Sufia Hena

Leonardo Gutierrez

Jean-Philippe Croué
email: jean.philippe.croue@univ-poitiers.fr

Removal of pharmaceutical and personal care products (PPCPs) from wastewater using microalgae: A review

Keywords: PPCPs, Microalgae, Hybrid system, Wastewater, Ecological risk

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Over the last few years, numerous studies have reported the presence of new pollutants in different compartments of the environment at trace concentrations ranging from ng/L to µg/L.

These pollutants have been termed emerging micro-pollutants (Kummerer, 2009). Emerging micro-pollutants are chemical compounds of no regulatory status. However, in 2000 the European Union (EU) confirmed that 33 priority substances were to be removed from surface water to achieve a satisfactory status of ecology within a period of 20 years. Few common pharmaceutical and personal care products (PPCPs) were added to this list in 2007 (e.g., diclofenac, ibuprofen, iopamidol, musks, carbamazepine, clofibric acid, triclosan, and phthalates).

While phthalates were declared as carcinogenic, the others were stated as toxic or endocrine disruptors [START_REF] Ebele | Pharmaceuticals and personal care products (PPCPs) in the freshwater aquatic environment[END_REF]. Micro-pollutants comprise natural and anthropogenic substances, including pharmaceuticals, personal care products, pesticides, and industrial chemicals. PPCPs are the largest group of chemical reagents used in varied fields such as livestock, agriculture, and the daily life of human beings. Specifically, pharmaceuticals are used for therapeutic purposes in humans and animals, while personal care products are used in personal hygiene and for beautification. Currently, the growing scientific attention towards PPCPs is due to their (i) ubiquitous presence in every compartment of the environment; (ii) ecological risk (i.e., mainly creating antibiotic resistance in bacteria and endocrine disruptor), and (iii) instrumental analysis, making it possible to detect PPCPs at trace levels. The ubiquitous presence of PPCPs in the environment is due to their excessive use, poor removal in wastewater treatment plants (WWTPs), and their stable structures that maximize their biological activity [START_REF] Jackson | Defining and measuring biological activity: applying the principles of metrology[END_REF]; thus, increasing their resistance to degradation [START_REF] Daughton | Pharmaceuticals and personal care products in the environment: agents of subtle change?[END_REF]. Interestingly, PPCPs have been present in the environment for a long time; however, their concentrations in surface water have recently risen to the level of identification and risk [START_REF] Jjemba | Excretion and ecotoxicity of pharmaceutical and personal care products in the environment[END_REF][START_REF] Fent | Ecotoxicology of human pharmaceuticals[END_REF]. The development of new coupled analytical techniques such as liquid chromatography (LC) and gas chromatography (GC) coupled with mass spectrometry (MS) has made possible the detection of compounds present in extremely low concentration in complex matrixes of aqueous and solid phases [START_REF] Ternes | Human Pharmaceuticals, Hormones and Fragrances: The Challenge of Micropollutants in Urban Water Management[END_REF]Diaz-Cruz and Barcelo, 2006), surface water [START_REF] Drewes | Fate of pharmaceuticals during ground water recharge[END_REF], and wastewater (Hernandez et al., 2007). Although PPCPs are continuously introduced into the environment in small amounts, they potentially affect the quality of drinking water, human health, and ecosystems (Sires and Brillas, 2012;[START_REF] Yuan | Degradation of selected pharmaceuticals in aqueous solution with UV and UV/H2O2[END_REF]. The potential chronic health effects associated with long exposures of combined low concentrations of each of the many PPCPs and their metabolites and conjugates in drinking water are not well known [START_REF] Stackelberg | Persistence of pharmaceutical compounds and other organic wastewater contaminants in a conventional drinking-watertreatment plant[END_REF][START_REF] Rivera-Utrilla | Pharmaceuticals as emerging contaminants and their removal from water. A review[END_REF]. Consequently, it is a rising priority of environmental science and engineering sectors to improve, modify, or couple the CWWTPs with advanced techniques to efficiently remove PPCPs (i.e., along with other regulated standard pollutants) from wastewater before they are discharged into nature.

Numerous review papers have summarized and discussed the occurrence of PPCPs in different compartments of the environment, such as groundwater [START_REF] Lapworth | Emerging organic contaminants in groundwater: a review of sources, fate and occurrence[END_REF] and wastewater [START_REF] Wang | Removal of pharmaceuticals and personal care products (PPCPs) from wastewater: A review[END_REF], as well as the instrumental analysis of PPCPs (Pérez-Lemus et al., 2019), and the treatment methods for their removal [START_REF] Bolong | A review of the effects of emerging contaminants in wastewater and options for their removal[END_REF]. [START_REF] Luo | A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment[END_REF] focused on PPCPs removal in conventional and advanced treatment processes, including membrane-based, advanced oxidation, adsorption, and coagulation-flocculation processes. [START_REF] Verlicchi | Occurrence of pharmaceutical compounds in urban wastewater: removal, mass load and environmental risk after a secondary treatment -a review[END_REF] summarized the comprehensive studies on pharmaceutical removal efficiency by conventional activated sludge in municipal wastewater treatment systems, while [START_REF] Liu | Removal mechanisms for endocrine disrupting compounds (EDCs) in wastewater treatment-physical means, biodegradation, and chemical advanced oxidation: a review[END_REF] covered biological, chemical, and physical methods of removal of endocrinedisrupting compounds. [START_REF] Leng | Use of microalgae based technology for the removal of antibiotics from wastewater: A review[END_REF] broadly discussed the mechanisms of antibiotics removal using microalgae and the combination of other technologies with microalgae.

Nevertheless, the contribution of microalgae in PPCPs removal and its comparison with conventional wastewater treatment have not been comprehensively reviewed. Consequently, the recent applications of microalgae in the removal of PPCPs, and the ecological risk of PPCPs on microalgae are summarized and discussed in this current review.

Mechanisms involved in the removal of PPCPs by microalgae

The mechanisms involved in the removal of PPCPs by microorganisms (bacteria, fungi, or microalgae) are similar and include biosorption, bioaccumulation, biodegradation, photodegradation, and volatilization (Fig. 1.). The detailed mechanisms of PPCPs removal by microalgae are discussed in this section. The differences in the physicochemical properties of PPCPs (Table 1) are the main cause for microalgae to select a particular or preferably combined mechanism (Table 2) to remove these pollutants from the water as their response for survival.

For instance, galaxolide and 4-octylphenol (OP) exclusively showed volatilization due to high

Henry law constant values in a High Rate Algal Pond (HRAP) system (Matamoros et al., 2016),

while Chlorella vulgaris remediates levofloxacine by bioaccumulation as well as biodegradation (Xiong et al., 2017a). Recently, [START_REF] Hena | Removal of metronidazole from aqueous media by C. vulgaris[END_REF] confirmed that metronidazole was removed by adsorption onto exo-polymeric substances of Chlorella vulgaris. [START_REF] Yu | Investigation of the removal mechanism of antibiotic ceftazidime by green algae and subsequent microbic impact assessment[END_REF] demonstrated that the removal of micropollutants by microalgae could be divided into three steps: 1) rapid adsorption due to physicochemical interactions between the cell wall and the pollutants, followed by a 2) comparatively slow transference of molecules via cell wall, and finally 3) either ending up as bioaccumulation, biodegradation, or both.

