in PVDF structure. Similar results were obtained by [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF], study in which the modifications in FTIR spectra at 1650 cm -1 were low but visible even if the hypochlorite solution pH studied were around the neutrality (6.0 and 7.5).

IMPACT OF AGEING ON MEMBRANE PHYSICAL-CHEMICAL PROPERTIES AT BENCH SCALE AND FULL SCALE

MEMBRANE SURFACE CHEMISTRY

The authors confirmed some crosslinking and chain scissions using size exclusion chromatography and nuclear magnetic resonance analyses [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]. [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF] also observed the appearance of a peak at 1650 cm -1 on PVDF membranes exposed to NaOCl 0.01 M for 60 min or 0.1 M for 20 min, but the authors underlined a low peak signal [START_REF] Robinson | Ageing of membranes for water treatment: Linking changes to performance[END_REF]. If now we consider Figure 3.5.D presenting a zoom in the 1500-1800 cm -1 interval for full-scale aged membranes, no peak appeared at 1650 cm -1 , indicating that at full-scale, on the range of exposure dose studied, PVDF chemical alteration is not observed.

By contrast, PVP is generally highly reactive in alkali conditions or in the presence of oxidative radicals. Moreover, it is known that the wave numbers between 1500 and 1800 cm -1 concentrate most of the signals related to PVP degradation products [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF]. Therefore, this zone is enlarged in Figure 3.5.C and D for both bench-scale and full-scale ageing, respectively. The PVP content, quantified through the intensity of the band at 1674 cm -1 , on bench-scale-and full-scale-aged membranes is presented with respect to the one on pristine membrane in decline in the 1674 cm -1 band for membranes that contained hydrophilic agent [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]. However, Figure 3.6 shows that for bench-scale samples after 78,000 ppm.h exposition, PVP content seemed to reach a plateau at around 50% of the initial value on pristine membrane.

The decline in PVP content for full-scale-aged membranes seemed to follow the same trend observed at bench scale for C × t values lower than 78,000 ppm.h, except for the first sample at 33,000 ppm.h, which presented higher PVP content. For higher C × t values, the PVP content continued to decrease reaching 25% of the initial content (Figure 3.6). Absorbance Wavenumber (cm -1 )

Pristine membrane -0 ppm.h Full-scale -33,000 ppm.h Full-scale -42,000 ppm.h Full-scale -98,000 ppm.h Bench-scale -78,000 ppm.h Bench-scale -500,000 ppm.h

Wave number (cm -1 ) Contact angle measurements using the captive bubble method were performed to evaluate the effects of hypochlorite exposure on wettability of pristine and bench-scale aged membranes.

CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE

The obtained value of contact angle is an average of ten measurements taken in different locations on the membrane skin peeled off from the support material. The pristine membrane presented a contact angle of 39 ± 4°. This value increased to 45 ± 4° at 12,000 ppm.h, then decreased to 40 ± 5 at 24,000 ppm.h and remained stable for higher doses. Statistical analysis showed that sodium hypochlorite exposure at bench-scale had no significant influence on the contact angle (Kruskal-Wallis test, p = 0.1041).

Contact angle measurements on full-scale samples were not performed as these analyses at macroscopic scale would not only characterize intrinsic wettability of membrane material, but would be also influenced by residual fouling. In addition, skin peeling off was not possible for membranes harvested from full-scale.

MORPHOLOGICAL PROPERTIES

Pore volume and pore size distribution

Regarding morphological properties, results of pore volume and pore size distribution measurements by nitrogen adsorption are presented in this section. The results of pore volume as a function of pore diameter (dp) are presented in Figure 3.7 for membranes aged at benchscale (A) and at full-scale (B). Regarding the pore volume of bench-scale aged membranes (Figure 3.7.A), an increase in porosity is first noted up to 78,000 ppm.h. For higher exposure doses to hypochlorite 125,000 and 500,000 ppm.h, it seems that a collapse or a reorganization in the membrane material structure promoted a decline in pore volume which takes a value lower than the one of the pristine membrane. Moreover, these changes in total pore volume

CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE

160 seem to be governed by an initial increase followed by a decline in the zone of small pores (dp < 20 nm, gray zone). Measurements are performed by nitrogen adsorption, which takes into consideration membrane surface and bulk material, so overestimation of these values are possible, however these overestimations are applied for all samples. For membranes ageing at full-scale (Figure 3.7.B), although samples harvested in the early phase of ageing (33,000 ppm.h) have a slightly lower total porous volume compared to pristine membrane, a huge increase in pore volume was observed in the whole range of exposure dose investigated. In parallel, similarly to bench-scale aged membrane in the exposure range 0-78,000 ppm.h, an increase in the number of small pores was observed. In addition, samples aged for 42,000 ppm.h showed an increasing number of larger pores (dp > 40 nm). These two zones are highlighted in gray in Figure 3.7.B.

SEM images

SEM images were performed for both sets of samples, which might provide some qualitative information about surface morphology of these membranes. It is worth noting that image analysis are not able to differentiate surface pits from true pores. Comparing SEM images of the pristine membrane (Figure 3.8.A) to the bench-scale aged sample at 78,000 ppm.h (Figure 3.8.B), an increase in the number of pores is noticeable. For the sample aged at 500,000 ppm.h the number of pores seemed to decrease. Roughness does not seem to change for bench-scale aged membranes. On the other hand, images from full-scale aged samples qualitatively confirmed the appearance of the larger pores (Figure 3.9.C). In addition, an increase in surface roughness can be observed when comparing full-scale aged samples to bench-scale-aged ones or to the pristine membrane. MPa). Therefore, the support used in ZeeWeed® membranes presents a mechanical resistance capable to withstand long exposure to hypochlorite.

Impact of NaOCl chemical ageing on intrinsic permeability CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 166

Changes in water permeability of bench-scale-aged membranes can be associated mainly with:

(i) changes in the surface physical-chemical characteristics, especially hydrophobicity and electrical charges and (ii) modifications in skin porosity (pore size and number).

The first phase of increasing permeability between 0 and 78,000 ppm.h presented in Figure 3 the formation of small pores, whereas a full PVP dislodgement would lead to the formation of macro-voids. By contrast, some works in the literature concluded that NaOCl exposure causes PVDF membrane permeability to increase as a result of a pore enlargement due to the oxidation and dislodgement of PVP from the fiber structure for exposure doses ranging from 10,000 ppm.h to 2,000,000 ppm.h [START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF][START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF]. These observations were found based on rejection tests and SEM images, which may be impacted by chemical interactions between probe molecules and membranes and characterize surface pits as true pores, respectively. However, Ravereau et al.

(2016) similarly to us reported an increasing volume of pores smaller than 10 nm, only for PVDF commercial membranes containing PVP when exposed to sodium hypochlorite at doses as high as 13,000,000 ppm.h, whereas PVDF membranes with no additive did not show any changes in pore size distribution [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]. Hence, our results demonstrated that in the range of exposure dose up to 78,000 ppm.h, ZeeWeed® membranes permeability increase is controlled by the appearance of small pores due to partial dislodgment of PVP and of its by-products.

After the exposure threshold of 78,000 ppm.h, the decline in Lp/Lp0 was no longer correlated with PVP content. It is then assumed that the degradation and dislodgment of the PVP within PVDF matrix increases not only the free volume of the membrane structure but also the molecular mobility [START_REF] Gao | Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes[END_REF]. PVDF chemical alteration being observed on these bench-scale aged membranes (Figure 3.5.C), we then conclude that at a certain exposure dose (here 78 000 ppm.h) the membrane structure changed possibly due to a reorganization of the polymeric chains in membrane skin, decreasing pore volume and Lp/Lp0 as a result. A statistically significant correlation was found between Lp/Lp0 and the volume of smaller pores for the whole range of C × t studied (r= 0.966, p = 0.008, Pearson correlation test). Pore volume did not correlate well with cleaning recovery (p> 0.05). This indicates that the increase in number of small pores observed up to 78,000 ppm.h of exposure dose did not change the fouling mechanisms. This make sense as these new pores are added to the main pore population around 10 nm in size present on pristine membrane, maintaining the same foulants size to pore size ratio.

Cleaning recovery can also be related to the interactions between foulants and membrane surface. Firstly, cleaning recovery slightly increased from 0 to 24,000 ppm.h of ageing at bench-scale then became stable after 24,000 ppm.h, indicating that foulants became less attached to the membrane surface, i.e., increasing fouling reversibility. Irreversible fouling is mainly related to organic matter adsorption on the membrane surface, which is explained by the extended Derjaquin-Landau-Verwey-Overbeek (DLVO) theory as a synergic work of Van der Waals force, polar force, and electrostatic interactions. Foulants in mixed liquor are mostly organic matter that are hydrophobic and negatively charged at neutral pH [START_REF] Matar | Temporal changes in extracellular polymeric substances on hydrophobic and hydrophilic membrane surfaces in a submerged membrane bioreactor[END_REF]. As previously mentioned, aged membranes hydrophobicity may increase due to PVP content decrease. This should contribute to higher adsorption of organic matter. However, at the same time, negative charges on aged membranes may increase, leading to higher electrostatic repulsions between the membrane surface and foulants and weakening the membrane-foulants interactions. The initial slight increase in cleaning recovery observed in the present study on samples aged at bench-scale (from 0 to 25,000 ppm.h) seems to show that the effect of electrostatic interactions is predominant by comparison to hydrophobic interaction one.

CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 169

For samples exposed to higher exposure dose (500,000 ppm.h), cleaning recovery dropped to 51%. Although they are less apparent on bench-scale samples than on full-scale one (Figures 3.8 and 3.9 respectively), we hypothesize that topographic changes (potential increase in roughness) may be playing a bigger role favoring foulant adsorption, as reported by [START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF]. These authors observed that this increase in roughness correlated well with an increase in protein adsorption onto the membrane surface [START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF].

MEMBRANE AGEING UNDER MBR OPERATING CONDITIONS AT FULL

SCALE

Impact of full-scale operation on mechanical resistance

Regarding mechanical resistance, membranes aged by single soaking in hypochlorite solutions did not experience the weakening observed for the membranes under full-scale operation (Figure 3.1). These findings suggest that an effect of mechanical stress (under aeration and/or filtration conditions) in MBR may be the reason for the decline in ultimate tensile strength.

However, the precise mechanism behind these changes could not be assessed and further studies are needed.

Concerning the membrane skin, we remind here that although other studies reported embrittlement of PVDF due to dehydrofluorination and chain scission after exposure to NaOCl (70,000 -12,000,000 ppm.h) [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF][START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF][START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF], in the present study FTIR spectra failed to confirm changes in the PVDF functional groups at full-scale (0 ppm.h -750,000 ppm.h, Figure 3.5.B and D) while PVDF chemical changes were visible at bench-scale (Figure 3.5.C).

CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE

170 Concerning the membrane support, which is the main responsible for membrane mechanical resistance, the functional groups showed no changes on the PET fingerprint (1124 and 1240 cm -1 ) in the IR spectra (Figure 3.10).

Figure 3.10: Infrared spectra of support material of pristine and full-scale aged ZeeWeed® membranes after peeling off PVDF skin.

However, alterations in the support polymer crystalline structure, crosslinking/chain breakage, and material fatigue could be the reasons for mechanical resistance decline without significant modification of the polymer fingerprint on its IR spectra. Techniques such as, size exclusion chromatography, thermogravimetric analysis and differential scanning calorimetry were already reported as effective tools to measure these kind of changes [START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF][START_REF] Gao | Effects of sodium hypochlorite on structural/surface characteristics, filtration performance and fouling behaviors of PVDF membranes[END_REF] and would have to be considered for go further in the analysis.

The membrane weakening observed on samples aged at full-scale could lead to fiber breakage, which could degrade the filtration quality. Nevertheless, operators reported no changes in the ) was strongly suppressed, reaching 20% of the initial content at an exposure dose of 98,000 ppm.h (Figure 3.6.B). Additionally, the typical peaks associated with by-products did not seem to increase to the same extent, with only a slight appearance of the succinimide peak (1700 cm -1 ) being observed, suggesting that part of the PVP was oxidized into succinimide and then removed from the membrane structure by leaching. In addition, a peak at 1730 cm -1 (carboxylic acids) was formed, which can be assigned either to a PVP degradation product or to residual fouling. For the former, [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF] reported that carboxylates (1590 cm -1 ) are favored under alkali conditions [START_REF] Prulho | Ageing of polyethersulfone/polyvinylpyrrolidone blends in contact with bleach water[END_REF], which is the case in the present study for the whole ageing process at bench-scale. At full-scale, membranes are cleaned by sodium hypochlorite followed by citric acid solutions at a pH between 2.5 and 3.0, which might favor the formation of carboxylic acids versus carboxylates in the PVP degradation process. In addition, the remarkable increase of the peak at 1730 cm -1 for samples aged at 98,000

ppm.h at full-scale might be associated to the extensive PVP degradation and formation of carboxylic acids. For the latter, it is worth noting that this carboxylic acid peak may be assigned Several studies compared PVP degradation in different ageing protocols. For instance, [START_REF] Causserand | Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation[END_REF] found no significant differences in PVP content decline between single-soaked membranes and membranes submitted to filtration/backwash cycles using NaOCl solution at bench scale [START_REF] Causserand | Effects of sodium hypochlorite exposure mode on PES/PVP ultrafiltration membrane degradation[END_REF]. In addition, Robinson and Bérubé (2021) investigated the hydrophilic agent content of (i) membranes single-soaked, (ii) membranes submitted to cyclic NaOCl filtration at bench-scale and to (iii) full-scale ageing in drinking water facilities. These authors also reported no significant differences in the decrease of hydrophilic agent content whatever the ageing procedure [START_REF] Robinson | Seeking realistic membrane ageing at bench-scale[END_REF]. However, the impacts related to specific MBR operating conditions were not assessed in such ageing studies. In direct filtration facilities for drinking water production, membranes are usually trapped inside their housing. In comparison, ZeeWeed® fibers in MBR modules are potted on both ends but not stretched under tension, thus being able to oscillate when aeration cycles are performed. These cycles usually occur for 10 s with aeration and between 10 -30 s with no aeration to remove foulants during filtration and cleaning protocols, by increasing shear stress at the membrane surface and convective forces. These operating conditions might also enhance dislodgement of hydrophilic agent and its degradation products from the membrane structure.

Impact of full-scale operation on intrinsic permeability

Fibers harvested from a full-scale MBR seemed to experience an increasing Lp/Lp0 for the whole study period (Figure 3.2), highlighting two main differences when compared to the sole hypochlorite action at bench scale: (i) permeability increased to a lesser extent and (ii) no (i) For the first point, membranes operating at full scale still showed high amounts of PVP in the early stages (up to exposure dose 33 000 ppm.h), which was considered the main driver for permeability changes. Moreover, these membranes experience fouling, which is partially removed by chemical cleanings. However, the later appearance of bigger pores may enhance residual fouling in the long term as more organic matter is able to flow through and be adsorbed into the membranes (Figure 3.7). This may increase membrane hydrophobicity [START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF] and despite the increasing porosity, water is hampered to flow through membranes, whereas membranes aged by single-soaking in NaOCl solution did not experience fouling, which enables the permeability to increase to higher values. By comparing the changes in permeability of membranes exposed only to sodium hypochlorite with membranes that went through cycles of a model fouling solution filtration and sodium hypochlorite cleaning at bench scale, [START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF] confirmed the smaller increases among the latter samples, assigning this difference to residual fouling [START_REF] Hajibabania | Relative impact of fouling and cleaning on PVDF membrane hydraulic performances[END_REF].

In addition to the previous discussion, the C × t value is used as a reference for comparison between bench-scale and full-scale ageing, but it is worth noting that at full-scale, NaOCl species are reacting not only with membrane polymers but also with the organic matter. In addition, plant operators reported declines in free chlorine concentrations of up to 30% over CIP soaking time. As in this study the C × t was calculated considering a stable free chlorine concentration over the CIP soaking time, this may have led to an overestimation of the actual C × t to which the membranes are really exposed at full-scale. To avoid this probable overestimation in C × t for full-scale samples, the relationship between PVP content and Lp/Lp0 is plotted in Figure 3.11 for both sets. For the same PVP removal, the increase in Lp/Lp0 in The lower overall values of cleaning recovery for full-scale-aged membranes and its declining trend (Figure 3.3) can be ascribed to: (i) stronger interactions between foulants in mixed liquor and with residual fouling remaining on the membrane surface; (ii) much rougher surface as shown on SEM images (Figure 3.9), which enhances fouling especially by adsorption [START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF]; (iii) and the appearance of larger pores that can lead to higher amounts of matter inside the membrane structure causing more severe pore clogging, which is a less reversible form of fouling than floc adhesion or cake formation [START_REF] Lin | A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: Characteristics, roles in membrane fouling and control strategies[END_REF].

Statistical analysis were performed for changes in bigger pore volumes (>40 nm) and smaller pore volumes (<20 nm), separately, with respect to cleaning recovery and no significant linear correlations were found (p > 0.05). Ultimately, changes in cleaning recovery correlated well with the loss of hydrophilic agent and with the increase in Lp/Lp0 (r = 0.819, p = 0.046 and r = -0.897, p = 0.015, Pearson's test, respectively).

