S Kdidi

G Vaca-Medina

J Peydecastaing

N A Oukarroum Ct

Fayoud

A Barakat

Electrostatic separation for sustainable production of rapeseed oil cake protein concentrate: Effect of mechanical disruption on protein and lignocellulosic fi.ber separation

Keywords:

Rapeseed oil cake (ROC) is a biomass of particular interest for biorefinery and food activities, due to its high pro tein, lignin. polyphenol and fiber contents. An environmental separation prooess combining ultrafine milling and electrostatic separation was reoently developed for proteo lignocellulosic plant fractionation. Severa( of the pa rameters of the fractionation prooess (milling mode, electrode voltage, charge, partide size, etc.) can affect the efficiency of protein separation and extraction. We assessed the impact of three different mechanical modes of milling ball milling. impact grinding and jet milling on the electrostatic separation (ES) of lignin and protein from ROC biornass. The initial biomass and the ES fractions were characterized and correlations were identified between particle colors and sizes and their protein and fiber contents. Mechanical grinding had a significant im pact on the surface properties of the particles, influencing their charge and the electrostatic separation of lignin and protein from biomass. The combination of impact grinding and electrostatic separation appear the most ef fident and effective technologies for rapeseed oil cake protein concentrate.

Introduction

Rapeseed oil cake (ROC) is a by product of oil extraction. It is produced in large quantities, with worldwide production estimated at 35 million ton in 2012. ROC has potential for use as a raw material in the production of rnaterials, antioxidants, arornatic compounds, feed/food and energy, due to it high protein, polyphenol and fi ber con tents. ROC biomass typically contains about 30 40% protein and 15 20% lignin [START_REF] Basse� | Chemicaland solvent-free mechanophysical frac tionation of biomass induced by tribo�ectrostatic charging: separation of proteins and lignin[END_REF][START_REF] Am | Characterisation of cell wall poly saccharides fr om rapeseed (Brassica napus) meal[END_REF]. It is also rich in carbohydrates, including pectins, hemicellu !oses and cellulose, which together form a complex network within plant cell walls [START_REF] Am | Characterisation of cell wall poly saccharides fr om rapeseed (Brassica napus) meal[END_REF].

ROC is currently used in the production of animal feed, due to its high protein content. However, ROC biornass could also be used as an ingredient in the manufacture of human foods, due to its high antioxi dant and fiber contents. This new valorization of ROC valorization will depend on the quality and quantity of the proteins extract, which will , in turn, depend on the extraction and purification methods used. Vari ous studies have shown that rapeseed proteins have several very useful properties (3 6]. ROC protein extraction methods should conserve pro tein structure and function, so as to preserve the organoleptic quality of ROC proteins. Chemical methods are currently used to extract proteins from ROC [START_REF] Pedroche | Brassica carinata protein isolates: chernical composition, protein characterization and im provement of functional properties by protein hydrolysis[END_REF]. These methods generate effluent and may modify the functional proprieties of the se macromolecules, which may be highly problematic. Most of the fractionation and separation operations used are based on expensive chemical processes (pulping, hydrolysis, sol vent extraction, steam and ammonia explosion, ionic liquid extraction etc.) that cannot isolate the main compounds (polyphenols proteins, carbo hydrates etc.) without a Joss of integrity. Indeed, plant materials have a robust supramolecular structure, the deconstruction of which often requires harsh conditions. In such conditions, the native functions of the compounds may not be fully preserved [START_REF] Mai | van der Goo� The potential of dry fractionation prooesses for sustainable plant protein production[END_REF]. Furthermore, these pro cesses require large amounts of energy, water and/or solvent. The downstream steps in purification and processing involve solvent re moval, which also greatly increases the final cost of the extracted bio molecules and biopolymers. In this context, several processes have been proposed for the extraction and concentration of proteins and for the removal of lignin polymers by alternative technologies not re quiring the use of chemical compounds and generating no effluent (9 12].