Bioadsorption

Bioadsorption is a passive process in which the soluble molecules or ions of PPCPs are distributed between the liquid phase and the cell wall or the extracellular polymeric substances (EPS) of microalgae. This distribution of PPCPs mainly occurs due to the hydrophobic or ionic interaction between the pollutants and the solid surface of microalgae. The cell wall of the microalgae consists of cellulose, hemicelluloses, alginate, chitin, glycan, lignin, and pectin, which are encapsulated by EPS. EPS are mainly a combination of polysaccharides, lipids, proteins, and nucleic acids, generally protecting the cell from harsh environments. There are two types of EPS: bonded EPS are attached to microalgal cells, while free EPS are excreted by microalgae in suspension, both providing available sites for pollutants to adsorb [START_REF] Fomina | Biosorption: current perspectives on concept, definition and application[END_REF]. The bioadsorption of PPCPs depends upon its structure, hydrophobicity, the presence of a functional group, and microalgal species as well. The cell wall and the EPS of microalgae mainly carry negative charges due to the presence of carboxyl, hydroxyl, and phosphoryl functional groups, which favor electrostatic interactions with positively charged PPCPs molecules. The ionization or dissociation of PPCPs in water mostly depends upon the pH of the aqueous media. De [START_REF] De Godos | Tetracycline removal during wastewater treatment in high-rate algal ponds[END_REF] observed that 50-71% tetracycline could be removed by Chlorella vulgaris in HRAP through an adsorption mechanism within 43 hours under different biomass of inoculation ranging from 44 to 355 mg TSS L -1 . The removal of tetracycline increased with increasing inoculum size. According to De [START_REF] De Godos | Tetracycline removal during wastewater treatment in high-rate algal ponds[END_REF], the pH of the culture varied from 7.5 to 8, allowing tetracycline to exist as zwitterions which favored strong adsorption onto the surface of microalgae in HRAP. [START_REF] Carrasquillo | Sorption of ciprofloxacin and oxytetracycline zwitterions to soils and soil minerals: influence of compound structure[END_REF] and [START_REF] Vasudevan | pH-dependent ciprofloxacin sorption to soils: interaction mechanisms and soil factors influencing sorption[END_REF] stated the same mechanism for the removal of tetracyclines and fluoroquinolones via soil surface. Also, the adsorption of pollutants onto the cell wall and EPS is a rapid process, as proven by many studies. Adsorption phenomena generally take place immediately and passively onto the cell wall of both viable and dead cells and on EPS and cell debris. [START_REF] Wang | Removal and reductive dechlorination of triclosan by Chlorella pyrenoidosa[END_REF] reported that Chlorella pyrenoidosa was capable of removing 50% of triclosan from aqueous media present in the range of 100 to 800 µg/L within 1 hour of contact. [START_REF] Guo | Removal of cephalosporin antibiotics 7-ACA from wastewater during the cultivation of lipid-accumulating microalgae[END_REF] evaluated the feasibility of three freshwater green microalgae (Chlorella sp., Chlamydomonas sp., and Mychonastes sp.) to remove 7-amino-cephalosporanic acid from wastewater within 10 minutes. Among all three microalgae, Mychonastes sp. exhibited the lowest adsorption capability (2.95 mg/g) of antibiotic 7-ACA followed by Chlamydomonas sp.

(3.09 mg/g) and Chlorella sp. (4.74 mg/g).

In general, the literature has shown that PPCPs are removed by combined mechanisms, where bioadsorption is one of the most common. However, the contribution of bioadsorption depends upon the microalgae and the micropollutant. Chlorella sorokiniana can remove 62% of mixed pharmaceuticals (estrone (E1), β-estradiol (E2), ethynylestradiol (EE2), carbamazepine, ibuprofen, diclofenac, metoprolol, paracetamol, and trimethoprim) by combined mechanisms, while 34% were removed by adsorption (De Wilt et al., 2016). Amongst all the pharmaceuticals above mentioned, carbamazepine was adsorbed the most onto microalgal biomass and accounted for the highest percentage (16.7%), followed by diclofenac (9.1%) and trimethoprim (7.6%), whereas the rest showed minimal contribution. Several results indicate that the adsorption of micropollutants is species-specific. Briefly, Selenastrum capricornutum showed 46% and 40% adsorption of the maximum removal of E2 and EE2, respectively, while Chlamydomonas reinhardtii removed 86% and 71% of E2 and EE2, respectively, by adsorption from anaerobic sludge in 7 days [START_REF] Hom-Diaz | Microalgae cultivation on wastewater digestate: β-estradiol and 17α-ethynylestradiol degradation and transformation products identification[END_REF]. Scenedesmus dimorphus adsorbed <10% of the maximum removal (85%) of E2 and EE2 in 8 days [START_REF] Zhang | Evaluating removal of steroid estrogens by a model alga as a possible sustainability benefit of hypothetical integrated algae cultivation and wastewater treatment systems[END_REF].

The adsorption of micropollutants onto the surface of microorganisms/solid phase mainly depends upon the hydrophobicity of the compounds. [START_REF] Rogers | Sources, behavior and fate of organic contaminants during sewage treatment and in sewage sludges[END_REF] discussed the lipophilicity or hydrophobicity of a substance in terms of log Kow (i.e., the octanol and water partition coefficient of the compounds). It has been estimated that the higher the log Kow value, the higher the adsorption of compounds onto the solid phase or the surface of microorganisms. [START_REF] Avdeef | Assessment of distribution-pH profiles[END_REF] reported that ionized species are more water-soluble and, thus, less available to solid-phase than its neutral form. Trimethoprim produces cationic species in aqueous media. However, its adsorption on different microalgae, i.e., Chlorella sorokiniana and Nannochloris sp., has been reported as either considerably low (De Wilt et al., 2016) or zero (Bai and Acharya., 2016), respectively, due to its low log Kow of 1.4 and ionized species. The solid-water distribution coefficients (Kd) of compounds also play a role in the sorption mechanism along with hydrophobicity. For instance, the moderate octane-water partition coefficient of ibuprofen (3.5-4.5) does not allow it to adsorb on solid surfaces due to its low Kd value (>0.3). Also, [START_REF] Tadkaew | Removal of trace organics by MBR treatment: the role of molecular properties[END_REF] reported that compounds of log Kd > 3.2 could be easily removed from water. [START_REF] Ding | Toxicity, degradation and metabolic fate of ibuprofen on freshwaterdiatom Navicula sp[END_REF] reported adsorption as high as 26% of the initial concentration of ibuprofen onto the surface of a freshwater diatom Navicula sp. over four days of inoculum. [START_REF] Kang | Removal of nutrients and pharmaceuticals and personal care products from wastewater using periphyton photobioreactors[END_REF] measured the adsorption capacity of ibuprofen onto periphyton (mixed microalgae and bacterial consortium) as less than 20% of the initial concentration, while the pH of the periphyton culture was 8. This pH condition was high enough to deprotonate ibuprofen and induces hindrance in the adsorption of negatively charged microalgae. Some lipid-containing microalgae could effectively remove antibiotics by adsorption. The biomass of lipid-containing Scenedesmus quadricauda has shown considerably high adsorption capacity (295 mg/g) to remove tetracycline [START_REF] Daneshvar | Versatile Applications of Freshwater and Marine Water Microalgae in Dairy Wastewater Treatment, Lipid Extraction and Tetracycline Biosorption[END_REF]. Likewise, the biomass of Chlorella sp. could remove 82.7% of cefalexin from wastewater before lipid extraction, while 71.2% removal was observed after lipid extraction [START_REF] Angulo | Bioremediation of Cephalexin with non-living Chlorella sp., biomass after lipid extraction[END_REF]. Hom-Diaz et al. (2017a) demonstrated that the dark and light cycle also regulates the mechanisms. During the dark phase, adsorption was the only phenomenon, while during light conditions, both photodegradation and adsorption played a role. However, as biomass increased, the adsorption was observed as the dominant mechanism due to the shading effect of biomass in which microalgal biomass prevented light from penetrating deeper within the bioreactor. It is clear from the above discussions that solely one factor does not dictate the adsorption mechanism of micropollutants onto microalgae.