Since permeability correlated well with all other parameters, MBR operators should apply the measurement of Lp after CIP as an in situ diagnostic tool for membrane ageing. For increasing values, operators should expect a build-up of hydraulic irreversible fouling, which would lead to an increasing requirement for chemical cleanings. In addition, hydrophilic agent degradation seems to be the primary cause for all the other changes and as in situ FTIR measurements are becoming available, it could also provide important information on membrane ageing in MBR operation.

Figure 3.12, below, illustrates the changes in membrane properties during ageing at bench-scale and at full-scale found in this research. In addition, the conclusions that could be drawn from this chapter are presented afterwards.

CONCLUSIONS

The main conclusions deduced from the results obtained and discussed in this chapter can be summarized as:





In contrast to bench-scale ageing, full-scale-aged membranes experienced a decline in mechanical resistance due to MBR operating conditions, i.e., aeration, filtration and backwash. In addition, bigger pores are formed as a result of a more pronounced degradation of PVP when compared to bench-scale ageing, which ultimately led to an increasing Lp/Lp0 during the study period. At the same time, in the range of exposure dose investigated, no PVDF chemical alteration was observed.



The increase in Lp/Lp0 with age in full-scale samples is less intense than in bench-scale samples for the same PVP removal as a result of residual fouling on the membrane surface that also led to a decline in hydraulic cleaning recovery. Thus, the presence of organic matter at the membrane surface would play a protection role when the membrane is exposed to hypochlorite.



Lp/Lp0 and PVP content correlated well with all ageing changes among fullscale samples and can be applied for full-scale MBR operators as nondestructive in situ tools to monitor membrane ageing.



Since the major changes seemed to be related to PVP oxidation, efforts are needed to change membrane formulations in order to achieve a stronger bond between PVP or other hydrophilic agents and the main polymer.



Since bench-scale ageing protocols are not fully representative of full-scale MBR ageing, an accelerated ageing protocol at pilot-scale would be useful for validating long-term membrane performance.



The correlation between membrane properties changes and filtration performance data directly from process sensors would be useful to monitor ageing from an operational standpoint. Firstly, chronological data regarding operating conditions are presented for both MBRs, then cumulative indicators regarding membrane operation are calculated. This is followed by the analysis of the changes in membrane properties for membranes harvested during the period of this research. Finally, the filtration performance in the long-term is assessed directly by process data (mainly permeate flux and transmembrane pressure) to establish process oriented indicators to monitor membrane ageing.

MBR CHARACTERISTICS AND OPERATING CONDITIONS

CHARACTERISTICS OF INFLUENTS

The quality characteristics of influents and effluents and performances of the studied MBRs are compared in this section. SEM presented a more concentrated influent, in terms of carbon and nitrogen. The BOD/TKN ratio (Figure 4.1) from SEM is also higher than SAV (4.4 ± 0.9 and 2.6 ± 0.6, respectively).

These differences are mainly linked to the differences in pre-treatment processes applied on each WWTP, since SAV WWTP is composed of a higher performance (in terms of solid/liquid

MBR OPERATING CONDITIONS

MBR's operating conditions are also important in membrane ageing studies, since they are related to the characteristics of fouling, which triggers chemical cleanings and the combination of all these events lead to membrane ageing. The chronological behavior of the main operating conditions for both MBRs related to biological basin are presented in Figure 4. The averages and standard deviations of these parameters for the whole period are summarized in Table 4.2.

COD organic rate (

MBR BIOLOGICAL PERFORMANCES AND EFFLUENT QUALITY

Regarding performances, MBR are known to be an efficient, reliable and robust process. The results in terms of performances and effluent quality are presented for each studied WWTP in the following.

 SAV's MBR

The performances of SAV-MBR were determined in terms of TSS, COD, BOD, TN, NH4, TP and PO4 during the studied period (Figure 4.4). Averages and standard deviations for all these removals are presented in Table 4.3. TSS and BOD removals were kept stable and around 98 ± 1%, which is expected for an MBR ( 4.3). TSS and BOD were comparable to SAV and above 99% (Figure 4.5.A). COD removal also remained highly efficient at 97 ± 4% (Figure 4.5.A). Considering nitrogen treatment, NH4 removal was at 98 ± 12% and TN removal was kept at 84 ± 10 % due to the ratio BOD/TKN more optimal for denitrification process than for SAV-MBR. Regarding phosphorus-based compounds, TP and PO4 removals were at 97 ± 9% and 95 ± 11%, respectively. These eliminations were mostly stable over time. While SAV's plant operators reported inconsistent injection of ferric chloride throughout this period, SEM's plant operators reported over injection in comparison to the required dose. Since SEM-MBR operates at a lower flowrate than the designed one, the minimal flowrate of the pumps used to inject ferric chloride is still too high, resulting in ferric chloride overdoses.

OPERATING CONDITIONS OF MEMBRANE TANKS

Comparison of the operating conditions of the membrane tanks for the two studied

MLSS in membrane tank

(g / L) 6.6 ± 0.9 6.3 ± 0.8 6.4 ± 0.9 Analysis of the heterogeneity of operating conditions between the membrane tanks of

SAV-MBR and SEM-MBR

The chronological behavior of permeate flux was assessed to compare the membrane tanks each other for both MBRs.

CUMULATIVE PROCESS INDICATORS

MBR operating conditions cause changes in membrane properties and the mixed liquor filtration over time promotes the accumulation of residual fouling which ultimately affects membrane's filtration ability. In order to establish a historic of membranes based on factors that may impact its long-term performance, seven process indicators were continuously cumulated since the commission of both MBRs according to their availability and potential impact on the membrane lifetime. These indicators are selected in order to allow to distinguish the operation of each individual membrane tank. Comparing these graphs, it is clear that:

1. Exposure dose: SAV's membranes are exposed to a slightly lower annual exposure doses than SEM's membranes. Some heterogeneities appears for the different membrane tanks of each MBR. SAV's membranes from tanks SAV-A5, SAV-B5 and SAV-B10 were the least exposed during the studied period (around 20 000 ppm.h after 1533 days). SEM's membranes from tank UF4 were the least exposed to chlorine remaining under 60 000 ppm.h after 2254 days. Membrane tanks SAV-B8 and SAV-B13 presented the highest annual exposure dose rate of 9 290 and 9 226 ppm.h/year, respectively, and for SEM's membranes, tank UF8 presented the highest annual exposure dose rate of 15 086 ppm.h/year; 2. Specific permeate volume: According to the difference in operation of the MBR in terms of filtration, SAV's membranes filtered more annually (130.8 -171.5 m 3 /m 2 /year) than SEM's ones (39 -57.6 m 3 /m 2 /year). Membrane tanks SAV-A2 and SAV-B10 filtered the least in SAV-MBR, mainly due to longer downtime periods. For SEM-MBR, UF2, UF6 and UF8 filtered the least, also due to longer downtime periods.

3. Specific aeration demand (SADm): The cumulated specific aeration demand is also highly different between the two WWTPs, with a mean value of 7,500, 6,500 and

MEMBRANE AGEING BASED ON AUTOPSIES CHARACTERIZATION

According to chapter 3, hollow-fibers became weaker as the chlorine exposure dose increased in full-scale operation. Additionally, intrinsic permeability and PVP content are important characteristics to monitor membrane ageing. An increase in intrinsic permeability was observed as a result of formation of bigger pores due to PVP removal for samples aged at full-scale MBR.

Thus, samples harvested from SAV-MBR and SEM-MBR were characterized according to these properties and results are shown with respect to the exposure dose and the specific permeate volume hereby. The operating mode and its impacts on membrane properties for both MBRs are discussed as well.

MECHANICAL RESISTANCE

Regarding ultimate tensile strength (Figure 4.13), it seems that there are two different groups of membranes with different mechanical resistance: □ The first one presented higher ultimate tensile strength (H-UTS), over 300 MPa, namely, SAV-A1; SAV-A8; SAV-B1; SAV-B5; SAV-B8; SAV-B10; SAV-B11; SAV-B14, SEM-UF1; □ The other group with lower ultimate tensile strength (L-UTS), below 300 MPa, which are SAV-A5; SEM-UF3; SEM-UF4; SEM-UF5; SEM-UF7; SEM-UF8. In order to understand the two distinct range of ultimate tensile strength found in Figure 4.13, the uniaxial stress-strain curves of a pristine membrane, two ages membranes, which present characteristic evolutions from each groups (H-UTS with higher ultimate tensile strength [SAV-A8 exposed to 24,740 ppm.h] L-UTS with lower ultimate tensile strength [SEM-UF3 exposed to 100,000 ppm.h]) are presented in Figure 4.14 to illustrate these differences. The stress-strain curves from all membrane tanks studied in this research are presented in Appendix C.

A difference in the shape of the curve in the highlighted region of low deformations is observed.

The group of higher ultimate tensile strength presents a similar curve to the pristine membrane, indicating roughly the same rigidity. On the other side, the group of lower ultimate tensile strength presented a progressive deformation as tensile strength increased up to the break, which indicates progressive chain scission of the support polymer structure under tension until fissure and ultimately breakage. Nevertheless, since operators did not report fiber breakage, we may extrapolate the data from membrane tanks with low ultimate tensile strength and consider that even though ultimate tensile strength decayed to around 180 MPa, hollow-fibers would still be able to withstand stressed conditions in MBR conditions.

INTRINSIC PERMEABILITY

Modules of 18 cm of length were built with fibers harvested from both full-scale MBRs and intrinsic permeability was measured by water filtration after a cleaning of fibers in similar CIP conditions according to each MBR. These results are shown with respect to the exposure dose for the SAV-MBR group in Lp/Lp0 SAV-MBR samples presented a higher variation, fluctuating mostly between 1.0 and 1.5. For SEM-MBR, Lp/Lp0 values were higher and presented an increasing trend with both exposure dose and operating time. Since these membranes are weaker, they might be more increase of 17% in intrinsic permeability when cleaning exposure time increased from 6 h to 24 h [START_REF] Abdullah | Filtration and cleaning performances of PVDF membranes aged with exposure to sodium hypochlorite[END_REF]. In an earlier study, the authors showed that for equivalent C x t, longer exposures are more detrimental to the membrane material than higher concentrations [START_REF] Abdullah | Assessing the effects of sodium hypochlorite exposure on the characteristics of PVDF based membranes[END_REF].Thus, we may expect that the cleaning protocol from SEM-MBR is more aggressive to the membrane material and effective in removing foulants than the one from SAV-MBR, resulting in higher Lp/Lp0.

PVP DEGRADATION

ATR-FTIR measurements were carried out to quantify the changes in PVP content in these samples. The ratio between the characteristic peak of PVP (1674 cm -1 ) and the characteristic peak of PVDF (1403 cm -1 ) was used as a percentage of the value from the pristine membrane.

Thus, PVP content as a function of exposure dose is presented in Since changes in membrane properties are expected to alter the MBR's filtration performance, the following section describes membrane ageing based on data generated directly from process sensors from both MBRs.

MEMBRANE AGEING BASED ON FULL SCALE INDICATORS

NORMALIZED PERMEABILITY INDEX

Using permeability index to describe membrane ageing in MBR was reported by Fenu et al.

(2012) [START_REF] Fenu | Elaborating the membrane life concept in a full scale hollow-fibers MBR[END_REF]. This parameter corresponds to the ratio between permeate flux and transmembrane pressure during the process after each cleaning-in-place protocol. It differs from intrinsic permeability as it combines the mechanisms related of the evolution membrane properties to its interaction with the activated sludge, since it is measured during a 7 day-period of mixed liquor filtration in the full membrane tank, whereas intrinsic permeability is measured by filtering pure water in lab modules built with membrane samples harvested. They observed that the residual fouling accumulation could be one of the main responsible for membrane replacement.

Therefore, the normalized permeability index was calculated for SAV-MBR and SEM-MBR and is presented with respect to the C x t and the operating time in Regarding SEM-MBR, correlations with C x t and operating time were not statistically significant, but the C x t presented the higher correlation coefficient (-0.557). This MBR presented lower residual fouling rate than SAV-MBR, which might be related to the lower total filtration time, and associated total permeate volume. After the same 1400 operating days, SEM-MBR reached an average normalized permeability index of 0.84. However, this decline is not homogeneous between the different membrane tanks: as example, SEM-UF8 reached 0.75 after the same period.

A further analysis regarding all cumulative process indicators was performed in the following to determine the impacts of the other operating conditions in membrane ageing of each MBR and their membrane tanks.

CORRELATION BETWEEN PERMEABILITY INDEX AND PROCESS CUMULATIVE INDICATORS

The normalized permeability index was determined and correlated with the seven cumulative process indicators (Table 4.5). All indicators presented statistically significant correlations to the normalized permeability index for SAV (p-value< 0.05), but among those that are measured for each individual membrane tank, the operating days presented a slightly higher correlation coefficient. Regarding the sludge quality parameters, MLSS stood out over supernatant COD or ∆COD. This higher correlation may indicate that sludge flocs and particulates may play a bigger role in membrane ageing than colloidal fractions. However, this slight difference may Additionally, SAV-MBR presented high variations of non-filtration time according to each membrane tank. A group of membrane tank with higher non-filtration time was compared to a group presenting lower non-filtration time and the differences in normalized permeability index were not statistically significant. These results are presented in Appendix F.

As a conclusion, it appears that the decline in permeability index at macro-scale may be linked to the residual fouling, related to the cumulated permeate volume, and these evolutions are equivalent in terms of order of magnitude between the two studied MBRs. However, it could be considered that the significant evolutions observed at micro scale do not affect at this stage the filtration performance at the industrial scale regarding the relative short period of time of the study compared to the membrane life-time expectations. The study of the evolution at fullscale, in parallel of the characterization of sampled membranes, must be continued in order to evaluate the consequences of these micro-scale observations on full-scale performances and the potential difference in impact between MBRs, linked to operating conditions.

CORRELATIONS BETWEEN MEMBRANE AUTOPSIES PROPERTIES AND NORMALIZED PERMEABILITY INDEX

Membrane properties is expected to affect their ability to filter organic matter. Since membrane were harvested just before each CIP protocol and characterized in the lab after cleaning at Authors measured fouling rate of harvested samples by filtration of a model solution and found higher fouling rates for membranes presenting lower hydrophilic agent contents.

Therefore, hydrophilic agent content and intrinsic permeability, in a minor precision, have proven to be key parameters to monitor membrane ageing and should be prioritized in the case of membrane autopsies.

These results also showed that efforts are needed into the conceptualization of more stable membrane compositions, especially, with stable hydrophilicity and pore size under MBR operating conditions.

CONCLUSIONS

Membrane autopsies

 Mechanical resistance is not a concern in full-scale MBR equipped with Zeeweed membranes, which are supported PVDF hollow-fibers. No signs of suspended solids are found in permeate over the duration of this research.

 Two MBRs with distinctive operating conditions were analyzed in long-term operation.

Membranes from SEM-MBR are exposed to higher chlorine doses and less filtration, which contributed to higher intrinsic permeabilities than SAV-MBR. PVP content was lower for membranes from SAV-MBR at a same C x t, but equivalent for the same permeate volume produced. This indicates that not only chlorine exposure is responsible for the degradation of PVP, but also hydrodynamic conditions (linked to aeration and filtration dynamics) are likely to affect this degradation.

Process indicators

 For SAV-MBR, sludge flocs and particulates seemed to contribute more to the decline in permeability index than the colloidal fractions, since cumulated filtered MLSS presented higher correlation coefficient than cumulated filter COD or ∆COD.

 SEM-MBR decline in permeability index correlated better with the exposure dose, probably due to higher chlorine exposure and less permeate filtration rates.

 SAV-MBR's strategy of conserving membranes in low chlorine levels (3 -15 ppm) during non-filtration periods seemed to not affect permeability index decline.

 Decline in normalized permeability index for SAV-MBR and SEM-MBR were similar with respect to specific permeate volume.

Coupling process indicators and membrane autopsies

 Permeability index correlated well to the hydrophilic agent content and intrinsic permeability of harvested membranes. Thus, these measurements should be a priority when autopsies are carried out. In addition, efforts are needed into the conceptualization of more stable membrane compositions, especially, with stable hydrophilicity under chlorine exposure and hydraulic dynamics.



The decline in permeability index at macro-scale is mainly linked to the residual fouling, related to the cumulated permeate volume, and these evolutions are equivalent in terms of order of magnitude for both MBRs studied. The significant evolutions observed at micro scale do not affect at this stage the filtration performance at the industrial scale regarding the relative short period of time of the study compared to the membrane life-time expectations. The study of the evolution at full-scale, in parallel of the characterization of sampled membranes, must be continued in order to evaluate the consequences of these micro-scale observations on full-scale performances and the potential difference in impact between MBRs, linked to operating conditions. Only one study monitored ageing of PVDF membranes in MBR considering rather data generated from sensors in the process. Therefore, this thesis focused on addressing the lack of knowledge regarding supported PVDF membrane ageing in MBR conditions treating urban wastewater.

GENERAL CONCLUSIONS AND

Effect of chlorine exposure on supported PVDF membrane by single soaking in NaOCl at bench-scale Supported PVDF membranes were single soaked in 1000 ppm NaOCl solutions at pH 9.0 for C x t ranging from 0 to 750 000 ppm.h. These samples were characterized according to a wide range of properties. Regarding mechanical resistance, chlorine exposure did not affect significantly ultimate tensile strength and elongation at the breaking point; however, hollowfibers seemed to become slightly more rigid in their elastic zone (low deformation zones). These properties are related to the support polymer, which is likely to be based on PET, a highly chlorine resistant polymer.