A process combining mechanical milling and electrostatic separation (ES) was recentiy proposed as a promising method forfractionating dif ferent plants materials to obtain protein rich fractions [START_REF] Jafari | Physicochemical characterization of a navy bean (Phaseolus vulgaris) protein fraction produced using a solvent-free method[END_REF]. Electrostatic separation processes Jead to enrichment in proteins, due to the removal of lignin from the protein fraction. However, several parameters can in fluence the fractionation of proteins, lignin and lignocellulosic materials from different plants materials [14 16]. Particle charge plays a major role in separation, as highlighted by Wang and coworkers in a theoret ical study of the application of ES to a gluten starch mixture [START_REF] Wang | MAL Schutyser, Chargjng and separation behavior of gluten-starch mixtures assessed with a custom-built electrostatic separator[END_REF]. The conductivity of a particle depends on its chemical composition, surface properties, moisture content, size, shape and density. Some of these physicochemical proprieties also depend on the type of milling [START_REF] Pelgrom | Schutyser, Ory fractionation for pro duction of functional pea protein concentrates[END_REF], the first step in the electrostatic fractionation process.

This study investigated the effects of three mechanical dissociation modes: ball milling (BM), impact grinding (IG) and jet milling (JM) on the electrostatic fractionation of proteins and lignocellulosic polymers from ROC biomass, to generate fractions highly enriched in protein or fiber (Fig. 1).

Materials and methods

Raw material

Rapeseed oilcake with moisture content of 8.1% was obtained from a French commercial retailer specializing in oil meal production after the cold pressing of partially dehulled rapeseed (B. napus L.), and dried.

Mechanical dissociation modes

A knife milling system (Retsch SM 100, Germany) was initially used to reduce the particle size of ROC pellets, using a grid with a screen size of 2 mm. The knife milled ROC particles were then ground ultrafine by one of three different mechanical dissociation methods operating at pilot scale (1 10 kg/h). The modes of grinding tested were: impact grinding (100 UPZ, Hosokawa Alpine, Germany) with a 0.1 mm screen size grid operating at room temperature and 18,000 rpm; jet milling (Hosokawa Alpine, Germany) with a feed rate of 1.1 kg/h at 2000 rpm; and ball milling with a 2 L porcelain jar containing 70 balls (35 balls of 25 mm in diameter and 35 balls of 20 mm in diameter). Ball milling was then conducted in a planetary ball mill at 51 rpm for 1 to 20 h with a ball: powder: empty space ratio of 1:1:1.

Electrostatic separation (ES) technology

Fine milling was coupled with electrostatic separation technology, to separate proteins and lignin from ROC biomass. A pilot scale electro static separator (TEP System, Tribo Flow Separations, Lexington, USA) was used to fractionate ultrafine ROC biomass powder (F0) into two fractions of different compositions (F+ and F-) [START_REF] Basse� | Chemicaland solvent-free mechanophysical frac tionation of biomass induced by tribo�ectrostatic charging: separation of proteins and lignin[END_REF]. The feed system of the ES was operated at 150 rpm; ROC particles were conveyed by compressed air into a charging line, and charged by triboelectric effects, due to the impact of ROC particles on each other and with the charging line walls. The charged particles were then introduced into a separation chamber containing two high voltage electrodes (10,000 V), in which the positively charged particles (F+) were attracted to the negative electrode and the negatively charged particles (F-) to the positive elec trode. A particle recovery system equipped with two cyclones was used to recover two fractions: one consisting of the positively charged parti cles (F+) and the other of the negatively charged particles (F-).

Rapeseed

Particle size and color

Following ROC biomass fractionation, the particle sizes of the F0, F+ and F-fractions were determined by laser diffraction (Mastersizer 2000, Malvern Instrument) at room temperature in ethanol. All frac tions were characterized in terms of median particle diameter (d50) and granulometric distribution (expressed as a percentage by volume, vol%). The color of each fraction was determined with a colorimeter (Minolta CR 410 Colorimeter, Nieuwegein, the Netherlands) and the CIELAB color space system: L* provides information about luminance, and a* and b* provide information about sample color (a*: from green to red, and b*: from dark blue to yellow). Measurements were made in duplicate, and the mean value was calculated for each of the three pa rameters (L*, a*, and b*).