Bioaccumulation

Unlike bioadsorption, bioaccumulation is an active process that consumes energy and is relatively slower than the adsorption process. Along with nutrients, microalgae can uptake and accumulate pollutants within the cell for growth processes. EPS and the cell wall act as front line barriers. Numerous microalgae can accumulate pollutants against the concentration gradient difference between the cytoplasm and external environment, e.g., Spirogyra can accumulate 850 times more radio-phosphorus than that of water [START_REF] Sen | Relationship of Algae to Water Pollution and Waste Water Treatment. Water Treatment[END_REF]. Although the mechanisms of bioadsorption and bioaccumulation of pollutants are completely different, the quantification of bioaccumulation and bioadsorption of pollutants is a difficult task since both processes are continuous. To decrease this uncertainty, many researchers have used general terms such as biosorption and uptake of pollutants, which includes both bioadsorption and bioaccumulation.

Several strategies have been analyzed in section 3 to differentiate the contribution of pollutants removal via bioadsorption and bioaccumulation inside the cells of microalgae. [START_REF] Adams | Aquatic toxicology test methods[END_REF] described bioaccumulation as the accumulation of pollutants inside the cytoplasm of the cell from an external environment. Bio-concentration has been explained as the accumulation of pollutants into and onto the organism. However, in this review, the term bioaccumulation specifically refers to pollutants exclusively accumulated into the cell, while the terms uptake or biosorption refers to pollutants quantified into the cell and onto the surface of microalgae. Bioadsorption is the first step of bioaccumulation. Hence, to bio-accumulate inside the cell, the molecules must first fulfill the necessary key criteria for bioadsorption. However, not all pollutants adsorbed onto the surface of microalgae can enter into the cell to bioaccumulate [START_REF] Wu | Mechanisms of removing pollutants from aqueous solutions by microorganisms and their aggregates: A review[END_REF]. Polysaccharides are the most common compounds stored inside the microalgal cell during photosynthesis. Once the concentration of these stored compounds decreases to a certain level, the adsorbed compounds onto the cell or bonded EPS enter into the cytoplasm or intracellular fractions through ion channels, carriers present in cell membranes, or simply via cell membrane diffusion [START_REF] Yu | Investigation of the removal mechanism of antibiotic ceftazidime by green algae and subsequent microbic impact assessment[END_REF]. [START_REF] Bai | Algal-mediated removal of selected pharmaceutical and personal care products (PPCPs) from Lake Mead water[END_REF] demonstrated green microalgae Nannochloris sp. low uptake of trimethoprim (11%), sulfamethoxazole (11%), and carbamazepine (13%) under dark conditions over 14 days, while 27% of triclosan was uptaken in 7 days. The above results were considerably close to those shown for these compounds following bioadsorption mechanism; thus, reflecting the dependence of biosorption or uptake on bioadsorption. In general, all microalgae store their food as lipid inside the cell, which facilitates the bioaccumulation of lipophilic compounds and assists in the trophic transfer.

A freshwater green microalga Cladophora sp. could accumulate triclosan (TCS), triclocarban, and methyl-triclosan as high as 50-400 ng/g fresh biomass of microalgae, while the environmental concentration was in a range of 50-200ng/L [START_REF] Coogan | Algal bioaccumulation of triclocarban, triclosan, and methyl-triclosan in a North Texas wastewater treatment plant receiving stream[END_REF]. In microalgae, some bio-accumulated pharmaceuticals at a low concentration assist in the production of reactive oxygen species inside the cell, which essentially control the cellular metabolism (Xiong et al., 2018a).

Biodegradation

Biodegradation is a metabolic degradation or breakdown of compounds catalyzed by enzymes.

Generally, microalgae turn complex parent compounds into simpler compounds. This process might involve several enzymatic reactions (e.g., hydrogenation, dehydrogenation, hydroxylation, dehydroxylation, carboxylation, decarboxylation, oxidation, reduction, hydrolysis, ring cleavage, demethylation, and glycosylation). Microalgae biodegrade xenobiotic compounds via series of enzymes through three phases [START_REF] Peng | Biotransformation of progesterone and norgestrel by two freshwater microalgae (Scenedesmus obliquus and Chlorella pyrenoidosa): transformation kinetics and products identification[END_REF][START_REF] Maes | Uptake, elimination, and biotransformation of 17α-ethinylestradiol by the freshwater alga Desmodesmus subspicatus[END_REF][START_REF] Xiong | Biodegradation of carbamazepine using freshwater microalgae Chlamydomonas mexicana and Scenedesmus obliquus and the determination of its metabolic fate[END_REF]Matamoros et al., 2016;[START_REF] Ding | Toxicity, degradation and metabolic fate of ibuprofen on freshwaterdiatom Navicula sp[END_REF]Xiong et al., 2017a). In the first phase, the detoxification of xenobiotic compounds by the enzyme Cytochrome P450 involves oxidation, reduction, or hydrolysis processes. During this phase, the lipophilic compounds are turned into hydrophilic compounds via the incorporation of hydroxyl groups. In the second phase, the compounds with electrophilic groups (CONH2, epoxide ring, COOH) form a conjugate bond with glutathione in the presence of enzyme glutathione-S-transferases or glucosyltransferases [START_REF] Nakajima | Glycosylation of bisphenol A by freshwater microalgae[END_REF] to protect the cell from oxidative damage. The third phase involves several enzymes (e.g., carboxylase, glutamyl-tRNA reductase, dehydrogenase, transferase, laccases, hydrolases, pyro-phosphatase, decarboxylase, mono(di)oxygenase, and dehydratase) to bio-transform the xenobiotic compounds into simpler and detoxified forms [START_REF] Petroutsos | Detoxification of 2,4-dichlorophenol by the marine microalga Tetraselmis marina[END_REF][START_REF] Otto | First laccase in green algae: purification and characterization of an extracellular phenol oxidase from Tetracystis aeria[END_REF]. [START_REF] Ding | Toxicity, degradation and metabolic fate of ibuprofen on freshwaterdiatom Navicula sp[END_REF] confirmed the glycosylation reaction with ibuprofen along with eight bio-transformed products in freshwater diatom Navicula sp. Green microalgae S. obliquus and C. pyrenoidosa, showing 95% biodegradation or biotransformation of progesterone and norgestrel present in an aqueous medium via hydroxylation, reduction, and oxidation pathways [START_REF] Peng | Biotransformation of progesterone and norgestrel by two freshwater microalgae (Scenedesmus obliquus and Chlorella pyrenoidosa): transformation kinetics and products identification[END_REF]. Biodegradation is one of the most effective mechanisms by which microalgae remove micropollutants from aqueous phase as well from biomass (intracellular or intercellular). Compounds biodegrading at an intracellular or intercellular phase bioaccumulate and adsorb onto the biomass, respectively. [START_REF] Naghdi | Removal of pharmaceutical compounds in water and wastewater using fungal oxidoreductase enzymes[END_REF] observed that some toxic antibiotics are broken down into less or non-toxic intermediates by extracellular enzymes produced by microalgae, which are further bioaccumulated and biodegraded by intracellular enzymes. The complexity of the structures also regulates the biodegradability of compounds. A compound with linear, unsaturated, and electron-donating groups was found to biodegrade more readily than compounds possessing cyclic structure, saturated long linear carbon chain, and electron-withdrawing groups in their structure. However, some pharmaceutical compounds do not follow these trends. Two structurally similar non-steroidal anti-inflammatory drugs (NSAIDs), i.e., ketoprofen and ibuprofen, performed differently during removal from wastewater.

Ibuprofen was removed more efficiently than ketoprofen by CWWTPs as well as microalgaemediated treatment (Hom-Diaz et al., 2017b). Other structurally similar NSAIDs, i.e., paracetamol and ibuprofen, showed similar removals (100%) while treated with Chlorella sorokiniana (De Wilt et al., 2016). EPS are hygroscopic in nature and form a matrix around the cell that assists compounds to adsorb and accumulate and eventually biodegrade due to the presence of extracellular enzymes.