Chlorine exposure highly influenced filtration properties of these membranes. Intrinsic permeability presented a three-phase behavior. At first, it increased up to 90% of initial intrinsic permeability for a C x t of 78,000 ppm.h, and then it decreased to the range of its initial value between 78,000 ppm.h and 150,000 ppm.h. After 150,000 ppm.h, intrinsic permeability fluctuated around its initial value. It was demonstrated that the first increasing phase correlated well to the increasing membrane porosity with formation of small pores (< 20 nm), which is likely to be caused by the degradation of PVP into secondary products. The following decreasing phase might be related to changes in PVDF chains observed with the appearance of a band at 1650 cm -1 in FTIR spectra related to a carbonyl group, usually formed after defluorination. It seems that PVP should be eliminated up to a certain point, and then this restructuration of PVDF takes place. Further analysis are necessary to establish the exact mechanism behind this decreasing permeability.

Cleaning recovery increased from 56% to 65% after ageing membranes up to 125,000 ppm.h and should be related to a higher negatively charged surface, caused by the degradation of PVP into secondary products. After that, cleaning recovery decreased back to 51% probably due to topographic changes, with a potential increase in roughness contributing to foulant adsorption.

Representability of accelerated ageing protocols at bench-scale towards membrane ageing in full-scale MBR conditions Contrarily to bench-scale ageing, membranes harvested from full-scale MBR presented a decline of 45% in ultimate tensile strength and relatively stable elongation at the breaking zone.

At their elastic zone, membranes presented lower rigidity (Young's modulus decreased by 33%). However, fiber breakage was never reported by the operators of the studied WWTP.

Furthermore, intrinsic permeability increased during the studied period reaching 1.6 times the initial value at 98,000 ppm.h. This increasing trend in permeability is also linked to the PVP degradation and an increase in porosity with appearance of smaller (< 20 nm) and bigger (>40 nm) pores. PVP content reached lower levels for full-scale MBR ageing than at bench-scale (25% against 40%). Based on FTIR spectra, it seems that PVP is dislodged from membrane structure, creating bigger pores and contributing to the increase in permeability.

However, when this parameter is plotted against filtration related indicators (cumulated specific permeate volume or specific MLSS mass), both MBRs presented the same rate of decline. This reinforces that operating conditions (filtration, aeration, biomass presence)other than chlorine exposure play an important role in membrane ageing.

Data generated directly from the process sensors were collected and filtration performance of MBRs was measured based on the permeability index, which is the ratio between permeate flux at 20°C and transmembrane pressure during filtration after CIP protocol. This indicator decreased faster for SAV-MBR than SEM-MBR in average with respect to the operating time (27% against 16% decline in 1400 days). However, the membrane tank SEM-UF8 decreased by 25% at the same period but this membrane tank presented a much higher chlorine exposure per permeate volume than all other membrane tanks due to accidentally prolonged CIP protocols. This data suggests that SEM-MBR choice to keep all membrane tanks filtering during a shorter period daily is efficient in maintaining permeability index at high levels over time, but special attention should be paid to control chlorine exposure.

Depending on the MBR operating conditions, some factors seemed to play bigger roles on the decline of permeability index. For SAV, the decline correlated well with cumulated filtered MLSS, whereas for SEM, chlorine exposure dose presented the best correlation. This might be related to the fact that SAV's membranes presented much higher cumulated permeate volume for the same exposure dose than SEM's membranes.

Whenever a membrane tank from SAV-MBR was not available for filtration for several days, membranes were kept in NaOCl solutions at low concentrations. This choice did not affect permeability index. Data in the later stages of membrane's life are needed to validate this operating choice.

Key indicators to monitor membrane ageing in full-scale MBR

To monitor membrane ageing, key indicators should provide information about filtration capacity and membrane properties. Permeability index presented statistically significant correlation to PVP content and intrinsic permeability. Therefore, these indicators are suitable to monitor membrane ageing, when PVP is degraded, intrinsic permeability is likely to increase and permeability index decreases.

Regarding cumulated process indicators, the C x t did not present the best correlations with membrane properties or permeability index. On the other hand, data related to filtration such as cumulated permeate volume, cumulated filtered MLSS mass and supernatant COD are promising to monitor membrane ageing and become new benchmarks for membrane lifespan.

PERSPECTIVES

Implications in terms of membrane lifespan

During the studied period, although bigger pores may be observed on the membrane surface, no signs of degradation in permeate quality were observed. Therefore, integrity issues did not promote membrane replacement over the 7 years considered in this research.

Based on the decline in permeability index with respect to the cumulated process indicators and assuming that the linear trends observed up to this date remain until membrane replacement, membrane end-life could be estimated considering average and peak filtration conditions. Endlife was predicted for SAV-MBR and SEM-MBR in 11 -13 and 18 -22 operating years, respectively. However, membranes from SAV-MBR would have filtered around 1,900 -2,300 m3/m2 (9,000 -11,000 kg MLSS/m2) as opposed to 1,100 -1,300 m3/m2 (6500 -7800 kg MLSS/m2) for SEM-MBR. Further researches considering data from a full cycle of life for these membranes are still necessary to validate this approach.

Effect of chlorine exposure on supported PVDF membrane by single soaking in NaOCl at bench-scale A further characterization of PVDF chains during chlorine exposure would provide useful information about the changes in membrane material and could help to validate the assumptions on the filtration properties evolution of these membranes over soaking in chlorine. Size exclusion chromatography might be able to determine crosslinking and chain scission.

Crystallinity can be measured by differential scanning calorimetry and more information about functional groups may be obtained by nuclear magnetic resonance.

Representability of accelerated ageing protocols at bench-scale towards membrane ageing in full-scale MBR conditions Based on the differences observed in membrane ageing by single soaking and in full-scale MBR, establishing an accelerated ageing protocol that takes into consideration filtration cycles, aeration and organic matter presence in addition to chlorine exposure would be an interesting tool to predict long-term performance. Different membranes could be tested, compared and these data could be taken into consideration when designing a full-scale MBR or replacing membranes in an already built one.

Preliminary tests were performed in a 2-m 3 membrane pilot installed in SAV WWTP (Figure during filtration between two CIP protocols would be useful to monitor membrane ageing.

Appendix A:

Specific aeration demand with respect to membrane surface (SADm) for the 14 membrane tanks studied from SAV-MBR

Figure 1 . 1 :

 11 Figure 1.1: Summarized scheme of a wastewater treatment plant. ..........................................

Figure 1 . 2 :

 12 Figure 1.2: World market value of MBR technology as a function of years (Based on data from [2,41]). ......................................................................................................................

Figure 1 . 3 :

 13 Figure 1.3: Schematic representation of (A) side-stream MBR and (B) submerged MBR and (C) membrane dedicated tank MBR. .................................................................................

Figure 1 . 4 :

 14 Figure 1.4: Illustration of hollow-fiber filtration from outside to inside..................................

Figure 1 . 5 :

 15 Figure 1.5: Membrane process classification based on pore size.............................................

Figure 1 . 6 :

 16 Figure 1.6: Fouling mechanisms (based on [52]) .....................................................................

Figure 1 . 7 :

 17 Figure 1.7: Different components in the mixed liquor. ............................................................

Figure 1 . 8 :

 18 Figure 1.8: Cake formation on membrane surface and its effects on TMP over time. Figure from [55]. ...........................................................................................................................

Figure 1 . 9 :

 19 Figure 1.9: Correlation between performance factors, membrane characteristics and analytical techniques established in the literature. The line thickness corresponds to the number of references that suggest the particular relation (from [8]). .................................................

Figure 1 . 10 :

 110 Figure 1.10: Interactions among chemical properties (purple), physical and structural characteristics (orange) and filtration performance (blue) used to explain results in literature. Numbers and thickness of arrows represent the amount of publications that discuss the relation between these properties. ...................................................................

Figure 1 . 11 :

 111 Figure 1.11: Formation of phenol groups in the PES chain and PES chain scission to form sulfonic acid groups and chlorinated phenols. Based on [74,75]. .....................................

Figure 1 . 12 :

 112 Figure 1.12: Defluorination mechanism of PVDF in alkali condition Based on [80]. ............

Figure 1 . 13 :

 113 Figure 1.13: Determination of end-point of operation in MBR. Red squares represent maximum permeability index after cleaning and green triangles represent minimum permeability index before cleaning. From: [6]. .................................................................

Figure 2 . 1 :

 21 Figure 2.1: Infrared spectra for the ZeeWeed® support membrane material (A) and Polyethylene terephthalate (B) sample from HR Nicolet sampler library extracted from OMNIC software (Thermo fischer, USA). ........................................................................

Figure 2 . 2 :

 22 Figure 2.2: Pictures taken of intact ZeeWeed® hollow-fiber (left) and a hollow-fiber peeled off with its PET support material visible (right)................................................................

Figure 2 . 3 :

 23 Figure 2.3: ZeeWeed® 500D membrane module. ...................................................................

Figure 2 . 4 :

 24 Figure 2.4: Schematic representation of Seine Morée WWTP. ...............................................

Figure 2 . 5 :

 25 Figure 2.5: Soaking time during CIP protocols for SEM-MBR. .............................................

Figure 2 . 6 :

 26 Figure 2.6: Schematic representation of SAV WWTP. ...........................................................

Figure 2 . 7 :

 27 Figure 2.7: Schematic representation of SAV-MBR (A) and photo of anoxic zone from one of the biological basins (B). .................................................................................................

Figure 2 . 8 :

 28 Figure 2.8: Schematic representation of a membrane tank, cassette and module of ZeeWeed® 500D hollow-fibers. .........................................................................................................

Figure 2 . 9 :

 29 Figure 2.9: Chlorine concentrations of soaking solutions harvested at the beginning of CIP protocols for SAV-MBR and SEM-MBR (A) and pH of these solutions for SAV-MBR (B). ...................................................................................................................................

Figure 2 . 10 :

 210 Figure 2.10: Schematic representation of membrane harvesting place for SAV-MBR (A) and SEM-MBR (B). ...............................................................................................................

Figure 2 . 11 :

 211 Figure 2.11: Cleaned permeate flowrates from membranes tank SAV-A1 from 16/04/2018 to 30/04/2018. ......................................................................................................................

Figure 2 . 12 :

 212 Figure 2.12: Permeability index and flux at 20°C for membranes tank SAV-A1 from 16/04/2018 to 30/04/2018. ...............................................................................................

Figure 2 . 13 :

 213 Figure 2. 13: Permeability index and permeate fluxes for membranes tank SAV-A1 as a function of time with periods of flux variations highlighted in violet. ............................

Figure 2 . 14 :

 214 Figure 2.14: Variations of permeability index (A) and TMP (B) as a function of flowrate for membranes tank SAV-A1 before and after CIP protocols in 2018 and 2019. ................

Figure 2 . 15 :

 215 Figure 2.15: Permeability indexes after the first CIP protocol in April 2018 (A) and the third one in October 2019 (B) for membranes tank SAV-A1. .................................................

Figure 2 . 16 :

 216 Figure 2.16: Boxplot of permeability indexes according to their range of flux for short period just after the first CIP protocol in April 2018 (A) and the third one in October 2019 (B) from membranes tank SAV-A1. ......................................................................................

Figure 2 . 17 :

 217 Figure 2.17: Referential J/TMP for membranes tank SAV-A1 (A) and membranes tank SEM-UF1 (B). ...........................................................................................................................

Figure 2 . 18 :

 218 Figure 2.18: Normalized permeability index for different permeate fluxes as a function of the operating days for membranes tank SAV-A1 (A) and SEM-UF1 (B). ...........................

Figure 2 . 19 :

 219 Figure 2.19: Averages of normalized permeability index for membrane tanks SAV-A1 (A) and SEM-UF1 (B). ..........................................................................................................

Figure 2 . 20 :

 220 Figure 2.20: Permeability index of membrane tank SAV-A1 around the first CIP protocol.

Figure 2 . 21 :

 221 Figure 2. 21: Stress-strain curve of pristine ZeeWeed® membrane. .....................................

Figure 2 . 22 :

 222 Figure 2.22: Standard behavior of the fouling reversibility test. ...........................................

Figure 2 . 23 :

 223 Figure 2.23: Filtration and backwash cycles in the cleaning recovery assay. ........................

Figure 2 . 24 :

 224 Figure 2.24: FTIR spectra of a pristine ZeeWeed® membrane. ............................................

Figure 3 . 1 :

 31 Figure 3.1: Ultimate tensile strength (A), Ultimate elongation (B) and Young's modulus (C) for bench-scale-and full-scale-aged membranes as a function of the cumulative exposure dose to hypochlorite.........................................................................................................

Figure 3 . 2 :

 32 Figure 3.2: Changes in intrinsic permeability of samples aged at bench-scale and full-scale with respect to virgin membrane permeability (Lp/Lp0 ratio) as a function of the cumulative exposure dose to hypochlorite. .....................................................................

Figure 3 . 3 :

 33 Figure 3.3: Cleaning recovery (%) after two cycles of filtration and hydraulic backwash on membranes aged at bench scale and at full scale as a function of the cumulative exposure dose to hypochlorite. Error bars represent experimental error. .......................................

Figure 3 . 4 :

 34 Figure 3.4: IR spectrum of a pristine ZeeWeed® membrane. ...............................................

Figure 3 . 5 :

 35 Figure 3.5: Full-range FTIR spectra (600-1800 cm-1) of membranes aged at bench-scale and full-scale (A) and enlarged view of the characteristic region of PVDF (B) and PVP and its degradation products (1500-1800 cm-1) for samples aged at bench-scale (C) and at fullscale (D). ..........................................................................................................................

Figure 3 . 6 :

 36 Figure 3.6: Change in PVP content in bench-scale and full-scale-aged membranes as a function of the full range of exposure dose (A) and a zoom in the zone between 0 and 100,000 ppm.h (B). ..........................................................................................................

Figure 3 . 7 :

 37 Figure 3.7: Pore volume and size distribution of aged membranes at bench-scale (A) and at full-scale (B) as a function of the exposure dose to hypochlorite. ..................................

Figure 3 . 8 :

 38 Figure 3.8: SEM images zoomed at x500 for pristine membranes (A), bench-scale aged membranes at 78,000 ppm.h (B), and 500,000 ppm.h (C). .............................................

Figure 3 . 9 :

 39 Figure 3.9: SEM images of full-scale aged samples harvested after 42,000 ppm.h (A), 62,000 ppm.h (B) and 98,000 ppm.h (C) chlorine exposure doses. ............................................

Figure 3 . 10 :

 310 Figure 3.10: Infrared spectra of support material of pristine and full-scale aged ZeeWeed® membranes after peeling off PVDF skin. ........................................................................

Figure 3 . 11 :

 311 Figure 3.11: Relationship between PVP content removal and the changes in Lp/Lp0 for membranes aged at bench-scale and at full-scale. ...........................................................

Figure 3 . 12 :

 312 Figure 3. 12: Illustration of changes in membrane properties during ageing at bench-scale by NaOCl single soaking and at full-scale MBR. ................................................................

Figure 4 . 1

 41 Figure 4.1 Ratio BOD5/TKN for SEM (A) and SAV (B) MBR. ...........................................

Figure 4 . 2

 42 Figure 4.2 Chronological data of COD organic rate (A for SEM and D for SAV), N-NH4 to sludge ratio (B for SEM and E for SAV) and MLSS concentration in the aerated zone (C for SEM and F for SAV) since commissioning of both MBRs. ......................................

Figure 4 . 3 :

 43 Figure 4.3: Chronological data of SRT (A for SEM and C for SAV), and Temperature (B for SEM and D for SAV). .....................................................................................................

Figure 4 . 4 :

 44 Figure 4.4 : SAV-MBR removal in terms of COD, BOD and TSS (A), TN and NH4 (B) and TP and PO4 (C). ...............................................................................................................

Figure 4 .

 4 Figure 4.5 SEM-MBR removal in terms of COD, BOD and TSS (A), TN and NH4 (B) and TP and PO4 (C). .....................................................................................................................

Figure 4 . 6 :

 46 Figure 4.6 : Total suspended solids in permeate from SAV-MBR (A) and SEM-MBR (B).

Figure 4 . 7

 47 Figure 4.7 Supernatant COD in recirculation channel from membrane tanks to aerated zone for SEM-MBR (A) and SAV-MBR (B). .........................................................................

Figure 4 . 8

 48 Figure 4.8 MLSS in membrane tanks for SEM-MBR (A) and SAV-MBR (B). ....................

Figure 4 . 9 :

 49 Figure 4.9: Permeate instantaneous flux at 20°C for SEM-MBR (A) and SAV-MBR (B). ..

Figure 4 .

 4 Figure 4.10 Exposure dose (C x t) for membranes tanks from SEM-MBR (A), SAV-MBR row A (B) and SAV-MBR row B (C). ....................................................................................

Figure 4 . 11 :

 411 Figure 4.11 : Specific permeate volume evolutions for membranes tanks from SEM-MBR (A) SAV-MBR row A (B) and SAV-MBR row B (C). .........................................................

Figure 4 . 12 :

 412 Figure 4.12: Specific aeration demand for membranes tanks from SEM-MBR (A), SAV-MBR row A (B) and SAV-MBR row B (C) with respect of days since commissioning date. .