Analysis of the composition of ROC fractions

The dry matter and ash contents of the various fractions were deter mined. Biomass was dried at 60 °C for 2 h and then incinerated at 900 °C for 1 h. Protein content was determined by the Kjeldahl method. Cellu lose and hemicellulose contents were determined by high performance anion exchange chromatography with pulse amperometric detection (HPAEC PAD) on an ICS 5000 ion chromatography system equipped with a CarboPac PA20 column (Dionex, Sunnyvale, CA), after two step hydrolysis with sulfuric acid [START_REF] Amamou | A Baraka� Mechano=zymatic deconstruction with a new enzymatic cocktail to enhance enzymatic hydrolysis and bioethanol fermentation of two macroalgae speàes[END_REF]. Briefly, ROC biomass was hydrolyzed, in duplicate, with 70% H 2 SO 4 for 1 h at 30 °C and then with 4% H 2 SO 4 for 65 min at 120 °C. Klason lignin was determined as the insoluble residue remaining after the hydrolysis of cell wall polysaccharides with sulfuric acid [START_REF] Basse� | Chemicaland solvent-free mechanophysical frac tionation of biomass induced by tribo�ectrostatic charging: separation of proteins and lignin[END_REF]. For the extraction of water soluble carbohydrates, ROC frac tions were mixed with water (10%, w/v) and incubated for 1 h at 60 °C, in duplicate. Monosaccharides (glucose, fructose, galactose, arab inose, xylose, and mannose) and sucrose were identified directly, by high performance anion exchange chromatography with pulse amper ometric detection (HPAEC PAD) on an ICS 5000 ion chromatography system equipped with a CarboPac PA20 column (Dionex, Sunnyvale, CA).

Scanning electron microscopy

Scanning electron microscopy (SEM) was performed with a Phenom G2 Pure electron microscope (Phenom World BV, Eindhoven, the Netherlands), for observation of the ultrafine RSM and SFM fractions. The samples were placed on double sided adhesive tape and fixed to SEM stubs. Excess particles were removed with a dry airstream. No sam ple pretreatment was required.

Measurement of specific energy consumption

The total energy (E TER ) consumed during the fractionation process was defined as the sum of the energies required for milling (E M ) and electrostatic separation (E ES ). Total energy consumption (expressed in Wh.kg -1 ROC) was determined as follows:

E TER E M þ E ES ð1Þ
E M and E ES were measured with a wattmeter [START_REF] Barakat | Eco-friendly dry chemomechanical pretreatments of lignocellulosic biomass: Impact on energy and yield of the enzymatic hydrolysis[END_REF]). Energy consumption was calculated as follows:

E M R t 0 P t -P 0 ð Þ dt m R t 0 ΔP t dt m ð2Þ
Where Pt is the power (watts) consumed at time t, P0 is the mean power consumption (watts) under idle conditions (without biomass), and m is the mass (kg) of biomass to be ground. Three power measurements (P0 and Pt) were obtained for each sample.

Statistical analysis

The results presented are the means of duplicate analyses. All data were subjected to analysis of variance with XLstat® (Addinsoft, Brook lyn, USA). Significant differences (P b 0.05) between individual means were identified in Tukey's test.

Results and discussion

Ultrafine milling of ROC biomass

The ROC used in this study consisted of 35.8 g protein/100 g dry bio mass, 6.1 g ash/100 g dry biomass and 13.4 g lignin/100 g dry biomass. These findings are consistent with previous reports, indicating values of between 30 and 40 g protein/100 g dry biomass, between 10 and 24 g lignin/100 g dry biomass (Pustjens et al., 2013; Jeong et al., 2014) and between 5 and 7 g ash/100 g dry biomass [START_REF] Basse� | Chemicaland solvent-free mechanophysical frac tionation of biomass induced by tribo�ectrostatic charging: separation of proteins and lignin[END_REF][START_REF] Am | Characterisation of cell wall poly saccharides fr om rapeseed (Brassica napus) meal[END_REF][START_REF] Jeong | Optirnization of enzymatic hydrolysis ronditions li>rextraction ofpectin from rapeseed cake (Brassica napus L.) using commercial enzymes[END_REF][START_REF] Caine | ln fluence of guanidin ation on apparent ileal digestibility of amino acids in pigs fed diets with soybean meal, rapeseed meal or peas as a protein source[END_REF]. Raw ROC (after knife milling with a 2 mm screen size grid) was subjected to three different types of milling, to dissociate the various constituents (protein, lignin, cellulose and ash) before electrostatic separation (Fig. 1).