Photo-biodegradation

Many PPCPs have been observed to degrade under sunlight irradiation, which refers to as photodegradation or photolysis [START_REF] Matamoros | Behaviour of pharmaceutical products and biodegradation intermediates in horizontal subsurface flow constructed wetland. A microcosm experiment[END_REF][START_REF] Hanamoto | Evaluation of the photolysis of pharmaceuticals within a river by 2 year field observations and toxicity changes by sunlight[END_REF]. This process is effective, environmentally friendly, economical, and easy to optimize. There are two types of photolysis involved in the degradation of PPCPs in wastewater: direct and indirect photolysis.

During direct photolysis, the targeted pollutant molecules with conjugated π bond, aromatic rings, and other functional groups can directly absorb UV-range sunlight irradiation and eventually break down without any involvement of microalgae [START_REF] Challis | A critical assessment of the photodegradation of pharmaceuticals in aquatic environments: defining our current understanding and identifying knowledge gaps[END_REF].

During indirect photolysis, the free radicals singlet oxygen ( as depicted in Fig. 2. During irradiation, algal cells gradually get destroyed and released carboxylic acid, which is associated with the cell wall and membranes of microalgae. The released carboxylic acid forms another reactive oxygen species: peroxy radical (ROO•). In microalgae culture, these reactive oxygen species take part in the photolysis of recalcitrant compounds in the culture medium. Singlet oxygen is detected by furfuryl alcohol produced in the algal suspension. Furfuryl alcohol traps singlet oxygen in water (Haag et al., 1984). The photolysis of PPCPs is an essential mechanism due to the ubiquitous presence of sunlight and microorganisms. [START_REF] Bai | Algal-mediated removal of selected pharmaceutical and personal care products (PPCPs) from Lake Mead water[END_REF] reported a 100% removal of ciprofloxacin and triclosan in Nannochloris sp. mediated culture over 7 days of incubation under 24 h of irradiation via photolysis mechanism. The higher the concentration of microalgae, the higher the generation of free radicals; thus, enhances photolysis degradation [START_REF] Liu | Photoproduction of hydroxyl radicals in aqueous solution withalgae under high-pressure mercury lamp[END_REF]. [START_REF] Zhang | Photodegradation of Norfloxacin in aqueous solution containing algae[END_REF] found that the photo biodegradation of norfloxacin increased with the concentration of microalgae. The seepage of chlorophyll from dead cells of microalgae acted as a photosensitizer and enhanced the photodegradation of benzo[a]pyrene [START_REF] Luo | Chlorophyll catalyse the photo-transformation of carcinogenic benzo[a]pyrene in water[END_REF]. [START_REF] Huang | Interactions between the antimicrobial agent triclosan and the bloom-forming cyanobacteria Microcystis aeruginosa[END_REF] observed a better photodegradation of triclosan in the presence of EPS from Microcystis aeruginosa.

Volatilization

There are minor differences between volatilization and stripping. Stripping is the removal of gaseous compounds from (WWTPs), while volatilization is the removal of volatile compounds.

Stripping or volatilization is more prominent in the aerobic chamber or the bioreactor tank and depends upon the airflow in contact, the vapor pressure of the liquid, Henry coefficient (H), and hydrophobicity (Kow [START_REF] Du | Understanding the algal contribution in combined UV-algae treatment to remove antibiotic cefradine[END_REF] used self-degradation as terminology for direct photolysis.

Quantification of adsorption mechanism

(i) Adsorption mechanism on live cells of microalgae: A fixed volume of microalgal suspension is withdrawn from the microalgae-mediated treatment flask. The cells are then separated from culture media using centrifugation (i.e., harvesting process). The cell pellets are re-suspended in ultrapure water and re-centrifuged to desorb the compounds that have been adsorbed onto the cell wall of microalgae. The supernatant is collected for further adsorbate analysis. The rpm values during the first and second centrifugation steps should not be high enough to desorb the adsorbate from the cell wall and disrupt the cell, respectively. The integrity status of the microalgal cell wall can be determined by treating the cells with 100 μL of 1 % Evans Blue solution followed by incubation for 10 min at room temperature and washed with deionized water to remove excess stain. The microalgal cells are then inspected under a light microscope, where cells with broken cell walls would appear blue as Evans Blue solution diffuses into the protoplasm region and stains the cells (Hena et al., 2015a (ii) Adsorption mechanism on dead biomass of microalgae: The untreated biomass of microalgae is harvested by centrifugation and washed with ultrapure water to remove salts in excess, followed by freeze-drying, and finally ground to obtain a uniform particle size to be used as an adsorbent for PPCPs from aqueous media. Then, the adsorption capability qe (mg/g) of particular microalgae towards certain PPCPs is calculated as Equation 1; where Ci and Cf are the initial and equilibrium concentrations (mg/L) of solute present in solution, respectively, m is the dry weight (g) of algal biomass (adsorbent), and V is the working volume of the solution (L) qe = V(Ci-Cf)/m Eq. 1 [START_REF] Guo | Removal of cephalosporin antibiotics 7-ACA from wastewater during the cultivation of lipid-accumulating microalgae[END_REF] used this protocol to analyze the adsorption removal of antibiotic 7-amino cephalosporanic acid onto Chlorella sp., Chlamydomonas sp., and Mychonastes sp. The comparison of these two approaches (i.e., adsorption of PCPPs on live/dead cells) has been previously discussed by [START_REF] Yu | Investigation of the removal mechanism of antibiotic ceftazidime by green algae and subsequent microbic impact assessment[END_REF]. The membranes of live cells control the permeability of external compounds, which is lost due to damage in the structures of cell membranes when cells are dead [START_REF] Tsezos | Comparison of the biosorption and desorption of hazardous organic pollutants by live and dead biomass[END_REF]. Consequently, the accessibility of external compounds to extra membrane-bound organelles and intracellular compounds increases in dead cells; hence, increasing the adsorption. However, if biodegradation is the dominant pathway of removal, then the total removal of pollutants would be higher in live cells than dead cells.

Quantification of absorption or bio-accumulation mechanism (i)

In the thawing process, the cell pellet after second centrifugation (section 3. (ii) Sonication is a mechanical method widely used to disrupt the microalgal cell. The sonicator generates intense sonic pressure waves, which create microbubbles in the liquid that amalgamates and vibrates. The conversion of mechanical energy (i.e., due to the oscillation of the probe) to heat energy causes implosion, resulting in high-speed bursts of liquid that collide to the surface of the cell wall and generate high energy shock waves that are sufficient to disrupt the cell [START_REF] Middelberg | Process-scale disruption of microorganisms[END_REF]. For sonication, the cell pellet is immersed into a mixture of dichloromethane:methanol (1:2, v/v) and sonicated for 1 h. The supernatant is used to determine the concentration of the compound inside the microalgae. [START_REF] Ding | Toxicity, degradation and metabolic fate of ibuprofen on freshwaterdiatom Navicula sp[END_REF] preferred the sonication method to released bio-accumulated non-steroidal anti-inflammatory drug naproxen from microalgae Cymbella sp. and Scenedesmus quadricauda.

(iii) High-pressure homogenization (HPH) is a promising technique to disrupt microalgal cells effectively in an aqueous environment. In HPH, the cells are forced to pass through a narrow orifice, which causes disruption in the cell wall of the microalgae due to the turbulence, cavitation, and shear stress. High pressure is the main parameter of HPH, which can be optimized as a function of the structure or the toughness of the cell wall of microalgae, e.g., Nannochloropsis sp. requires higher pressure (28k-30kpsi) than Chlorella sp. (15k-17kpsi).