Figure 4 . 13 :

 413 Figure 4.13 : Ultimate tensile strength of membranes harvested from both MBRs with respect to exposure dose (A) and operating days (B). .................................................................

Figure 4 . 14 :

 414 Figure 4.14: Uniaxial stress-strain curves for a pristine membrane (0 ppm.h), SAV-A8 at 24,740 ppm.h representing the higher ultimate tensile strength and SEM-UF3 at 100,000 ppm.h representing the lower ultimate tensile strength. ..................................................

Figure 4 . 15 :

 415 Figure 4.15 : Lp/Lp0 with respect to the exposure dose for membrane tanks from SAV-MBR (A) and SEM-MBR (C) and with respect to the time of operation from SAV-MBR (B) and SEM-MBR (D). ........................................................................................................

Figure 4 . 16 :

 416 Figure 4.16: PVP content of membranes harvested from SAV-MBR with respect to exposure dose (A) and operating time (B) and from SEM-MBR with respect to the exposure dose (C) operating time (D). ....................................................................................................

Figure 4 . 17 :

 417 Figure 4.17 : Relationship between cumulated permeate volume and exposure dose for individual membrane tanks from SEM-MBR and overall tanks from SAV-MBR. ........

Figure 4 . 18 :

 418 Figure 4.18: PVP content from membranes harvested from SAV-MBR, SEM-MBR and from bench-scale with respect to the C x t (A) and for SAV-MBR and SEM-MBR with respect to the specific permeate volume (B). ...............................................................................

Figure 4 . 19 :

 419 Figure 4.19: Normalized permeability index for SAV-MBR with respect to C x t (A) and to operating time (B) and similarly for SEM-MBR with respect to C x t (C) and to operating time (D)............................................................................................................................

Figure 4 . 20 :

 420 Figure 4.20: Normalized permeability index as a function of cumulated specific permeate volume (A), specific filtered MLSS (B) for SAV-MBR and specific permeate volume (C) and specific filtered MLSS (D) for SEM-MBR. .............................................................

Figure 4 . 21 :

 421 Figure 4.21: Normalized permeability index with respect to hydrophilic agent content and intrinsic permeability. ......................................................................................................

Figure GC. 1 :

 1 Figure GC.1: Pilot-scale membrane tank used in preliminary tests of accelerated ageing in onsite conditions. .................................................................................................................

Figure B. 1 :

 1 Figure B.1: Specific filtered MLSS for membranes tanks from SEM-MBR (A) row A (B) and row B (C) in SAV-MBR with respect of days since commissioning date. .....................

Figure B. 2 :

 2 Figure B. 2: Specific filtered supernatant COD for membrane tanks in row A (A) and B (B) from SAV-MBR. .............................................................................................................

Figure B. 3 :

 3 Figure B. 3: Specific filtered supernatant ∆COD for membrane tanks in row A (A) and B (B) from SAV-MBR. .............................................................................................................

Figure C. 1 :

 1 Figure C.1: Uniaxial stress-strain curves from samples harvested from each membrane tank studied from SAV-MBR and SEM-MBR. Samples were separated according to group H-UTS (A) and L-UTS (B). .................................................................................................

Figure D. 1 :

 1 Figure D.1: Ultimate tensile strength with respect to specific ∆COD (A), Lp/Lp0 (B) and PVP content (C) with respect to specific filtered MLSS. ........................................................

Figure E. 1 :

 1 Figure E.1: Normalized permeability index with respect to specific aeration demand for SAV-MBR (A) and SEM-MBR (B) and with respect to the filtered supernatant COD (C) and ∆COD (D), specifically for SAV-MBR. .........................................................................

Figure F. 1 :

 1 Figure F.1: Normalized permeability index with respect to the number of days since commissioning date (A) and specific permeate volume (B) and exposure dose (C) from SAV-MBR. ......................................................................................................................

Figure F. 2 :

 2 Figure F.2: Normalized permeability index as a function of the cumulated specific permeate volume produced for membrane tanks from SAV-MBR and SEM-MBR. .....................

Table 4 . 2 :

 42 Averages and standard deviations of the main operating conditions of SAV and SEM MBRs. * means differences between these values are not statistically significant. ......................................................................................................................................... Table 4.3 Quality characteristics of effluents along with performances from SEM-MBR (data from May/2014 to January/2021) and SAV-MBR (data from November/2017 to June/2021). ......................................................................................................................

  Fenu et al. (2012) confirmed that increase in operating pressure was the main end-of-life trigger assessing flowrate and pressure date from an MBR treating up to 250 m 3 /h in Schilde wastewater treatment plant equipped with PVDF membranes. Authors went further into defining an end-of-life concept based on the permeability index (ratio of permeate flux and TMP) decline over time. This decline was attributed to residual fouling that accumulates in membranes even though cleaning protocols are performed. However, no membrane characterization was performed to identify changes in the physical-chemical properties and to quantify this accumulation of residual fouling [6]. Mailler et al. (2021) also determined the lifespan of membranes in an MBR equipped with polyether sulfone (PES) membranes based

3 ) 4 )

 34 Describe membrane ageing based on membrane properties and in-situ filtration performance at different full-scale MBR conditions: Changes in membrane properties and permeability index generated from process sensors will be analyzed since the commissioning date of each MBR; Determine the effects of different operating conditions on membrane ageing in MBR based on changes in membrane properties and on in-situ filtration performance: Changes in filtration performance obtained from full-scale data will be correlated to the changes in membrane properties and key properties will be identified. To this aim, chapter 1, includes a bibliographic study, contextualizes the importance of understanding membrane ageing and highlights the scientific bottlenecks related to supported PVDF membrane ageing. Then, chapter 2 describes the materials and methods employed in the thesis to achieve the four objectives mentioned above. Chapter 3 describes supported PVDF membrane ageing by chlorine exposure at bench-scale and compares this accelerated ageing protocol to ageing at full-scale MBR, answering objectives (i) and (ii). These results were published in Separation & Purification Technology [29]. Afterwards, results presented in chapter 4 characterize membrane ageing at different operating conditions at full-scale MBR and determine key indicators related to these on-site conditions to monitor ageing in MBR, elucidating objectives (iii) and (iv). Finally, a general conclusion presents the main scientific contributions and operational benefits from this research, with open perspectives. The present thesis was developed in a partnership between Laboratoire de Génie Chimique (LGC) from Toulouse, Institut national de recherche pour l'agriculture, l'alimentation et l'environnement (INRAE) from Antony and Service Public de l'Assainissement Francilien (SIAAP). This partnership was built in the scientific program Modélisation, Contrôle, Optimisation des Procédés d'Epuration des Eaux (MOCOPEE), which brings together both scientific and operational partners in order to address actual operational difficulties and future changes in the wastewater industry With population growth, climate change, emerging pollutants and poor sanitation, water scarcity already affects more than 40% of the global population and the gap between freshwater

Figure 1 . 1 :

 11 Figure 1.1: Summarized scheme of a wastewater treatment plant.

Figure 1 . 2 :

 12 Figure 1.2: World market value of MBR technology as a function of years (Based on data

Figure 1 . 3 :

 13 Figure 1.3: Schematic representation of (A) side-stream MBR and (B) submerged MBR and

Figure 1 . 4 :

 14 Figure 1.4: Illustration of hollow-fiber filtration from outside to inside.

Figure 1 . 5 :

 15 Figure 1.5: Membrane process classification based on pore size.

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 38 structure contains 59.4 wt% fluorine and 3%wt hydrogen with a degree of crystallinity from

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 40 CP : concentration of the species in permeate; C0: concentration of the species in the feed.

Figure 1 . 6 :

 16 Figure 1.6: Fouling mechanisms (based on [52])

Figure 1

 1

Figure 1 . 7 :

 17 Figure 1.7: Different components in the mixed liquor.

Figure 1 . 8 .

 18 Firstly, the passive adsorption of high molecular weight polysaccharides (less biodegradable), proteins and colloids causes a rapid, but short increase in TMP in the early stage. After this sudden

CHAPTER

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 51 directly injected by backwashing into the membranes or membranes can simply be soaked into cleaning solutions for a determined time.

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 52 Studies presented in the Wang et al. (2014) suggested that the use, either jointly or sequentially of reagents have a positive effect on membrane recovery [65]. It was shown that a solution composed of NaOCl and NaOH could increase relative flux, i.e. flux of cleaned membrane related to flux of fouled membrane, up to 150%, whereas the increase using the isolated solutions was only 60% for NaOCl and 10% for NaOH. Furthermore, the long-term cleaning with hypochlorite and acids is widely used in wastewater treatment plants as they affect inorganic and organic fouling.

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 57 hydrophobicity, retention, permeability and fouling rate [66]. As already discussed, ageing is based on the evolution of several properties followed by different techniques in published studies. In Robinson et al. (2016), authors presented the Figure 1.9 below, which correlates performance factors with membrane properties and the possible analytical methods to determine these values established from the membrane literature [8].

Figure 1 . 9 :

 19 Figure 1.9: Correlation between performance factors, membrane characteristics and analytical

Figure 1 . 10 :

 110 Figure 1.10: Interactions among chemical properties (purple), physical and structural

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 65 integrity faults even at small hypochlorite exposure concentrations. Besides, changes in pore size distribution, decrease in breaking force (from 240 to 80 MPa), formation of carboxyl, aldehydes and ketones end groups due to cellulose oxidation and departure of additives, such as PVP, were confirmed by ATR-FTIR spectra and SEM images. CE membranes also increased intrinsic permeability when cleaned by NaOCl (40 ppm) or NaOH (300 ppm) for 72h, probably due to pore enlargement. For PVC membranes, pristine and fouled membranes exhibited NaOCl sensitivity. Enlargement of pore size during exposure to hypochlorite solutions of concentrations as low as 1% was observed. For higher concentrations, permeability first increased followed by a decline caused by twists and wrinkles.

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 66 range of pH 6.0 -8.0. HClO predominance (pH 6.0) and its coexistence with OCl -(pH 8.0) promotes: (1) the formation of phenol groups in the PES chain by radical oxidation (OH•) (Figure 1.11.A) confirmed by measurements of tangential streaming current measurements; (2) PES chain scission forming sulfonic acid groups (Figure 1.11.B) confirmed by the presence of a C-Cl covalent bond in XPS spectra [75].

Figure 1 . 11 :

 111 Figure 1.11: Formation of phenol groups in the PES chain and PES chain scission to form

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 67 Different characteristics in PES/PVP membranes ageing in contact with hypochlorite at pH 8.0 were assessed by [70,71]. Both studies observed PVP dislodgement and degradation, which resulted in an increase in pore size. In Hanafi et al. (2016), the authors aged membranes by

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 68 membranes exhibited a similar increase in permeability. However, the dynamic exposure by filtration and backwashing of NaOCl solution promoted a slightly greater degradation of PVP and decline in elongation at break. It is worth noting that authors considered that the number of experiments carried out with filtration/backwashing cycles was not enough to conclude an effect of these exposure modes on membrane ageing. Exposure dose = [Total free chlorine]×t n Eq. 12 In PVDF membranes with hydrophilic additive, Marbelia et al. (2019) observed the same effect of pore enlargement and increasing water permeability as the hydrophilic agent leached out [76]. Contrastively, Ravereau et al. (2016) found a slight decrease in pore size for PVDF/PVP

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 69 them assess ageing by soaking membranes in hypochlorite solutions to simulate exposure. Only four of them aged membranes by cycles of filtration of model solutions and cleaning with

  tensile strength with NaOCl contact[START_REF] Le-Clech | Membrane bioreactors and their uses in wastewater treatments[END_REF][START_REF] Fenchel | Chapter 1 -Bacterial Metabolism[END_REF][START_REF] Habib | Influence of relaxation modes on membrane fouling in submerged membrane bioreactor for domestic wastewater treatment[END_REF][START_REF] Robles | Factors that affect the permeability of commercial hollow-fibre membranes in a submerged anaerobic MBR (HF-SAnMBR) system[END_REF][START_REF] Zsirai | Efficacy of relaxation, backflushing, chemical cleaning and clogging removal for an immersed hollow fibre membrane bioreactor[END_REF]. However,[START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF] obtained little or no impact of NaOCl on mechanical properties for non-supported PVDF membranes either with or without hydrophilic agent at pH ranging from 6.0 to 11.5[START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF].[START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF] aged non-supported PVDF membranes in NaOCl solutions ranging from pH 3 to 11 and only observed weakening of fibers at pH 11[START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. This decline in tensile strength was comparable to the weakening of membranes exposed to NaOH at pH 11, which is known to cause defluorination in the PVDF matrix[START_REF] Menon | Impact of cleaning agents on properties of tubular polyvinylidene fluoride (PVDF) membrane[END_REF]. Thus, they attributed this effect to defluorination of PVDF molecules rather than any changes in additives. Both studies aged their membranes at similar NaOCl concentrations (4000 and 5000 ppm, respectively), butLi et al. (2021) used NaOH to adjust pH at 11 in their research, whereas Ravereau et al. (2016) only mentioned the use of NaOCl [13,17].

  observed after extensive exposition to hypochlorite. Depending on pH, the PVP degradation rate and degradation products would change.[START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF] aged PVDF/PVP membranes for 500,000 ppm.h at pH from 3 to 11 and found higher PVP degradation in the pH zone of 7.0 -9.0[START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. At pH > 9.0, PVP underwent ring opening, forming carboxylic acids, whereas at lower pH, the formation of succinimide was preferred. Ren et al. (2021) exposed membranes to 200 ppm NaOCl at pH 11 at doses ranging from 4,800

  Vanysacker et al. (2014) found no changes in contact angle measured on aged membranes for 300,000 ppm.h [19]. Hajibabania et al. (2012) attributed the decreasing contact angle they observed to the impact of roughness and pore size on the analytical method [23]. Puspitasari et al. (2010) and Zhang et al. (2017) obtained increasing contact angle at first, followed by a

Figure 1 74 Figure 1 . 12 :

 174112 Figure 1.12.

  these compounds. Hajibabania et al. (2012) filtered a model solution composed of 20 mg.L -1

Despite 87 Figure 1 . 13 :

 87113 Figure 1.13: Determination of end-point of operation in MBR. Red squares represent

CHAPTER 1 :

 1 BIBLIOGRAPHIC STUDY 88 The same conclusions about permeability decay were draw by Cote et al. (2012) in a study using the data set of ZeeWeed® membranes shipped across North America in plants bigger than 200 m 3 .d -1 over 15 years

CHAPTER 2 : 90 CHAPTER 2

 2902 MATERIAL AND EXPERIMENTAL METHODS

Figure 2 . 1 :

 21 Figure 2.1: Infrared spectra for the ZeeWeed® support membrane material (A) and Polyethylene terephthalate (B) sample from HR Nicolet sampler library extracted from OMNIC software (Thermo fischer, USA).A skin of polyvinyl difluoride (PVDF) covers the outer surface of this support; this is the main

Figure 2 . 2 : 2 .

 222 Figure 2.2: Pictures taken of intact ZeeWeed® hollow-fiber (left) and a hollow-fiber peeled off with its PET support material visible (right).

Figure 2 . 3 :

 23 Figure 2.3: ZeeWeed® 500D membrane module.

CHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS94sodium hypochlorite is the most used in MBR, this study focused on the effects of sodium hypochlorite on these membranes. Hypochlorite solutions used for the bench-scale ageing experiments were obtained by dilution of commercial sodium hypochlorite at 14-wt% (VWR chemicals, USA). In addition, concentrated sulfuric acid (analytical grade 95%, VWR chemicals, USA) used for pH adjustment and sodium bisulfite (Honeywell Research Chemicals, USA) used for membrane storage were of analytical grade. Aqueous solutions were all prepared using pure water from Milli-Q TM equipment (Thermo Fisher Scientific, USA).

Figure 2 . 4 :

 24 Figure 2.4: Schematic representation of Seine Morée WWTP.

CHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS98Cleaning-in-place (CIP) is performed once a year. Ideally, membranes are submerged in a solution of 800 ppm NaOCl, and after rinsing they are then submerged in a solution of citric acid at a pH around 2.7. Each soaking is performed for 20 h with cyclic aeration every hour; giving a theoretical annual NaOCl C × t of 16,000 ppm.h related to CIP protocols. The soaking solution is collected at the beginning of soaking and analyzed in laboratory for chlorine concentration as described in section 2.1.2. In addition, SEM-MBR presented variations in soaking times for each CIP protocol, since the start and end of these protocols are controlled manually (Figure2.5). Soaking time could vary between 17 h and 28 h for SEM-MBR.

Figure 2 . 5 :

 25 Figure 2.5: Soaking time during CIP protocols for SEM-MBR.

  Seine aval's (SAV) WWTP is the oldest one in Paris region treating wastewater since 1940 with a treatment capacity of 200,000 m 3 /d, increasing to 2,100,000 m 3 /d by 1980 with a biological treatment, i.e., classic activated sludge, and anaerobic digestion for the produced then, Paris' wastewater treatment was decentralized and SAV decreased its capacity to 1,500,000 m 3 /d. Its pretreatment unit is composed of screening, grit and grease removal. Then, the effluent is sent to primary decantation before biological treatment. Since 2013, the biological treatment was largely modernized with 80% of this wastewater being treated by biofiltration and 20% by MBR. The schematic representation of this WWTP is shown in Figure2.6.