The objective was to produce the particles of the same size (D 50 ) by the various milling modes and to assess the impact of the mode of dis sociation on the fractionation of proteins and fiber from biomass. The raw material was fragmented and deconstructed by the application of different mechanical stresses: impact, compression, friction, and shear ing (Fig. 1). For example, in jet milling (JM), the particles are projected against each other in an air stream, and the major mechanical stresses generated are impact and friction between particles (Fig. 1).

Knife milling (KM) involves the use of a rotor to drive different tools; rotor speed is generally adjustable. A sieve or a screen is used to control particle size in the final product. Such mills mostly generate impact and shear stresses (Fig. 1). During ball milling, the biomass is exposed to im pact and compression stresses induced by collisions between the balls, particles and container walls [START_REF] Licari | Comparison ofvarious rnilling modes combined to the enzymatic hyd rolysis of lignocellulosic biomass for bioenergy production: Glucose yield and energy efficiency[END_REF]. The particle size distributions of sam ples prepared by different milling modes are presented in Fig. 2. Median particle diameters of 42, 43 and 42 μm were obtained by impact grind ing, ball milling and jet milling, respectively. Mechanical dissociation mode had a significant effect on particle size distribution (Fig. 1), even though the three milling modes produced particles with almost identi cal d 50 values (i.e. 42 μm). Particle size distribution was bimodal for BM but not for the other milling modes. Samples generated by BM may, This effect can be explained by the greater damage to ROC particles caused by the stresses generated during BM (impact and compression). Silva and coworkers investigated the effect of milling mode on wheat straw structure [START_REF] Silva | Effects of grinding pro cesses on enzymatic degradation of wheat straw[END_REF][START_REF] Silva | Successive centrifugal grinding and sieving of wheat straw[END_REF]. They showed that BM was the only milling method capable of generating significant changes in macromolecular structure. SEM of the separated fractions (Fig. 3) revealed clear differ ences between the negatively (F-) and positively (F+) charged frac tions and the raw ROC biomass (F0). SEM of the ROC raw material (F0) showed it to consist of a mixture of granular globular bodies and cellular matrices of fiber rich constituents. Following ultrafine milling and electrostatic separation, the globular protein bodies were concen trated in the positive fraction, whereas the cellular and fibrous matrices were recovered in the negatively charged fraction (F-). These findings clearly confirm the impact of milling mode on the fragmentation and deconstruction of the supramolecular structure of ROC, which can influ ence directly the physicochemical proprieties of particles and, thus, the separation of protein and fiber from ROC biomass.

Electrostatic fractionation of proteins and lignocellulose from ROC biomass

Electrostatic separation (ES) has been used in several studies for the efficient fractionation of different biomasses, such as wheat bran and sunflower oil cakes, to separate cellulose, proteins, lignin and ash with out water or effluent [START_REF] Basse� | Chemicaland solvent-free mechanophysical frac tionation of biomass induced by tribo�ectrostatic charging: separation of proteins and lignin[END_REF][START_REF] Chuetor | Innovative combined dry fractionation technologies for rice straw valorization to biofuels[END_REF][START_REF] Hemery | Electrostatic properties of wheat bran and its constitutive layers: influence of particle size, composition, and moisture content[END_REF]. Electrostatic separation was performed on each of the milled ROC sample prepared with the different dissocia tion modes, with F0 denoting the initial material (Fig. 1). Samples were separated into two fractions: positively charged particles (F+) and neg atively charged particles (F-). The particle size distribution, color and biochemical composition of the three samples corresponding to the three different milling modes were then determined.