Multiple circulations or passes of the cells through orifice increases the degree of cell disruption. [START_REF] Bernaerts | Evaluating microalgal cell disruption upon ultra high pressure homogenization[END_REF] observed that higher pressure (e.g. 36kpsi) reduced the number of passes required to obtain the same level of cell disruption compared to lower pressure (e.g. 14.5kpsi) for Nannochloropsis sp. suspension.

(iv) Enzymatic hydrolyzation is a biochemical process to disrupt the microalgal cell wall, which requires much less energy compared to other mechanical techniques (e.g., sonication and HPH). The main constituents of the cell wall of microalgae are cellulose incorporated with glycoprotein. Cellulases and protease are the most common enzymes combination used to disrupt the cell wall effectively. [START_REF] Wang | Combined enzymatic and mechanical cell disruption and lipid extraction of green alga Neochloris oleoabundans[END_REF] found with the almost equal amount of energy consumption, HPH can obtain nearly double the cell disruption efficiency than sonication. Enzymatic hydrolysis and thawing processes are relatively time-consuming process in comparison to sonication and HPH.

Quantification of abiotic mechanism

The direct photolysis of the PPCPs is calculated by adding a known amount of PPCP in ultrapure water. Degrading under radiation is allowed for a certain time in the absence of microalgae or any other microorganisms. Ultrapure water is used as a medium to avoid any other means of degradation or removal of PPCP. Similarly, hydrolysis is assessed by adding a known amount of PPCP in ultrapure water, where degrading is allowed under dark conditions and in the absence of algal cells for a specific period. A sterile medium is used to confirm the contribution of the hydrolysis pathway in wastewater. The volatilization is estimated by comparing the PPCP present in aerated and non-aerated reactors. The evaporation of water during volatilization is balanced by adding ultrapure water. However, due to the presence of microorganisms in real wastewater, direct measurements of hydrolysis and direct photolysis are not possible. The quantifications of hydrolysis and direct photolysis are conducted by indirect measurement, as described at the beginning of the current section.

Quantification of indirect photolysis and biodegradation mechanism

The indirect photolysis by microalgae is estimated by comparing the concentration of PPCPs present in the reactors fed with microalgae in wastewater under light and dark conditions. The biodegradation (Rb) of PPCPs by microalgae is calculated according to the following equation (Xiong et al., 2017a):

Rb (%) = (At -Ar -Rad -Rab -Ra -Rinp) x 100/At
Where At is the initial amount of PPCP added to the medium, Ar is the residual amount in the medium, Rad is the amount of PPCP adsorbed on the microalgal cell surface, Ra is the amount removed by abiotic processes, Rinp is the amount of PPCP removed by indirect photolysis, and Rab is the amount of PPCP accumulated in the algal cells.

The biodegradation of PPCPs is considered as the predominant or sole mechanism if it ceases at the addition of nutrients due to the preference of microalgae to consume nutrients over microcontaminants when both are present in the medium (Matamoros et al., 2016).

Factors affecting the removal of PPCPs

The performance of a wastewater treatment system in terms of removal of PPCPs is considerably dependent on abiotic and biotic factors. Hydraulic retention time, light intensity, temperature, and seasonality are considered as abiotic factors, while phylogenetic diversity of microorganisms is known as biotic factor that affects the removal of PPCPs from the wastewater treatment system. It is anticipated that the optimum conditions of abiotic factors favor a higher specific growth rate of microalgae, which induces the system to quickly and more efficiently uptake the nutrients N & P and PPCPs (i.e., as a source of carbon).

Temperature and seasonality

Temperature and season are interrelated factors, where open and upscale plants come in direct influence, while the temperature of a closed and small treatment system can be controlled.

Temperature regulates the metabolic process and enzymatic activity in microalgae. Any fluctuation in temperature can affect the biodegradability of PPCPs, which is directly related to the enzymatic actions. The optimal temperature for microalgae is reported in the range of 25-30°C [START_REF] Bamba | Effects of temperature and other operational parameters on Chlorella vulgaris mass cultivation in a simple and low-cost column photobioreactor[END_REF][START_REF] Chinnasamy | Biomass production potential of a wastewater alga Chlorella vulgaris arc 1 under elevated levels of CO2 and temperature[END_REF]. Higher than optimum temperature causes a decrease in chlorophyll-a content in microalgae cells, which decreases the photosynthesis process and consequently affects the growth. Thus, a high temperature is not suitable for those PPCPs which undergo biodegradation and sorption; however, high temperature promotes indirect photolysis up to some extent. Microalgae cells get injured at high temperatures (e.g. >30ºC) and consequently release low molecular organic acids into the culture media, which favor indirect photolysis degradation of PPCPs. Similarly, Zhang et al. ( 2012) reported higher photolysis degradation of norfloxacin at 35ºC than at 25ºC. Among many recalcitrant compounds, carbamazepine is one of the most troublesome pharmaceuticals showing extremely low removal at CWWTPs [START_REF] Luo | A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment[END_REF] as well as from algal photo-bioreactor [START_REF] Xiong | Biodegradation of carbamazepine using freshwater microalgae Chlamydomonas mexicana and Scenedesmus obliquus and the determination of its metabolic fate[END_REF] during winter. However, [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: A pilot-scale study[END_REF] observed a 30% increase in the biodegradation of carbamazepine in the presence of microalgae at warmer weather than cold conditions. Warmer temperature (Mediterranean climate of average temperature 26ºC) favors the removal of biodegradable, hydrophobic, and photodegradable compounds that are associated with more prolonged average solar radiation [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: A pilot-scale study[END_REF]. Also, Scenedesmus sp. showed better removal of NH4-N at a warmer climate [START_REF] Ruiz-Martínez | Effect of temperature on ammonium removal in Scenedesmus sp[END_REF]. Microalgae produce excess EPS in cold climates to withstand harsh weather, which could be beneficial for indirect photodegradation. However, short and low irradiation does not favor photodegradation [START_REF] Wang | Extracellular polymeric substances (EPS) properties and their effects on membrane fouling in a submerged membrane bioreactor[END_REF]. [START_REF] Gao | Removal and biodegradation of Nonylphenol by different Chlorella species[END_REF] demonstrated that four species of Chlorella, i.e., Chlorella vulgaris, Chlorella sp. (1uoai), Chlorella sp. (2f5aia), and Chlorella miniata biodegraded nonylphenol more efficiently at elevated temperature (30°C) and high light intensity; thus, proving high photosynthetic and metabolic activities at high temperature and intense light.

Hydraulic retention time (HRT)

The HRT is a key parameter that should be optimized for better performance of photobioreactors in terms of removal efficiency of PPCPs from wastewater. In general, the HRT used for wastewater treatment in photobioreactors and HRAP varies from 2-10 days [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: A pilot-scale study[END_REF][START_REF] Chu | Continuous cultivation of Chlorella pyrenoidosa using anaerobic digested starch processing wastewater in the outdoors[END_REF][START_REF] Rasoul-Amini | Removal of nitrogen and phosphorus from wastewater using microalgae free cells in bath culture system[END_REF]. Longer HRT increases the removal of PPCPs during treatment by providing enough time for biodegradation, sorption, and indirect photodegradation. On the other hand, longer HRT causes an increase in the biomass of microalgae due to cell division, increasing the chance of micropollutants to be biodegraded and sorbed by more cells. [START_REF] Matamoros | Capability of microalgae-based wastewater treatment systems to remove emerging organic contaminants: A pilot-scale study[END_REF] suggested that during warm or summer seasons, longer HRT does not play a significant role. However, a remarkable difference in PPCPs removal was observed during a cold season when the temperature was considered as limiting condition. Four days of HRT is considered enough for removing a long list of PPCPs in HRAP.