Figure 2 . 6 :

 26 Figure 2.6: Schematic representation of SAV WWTP.

Figure 2 . 7 :

 27 Figure 2.7: Schematic representation of SAV-MBR (A) and photo of anoxic zone from one of the biological basins (B).

FlowmeterCHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS101Each membrane tank has 300 m 3 capacity and is equipped with 10 cassettes, composed of 48 membrane modules of 34.4 m 2 of filtration surface (total filtration surface of a membrane tank is 16,512 m 2 ) (Figure2.8). In addition, mixed liquor is recirculated from membrane tanks to aerated zone.

Figure 2 . 8 :

 28 Figure 2.8: Schematic representation of a membrane tank, cassette and module of ZeeWeed®

Feed

  MATERIAL AND EXPERIMENTAL METHODS102Cleaning in-place (CIP) is usually performed twice a year. Firstly, membranes are submerged in a NaOCl solution at 1000 ppm for 6 h, then they are rinsed before being soaked in a citric acid solution at pH around 2.7. The annual NaOCl C x t related to CIP is estimated to 12,000 ppm.h. Therefore, the total annual theoretical chlorine exposure (CEB + CIP) is estimated at 13,000 ppm.h.Specific conditions for membrane tanks in SAV-MBRConcerning the filtration and aeration cycles among membrane tanks in SAV-MBR, this MBR is composed of 28 membrane tanks, thus data treatment and graphical representations are challenging and time consuming. All membrane tanks presented similar permeate flux (Table

CHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS103

Figure 2 . 9 :

 29 Figure 2.9: Chlorine concentrations of soaking solutions harvested at the beginning of CIP

at 1 gFigure 2 . 10 :FeedCHAPTER 2 :

 12102 Figure 2.10: Schematic representation of membrane harvesting place for SAV-MBR (A) and SEM-MBR (B).

CHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS 118 These values mostly represented times of transition between filtration and relaxation/backwash. The cleaned data for the first period (16/04/2018 -30/04/2018) is presented in Figure 2.11.

Figure 2 . 11 :

 211 Figure 2.11: Cleaned permeate flowrates from membranes tank SAV-A1 from 16/04/2018 to

Figure 2 . 12 :

 212 Figure 2.12: Permeability index and flux at 20°C for membranes tank SAV-A1 from

Figure 2 . 13 :Figure 2 . 14 :

 213214 Figure 2. 13: Permeability index and permeate fluxes for membranes tank SAV-A1 as a

Figure 2 .

 2 Figure 2.14.B;

CHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS 123 the range of values between the 10th and 90th percentile of permeate flux observed in this study, and their corresponding permeability indexes were calculated, whenever the installation was operated at such fluxes. To illustrate this procedure, the permeability indexes are presented for the first (Figure 2.15.A) and third (Figure 2.15.B) CIP of membrane tank A1 in April 2018 and October 2019, respectively.

Figure 2 . 15 :

 215 Figure 2.15: Permeability indexes after the first CIP protocol in April 2018 (A) and the third

FluxFigure 2 . 16 :Figure 2 . 17 :

 216217 Figure 2.16: Boxplot of permeability indexes according to their range of flux for short period

  is performed, the normalized permeability indexes are in a common scale. For instance, these values for each CIP can be plotted as a function of the operating days, as shown in Figure2.18 for membrane tanks SAV-A1 and SEM-UF1.

Figure 2 . 18 :

 218 Figure 2.18: Normalized permeability index for different permeate fluxes as a function of the

Figure 2 . 19 :

 219 Figure 2.19: Averages of normalized permeability index for membrane tanks SAV-A1 (A) and SEM-UF1 (B).

  Although the categorization of permeability index by permeate flux decreases variability related to other factors than the membrane stage, after CIP protocols the first values of permeability index may be influenced by the mixed liquor quality, residual chemical cleaning agents, among other. For instance, these dynamics after CIP can be observed in Figure2.15 in the previous section and a zoom in the first CIP of membrane tank SAV-A1 is illustrated in Figure2.20.

Figure 2 . 20 :

 220 Figure 2.20: Permeability index of membrane tank SAV-A1 around the first CIP protocol.

Figure 2 .

 2 Figure 2. 21: Stress-strain curve of pristine ZeeWeed® membrane.

Figure 2

 2 Figure 2.22.

Figure 2 . 22 :CHAPTER 2 :

 2222 Figure 2.22: Standard behavior of the fouling reversibility test.

Figure 2

 2 Figure 2.23.

Figure 2 . 23 :

 223 Figure 2.23: Filtration and backwash cycles in the cleaning recovery assay.

CHAPTER 2 :

 2 MATERIAL AND EXPERIMENTAL METHODS141 irreversible fouling fraction. Cleaning recovery c3 takes into account the interactions between membrane surface and foulants, but also residual foulants and additional foulants during the second cycle of filtration, which is a more realistic approach to the behavior expected at fullscale.

Figure 2 . 24 : 1 CHAPTER 2 :

 22412 Figure 2.24: FTIR spectra of a pristine ZeeWeed® membrane.

CHAPTER 3 :CHAPTER 3

 33 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE145

CHAPTER 3 :Figure 3 . 1 :CHAPTER 3 :

 3313 Figure 3.1: Ultimate tensile strength (A), Ultimate elongation (B) and Young's modulus (C) for bench-scale-and full-scale-aged membranes as a function of the cumulative exposure dose to hypochlorite.

Figure 3 . 2 :

 32 Figure 3.2: Changes in intrinsic permeability of samples aged at bench-scale and full-scale

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE152 value. In addition, full-scale harvested membranes showed a decline from 39% to 32% in cleaning recovery when the C × t increased from 33,000 to 100,000 ppm.h.

Figure 3 . 3 :

 33 Figure 3.3: Cleaning recovery (%) after two cycles of filtration and hydraulic backwash on

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 153 Surface functional groups by ATR-FTIR The external skin of the pristine ZeeWeed® membrane was analyzed by ATR-FTIR and the IR spectrum is presented in Figure 3.4. The spectrum presented confirmed the PVDF fingerprint,

Figure 3 . 4 : 1 )

 341 Figure 3.4: IR spectrum of a pristine ZeeWeed® membrane.

Figure 3 .

 3 6 as a function of C × t. It is worth noting that, although full-scale samples may present residual fouling (1730 CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 155 cm -1 ) [12], measurements of PVP and degradation product peaks, except for by-products presenting carboxylic acids (1730 cm -1 ), were still possible. Both sets of membranes (bench and full-scale) showed a decline in PVP content that can be associated with either its removal from the membrane structure and/or to its degradation into other by-products. Pellegrin et al. (2013) among others also observed decreasing PVP content in PES/PVP membranes during exposure to hypochlorite solution (pH 6, 8 and 11; NaOCl concentration 350 ppm) [74,81]. In addition, Ravereau et al. (2016) aged PVDF membranes with and without hydrophilic agent in sodium hypochlorite (pH 6 -11.5 concentration of 4000 ppm NaOCl) and also obtained a

CHAPTER 3 : 1 ) 1 )

 311 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-Pristine membrane -0 ppm.h Full-scale -33,000 ppm.h Full-scale -42,000 ppm.h Full-scale -98,000 ppm.h Bench-scale -78,000 ppm.h Bench-scale -500,000 ppm.hWave number (cm -

Figure 3 . 5 :

 35 Figure3.5: Full-range FTIR spectra (600-1800 cm-1) of membranes aged at bench-scale and full-scale (A) and enlarged view of the characteristic region of PVDF (B) and PVP and its degradation products (1500-1800 cm-1) for samples aged at bench-scale (C) and at full-scale (D).

CHAPTER 3 :Figure 3 . 6 :CHAPTER 3 :

 3363 Figure 3.6: Change in PVP content in bench-scale and full-scale-aged membranes as a function of the full range of exposure dose (A) and a zoom in the zone between 0 and 100,000 ppm.h (B).

CHAPTER 3 :Figure 3 . 7 :

 337 Figure 3.7: Pore volume and size distribution of aged membranes at bench-scale (A) and at

CHAPTER 3 :Figure 3 . 8 :CHAPTER 3 :Figure 3 . 9 :

 338339 Figure 3.8: SEM images zoomed at x500 for pristine membranes (A), bench-scale aged

FullCHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE165Impact of NaOCl chemical ageing on mechanical resistanceIn the case of supported membranes, mechanical resistance is mainly linked to the material used to support PVDF. By ATR-FTIR analysis, it was found that the supporting material of these membranes had 68% accordance with PET. PET is often used as a support material for ultrafiltration membranes. However, only few studies of membrane ageing characterized its mechanical resistance. Pulido et al. (2019) evaluated the properties of recycled PET-based membranes and concluded that the material was chlorine resistant after a 2-h exposure to a 5000 ppm hypochlorite solution [90]. However, the effects of long-term exposure were not determined. The results of bench-scale-aged samples in the present research indicate that the long-term exposure to NaOCl (static soaking of 20 days in 1000 ppm NaOCl) does not impact the mechanical properties at break (ultimate tensile strength and ultimate elongation) of these supported hollow fibers, but fibers became a slightly more rigid with chlorine exposure. Conversely, Wang et al. (2018) characterized hypochlorite aging of membranes reinforced with PET nonwoven fabric and found a slight decline in ultimate tensile strength. It is worth noting that these membranes when pristine exhibited ultimate tensile strength in the range of 40 MPa, which is around 10 times less resistant than the fibers used in the present study (395.8 ± 13.7

. 2 was

 2 found to be linked to the decline in PVP content observed in Figure3.6. Statistical analysis confirmed a significant correlation in this dose range between both parameters, giving a correlation coefficient (r) of -0.951 (p = 0.0003, Pearson correlation test). Despite the decline in PVP content, the contact angle values did not change significantly. In fact, this parameter is not only influenced by hydrophilic agent content, but also by electrical charges on the membrane surface and by morphological aspects such as surface roughness. When in contact with NaOCl, the PVP cyclic group, pyrrolidone, is opened forming carboxylates. As a result, membranes may increase their negative charges as they age. Recently,[START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF] confirmed the increase in negative charges on PVDF/PVP membranes during exposure to NaOCl[START_REF] Li | Aging of PVDF and PES ultrafiltration membranes by sodium hypochlorite: Effect of solution pH[END_REF]. Thus, changes in membrane wettability due to an increase in hydrophobicity may be counter-balanced by an increase in electrical surface charges. This was observed by[START_REF] Pellegrin | Filtration performance and pore size distribution of hypochlorite aged PES/PVP ultrafiltration membranes[END_REF] on hypochlorite aged PES/PVP membrane[START_REF] Pellegrin | Filtration performance and pore size distribution of hypochlorite aged PES/PVP ultrafiltration membranes[END_REF]. As a consequence of this combined effect, membrane permeability is not expected to be significantly impacted by changes in surface physical-chemical properties, which oblige to look after another source of evolution in permeability observed in the present study. Morphological properties were thus evaluated. Regarding the change in membrane skin porosity for exposure dose up to 78,000 ppm.h, a formation of small pores (dp < 20 nm) is observed in Figure3.7.A, enhancing overall membrane porosity. In fact, FTIR spectra (Figure3.5.C) showed that PVP molecules are not fully dislodged from the membrane structure. They are instead oxidized into succinimide (at 1700/1770 cm -1 ) and carboxylates (1590 cm -1 ) CHAPTER 3: COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 167 following the PVP degradation mechanism proposed by Prulho et al. (2013) [82]. These molecules remained (totally or partially) in the membrane structure, possibly leading only to

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 168 Impact of NaOCl chemical ageing on cleaning recovery Similarly to water permeability, cleaning recovery is also impacted by the physical-chemical properties of the membrane surface and by its porosity.

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 171 permeate quality (no suspended solids measured in permeate) over 7 years of operation despite the decline in mechanical resistance. Impact of full-scale operation on PVP degradation The mechanisms of PVP degradation are also different between bench-scale and full-scale ageing (Figure 3.5.C and D, respectively). At bench scale (Figure 3.5.C), the sole effect of hypochlorite (pH 9.0) induced the production of carboxylate and succinimide from the chemical attacks to PVP molecules. At exposure doses between 100,000 and 500,000 ppm.h, these degradation products remained in the membrane structure while the non-oxidized PVP was maintained around 40 -50% of the pristine membrane PVP content (Figure 3.6). At full scale (Figure 3.5.D), the PVP peak (1674 cm -1

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 172 to residual proteins, which are present in the mixed liquor and contribute to the irreversible membrane fouling, as reported by Levitsky et al. (2011) [12].

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE173 decline in Lp/Lp0 was experienced in the exposure dose range studied. Both residual fouling and the differences in porosity changes can be related to these results.

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE174 samples aged at bench scale is higher than for full-scale-aged membranes. This illustrates the role of the different pore size distributions and residual fouling, which impairs the increase in Lp/Lp0.It is worth noting that the lack of decline in permeability at full-scale should be confirmed by additional samples in the later phases of membrane's lifetime.

Figure 3 . 11 :

 311 Figure 3.11: Relationship between PVP content removal and the changes in Lp/Lp0 for

CHAPTER 3 :Figure 3 . 12 :

 3312 Figure 3. 12: Illustration of changes in membrane properties during ageing at bench-scale by

CHAPTER 3 :

 3 Differences in the membrane ageing process for membranes aged by single soaking in NaOCl solution and during a 7-year period in an MBR treating municipal wastewater are highlighted by applying the same methodological protocol. A nonnegligible contribution of filtration conditions, mechanical stress due to aeration and backwashes, and residual fouling, specific to full-scale operating conditions, may significantly change ageing mechanisms. This study showed that understanding the mechanisms behind the action of NaOCl on PVDF membranes may not represent what actually occurs at full-scale operation and does not replace continuous autopsies of harvested fibers over the years or monitoring of consistent full-scale operation indicators. At bench scale, NaOCl species caused PVP oxidation, which led to the formation of small pores causing an increase in Lp/Lp0 in contrast to the generally accepted COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE 178 hypothesis of pore enlargement reported in the literature. For exposure doses higher than 78,000 ppm.h, the membrane material seems to submit a re-arrangement, which is made possible by PVDF chemical alteration, leading to a decrease in Lp/Lp0.

CHAPTER 3 :

 3 COMPREHENSIVE STUDY OF SUPPORTED PVDF MEMBRANE AGEING IN MBR: A DIRECT COMPARISON BETWEEN CHANGES AT BENCH-SCALE AND FULL-SCALE179 Changes in membrane properties from full-scale MBR are becoming available.Thus, efforts into membrane ageing modelling are needed to perform predictive calculations of the long-term ageing effects in MBR operation.

  Monitoring membrane ageing is crucial for plant operators, in order to schedule WWTP's downtime due to membrane replacement. Depending on the operating conditions in MBRs, we expect that the changes in membrane properties may differ, but results are lacking in the scientific literature. Studies about membrane ageing hardly ever consider the chronological operating conditions that membranes experience during their lifetime. Therefore, the aim of this chapter was to evaluate the impacts of operating conditions on membrane properties and create oriented process indicators to monitor membrane ageing. Samples of membranes were harvested from two similar MBRs in the wastewater treatment facilities of Seine Aval (SAV), designed to treat 200 000 m 3 /d since November/2017, and Seine Morée (SEM), designed to treat 50 000 m 3 /d since May/2014, and operated by SIAAP.

Figure 4 . 1

 41 Figure 4.1 Ratio BOD5/TKN for SEM (A) and SAV (B) MBR.

  tank based on the injection of microsand and coagulants (Actiflo®, Veolia water technologies, France), compared to a lamellar settling tank.

  2 and Figure 4.3.

Figure 4 . 2 .

 42 A and D) was slightly lower for SEM, mainly because of higher MLSS concentrations in this MBR (Figure4.2.C and F). However, the BOD loading rate is quite equivalent for both WWTPs (0.05 and 0.04 kgBOD5.kgMLSS -1 .d -1 for SEM and SAV respectively). Similarly, SEM-MBR is operated at lower N-NH4 to sludge ratio (Figure 4.2.B and E). Besides, SEM presents a lower SRT than SAV (Figure 4.3.A and C), varying between 10 and 30 days (with an average value of 16 d during the studied period), whereas SAV presented a first period of narrow SRT between 25 -30 days and after the day 800, SRT variations were wider between 25 -40 days (with an average value of 29 days during the studied period). Variation in SRT are common since solid extraction was usually performed 4-5 times a week. Temperatures were similar for both MBRs over the whole period (Figure 4.3.B and D).

Finally, the influent

  flowrate normalized by the total membrane surface of each MBR is highly different between the two WWTPs (0.15 and 0.40 m 3 .mmembrane -2 .d -1 for SEM and SAV respectively), since SEM-MBR operated at around 30% of its designed flowrate.

Figure 4 . 2

 42 Figure 4.2 Chronological data of COD organic rate (A for SEM and D for SAV), N-NH4 to sludge ratio

Figure 4 . 3 :

 43 Figure 4.3: Chronological data of SRT (A for SEM and C for SAV), and Temperature (B for SEM and D

Figure 4 .Figure 4 . 4 :

 444 Figure 4.4 : SAV-MBR removal in terms of COD, BOD and TSS (A), TN and NH4 (B) and



  SEM's MBR SEM's MBR presented overall greater performances than SAV (Table

Figure 4 .

 4 Figure 4.5 SEM-MBR removal in terms of COD, BOD and TSS (A), TN and NH4 (B) and TP

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 192 permeate collection tank when the MBRs presented long non-operating time, indicating the presence of microorganisms in permeate.