The chemical composition and structural features of these fractions are presented in Table 1. The ROC, (F+) and (F-) fractions differed in terms of their appearance; "L*", "a*" and "b*" values were higher for positively charged fractions. F+ fractions were more "yellowish" than F-fractions, which were almost brown. F+ and F-yields were similar for JM. By contrast, F+ yields were higher than F-yields for the impact and ball milling techniques (about 60% F+ and 40% F-). As there was no significant difference in particle sizes between F0 samples, these dif ferences in yield can be assumed to reflect physicochemical differences between particles due to the mode of dissociation. Protein content was higher in the positively charged fractions, whereas lignin content was higher in the negatively charged fractions, confirming published results [START_REF] Basse� | Chemicaland solvent-free mechanophysical frac tionation of biomass induced by tribo�ectrostatic charging: separation of proteins and lignin[END_REF][START_REF] Chuetor | Innovative combined dry fractionation technologies for rice straw valorization to biofuels[END_REF]. These results confirm the SEM observations, which clearly indi cated a more lignocellulosic structure for F-than for F+. The highest protein concentration (44.8%) and the lowest lignin con tent (4.3%) were obtained with KM technology (Table 1). By contrast, the highest lignin concentration (28%) was obtained with JM. This con tent is twice that of the raw ROC material (13.4% for F0). The lignin con tents of the negatively charged fractions (F-) obtained by KM and JM were not significantly different.

The triboelectric charging of particles is, therefore, controlled exclu sively by the properties of the surface layer. The positive or negative sur face charge density acquired during triboelectric charging is thus determined by the ability of the surface of the material to donate or accept electrons when it comes into contact with another material [START_REF] Mazumder | Twenty-first century research needs in electrostatic processes applied to industry and medicine[END_REF].

Influence of the fractionation process on physicochemical properties

Principal Component Analysis (PCA) and Hierarchical Archimedean Couplae (HAC) analysis were performed to assess the correlations be tween the physical and chemical properties of fractions obtained by electrostatic separation (Fig. 4). These analyses were performed on data for all fractions, focusing on six variables: three biochemical attri butes (protein, ash and lignin contents) and L*, a*, b* values. These anal yses classified samples into groups with significantly different biochemical compositions.

The HAC performed on the PCA data identified three different groups of samples (Shown by ellipses in Fig. 4B): the negatively charged sam ples, the positively charged samples and starting materials. The nega tively charged samples had high lignin content. The group corresponding to the starting material was located in the middle of the map as these samples had intermediate contents.

Comparison of milling modes in terms of energy requirement and efficiency

The total energy requirement (E TER ) of the different milling modes was calculated according to Eqs. ( 1) and (2) [ 28 30]. The total energy requirement (E TER ) of milling processes depends on the physicochemi cal proprieties of the biomass used and the distributions of particle size before and after milling, which are also highly dependent on the equipment or machine used. Generally, at the laboratory scale, E TER or specific energy can be measured only with size reduction machines equipped with a size screen, which is not possible for BM and JM. Thus, total energy (E TER ) alone, rather than specific energy, was used to compare the various milling modes (Fig. 5).

The specific energy requirements for the reduction of particle size in ROC biomass were estimated at 1.73, 3.54 and 4.72 kWh.kg -1 ROC for KM, BM and JM, respectively (Fig. 5). The energy consumed for electro static separation was estimated at 1.64 kWh.kg -1 ROC. Energy con sumption was clearly lower for KM than for the other two milling modes. BM and JM required more energy than IG to achieve a similar final particle size of 42 μm. We conclude from these data that E TER is highest for JM, followed by BM and, finally, KM, which has the lowest E TER . Lignin extraction yields of 96, 65 and 69% were obtained for JM, BM and KM, respectively. Thus, the extraction yields for lignin and protein clearly depend on the mode of milling. JM technology extracted lignin more efficiently than the other techniques, with yields reaching 96% without the addition of water, chemicals or solvents. By contrast, KM was the most efficient technique for protein extraction, with a yield of 74%.