This HRT is comparatively longer than that of CWWTPs, generally operating from 12-24h.

Light intensity

Light is a vital environmental factor for the survival of photoautotrophic and photoheterotrophic microalgae. The microalgae culture system for metabolite production or wastewater treatment is commonly designed to maximize the use of light by providing high surface/volume ratios. Light intensity affects the growth of microalgae and, consequently, nutrients and pollutants removal from wastewater. It has been demonstrated that increasing light intensity enhances the production of biomass [START_REF] Luo | Simultaneous microalgae cultivation and wastewater treatment in submerged membrane photobioreactors: A review[END_REF][START_REF] Hena | Dairy farm wastewater treatment and lipid accumulation by Arthrospira platensis[END_REF]. The exposure to significantly high illumination damages the photosystem II of chloroplast in microalgae; consequently, decreasing the metabolic activities of microalgae and the efficiency of PPCPs removal. The damaged photosystem II in chloroplast eventually gets repaired; nevertheless, cells take a longer time to reach its exponential phase and hence decrease the efficiency of nutrients uptake as well as micropollutants removal. It has also been reported that the maximum nutrients and organic compounds removal occur during the exponential phase of cells. In the case of PPCPs removal, high biomass of microalgae favors biodegradation and sorption; however, the photolysis mechanism has a slightly different trend. [START_REF] Zhang | Photodegradation of Norfloxacin in aqueous solution containing algae[END_REF] Occasionally, the presence of microalgae causes a decrease in photodegradation because of the adsorption of the targeted compounds; thus, reducing the availability of compounds for photooxidation. Interestingly, the same mechanisms (i.e., hydrolysis, photolysis, and adsorption) were involved in removing 7-amino cephalosporanic acid by different microalgae [START_REF] Guo | Removal of cephalosporin antibiotics 7-ACA from wastewater during the cultivation of lipid-accumulating microalgae[END_REF]. Similarly, diclofenac (i.e., NSAID), behaves as compound-specific with different species of Chlorella sp.

Phylogenetic diversity of microorganisms

The two different species of Chlorella (i.e., C. sorokiniana and C. vulgaris) have shown an almost equal removal of diclofenac (i.e., 29% and 21%, respectively) in 9 days [START_REF] Escapa | Comparative assessment of diclofenac removal from water by different microalgae strains[END_REF].

Species-specific compounds show different removal efficiencies with different microalgae. For example, green microalgae Nannochloris sp. can completely remove triclosan (Bai and Acharya 2016), while cyanobacteria Microcystis aeruginosa can remove only 46% [START_REF] Huang | Interactions between the antimicrobial agent triclosan and the bloom-forming cyanobacteria Microcystis aeruginosa[END_REF] from aqueous media within 7-8 days. Diclofenac also acts as species-specific for Chlorella sp.

and Scenedesmus obliquus, which removed 21-29% and 79%, respectively, in the same time span [START_REF] Escapa | Comparative assessment of diclofenac removal from water by different microalgae strains[END_REF]. Natural and synthetic hormones also behave differently during their removal by microalgae. Progesterone is a natural hormone, while norgestrel is a synthetic hormone. They have a similar structure and almost the same molecular weight; however, they are biodegraded at different rates. Progesterone biodegrades faster than norgestrel by microalgal strains Scenedesmus obliquus and Chlorella pyrenoidosa. Additionally, Scenedesmus obliquus can biodegrade both hormones up to 95%, while Chlorella pyrenoidosa can biodegrade only progesterone up to 95%. Nevertheless, norgestrel can only be removed up to 60% [START_REF] Peng | Biotransformation of progesterone and norgestrel by two freshwater microalgae (Scenedesmus obliquus and Chlorella pyrenoidosa): transformation kinetics and products identification[END_REF]. Briefly, progesterone is an example of compound-specific, while norgestrel is an example of species-specific for Scenedesmus obliquus and Chlorella pyrenoidosa. A few interesting comparative studies of PPCPs removal via microalgae have been listed in Table 3.

It is obvious from the above discussion that it is practically impossible to efficiently remove different varieties of PPCPs from real wastewater using an axenic (mono) culture system of microalgae. The phylogenetically diversified culture of microalgae also known as consortium cultures or co-cultivation is defined as a mixed culture of two or more types of strains of microalgae, or microalgae associated with other microorganisms in cultures such as bacteria or fungi are more popular for the treatment of wastewater containing a range of PPCPs (Matamoros et al., 2016;Liu et al., 2017;[START_REF] Munoz | Algal-bacterial processes for the treatment of hazardous contaminants: A review[END_REF]. Similar to microalgae, bacteria can also bio-adsorb PPCPs on the surface of the cells, which is commonly made up of peptidoglycan, teichoic acids, and lipoteichoic acids [START_REF] Flemming | The biofilm matrix[END_REF]. These chemicals offer spots for the bioadsorption of PPCPs [START_REF] Fomina | Biosorption: current perspectives on concept, definition and application[END_REF]. Simultaneously, bacterial EPS also helps in adsorption of PPCPs with the same mechanism as microalgae do. During wastewater treatment, bacteria perform metabolic as well as co-metabolic degradation (Table 4).

In co-metabolic degradation, the degradation of PPCPs depends on the presence of non-specific enzymes catalyzing the metabolism of other substrates [START_REF] Tiwari | Review on fate and mechanism of removal of pharmaceutical pollutants from wastewater using biological approach[END_REF].

A comparison between a microalgal consortium and microalgae-bacterial consortium is summarized in Table 5. A mixed culture of Chlorella sp. and Scenedesmus sp. was used for removing a range of NSAIDs from water (Matamoros et al., 2016). The reason for the increased nutrients and PPCPs removal efficiency of the co-cultures of microalgae is still not well understood; nevertheless, it could be due to the competitive/inhibitive and cooperative interactions between different strains of microalgae, as shown in Fig. In microalgae-bacteria consortium, both cooperative and inhibitive aspects are observed.

Metabolites produced by microalgae could be allelochemicals or growth inhibitors for bacteria.

Chlorellin performed as bactericidal for several bacteria, such as Pseudomonas aeruginosa, Bacillus subtilis, Staphylococcus aureus, and Escherichia coli. This is the reason why microalgae-mediated treated wastewaters have been reported as free from the presence of active E. coli or fecal coliform bacteria [START_REF] Ansa | The role of algae in the removal of Escherichia coli in a tropical eutrophic lake[END_REF]. Besides, high pH and elevated temperature of culture (i.e., caused by photosynthetic processes of microalgae) act as bactericides. On the other hand, the counterpart bacteria also excrete metabolites, which would be algaecides and control the microalgal bloom naturally [START_REF] Fuentes | Impact of Microalgae-Bacteria Interactions on the Production of Algal Biomass and Associated Compounds Mar[END_REF]. Despite the competition between microorganisms, the symbiotic relationships in many microalgae and bacteria have been observed to biodegrade pollutants cooperatively. Bacteria capture the organic compounds and O2

released by microalgae as electron acceptors, reducing photosynthetic oxygen stress among photoautotroph microalgae [START_REF] Park | Growth promotion of Chlorella ellipsoidea by coinoculation with Brevundimonas sp isolated from the microalga[END_REF]. However, in return, vitamin B12 produced by bacteria promotes the growth in microalgae [START_REF] Kazamia | Mutualistic interactions between vitamin B12-dependent algae and heterotrophic bacteria exhibit regulation[END_REF]. Besides, in iron-deficient environments, microalgae uptake siderophores produced by some bacteria as iron substitutes for growth [START_REF] Amin | Review on biofuel oil and gas production processes from microalgae[END_REF]. Bacillus licheniformis was found to enhance the content of chlorophyll-a in Chlorella vulgaris, as well as the removal efficiency of nitrogen and phosphorus from wastewater [START_REF] Liang | Efficiency assessment and pH effect in removing nitrogen and phosphorus by algae-bacteria combined system of Clorella vulgaris and Bacillus licheniformis[END_REF]. Similarly, few freshwater microalgae and diatoms showed better pollutant removal efficiency when cultured with bacteria Neochloris oleoabundans, and have accumulated more cellular metabolites (lipid) than when cultured alone [START_REF] Olguin | Population dynamics in mixed cultures of Neochloris oleoabundans and native microalgae from water of a polluted river and isolation of a diatom consortium for the production of lipid rich biomass[END_REF]. Chlorella sp. with bacterial consortium was used to remove ketoprofen [START_REF] Ismail | Biodegradation of ketoprofen using a microalgalbacterial consortium[END_REF].