Figure 4 . 6 :

 46 Figure 4.6 : Total suspended solids in permeate from SAV-MBR (A) and SEM-MBR (B).

WWTPCHAPTER 4 :

 4 Regarding membrane tanks, chronology of supernatant COD measured in the recirculation channels (membrane tank to aerated zone) and MLSS in membrane tanks for both MBRs are presented in Figure 4.7 and Figure 4.8, respectively. □ SAV's supernatant COD may be separated into two periods with different trends (Figure 4.7.B): Period I, when supernatant COD presented high variability ranging between 20 and 90 mg O2/L and period II, when this indicator decreased and was more stable around 20 mg O2/L. The increase in COD from the period I is related to issues in the settling tanks between 2018 and 2019. The average value of the MLSS in the membrane tanks during the overall studied period is 6.4 g.L -1 . □ SEM's supernatant COD started to be measured from Jan/2019 and for this period, values varied between 20 and 60 mgO2/L, with an average of 44 mg O2/L. Regarding IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 193 MLSS in membrane tanks, SEM usually operated at higher concentrations (7.6 g.L -1 ) than SAV (Figure 4.8). Table 4.4 presents averages and standard deviations of the main operating data related to membrane tanks from both MBRs. SEM-MBR operates at MLSS concentrations and supernatant COD concentrations 19% and 40% greater than SAV-MBR, respectively. Moreover, both MBRs presented similar permeate instantaneous flux and SADm averages, but the daily filtration times of each MBR are highly different. Membrane tanks from SAV-MBR presented 20.7 h of filtration daily in average, whereas membrane tanks from SEM-MBR only filtered around 5.4 h per day, due to its operation under the nominal capacity (further described in chapter 2).

Figure 4 . 7

 47 Figure 4.7 Supernatant COD in recirculation channel from membrane tanks to aerated zone for SEM-MBR (A) and SAV-MBR (B).

Figure 4 . 8

 48 Figure 4.8 MLSS in membrane tanks for SEM-MBR (A) and SAV-MBR (B).

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS194Table 4.4 Averages and standard deviations of operating conditions from SAV-MBR and SEM-MBR.

Figure 4 .

 4 9 presents chronological data of instantaneous permeate flux at 20°C for 14 membrane tanks from SAV-MBR and 8 from SEM-MBR over the studied period.For the first year, SEM's influent flowrate increased progressively and as a result, membrane tanks were initially operated with low permeate flux around 12 LMH, which has been progressively increased to reach between 20 and 30 LMH (Figure4.9.A).

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 195 Regarding SAV's chronology (Figure 4.9.B), permeate flux is ranged from 17 to 50 LMH homogeneously throughout all membrane tanks. Since this MBR is operated at an influent flowrates during raining weather, permeate flowrate oscillated more throughout the years comparing to SEM-MBR. Therefore, MBRs were operated at a similar permeate flux in average (26.6 and 26.4 LMH for SAV-MBR and SEM-SBR respectively), but the main difference was found in the chronological behavior. SEM-MBR presented a stable flux along the period with a maximal value of permeate flux around 30 LMH, in comparison with the more dynamic evolution of the permeate flux for SAV-MBR in the range 17 to 50 LMH during the studied period, link to the seasonal variations.

CHAPTER 4 :Figure 4 . 9 :

 449 Figure 4.9: Permeate instantaneous flux at 20°C for SEM-MBR (A) and SAV-MBR (B).

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 198 According to the availability of the data for the two studied WWTP, the selected cumulative indicators were: operating days, exposure dose, specific permeate volume (Vp), specific aeration demand and filtered MLSS mass, filtered supernatant COD and ∆COD. These cumulative indicators with respect to the time since the commissioning date are presented in

Figure 4 .

 4 Figure 4.10 to Figure 4.12. It is worth noting that Figure 4.10(A) represents the exposure doses

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 199 1,500 m 3 /mmembrane 2 for SAV-MBR-RowA, SAV-MBR-RowB and SEM-MBR respectively, after 1,440 days of operation. Moreover, important heterogeneities are observed between filtration zones for the SEM-MBR. 4. The other parameters (Figures presented in Appendix B) presented similar conclusions to the specific permeate volume as they are closely linked to this parameter, since they are mostly impacted by the downtime periods..

Figure 4 .

 4 Figure 4.10 Cumulative exposure dose (C x t) for membranes tanks from SEM-MBR (A), SAV-MBR row A (B) and SAV-MBR row B (C).

Figure 4 . 11 :Figure 4 . 12 :CHAPTER 4 :

 4114124 Figure 4.11 : Specific permeate volume evolutions for membranes tanks from SEM-MBR (A) SAV-MBR row A (B) and SAV-MBR row B (C).

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS203Considering membranes from tanks in the same group, ultimate tensile strength seemed to decrease homogeneously across the analyzed samples for both SAV-MBR and SEM-MBR. For instance, in the group of membranes with values comparable to the analyzed pristine membrane (396 ± 14 MPa), the ultimate tensile strength of the most aged sample (SEM-UF1 at 64,000 ppm.h) reached 315 ± 17 MPa, that is a decrease of 20%.

CHAPTER 4 :Figure 4 . 13 :CHAPTER 4 :Figure 4 . 14 :

 44134414 Figure 4.13 : Ultimate tensile strength of membranes harvested from both MBRs with respect

Figure 4 .

 4 15.A and SEM-MBR in Figure 4.15.C and with respect to the operating time for SAV-MBR in Figure 4.15.B and SEM-MBR in Figure 4.15.D.

  Pristine membraneGroup of lower ult tensile strength Group of higher ult tensile strength Pristine membrane L-UTS H-UTS susceptible to pore enlargement than the SAV-MBR group. During CIP protocols in this MBR, membranes are soaked in 800 ppm of NaOCl for 20 h, whereas in SAV-MBR, CIP is performed for 6 h with a hypochlorite solution at 1000 ppm. Abdullah and Bérubé (2018) obtained an

Figure 4 . 15 :

 415 Figure 4.15 : Lp/Lp0 with respect to the exposure dose for membrane tanks from SAV-

Figure 4 .

 4 [START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF].A for membranes samples from SAV-MBR and in Figure4.[START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF].C for the ones from SEM-MBR. As expected, PVP is degraded as membranes aged in full-scale operation for both groups and the main differences are related to each MBR. Comparing the decline in PVP content between SAV-MBR and SEM-MBR samples, it seems that the decrease in PVP content highly depend on the studied MBR and is faster among SAV-MBR samples with respect to the operating time (Figure4.16.B for SAV-MBR and Figure4.16.D for SEM-MBR). This important observation highlights that, although the modifications of the membrane properties, in particular the PVP content, depend on the exposure dose (as discussed in Chapter 3), these properties also strongly depends on the operating conditions (different between the two sites). Despite the hypothesis of a more aggressive cleaning protocol applied on SEM-MBR, the operating conditions of the SAV-MBR site induce a significantly faster aging than for the SEM-MBR site (potentially linked to the higher daily filtration time, greater flow variability or a more regular exposure to sodium hypochlorite). This difference in terms of operating conditions is highlighted by Figure4.17 which presents the cumulated permeate volume as a function of the exposure dose for each individual samples. Two different zones, associated to the two MBR are clearly identified. The Zone 1, corresponding to the SAV-MBR samples, presents a high filtration intensity to exposure dose ratio, contrary to SAM-MBR samples which are submitted to a low filtration intensity to exposure dose ratio. Moreover, this diagram could be considered in order to interpret the preponderance of each individual ageing mechanisms (mechanical vs. chemical) on the global CHAPTER 4: IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 208 ageing mechanism: although the chemical ageing mechanisms always plays an important role in the global ageing mechanism of the membrane properties, the global ageing mechanisms is clearly modulated and accentuated by the intensity of the mechanical stresses related to the filtration process (in terms of filtration time, flux and associated aeration).

Figure 4 .CHAPTER 4 :Figure 4 . 17 :

 44417 Figure 4.16: PVP content of membranes harvested from SAV-MBR with respect to exposure

1 CHAPTER 4 :Figure 4 . 18 :CHAPTER 4 :

 144184 Figure 4.18: PVP content from membranes harvested from SAV-MBR, SEM-MBR and from

Figure 4 .

 4 [START_REF] Vanysacker | Effect of chemical cleaning and membrane aging on membrane biofouling using model organisms with increasing complexity[END_REF]. A global analysis of the evolution of this permeability index highlights a low but regular decrease of this parameter with time (and associated exposure dose) with a similar order of magnitude for the two MBRs.

CHAPTER 4 :

 4 IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS212For SAV-MBR, normalized permeability index correlated slightly better to the operating time with a correlation coefficient of -0.836 (p-value < 0.05, Spearman). Data decreased in a residual fouling rate of -1.91E-4 d -1 , leading the normalized permeability index to reach 0.73 after 1400 days of operation, which is a decline of 27% of permeability index after almost 4 years of treatment.

Figure 4 . 19 :

 419 Figure 4.19: Normalized permeability index for SAV-MBR with respect to C x t (A) and to operating time (B) and similarly for SEM-MBR with respect to C x t (C) and to operating time (D).

  1.91E-04x + 1.0 r = -0.836 Normalized permeability index Normalized permeability index C x t (ppm.h) C x t (ppm.h) Time (days) Time (days)

Figure 4 . 20 :

 420 Figure 4.20: Normalized permeability index as a function of cumulated specific permeate

  similar concentrations and soaking time from each MBR (see Chapter 2), results from samples characterization, i.e. intrinsic permeability (Figure 4.15) and PVP content (Figure 4.16), were correlated to the normalized permeability index (Figure 4.19) obtained from operating data. These correlations are presented in Figure 4.21. PVP content and intrinsic permeability presented statistically significant correlations to the normalized permeability index. Moreover, hydrophilic agent content presented the highest CHAPTER 4: IMPACT OF FULL-SCALE MBR OPERATING CONDITIONS ON MEMBRANE AGEING AND MONITORING TOOLS 217 Spearman correlation coefficient (R = 0.598, p-value<0.001) compared to the intrinsic permeability (R= -0.476; p-value=0.006). Although it is still necessary to collect data in the later phases of membrane life to confirm these findings, these results showed a slight effect of the PVP content decrease on macroscopic indicators, such as the normalized permeability index.

Figure 4 . 21 :

 421 Figure 4.21: Normalized permeability index with respect to hydrophilic agent content and

  Membrane ageing studies mainly focused on understanding changes of membrane properties in contact with sodium hypochlorite. A few others increased the complexity by including cycles of a model solution filtration and chemical cleaning in the ageing procedure. More recently, these two approaches were used and compared for the characterization of PVDF membranes aged in drinking water facilities. All these studies monitored ageing by membrane autopsies, which means harvesting fibers and characterizing them with different analytical techniques.

GC. 1 )

 1 . The pilot MBR was equipped with ZeeWeed® 500 D membrane modules operating in similar conditions to the full-scale SAV-MBR (permeate flux, aeration demand, cleaning cycles). The tank is fed by mixed liquor directly from the end of aerated zone from the same biological tank that fed membrane compartments in the full-scale SAV-MBR.

Figure GC. 1 :

 1 Figure GC.1: Pilot-scale membrane tank used in preliminary tests of accelerated ageing in on-

Figure B. 2 :

 2 Figure B. 2: Specific filtered supernatant COD for membrane tanks in row A (A) and B (B) from SAV-MBR.

Figure B. 3 :

 3 Figure B. 3: Specific filtered supernatant ∆COD for membrane tanks in row A (A) and B (B) from SAV-MBR.

  SAV-B13 SAV-B14

Table 1 . 2

 12

: Fouling control strategies applied in MBRs treating wastewater. Based on

[START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF]

. ....

Table 1 .

 1 3: Characteristic peaks in an ATR-FTIR spectra, evaluated in PVDF/PVP membrane ageing studies. ...................................................................................................................

Table 1 .

 1 4: Results of studies on PVDF ageing in contact with hypochlorite from scientific literature. ............................................................................................................................

Table 2 .

 2 1: Nominal characteristics of ZeeWeed® membranes provided by the manufacturer. ...........................................................................................................................................

Table 2 .

 2

2: Process data collected for both SEM and SAV's MBRs. .......................................

Table 2 .

 2 3: Permeate instantaneous flux at 20°C for the 28 membrane tanks from SAV-MBR calculated between November/2017 and June/2021. ......................................................

Table 2 .

 2

4: SADm for the 28 membrane tanks from SAV-MBR calculated between November/2017 and June/2021. ......................................................................................

Table 2.5: Ratio of non-operating days since the commissioning date (in %) for each membrane tank from SAV. Bold membrane tanks are further analyzed. ........................ Table 2.6: Exposure doses of membrane samples aged at bench-scale. ................................ Table 2.7: Process indicators to monitor ageing for membrane samples harvested at full-scale. ......................................................................................................................................... Table 2.8: Impact of period length on J/TMP (CIP: April/2018, SAV-A1). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10. ......................................................................................................... Table 2.9: Impact of period length on J/TMP (CIP: October/2019, SAV-A1). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10. ......................................................................................................... Table 2.10: Impact of period length on J/TMP (CIP: October/2017, SEM-UF4). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10. .........................................................................................................

Table 2 .

 2 11: Impact of period length on J/TMP (CIP: June/2019, SEM-UF4). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10. .........................................................................................................

Table 2 .

 2 12: Cleaning recovery for different backwash duration. ...........................................

Table 4 .

 4

1 Quality characteristics of influents (after pre-treatment) from SEM-MBR (data from May/2014 to January/2021) and SAV-MBR (data from November/2017 to June/2021). .........................................................................................................................................

Table 4 .

 4 4 Averages and standard deviations of operating conditions from SAV-MBR and SEM-MBR. ......................................................................................................................

Table 4 .

 4 5 : Spearman correlation coefficients between each process indicator and the normalized permeability index (bold values are the highest correlation coefficients). ... Table A.1: Statistical summary of SADm for membrane tanks in row A from SAV-MBR.. Table A.2: Statistical summary of SADm for membrane tanks in row B from SAV-MBR. .

Table D.1: Spearman correlation coefficient and (p-value) between membrane autopsies properties and cumulative process indicators for membrane tanks in from SAV-MBR. Table D.2: Spearman correlation coefficient and (p-value) between membrane autopsies properties and cumulative process indicators for membrane tanks from SEM-MBR. Bold values are the highest correlation coefficients. ................................................................ Table F.1: Ratio of non-operating days since the commissioning date (in %) for each membrane tank from SAV. .............................................................................................. Table F.2: Daily filtration time for membrane tanks from SEM. ..........................................

Table 1 .

 1

	Parameters	Meaning	Characteristic values
	Mixed Liquor Suspended Solids (MLSS; 𝐿 ) 𝑔	solids in mixed liquor Concentration of suspended	1.5 -7.0
	Hydraulic Retention Time	Time of contact between	1 -20
	(HRT; h)	microorganisms and influent's
			organic matter
	Sludge Retention Time	Time that sludge remains in the	4 -20
	(SRT; days)	reactor
	Food to Microorganism ratio	Amount of substrate available	0.07 -0.5
	(F/M;	𝑘𝑔 𝐵𝑂𝐷 5 𝑘𝑔 𝑀𝑉𝑆𝑆.𝑑𝑎𝑦 )	to microorganism for time
	Sludge Recirculation Factor	Rate in which sludge is	0.25 -0.8
	(QR/Q; dimensionless)	recirculated
	Treatment yield (%)	Percentage of removed BOD5	50 ->95
	Kg O2/kg BOD5 removed	Amount of O2 needed to	0.8 -1.0
			remove 1 kg BOD5

1: Main parameters and their characteristic values for CAS process (Based on

[START_REF] Muralikrishna | Chapter Twelve -Wastewater Treatment Technologies[END_REF]

).

At the end of this process, treated water can reach often 10-15 mg BOD5/L and solid suspension concentrations as low as 20 mg/L, which are within the limits stablished by the European Directive for wastewater of 35 mg/L of DBO5

[START_REF] Scholz | Chapter 15 -Activated Sludge Processes[END_REF]

.

Table 1 .

 1 2: Fouling control strategies applied in MBRs treating wastewater. Based on[START_REF] Bagheri | Critical review of fouling mitigation strategies in membrane bioreactors treating water and wastewater[END_REF].