Dry:EEbiomass

Extracted lignin or protein yield kg=kg biomass

ð Þ Total energy kWh=kg ð Þ ð3Þ
E TER and energy efficiency (ɳ) are generally used to compare the per formance and efficiency of different pretreatment processes [START_REF] Barakat | Eco-friendly dry chemomechanical pretreatments of lignocellulosic biomass: Impact on energy and yield of the enzymatic hydrolysis[END_REF][START_REF] Alinia | Pretreatment of wheat straw by supercritical CO2 and its enzymatic hydrolysis for sugar production[END_REF]. However, few data have been published for comparisons of energy .. consumption and energy effiàency between chemical, physicochemical and mechanical treatments of lignocellulosic biomass. We calculated dry fractionation energy efficiency (DryEEbiomass) coefficient (Eq. ( 3) ), to compare the effiàenàes of different milling modes coupled to ES for the extraction and separation of proteins and Jignocel lulosic fi bers. The most effective dry fractionation process has the highest Dry. EEbiomass values. The highest DryEE biomass obtained here was 0.429 kg protein kWh-1 and 0.398 kg lignin kWh-1 , for KM, and the Jowest DryEE t,;omass was 0.142 kg protein kWh-1 and 0.137 kg lignin kWh-1 , for BM (Table 2). The high energy consumption of BM, which gave the Jowest DryEE biomass value, can be attributed Jargely to the much longer milling time than for KM.

These results indicate that the energy effiàency dry fractionation (DryEEbiomas s) can be used to compare various fractionation methods. However, it is not straightforward to compare the energy requirements of different milling modes at Jaboratory scale, as these requirements de pend on the type, power, and size of the milling apparatus, and the ex perimental conditions. In our study comparing different milling modes at Jaboratory scale, knife milling (KM) coupled with ES seemed to be the most efficient of the mechanical pretreatments tested, in terms of energy and the fractionation and separation of protein and Jignocellu Josie fibers.

Conclusion

We used electrosta tic se para tion to assess the effect of the mode of mechanical dissoàation on rapeseed oil cake fractionation and proteins concentrates. Three different modes of dissoàation were used to grind the raw material: ball, impact and jet milling technologies. Milling mode affected the protein contents of the fractions obtained by electro static separation (ES). The biochemical composition and physical pro prieties of ROC biomass fractions (protein, lignin, polysaccharides, color, particle size) and dry fractionation energy effiàency indicated that knife milling ( KM) coupled with ES was the most effective technol ogy for the fractionation and separation of proteins and lignocellulosic fibers.

Fig. 1 .

 1 Fig.1. Fractionation scheme for ROC biomass coupling electrostatic separation with various types of milling.

Fig. 2 .

 2 Fig.2. Impact of milling mode (ground by jet, ball and impact) on particle size distribution of ROC samples

Fig. 3 .

 3 Fig. 3. Scanning electron micrographs of the negatively (F ) and positively (F+) charged fractions and starting materials

  Finally, the posi tively charged samples had high protein concentrations, and high values for L*, a* and b*. Protein content was positively correlated with L* and b* values. Thus, b* and L* values can be used as indicators of the efficiency of ROC biomass fractionation by ES.

Fig. 5

 5 Fig. 5 also show the efficiencies of lignin and protein extraction and separation from lignocellulose biomass as a function of grinding mode in the absence of added water and chemical catalyst. Protein extraction yield was estimated at 55, 66 and 74% for JM, BM and KM, respectively.Lignin extraction yields of 96, 65 and 69% were obtained for JM, BM and KM, respectively. Thus, the extraction yields for lignin and protein clearly depend on the mode of milling. JM technology extracted lignin more efficiently than the other techniques, with yields reaching 96% without the addition of water, chemicals or solvents. By contrast, KM was the most efficient technique for protein extraction, with a yield of 74%.

AF2Fig. 4 .

 4 Fig. 4. PCA and HAC analyses of the biochemical composition of the fractions. A: correlation circle with the active variables (color) and supplementary variables (biochemical composition) and B: similarity map of factors 1 and 2, and clustering of the samples into three classes.

Fig. 5 .