Another consortium microorganism used for PPCPs removal along with microalgae is fungi.

Bodin et al. ( 2016) cultivated microalgae Chlorella vulgaris with fungus Aspergillus niger to remove ranitidine from water.

Acclimatization of microalgae

The improved response of microalgae in an extreme environment is crucial for wastewater treatment, especially when the influent carries PPCPs. A slow exposure of PPCPs to microalgae Although it is difficult to analyze the toxic effect of PPCPs to non-targeted microalgae in the environment, the measurement of hazard or risk quotients can be used to depict the potential ecological risk of a specific PPCP to microalgae [START_REF] Gros | Removal of pharmaceuticals during wastewater treatment and environmental risk assessment using hazard indexes[END_REF]. The hazard or risk quotient is the ratio of measured environmental concentration (MEC) and predicted no-effect concentration (PNEC) of a certain PPCP for a specific microalga [START_REF] Gros | Removal of pharmaceuticals during wastewater treatment and environmental risk assessment using hazard indexes[END_REF]. MEC

represents the maximum concentration level detected for each PPCP in water bodies (wastewater effluents and surface waters) while the PNEC values are obtained by specifically dividing the EC50 value of each PPCP by a safety factor of 1000 [START_REF] Gonzalez-Pleiter | Toxicity of five antibiotics and their mixtures towards photosynthetic aquatic organisms: Implications for environmental risk assessment[END_REF]. The value of risk quotient of a particular PPCP equals to or higher than one indicates its potential toxicity for an alga [START_REF] Grung | Environmental assessment of Norwegian priority pharmaceuticals based on the EMEA guideline[END_REF]. The toxicological effect of a PPCP can be assessed by studying different aspects such as growth rate and biochemical characteristics of the microalga, including pigments (chlorophyll and carotenoid) content, and the activity of antioxidant enzymes, i.e., malondialdehyde (MDA), catalase (CAT), and superoxide dismutase (SOD). However, the most common way for toxicity evaluation is to calculate the effective concentration (EC50) of the stressor (PPCPs) at which 50% of the growth rate of an organism (microalgae) is inhibited. It has been reported that the value of EC50 increased with an increasing period of cultivation; thus, anticipating the adaptation of microalgae. According to the Commission of the European Communities (1996), pollutants with EC50 >100 mg/L are classified as harmless substances, those with EC50 from 10-100 mg/L are included under the harmful category, while those with EC50 of toxic substances placed between 1-10 mg/L, and all pollutants having EC50 <1 mg/L are considered as very toxic substances. The toxicological effect of PPCPs on the growth rate, chlorophyll, and carotenoids content, and activity of antioxidant enzymes of microalgae are summarised in Table 6. Under normal culture and environmental conditions, the generation and consumption of reactive oxygen species (ROS) in microalgae are maintained in equilibrium.

However, the exposure of PPCPs tends to induce excess production of ROS in microalgal cells, such as hydroxyl radicals ( • OH), superoxide radicals (O2 •-), and hydrogen peroxide (H2O2). ROS are strong oxidants that can lethally damage the cell organelles by peroxidation of polyunsaturated fatty acids and proteins present in membranes, causing an increase in the permeability of the membrane [START_REF] Ke | Effects of metals on biosorption and biodegradation of mixed polycyclic aromatic hydrocarbons by a fresh water green alga Selenastrum capricornutum[END_REF][START_REF] Tang | Aerobic degradation of BDE-209 by Enterococcus casseliflavus: isolation, identification and cell changes during degradation process[END_REF]. The polyunsaturated fatty acids produce malondialdehyde (MDA) as a product after peroxidation, which can be analyzed in the lab to confirm the degree of structural and functional damages in the cell. Once the microalgae are exposed to the stressor or toxic compounds (PPCP), the cells start to counteract the toxicity of excess ROS by initiating the synthesis of anti-oxidative enzymes such as superoxide dismutase (SOD) as the first line of defense in the cell. SOD reacts with O2 •-and converts it to O2 and hydrogen peroxide, which are less oxidative than O2 •-. Catalase (CAT), a heme-containing enzyme produced at the chloroplast and cytoplasm along with SOD, interacts with H2O2 and produces O2 and H2O [START_REF] Jin | Bioaccumulation and catabolism of prometryne in green algae[END_REF][START_REF] Huang | Enantioselective toxic effects and biodegradation of benalaxyl in Scenedesmus obliquus[END_REF]. A steady decrease in SOD and CAT activity during assay tests is a sign of adaptation of microalgal cells, which gradually appears after a few days of exposure to stressors (PPCPs) (Xiong et al., 2017a).

Several microalgae show a low dose stimulation and high dose inhibition in growth in the presence of particular stressors, known as hormesis. Freshwater diatom Navicula sp. and green microalgae C. vulgaris exhibited better growth in the presence of low concentrations of ibuprofen due to hermetic dose-response [START_REF] Geiger | Single and mixture toxicity of pharmaceuticals and chlorophenols to freshwater algae Chlorella vulgaris, Ecotoxicol[END_REF][START_REF] Ding | Toxicity, degradation and metabolic fate of ibuprofen on freshwaterdiatom Navicula sp[END_REF]. Among ROS, superoxide (O2 •-) acts as a central factor of signal transduction, activating the gene expression to encoding SOD [START_REF] Liu | Combined effects of two antibiotic contaminants on Microcystis aeruginosa[END_REF][START_REF] Liu | Antioxidant responses and degradation of two antibiotic contaminants in Microcystis aeruginosa[END_REF]. However, in a worst-case scenario, if the DNA is damaged by oxidation, then the cell fails to organize its systems and is followed by cell death, also known as programmed cell death (PCD).