	Method

Strategy Application (Lab. vs. Industrial scale) Principle Examples Mechanical

	Cell	Passive immobilization	Lab scale	Use of surface to prevent accumulation of bound EPSs and SMPs	Granular activated carbon, PU, PEG, PVDF
	immobilization Biological	Aeration Cell entrapment Backwashing Quorum quenching	Industrial scale Lab scale Industrial scale Lab scale	Increase shear stress on membrane surface Artificially entraps cells in a porous polymeric matrix to prevent accumulation of bound EPSs and SMPs on membrane surface Reduction of gel polarization layer, prevent layer compaction and remove deposited layer Disruption of QS by blockage of production, interference with the signal receptor or inactivation of signal molecules.	Agar, alginate, carrageenan, polyvinyl alcohol, xanthan gum Upwards, turbulence fluctuations Air, clean water, permeate Lactonase and acylase or E. coli and Rhodococcus to degrade AHLs
		Biofilm carriers Enzymatic disruption of EPS	Lab scale Lab scale	Affect physiochemical characteristics of mixed liquor and mechanical effects on membrane surface Enzymes destroy the physical integrity of EPSs through degradation process and facilitate detachment of biofilm	Granular activated carbon, PU, PEG, PVDF Proteinase K, trypsin and subtilisin
		Vibration	Lab scale	Increase shear stress on membrane surface An anode and a cathode powered with direct current. Metal	-
		Electrocoagulation	Lab scale	cations are generated from the electrodissolution of anodes	Aluminum or iron as anodes
	Electrical	Ultrasonic waves Acids Bases Microbial Fuel Cell (MFC)	Lab scale Industrial scale Industrial scale Lab scale	Dissolve and displace soluble and insoluble to prevent in the MBR reducing fouling by charge neutralization. polarization layer and biofilm layer Remove inorganic foulants Remove organic foulants It promotes fouling mitigation through electrostatic radicals repulsion and by in situ generation of H2O2 and OH	-Oxalic, citric, sulfuric acids NaOH and KOH -
	Chemical cleaning	Oxidants Surfactants	Industrial scale Industrial scale	Oxidation of organic foulants and desinfection of biological foulants Solubilize foulants based on fat, oil and proteins	NaOCl, H2O2, PVP-iodine SDS
		Chelating chemicals	Lab scale	Affect intermolecular bridges within foulants releasing them to their original losen conformation	diamine tetraacetic acid, sodium tripolyphosphate
		Emerging chemicals	Industrial scale	Affect biofilm properties	Rhamnolipids, free nitrous acid
	Fouling	Active approach		Antimicrobial agents to eliminate proliferative fouling	Acrylic acid, cellulose, graphene NMs
	release membranes	Passive approach	Industrial scale	Prevent the adsorption of foulants on membrane surface without any impacts on the intrinsic features of the foulants	PEG, zwitterionic polymers, PVP and metals and carbon-based NMs

  1: BIBLIOGRAPHIC STUDY 50 Other mechanical approaches are being developed mostly at lab and pilot scale MBRs and presenting promising results, which is the case for biofilm carriers, vibrating membranes and ultrasonic waves. Biofilm carriers can reduce fouling by both affecting physicochemical

	properties of sludge and scouring the deposit on membrane surface. It was shown that biofilm
	carriers could affect the content of EPS and SMP and increased filtration time up to 8 times,
	however there are few information about the impact of configuration and type of biofilm
	carriers on MBR operation. Besides, vibrating membranes can act only enhancing shear stress
	on membrane surface, but it can also homogenate mixed liquor in submerged MBRs. Applying
	vibration or rotation allowed membranes to achieve higher levels of fouling mitigation than air
	scouring. Although promising, the major drawback of this technology is still the elevated
	energy cost. Ultrasonic waves are also being studied as they can act dissolving and displacing
	soluble and insoluble particles. Employing sonication can form cavitation bubbles that can
	remove biofilm and prevent cake deposition. Nevertheless, cleaning efficiency is reduced at
	high TMP levels and membrane damage is a frequent issue on practical applications [5].

Table 1

 1

.3 presents recurrent bands followed by ATR-FTIR spectra for the ageing of PVDF/PVP membranes. Besides ATR-FTIR, XPS and thermogravimetric analysis are widely used to determine hydrophilic agent and PVDF degradations.

Table 1 .

 1

	3: Characteristic peaks in an ATR-FTIR spectra, evaluated in PVDF/PVP membrane
		ageing studies.
	Group	Characteristic band	Reference
	C-F (PVDF)	3025; 1200; 471	[76]
	PVDF α-phase	762; 796; 875; 974; 1070;	[14,16]
		1181
	PVDF β-phase	841; 1274; 1403	[14,16]
	C=C (Dehydrofluorination)	1590 -1650	[16]
	C≡C (Dehydrofluorination)	2100	[16]
	C=O (Hydrophilic agent)	1670 -1800	[16,76]
	C-O (PVP amide)	1663; 1674	[70,73,81]
	Succinimide (Degradation	1700/1770	[81]
	product from PVP)
	Secondary amide	1540	[81]
	Carboxilic acid	1730/1720	[82]
	Carboxylate (-COO -)	1590	[13,82]

  Thus they proposed that hydrophilicity is governing BSA retention by adsorption, since BSA is often used as model of hydrophilic protein[START_REF] Levitsky | Understanding the oxidative cleaning of UF membranes[END_REF]. These results were corroborated by Zhang et al. (2017) more recently, indeed they proposed that electrostatic repulsion is probably another reason for the increased BSA retention, since membranes became more negatively charged as they aged and BSA is negatively charged in aqueous solutions at neutral

[START_REF] Wu | Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods[END_REF]

confirmed the decline in retention with ageing for raw water instead of model solutions as the case for

[START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF]

[START_REF] Gao | Interaction energy and competitive adsorption evaluation of different NOM fractions on aged membrane surfaces[END_REF][START_REF] Wu | Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods[END_REF]

. Moreover, they observed that pristine membranes present higher hydrophilicity, increasing repulsion between membrane-foulants. This promotes the formation of a cake layer denser than membrane pore structure. As membranes aged, hydrophilicity decreased and foulants are able to flow through the pores, decreasing retention. CHAPTER 1: BIBLIOGRAPHIC STUDY 80 Three other studies tested rejection of BSA by aged membranes (43, 63, 64). Levitsky et al. (2011) observed that membranes became more hydrophilic with ageing and BSA retention increased, although pore size increased. These changes in pore size did not correlate to BSA retention.

pH

[START_REF] Zhang | A comparison study: The different impacts of sodium hypochlorite on PVDF and PSF ultrafiltration (UF) membranes[END_REF]

. On the other hand,

[START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF]

obtained poorer BSA retention within aged membranes of bigger pores

[START_REF] Rabuni | Impact of in situ physical and chemical cleaning on PVDF membrane properties and performances[END_REF]

. Thus, they proposed that BSA retention was not only governed by membrane hydrophilicity, but size exclusion mechanism might have an effect on protein filtration.

Table 1 .

 1 4 presents the major changes in membrane properties found in PVDF membrane ageing studies during exposure to hypochlorite. All studies that analyzed additives content observed a decline, mainly due to the high reactivity of these agents in alkali conditions or with chlorine radicals. Hydrophilic agents are oxidized and can form secondary products or be dislodged from membrane structure generating mostly an increase in hydrophobicity, but other changes were

	membranes [20]. Actually both studies observed variations of less than 10%, however these
	variations were within experimental error for Ren et al. (2021), but not for Wang et al. (2018)
	[18,20].

not rare. This may lead to pore enlargement, which caused intrinsic water permeability to increase in most cases, but the combined effect of simultaneous increase in hydrophobicity and pore size in some studies have led to different conclusions. These two properties presented the most controversial results, partly due to a lack of reliable analytical method.

Mechanical properties were often tested and changes in the ultimate tensile strength are highlighted in this table. The most part of the studies observed decreasing ultimate tensile strength as membranes aged, however, two studies did not observe changes in this property.

Between these two studies, Ravereau et al. (2016) observed stable ultimate tensile strength for non-supported membranes without hydrophilic agents, whereas a decline was observed for membranes presenting these agents

[START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]

. In the second study, Ren et al. (2021) used supported membranes with PET, which explains the higher resistance and stability over chlorine exposure

[START_REF] Ren | Chemical cleaning reagent of sodium hypochlorite eroding polyvinylidene fluoride ultrafiltration membranes: Aging pathway, performance decay and molecular mechanism[END_REF]

. However,

[START_REF] Wang | Impact of sodium hypochlorite cleaning on the surface properties and performance of PVDF membranes[END_REF]

, using similar membranes and ageing conditions than Ren et al., claimed to have observed a decline in ultimate tensile strength of these supported

The chemical resistance of PVDF against NaOCl was observed in several studies, three of them observed stable PVDF fingerprint as membranes aged whereas ten observed signs of degradation. Ravereau et al. (2016) reported higher degradation rates at acidified conditions of NaOCl solutions (6.0 < pH < 7.5) than for solutions at pH of 11.5

[START_REF] Ravereau | Ageing of polyvinylidene fluoride hollow fiber membranes in sodium hypochlorite solutions[END_REF]

. Furthermore, the presence of hydrophilic agent favored PVDF chain scission. The differences observed in PVDF

Table 1 .

 1 4: Results of studies on PVDF ageing in contact with hypochlorite from scientific literature.

	Ageing
	effects

Table 2 .

 2 1: Nominal characteristics of ZeeWeed® membranes provided by the manufacturer.

	Reference	ZeeWeed® 500D
	Material	PVDF
	Type	Supported hollow fiber
	Filtration orientation	Outside-inside
	Nominal Pore size	0.04 µm
	Fiber diameters	1.9 mm Outer/ 0.8 mm Inner
	TMP range	-55 to 55 kPa
	pH	5.0 -9.5 (operation) 2.0 -10.5 (cleaning)
	Maximum Temperature	40 °C
	Maximum Cl2 Concentration	1000 ppm

Table 2 .

 2 2: Process data collected for both SEM and SAV's MBRs.

	Parameter	Units	Frequency
	TSS inlet	(mg / L)	Daily
	MLSS in recirculation channel	(mg / L)	Daily
	COD influent and permeate	(mg O2 / L)	Daily
	BOD5 influent and permeate	(mg O2 / L)	Daily
	TKN influent and permeate	(mg N / L)	Daily
	NH4 influent and permeate	(mg N / L)	Daily
	Total phosphorus (TP) influent and permeate	(mg P / L)	Daily
	PO4 3-influent and permeate	(mg P / L)	Daily
	Influent flowrate	(m 3 / day)	Averaged from instantaneous data
	Recirculation flowrate	(m 3 / day)	Averaged from instantaneous data
	Permeate flowrate	(m 3 / day) or (m 3 /h)	Averaged from instantaneous data
	Permeate temperature	(°C)	Averaged from instantaneous data
	pH of influent	-	Averaged from instantaneous data
	Supernatant COD in recirculation channel	(gO2/L)	Daily
	Aeration flowrate in membranes	(Nm 3 / dh)	Averaged from instantaneous data
	Indicator of sodium hypochlorite maintenance cleaning	-	Daily

Table 2 . 3 :

 23 Permeate instantaneous flux at 20°C for the 28 membrane tanks from SAV-MBR calculated between November/2017 and June/2021.

	Permeate

instantaneous flux 20°C (LMH)

	Membrane tanks	A1	A2	A3	A4	A5	A6	A7
	Average	26.6	27.1	26.6	26.6	26.9	26.0	25.7
	SD	6.3	6.0	6.4	6.4	6.5	6.2	6.4
	Membrane tanks	A8	A9	A10	A11	A12	A13	A14
	Average	26.8	27.1	26.8	27.0	27.2	27.0	26.5
	SD	6.3	6.4	6.3	6.4	6.4	6.4	6.5
	Membrane tanks	B1	B2	B3	B4	B5	B6	B7
	Average	27.0	26.6	27.0	26.8	26.1	26.6	26.4
	SD	6.4	6.4	6.9	6.6	6.1	6.5	6.6
	Membrane tanks	B8	B9	B10	B11	B12	B13	B14
	Average	26.8	26.3	25.2	26.6	26.2	25.9	26.3
	SD	6.7	6.4	6.2	6.3	6.2	6.3	6.4

Table 2 . 4 :

 24 SADm for the 28 membrane tanks from SAV-MBR calculated between November/2017 and June/2021. Some of these membrane tanks were stocked this way more often than others, thus the ratio of these non-operating days since the commissioning date (November/2017 to June/2021) were calculated and presented in Table2.5. Based on this ratio, two groups of membrane tanks were observed, group 1 of membrane tanks presenting under 10% of non-operating days since the commissioning date and group 2 of membrane tanks

				SAD (m 3 /m 2 /h)
	Membrane tanks	A1	A2	A3	A4	A5	A6	A7
	Average	0.22	0.21	0.22	0.21	0.22	0.20	0.21
	SD	0.06	0.05	0.06	0.06	0.05	0.05	0.06
	Membrane tanks	A8	A9	A10	A11	A12	A13	A14
	Average	0.22	0.21	0.22	0.21	0.22	0.22	0.19
	SD	0.06	0.05	0.05	0.05	0.05	0.05	0.06
	Membrane tanks	B1	B2	B3	B4	B5	B6	B7
	Average	0.20	0.21	0.22	0.22	0.21	0.21	0.22
	SD	0.05	0.05	0.05	0.05	0.05	0.05	0.05
	Membrane tanks	B8	B9	B10	B11	B12	B13	B14
	Average	0.23	0.23	0.22	0.20	0.22	0.20	0.22
	SD	0.05	0.05	0.05	0.05	0.05	0.05	0.05

Membrane tanks from SAV-MBR presented some particularities whenever they had to be shut down due to different reasons, e.g. unavailability of cleaning-in-place line, pump issues, maintenance, etc. During these non-operating days, these membrane tanks were stocked in low NaOCl concentration solutions

(3 -15 ppm)

.

presenting between 13% and 36% of non-operating days. Therefore, this is a parameter able to differentiate membrane tanks. Based on this ratio and in order to choose membrane tanks in CHAPTER 2: MATERIAL AND EXPERIMENTAL METHODS 105 different locations with respect to the feed, 14 membrane tanks were chosen and are highlighted in Table

2

.5 to further analysis of process data.

Table 2

 2

	.6 for bench-scale and 2.7

indicators calculated were: operating days, exposure dose, specific permeate volume (Vp), specific aeration demand and filtered MLSS mass, filtered supernatant COD mass and filtered supernatant ∆COD mass, which are calculated as ascribed in Eq. 2.1 -2.3. COD measurements were not available for the full operation of SEM-MBR, therefore, the associated indicators were not calculated. B) of 14 tanks. Regarding these process indicators, operating days, permeate volume, exposure dose and aeration volume are individually measured for each membrane tank, whereas filtered MLSS, COD and ∆COD are measured by row in the recirculation channel, which implies that we consider homogeneous concentrations among tanks from the same row. Similarly, to SAV-MBR, SEM-MBR membrane tanks presented the same individualized parameters, whereas only one MLSS value is available for all eight membrane tanks, implying the same hypothesis.

Table 2 .

 2 6: Exposure doses of membrane samples aged at bench-scale.

	Bench-scale single soaking

Table 2 .

 2 7: Process indicators to monitor ageing for membrane samples harvested at full-scale.

	Time since commissioning date (days)	C × t (ppm.

h) Specific permeate volume (m 3 /m 2 ) Specific filtered Supernatant COD (kg O2/m 2 )

	Time since commissioning date (days) Time since commissioning date (days) Time since commissioning date (days)	C × t (ppm.h) C × t (ppm.h) C × t (ppm.h)	Specific permeate volume (m 3 /m 2 ) Specific permeate volume (m 3 /m 2 ) Specific permeate volume (m 3 /m 2 )	Specific filtered Supernatant COD (kg O2/m 2 ) Specific filtered Supernatant COD (kg O2/m 2 ) Specific filtered Supernatant COD (kg O2/m 2 )	Specific filtered ∆COD (kg O2/m 2 ) filtered ∆COD Specific (kg O2/m 2 ) Specific filtered Specific filtered MLSS mass (kg MLSS/m 2 ) MLSS mass (kg MLSS/m 2 ) SADm SADm (m 3 /m ) (m 3 /m ) Specific filtered ∆COD (kg O2/m 2 ) Specific filtered MLSS mass (kg MLSS/m 2 ) SADm (m /m 2 ) Specific Specific filtered ∆COD MLSS/m 2 ) (kg O2/m 2 ) filtered MLSS mass (kg SADm (m 3 /m 2 )	Non-filtration time (days) Non-filtration time (days) Non-filtration time (days) Non-filtration time (days)
					SAV-A1 SAV-B5 SAV-B11 SEM-UF4
	314 515 606 1084	6,600 7,500 20,100 26,000	144 320 289 181	10.2 14.6 -	6.2		6.3 8.0 -	3.5	1002 1887 1015	404	1027	76 104 383 141
	667 802 1511	13,500 13,900 32,900	314 405 244	13.5 -	14.2 SAV-B14	7.3 -	8.7	1765 1401	1357	1621	89 350 338
	1053 303	26,500 6,800	494 138	4.7	18.0 SAV-A8 SEM-UF5	2.5	9.6	464	2319		104 18
	344 939 1359	6,800 20,300 45,900	170 443 234	6.9 15.6 -	SAV-B1	3.8 7.8 -		443 2328 1389		2074	24 30 244
	239 1017 1525	6,900 20,400 63,800	117 488 266	17.7 -	4.1 SEM-UF1	9.3 -	0.8	2203 1588	433	2328	95 88 275
	836 1199 991	19,900 26,800 22,900	401 560 165	19.3 -	14.2 SEM-UF8	9.7 -	3.9	2547 953	1983	597	126 101 113
	1090 1293 988	26,700 47,700 41,000	517 206 157	--	16.9 SAV-B8	--	4.2	1203 911	2583	845	237 288 124
	304 1606 1127	6,800 59,300 56,000	141 266 178	5.1 --	SAV-A5	2.7 --		484 1579 1038		1089	15 339 288
	349 755 1449	6,800 25,700 74,500	173 356 226	13.4 -	7.1 SEM-UF3	7.5 -	3.9	1667 1336	460	1416	21 53 353
	743 1081 816	13,700 26,600 33,000	364 495 140	16.2 -	15.7	8.2 -	9.2	2381 717	1542	718	40 61 65
	1086 1079	14,600 42,400	520 185	-	18.9 SAV-B10	-	10.0	1028	2380		54 158
	1270 433 1282	21,100 13,100 61,600	595 209 218	8.0 -	20.5	4.5 -	10.4	964 1206	2737		68 148 362
	756 1552	13,900 72,200	345 268	10.8 -			5.1 -		1683 1519			293 399
	1864	98,200	294	-			-		1943			409

Table 2 .