 5 Fig. 5. Energy requirement (kWhkg 1 ) and extraction yields for lignin and protein of the dillerent rniUing modes coupled to ES.

Table 1

 1 Biochemical composition of the ROC raw materials (F0) and electrostatically separated fractions (F and F+). Values in the same row followed by the same letter are not significantly different (P N 0.05).

	Ball milling	Jet milling	Knife milling
	(BM)	(JM)	(KM)

Tablez Extraction yields for lignin and protein fr om biomass. eneigy requirement and dry energy efficiency for the different modes of rniUing.

	Milling	Protein	Ugnin	En-•	Ory.EEblom.lSS
	mode	yield	yield	(kWh/kg)	(kg/kWh)
		(%)	(%)		Protein	Ugnin
	KM	7420	68.82	1.73	0.429	0.398
	JM	55.01	95.91	3.54	0.155	0271

BM 66.82 64.50 4.72 0.142 0.137

Acknowledgments

We would like to thank the 3BCAR Carnot Institute for providing fi nanàal support for this study (Sw:fDry Project).


Gaëlle Guyot
email: gaelle.guyot@cerema.fr

Lessons learned from a ten-year monitoring in residential buildings equipped with humidity based demand controlled ventilation in France

Keywords: Ventilation, performance, DCV, humidity, residential buildings

Humidity-based DCV systems have been widely used in France for 35 years and are considered as a reference system, including for low-energy residential buildings. Indeed, most of the new residential buildings, which must be low-energy buildings to comply with the RT 2012 energy performance regulation, are equipped with such systems. Feedbacks from two long-term studies show the durability of the humidity sensitive components and show the robustness of this system to bad maintenance or use by occupants. The on-going Performance 2 project delivers the first results of a ten-year monitoring in thirty social housing apartments. In the context of the increasing awareness about smart ventilation, from these feedbacks, it seems very important consider the durability of the systems and its components (including the sensors) and the robustness of the systems to a lack of maintenance by occupants.

INTRODUCTION

Regulatory context

The French dwellings airing regulation (J. O. 1983) requires a general and continuous airing system, describes the compulsory general layouts of ventilation installation and requires exhaust airflows in each humid room, depending on the total number of rooms in the dwelling. This regulation has been modified in 1983 in order to reduce these airflows in case of demand-controlled ventilation system (DCV), for instance based on humidity. In this case, manufacturers must follow a compliance procedure to ensure adequate ventilation, with a performance-based approach. Indeed, the procedure (CCFAT 2015) describes the common scenario used to evaluate the DCV systems using the multizone software MATHIS [START_REF] Demouge | Numerical Validation Fo Natural Ventilation Design[END_REF] and calculating two IAQ performance indicators: a condensation risk (number of hours with relative humidity higher than 75%) and a cumulative CO 2 exposure exceeding 2000 ppm, further described in [START_REF] Guyot | Performance Based Approaches in Standards and Regulations for Smart Ventilation in Residential Buildings: A Summary Review[END_REF]. Each room of the dwelling is modelled as single zone, with a time-step of 15 min. Once a system receives certification of compliance via this procedure, called "Avis technique", it can be used in new dwellings according to its specifications. For each type and size of dwelling, the agreement gives the references of inlets and outlets and the input data for energy calculation. As a result, humidity-based DCV systems have been widely used in France for 35 years. Most of the new residential buildings, which must be low-energy buildings to comply with the RT 2012 energy performance regulation, are equipped with such systems [START_REF] Mélois | Improving Building Envelope Knowledge from Analysis of 219,000 Certified on-Site Air Leakage Measurements in France[END_REF]. They are also considered today as a reference system.

Technological context: presentation of the reference humidity based DCV system

RH-MEV is a Demand Control Ventilation (DCV) system adjusting the airflows according to the estimated needs of the building and its occupants, with a direct relative humidity (RH) measurement in both the wet and dry rooms. The extensions and retractions of a hygroscopic fabric modify the cross-section of inlets and outlets upon hygrometric changes in their environment without the need for motors or electronic sensors. The reference system used in France includes as air terminal devices, humidity sensitive trickle ventilators in the dry rooms and humidity sensitive exhaust units in the wet rooms, except in the toilets which are equipped with an occupancy. Energy savings have been estimated about 30% to 50 % of the heating energy compared to constant airflows exhaust-only ventilation (Savin and Bernard 2009). They could be considered as a type of smart ventilation system, according to the definition given by [START_REF] Durier | VIP 38: What Is Smart Ventilation?[END_REF].

FIRST FEEDBACKS ON ROBUSTNESS AND DURABILITY AFTER PROLONGED IN SITU OPERATION

In 2006, before generalizing the usage of humidity based ventilation in French residential buildings, the French Ministry for Housing ordered an evaluation of these systems after prolonged on-site operation. For this purpose, after 6 years of in-situ functioning, an independent laboratory collected 57 exhaust units in 21 social housings [START_REF] Berthin | Feedback on Installation, Maintenance, and Aging of Mechanical Humidity-Controlled Ventilation Exhaust Units[END_REF]. Despite an absence of sufficient maintenance observed for 75% of the units (dust), tests in laboratory conditions showed that 46 % of the kitchen units complied with factory specifications despite the absence of maintenance, 100 % of the bathroom exhaust units showed an airflow reduced by 5 m 3 /h. 100 % of the non-damaged exhaust units exhibited a conform hygroscopic behavior or showed a slight shift of their characteristic when still soiled.

Once the devices have been cleaned and properly re-assembled, 75 % of the units complied with factory specifications. Among the other 25 %, the kitchen elements showed an increase of volume flow lower than 3 m 3 /h, while bathroom units exhibited a decrease lower than 2 m 3 /h. Results are further described in These first results show the reliability of the components including humidity sensors and the robustness of their operation to a bad maintenance and use by occupants.

FIRST RESULTS FROM THE PERFORMANCE 1&2 PROJECTS

The Performance 2 project is based on a 10 years' follow-up study on a large-scale monitoring on thirty occupied social housing apartments in two residential buildings. The former Performance project was an on-field evaluation of the reference humidity-based components and systems from an energy performance and an indoor air quality point of view, over two heating seasons (2007)(2008)(2009) [START_REF] Bernard | Performance de La Ventilation et Du Bâti -Phase 3 -Performance Énergétique et QAI Des Systèmes Hygroréglables[END_REF]. The ventilation terminals were instrumented, and the buildings were equipped with sensors during the construction phase, including temperature, humidity, and CO 2 in the different rooms of the dwellings, as well as pressure and volume flow sensors for monitoring the ventilation system. The Performance project showed the good IAQ in terms of CO 2 and humidity provided by the ventilation system, despite the over-occupation of some apartments. Ten years later, in the framework of the Performance 2 project, the data acquisition system is turned back on with the intention to assess the ventilation system behaviour/performance after a prolonged in-situ functioning period [START_REF] Jardinier | Durability of Humidity-Based Demand-Controlled Ventilation Performance: Results of a 10 Years Monitoring in Residential Buildings[END_REF]. This preliminary study results shows: -At start-up, more than 80 % of the metrology sensors is still in working conditions. -Using this rough data, the average in-situ drift of the hygroscopic devices after 9 years of operation is estimated below ± 1.5 %RH and is lower than the announced accuracy of the electronical humidity sensors at installation (± 1.8 %RH).

-

The observed drift of volume flows on some of the exhaust units is typical of an absence of maintenance.

-

The battery of the presence-based toilet exhaust is often (90 %) discharged. In July 2019, we organized a first meeting with the occupants, and got their first feedbacks. These first promising results on the durability and robustness of these systems need to be confirmed by the end of the Performance 2 project which shall include:

-The collection of the ventilation devices for a full quality control before and after the recommended cleaning. -

The collection of the metrology sensors for re-calibration and drift-correction on the measurements.

-A new set-up for each apartment including particle sensors to follow the latest interests of IAQ research.

PERSPECTIVES

In the context of the increasing awareness about smart ventilation, from these feedbacks, it seems very important to include in this definition the durability of the systems and its components (including the sensors) and the robustness of the systems to a lack of maintenance by occupants.