 2

	24

10: Impact of period length on J/TMP (CIP: October/2017, SEM-UF4). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10.

Table 2 .

 2

	Length	J/TMP28-30 (LMH/bar)	J/TMP30-32 (LMH/bar)	Filtration cycles (∑n)
	CIP+1	n = 2	n = 5	7
	CIP+2	n =9	498.3 ± 44.4	33
			n =24
	CIP+3	483.1 ± 45.5	503.5 ± 44.2	58
		n =19	n =39
	CIP+4	485.3 ± 43.3	491.4 ± 46.1	79
		n =23	n =56
	CIP+5	488.0 ± 50.0	489.6 ± 46.0	99
		n =30	n =69
	CIP+6	488.7 ± 46.6	492.8 ± 46.5	115
		n = 37	n =78
	CIP+7	485.0 ± 45.2	497.8 ± 47.8	139
		n =42	n =95
	CIP+1 to +7 483.2 ± 44.6	498.0 ± 48.4	132
		n =40	n =90
	CIP+2 to +7 482.7 ± 46.1	497.7 ± 49.1
		n =33	n =71

11: Impact of period length on J/TMP (CIP: June/2019, SEM-UF4). Averages and standard deviations calculated with "n" filtration cycles. J/TMP averages were only calculated if n ≥ 10.

Table 2 .

 2 12: Cleaning recovery for different backwash duration.

	Backwash	Cleaning
	duration (min)	recovery
	15	77 %
	30	82 %
	60	69%

Table 4 .

 4 Chronological data was collected from May/2014 to January/2021 for SEM-MBR and from November/2017 to June/2021 for SAV-MBR. The main characteristics of influents for both MBRs considered in this study are presented in Table 4.1. 1 Quality characteristics of influents (after pre-treatment) from SEM-MBR (data from May/2014 to January/2021) and SAV-MBR (data from November/2017 to June/2021).

			Influent's characteristics
	Parameter	Units	SEM-MBR	SAV-MBR
	TSS	(mg / L)	332.9 ± 107.5	123.0 ± 43.2
	COD	(mg O2 / L)	694 ± 166	311 ± 92
	BOD5	(mg O2 / L)	299.3 ± 74.6	123.0 ± 37.8
	Total Kjeldahl
	Nitrogen	(mg N / L)	68.3 ± 14.0	46.0 ± 9.8
	(TKN)
	NH4	(mg N / L)	49.8 ± 12.2	33.4 ± 8.5
	Total Nitrogen (TN)	(mg N / L)	69.3 ± 14.0	47.0 ± 9.7
	Total
	phosphorus	(mg P / L)	7.5 ± 1.6	4.5 ± 1.1
	(TP)
	PO4 3-	(mg P / L)	4.2 ± 1.2	2.2 ± 0.9
	pH	-	7.8 ± 0.2	7.7 ± 0.2
	Temperature	(°C)	-	19.1 ± 3.3
	TAC	(mg CO3/L)	-	216.2 ± 42.3

Table 4 .

 4 2 : Averages and standard deviations of the main operating conditions of SAV and SEM MBRs. * means differences between these values are not statistically significant.

			SEM	SAV
		Operating conditions -Biological Basin
	Influent flow rate	(m 3 /total m 2 of membrane/day)	0.15 ± 0.03	0.40 ± 0.02
	COD organic rate	(kg COD / kg MLSS / d)	0.06 ± 0.02	0.10 ± 0.05
	BOD5 organic rate	(kg BOD5 / kg MLSS / d)	0.05 ± 0.01	0.04 ± 0.01
	Organic loading rate	(kgCOD / m 3 / day)	0.75 ± 0.19	0.48 ± 0.19
	N-NH4 to sludge	(kg N-NH4 / kg MLSS / d)	0.008 ± 0.002	0.010 ± 0.003
	N-NH4 loading rate	(kgN-NH4 /m 3 aerated / d)	0.114 ± 0.04 *	0.117 ± 0.034 *
	MLSS in aerated zone	(g/L)	6.4 ± 0.9	5.0 ± 1.0
	HRT	(h)	4.3 ± 0.9	4.6 ± 0.9
	SRT	(d)	16 ± 4	29 ± 5
	Recirculation factor	(-)	4.7 ± 0.3	4.5 ± 0.5
	Temperature	(°C)	19.7 ± 3.1	19.1 ± 3.3

Table 4 .

 4 3 Quality characteristics of effluents along with performances from SEM-MBR (data from May/2014 to January/2021) and SAV-MBR (data from November/2017 to June/2021).

			Characteristics of effluent	Removal Efficiency (%)
	Parameter	Units	SEM-MBR SAV-MBR SEM-MBR SAV-MBR
	TSS	(mg / L)	<2.0	<2.0	99 ± 4	98 ± 1
	COD	(mg O2 / L)	14 ± 3	14 ± 5	97 ± 4	85 ± 7
	BOD5	(mg O2 / L)	1.1 ± 0.7	2.1 ± 0.9	99 ± 2	99.3 ± 0.4
	Total Kjeldahl				97 ± 11	96 ± 2
	Nitrogen	(mg N / L)	0.9 ± 0.5	1.6 ± 0.9
	(TKN)
	NH4	(mg N / L)	0.3 ± 0.4	0.6 ± 0.5	98 ± 12	98 ± 3
	Total Nitrogen (TN)	(mg N / L)	9.6 ± 3.0	14.1 ± 7.4	84 ± 11	70 ± 10
	Total				97 ± 9	76 ± 22
	phosphorus	(mg P / L)	0.1 ± 0.2	1.1 ± 1.0
	(TP)
	PO4 3-	(mg P / L)	0.2 ± 0.2	1.0 ± 0.9	95 ± 11	68 ± 26
	pH	-	7.7 ± 0.2	8.0 ± 2.5
	Temperature	(°C)	19.4 ± 3.4
	TAC	(mg CO3/L)		114.3 ± 23.2
	Permeate quality chronology
	Zeeweed membranes present a pore size diameter of 0.04 µm, which enables the retention of
	particles, bacteria, protozoa, some viruses and colloidal matter. Following permeate quality is
	an indirect way to monitor membrane ageing, since increase in membranes pore size over time
	leads to the formation of defaults enable bigger particles to flow through the membranes. Figure
	4.6 presents results of TSS concentration in permeate for both SAV-MBR (A) and SEM-MBR
	(B). Most of the time permeate was below the quantification limit of this analysis (2 mg/L),
	confirming membranes' integrity over time. Plant operators from SEM-MBR reported that this
	permeate presented slightly higher TSS concentrations in the beginning due to leaking in pipe
	connections. Moreover, plant operators from both MBR reported formation of biofilm in the

  To conclude, average values of permeate flux and SADm are similar for both MBRs, but some differences in membrane tank operation can be highlighted:

	(i)	Membrane tanks from SEM-MBR presented higher supernatant COD (43.9 and 31.3
	mg.L -1 respectively) and MLSS (7.6 and 6.4 g.L -1 respectively) concentrations than the ones
	from SAV-MBR (Table 4.4 and Figure 4.7 and Figure 4.8);
	(ii)	Permeate fluxes from SAV-MBR are more variable and reach highest peak values (up
	to 50 LMH) than the ones from SEM-MBR (with a relative stability over time and a highest
	value up to 30 LMH) (see Figure 4.9);
		Tank A1	Tank A2	Tank A3	Tank A5	Tank A8	Tank A12 Tank A13
		Tank B1	Tank B5	Tank B8	Tank B10 Tank B11 Tank B13 Tank B14
							197

(iii) SEM-MBR only presented 5.4 ± 1.9 h of daily filtration hours, whereas SAV-MBR presented 20.7 ± 1.7 h due to the adaptation of flowrate to treat only 30% of its nominal capacity (further explained in Chapter 2).

Table 4 .

 4 5 : Spearman correlation coefficients between each process indicator and the normalized permeability index (bold values are the highest correlation coefficients).

	Process indicators	Normalized permeability index -SAV	Normalized permeability index -SEM
	Operating time	-0.836	-0.189
	Permeate volume	-0.823	-0.075
	Exposure dose	-0.826	-0.557
	SupernatantCOD	-0.763	-
	∆COD	-0.667	-
	Filtered MLSS	-0.839	-0.111
	Aeration volume	-0.812	-0.129

Table A .

 A 1: Statistical summary of SADm for membrane tanks in row A from SAV-MBR.

			SADm (m 3 /m 2 /h)
	Membrane tanks	A1	A2	A3	A5	A5	A8	A12	A13
	Average	0.22	0.21	0.22	0.22	0.22	0.22	0.22	0.22
	SD	0.06	0.05	0.06	0.05	0.05	0.06	0.05	0.05
	1Q	0.18	0.17	0.18	0.18	0.18	0.18	0.18	0.19
	2Q	0.24	0.22	0.23	0.23	0.23	0.24	0.23	0.24
	3Q	0.27	0.25	0.26	0.26	0.26	0.27	0.26	0.27
	Min	0.04	0.04	0.04	0.04	0.04	0.04	0.04	0.04
	Max	0.32	0.28	0.31	0.31	0.31	0.31	0.30	0.32
	5th Percentile	0.11	0.11	0.11	0.12	0.12	0.10	0.13	0.14
	10th Percentile	0.15	0.14	0.14	0.14	0.14	0.14	0.15	0.15
	90th Percentile	0.29	0.26	0.28	0.28	0.28	0.28	0.27	0.28
	95th Percentile	0.30	0.27	0.29	0.28	0.28	0.29	0.28	0.28
	N	1357	1074	1298	1400	1400	1375	1340	1390

Table A .

 A 2: Statistical summary of SADm for membrane tanks in row B from SAV-MBR.

			SAD (m 3 /m 2 /h)
	Membrane tanks	B1	B5	B8	B10	B11	B13	B14
	Average	0.20	0.21	0.23	0.22	0.20	0.20	0.22
	SD	0.05	0.05	0.05	0.05	0.05	0.05	0.05
	1Q	0.16	0.17	0.19	0.17	0.17	0.17	0.18
	2Q	0.21	0.22	0.24	0.23	0.21	0.21	0.23
	3Q	0.24	0.25	0.27	0.26	0.24	0.24	0.26
	Min	0.04	0.04	0.04	0.04	0.04	0.04	0.04
	Max	0.29	0.30	0.31	0.30	0.29	0.29	0.31
	5th Percentile	0.09	0.12	0.13	0.12	0.12	0.11	0.12
	10th Percentile	0.13	0.14	0.16	0.14	0.14	0.14	0.14
	90th Percentile	0.25	0.27	0.28	0.27	0.25	0.26	0.27
	95th Percentile	0.26	0.28	0.29	0.28	0.26	0.27	0.29
	N	1153	1044	1341	994	1224	1169	1265

All statistical analyses were conducted in R software and XLStat (Microsoft Office, USA) [START_REF] Core | R: A Language and Environment for Statistical Computing[END_REF].

Normality tests (Shapiro-Wilk), principal compound analysis and correlation tests presented in

Chapter 3 were performed in R software for membrane autopsies. The normality tests (Shapiro-Wilk), PCA and correlation tests for the relationships among process data and autopsies presented in Chapter 4 were performed in XLStat. be related to the frequency of each measurement. MLSS data is obtained every day and COD is only measured twice a week. The higher correlation coefficient for SAV also suggested that normalized permeability index decreased linearly as a function of operating days and the cumulated specific filtered MLSS (closely linked to the cumulated permeate volume) with residual fouling rates of -1.905E-4 d -1 and -8.523E-5 (kg MLSS/m 2 membrane) -1 (Figure 4.19.B and Figure 4.20.B, respectively). The graphs between normalized permeability index and the other indicators are presented in

For SEM-MBR, correlation coefficients are lower than the ones observed SAV-MBR, with non-significant correlations (Table 4.5). This dispersion can be observed in Figure 4.20. C and D with the plot of the normalized permeability index as a function of the cumulated specific permeate volume and the specific filtered MLSS. This higher variation in the behavior of each membrane tank might be associated to differences in chlorine exposure (Figure 4.10) and a lower rate of filtration (Figure 4.11).

Globally, SAV-MBR presented a slightly faster rate of decline in normalized permeability index than SEM-MBR with respect to operating days (slopes in Figure 4. [START_REF] Vanysacker | Effect of chemical cleaning and membrane aging on membrane biofouling using model organisms with increasing complexity[END_REF].B and D, respectively), which might be linked to the higher filtration rate observed in SAV-MBR (Figure 4.17).

Cleaning recovery decreased from 56% for a pristine membrane to 32% for membranes harvested at 98,000 ppm.h. These results illustrate the stronger interactions between foulants and residual fouling from full-scale operation that remains in the membrane structure even after chemical cleanings.

Therefore, although accelerated ageing protocols by single soaking provide interesting information about membrane material sensitivity towards chlorine, the changes in membrane properties observed in this protocol did not fully represent results from membranes harvested at full-scale MBR. In both cases, PVP was oxidized, but different secondary products were found, besides different patterns in morphology, pore size, permeability and cleaning recovery.

Membrane ageing at full-scale MBR based on membrane properties and indicators of operation

Membrane ageing was studied in two full-scale MBR presenting slightly different conditions.

Based on membrane properties, one group of membranes presented ultimate tensile strength higher than 300 MPa and another presented values lower than 300 MPa. Samples from both MBRs were in both groups, which suggests that the reason for membranes presenting lower ultimate tensile strength is more likely to be related to to operating conditions or sampling method. However, further analysis and sampling is been carried out to investigate the origin of such differences.

Intrinsic permeability seemed to be more sensible to ageing for the group of lower mechanical resistance, increasing up to 2.5 of initial values. Although the great dispersion, samples from both MBRs in the two groups presented similar trends.

PVP content was one of the most sensible membrane properties to ageing regarding operating conditions. Actually, membranes harvested from SAV-MBR presented a faster decline in PVP content reaching around 35% of the initial value at an exposure dose of 20,000 ppm.h (1000 days of operation), whereas samples from SEM-MBR were around 80% at the same dose. Based on these results, it seems that the specific filtered MLSS might be a promising indicator for monitoring membrane ageing. This indicator provides information about filtration and organic matter that membranes face over time. As a perspective of this research, the filtered MLSS will continue to be followed during this membrane life cycle for both MBRs.

Appendix E:

Correlation between membrane properties and normalized permeability index

The normalized permeability index with respect to specific aeration demand for both MBRs and with respect to the filtered supernant COD and ∆COD, specifically for SAV, are presented

Appendix F:

Effects of non-operating periods on normalized permeability index of membrane tanks of SEM-MBR

Regarding some particularities among membrane tanks from SAV-MBR, whenever these membrane tanks were shut down due to different reasons, e.g. unavailability of cleaning-inplace line, pump issues, etc, these membrane tanks were stocked in low NaOCl concentration solutions (3 -15 ppm). Some of these membrane tanks were stocked more often than others, thus the ratio of these non-operating days since the commissioning date (November/2017 to June/2021) were calculated and presented in Table F.1. Membrane tanks were divided into group 1 for the ones that presented a ratio below 10% and group 2 for the ones above this value. The effect of these operating modes in membrane ageing are analyzed and presented hereby.

Each membrane tank presented highly different conditions over time due to operating issues, e.g. cleaning reagents lines unavailability, dry weather (lower inflow) and equipment related issues. When a membrane tank is unavailable for long periods, membranes are submerged in a diluted chlorine solution (3 -15 ppm) with 5 min aeration cycles every 60 min for SAV-MBR.

Thus, regarding the ratio between days that each membrane tank did not properly function since commissioning, membrane tanks can be separated into two groups (Table F.1). Group 1 for the membrane tanks with the most important time in operation, ranging from 3 to 9% of nonoperating days and group 2 with non-operation days ranging from 18 to 36% of the total days since November/2017.

A statistical analysis was carried out with these two separated groups in order to evaluate the effects of these conditions onto membrane ability to filter mixed liquor over time. The differences between group 1 and 2 are not statistically significant (p-value=0.786; Mann-Whitney test). This is also visible when normalized permeability indexes are plotted against filtered volume, the number of days since commissioning days and cumulated exposure dose in Similarly, normalized permeability indexes were calculated to SEM-MBR from 2015 to 2020.

As already observed, the main difference between these two MBRs is the daily filtration time.

The average filtration duration per day (with standard deviations) for each membrane tank from SEM-MBR between 2015 and 2020 are presented in Table F.2. This shows that each membrane tank operate roughly the same way daily. stronger decline in normalized permeability index was highly influenced by data from SEM-UF8, without considering this tank, SEM-MBR's slope would be closer to SAV-MBR (-5.565E-04) and p-value of the statistical analysis between these two MBRs would increase to 0.198, reinforcing their similarity. Therefore, long non-filtration periods may also be efficient to limit the residual fouling accumulation, reinforcing that dynamics of filtration play a major role on membrane ageing.