Photosynthesis is an essential process for the survival of microalgae. This process is related to the growth and several metabolic activities of microalgae. Thus, any alteration in growth in microalgae during PPCPs (stressors) exposure is related to the changes in chlorophyll. PPCPs damage thylakoid and chloroplast lipids by annihilating membranes functionally and structurally, consequently destroying photosystem II (PSII) and the content of chlorophylls and carotenoids [START_REF] Nie | Response of the freshwater alga Chlorella vulgaris to trichloroisocyanuric acid and ciprofloxacin[END_REF]. Low chlorophyll content as a stress response of PPCPs has been observed in microalgae Tetraselmis chui, Isochrysis galbana, and C. pyrenoidosa in the presence of thiamphenicol, florfenicol, and chloramphenicol [START_REF] Lai | Effects of chloramphenicol, florfenicol, and thiamphenicol on growth of algae Chlorella pyrenoidosa, Isochrysis galbana, and Tetraselmis chui[END_REF]. In 10 days, C. vulgaris started showing stress response for metronidazole [START_REF] Hena | Removal of metronidazole from aqueous media by C. vulgaris[END_REF]. In some cases, a gradual increase in chlorophyll has been observed with an increase of PPCP dosage with time. Chlorella pyrenoidosa and Microcystis aeruginosa showed an increase in chlorophyll in the presence of antibiotics in media, such as spiramycin, cefradine, and amoxicillin [START_REF] Chen | Algal feedback and removal efficiency in a sequencing batch reactor algae process (SBAR) to treat the antibiotic cefradine[END_REF][START_REF] Liu | Combined effects of two antibiotic contaminants on Microcystis aeruginosa[END_REF]. Xiong et al. (2017b) reported an increment in chlorophyll by 38% to 19% in Chlamydomonas mexicana with an increase in the concentration of ciprofloxacin from 60 mg/L to 100 mg/L, respectively, compared to control culture. Chlorophyll was observed to act as a protective mediator in the attenuation of excess ROS in the cell [START_REF] Xiong | Biodegradation of carbamazepine using freshwater microalgae Chlamydomonas mexicana and Scenedesmus obliquus and the determination of its metabolic fate[END_REF]. Carotenoids play important roles during the initial reactions of photosynthesis and possess enough antioxidative properties to quench the ROS produced during the exposure of pollutants [START_REF] Jahns | The role of the xanthophyll cycle and of lutein in photoprotection of photosystem II[END_REF]. Carotenoids increases in C. mexicana in the presence of enrofloxacin and ciprofloxacin (Xiong et al., 2017b). Increased content of carotenoid has also been observed in Dunaliella bardawil, Dunaliella tertiolecta, S. obliquus, and C. mexicana exposed to carbamazepine [START_REF] Xiong | Biodegradation of carbamazepine using freshwater microalgae Chlamydomonas mexicana and Scenedesmus obliquus and the determination of its metabolic fate[END_REF][START_REF] Tsiaka | Carbamazepinemediated pro-oxidant effects on the unicellular marine algal species Dunaliella tertiolecta and the hemocytes of mussel Mytilusgallo provincialis[END_REF][START_REF] Salguero | Carotenoids as protective response against oxidative damage in Dunaliella bardawil[END_REF]. The matrix of water plays a significant role in toxicity upon microalgae. Some quinolones acquire antagonistic characters in the presence of calcium and magnesium and decrease their effects on microorganisms and toxicity [START_REF] Li | Effects of brackish water and seawater on in vitro efficacy of antibacterial drugs used in aquaculture[END_REF]. [START_REF] Hagenbuch | Toxic effect of the combined antibiotics ciprofloxacin, lincomycin, and tylosin on two species of marine diatoms[END_REF] reported the same effect of tylosin, lincomycin, and ciprofloxacin towards C. closterium and N. ramosissima (i.e., both are from marine habitats). Also, a higher EC50 value was observed in saline water culture than those obtained in freshwater culture [START_REF] Halling-Sørensen | Reduced antimicrobial potencies of oxytetracycline, tylosin, sulfadiazine, streptomycin, ciprofloxacin, and olaquindox due to environmental processes[END_REF]. On the other hand, photodegradation products of sulfamethoxazole are more toxic and less biodegradable in seawater; thus, increasing the toxicity of compounds, and decreasing the EC50 value for microalgae [START_REF] Teixeira | Effects of environmentally-relevant antibiotic mixtures on marine microalgal growth[END_REF].

Closed and open culture techniques

Photo-bioreactors are closed systems where microalgae grow under controlled conditions. A tubular photo-bioreactor is one of the most efficient and promising designs of closed photobioreactor systems. The culture is circulated by pumping, and the system is incorporated with a gas exchange unit where a CO2 supply can be used by microalgae in photosynthesis and for removing O2 from the medium. Compared to open systems, closed systems are more efficient in terms of volumetric productivity (such as lipid and biomass) and pollutant removal efficiency.

However, difficulties in upscaling, overheating, and construction cost are clear drawbacks for implementing it as wastewater treatment in large scales unless the wastewater is containing highly hazardous pollutants that cannot be remediated in open systems. Volatilization mechanisms for PPCPs removal also get hindered by closed bioreactors. [START_REF] Kong | Culture of microalgae Chlamydomonas reinhardtii in wastewater for biomass feedstock production[END_REF] reported higher wastewater treatment efficiency in closed systems rather than open systems with C. reinhardtii. Closed culture system is more popular in the lab for small scale of real [START_REF] Zhou | Local bioprospecting for high-lipid producing microalgal strains to be grown on concentrated municipal wastewater for biofuel production[END_REF] or synthetic wastewater treatment to study the suitability of a particular microalgal strain for a specific micro-pollutant in a controlled atmosphere (Xiong et al., 2017b;[START_REF] Hena | Removal of metronidazole from aqueous media by C. vulgaris[END_REF][START_REF] Escapa | Paracetamol and salicylic acid removal from contaminated water by microalgae[END_REF]. [START_REF] Zhou | Local bioprospecting for high-lipid producing microalgal strains to be grown on concentrated municipal wastewater for biofuel production[END_REF] autoclaved the wastewater to check the stability of selected microalgal strains for particular wastewater, which is not feasible for real wastewater treatment at large scales.

HRAPs are a modified form of conventional stabilization ponds used for the treatment of urban wastewater. HRAPs are microalgal culture ponds with 0.2 to 1 m of depth, incorporating a paddlewheel to homogenize the culture and helping in mixing the atmospheric CO2 with media.

Generally, open systems are not equipped with air or a CO2 supply. WWTPs incorporated with HRAP for wastewater treatment are known as Advanced Pond System (APS). APS is comprised of anaerobic digestion chamber, HRAP, and followed by algal settling pond, as shown in Fig. 5.

Anaerobic ponds are commonly 2-5 m in depth and receive wastewater with high biological oxygen demand (BOD) and chemical oxygen demand (COD) values. In general, this pond is free of microalgae and dissolved oxygen. The anaerobic bacteria are involved in the BOD removal by anaerobic digestion, followed by a settling process. The treated wastewater is transferred to HRAP for microalgal treatment, which is inoculum with single or consortium of microalgae or microalgae-bacteria to remove COD, BOD, total nitrogen, phosphorus, and micro-pollutants (i.e., including PPCPs). The microalgal settling pond or microalgal biomass harvesting pond receives treated wastewater from HRAP, where different techniques are used to separate biomass from aqueous media. Since microalgae cannot settle by themselves as activated sludge or it may partially settle due to gravity after a long time. A flocculant is added to speed up this process.

The separated microalgal biomass can be used as a fertilizer [START_REF] Mulbry | Recycling ofmanure nutrients: use of algal biomass from dairy manure treatment as a slow release fertilizer[END_REF] or as feedstock for the energy [START_REF] Hena | Dairy farm wastewater treatment and lipid accumulation by Arthrospira platensis[END_REF]. The advantages and limitations of the processes using suspended and immobilized cell culture are listed in Table 7.

Suspended and immobilized culture techniques

The suspended microalgae culture is well established in wastewater treatment; however, the harvesting of microalgal cells is an extra step to finalize the treatment process. To overcome the time and energy-consuming issues, the immobilized culture technique of microalgae has shown as a promising alternative to suspended culture [START_REF] Pires | Wastewater treatment to enhance the economic viability of microalgae culture[END_REF][START_REF] He | Algal-based immobilization process to treat the effluent from a secondary wastewater treatment plant (WWTP)[END_REF]. In general, the immobilized technique is more effective in removing PPCPs than suspended

technique. An immobilized mono-culture of C. vulgaris on sodium alginate gel impregnated with Al2O3 nanoparticles was capable of removing 89.6% of carbamazepine from water in four days of treatment in a closed photo-bioreactor system, while only 48.5% was removed from the suspended culture of C. vulgaris (constant cell density; 3.0×10 6 cells/mL) [START_REF] Wu | Al2O3 Nanoparticles promote the removal of carbamazepine in water by Chlorella vulgaris immobilizedin sodium alginate gel beads[END_REF]. [START_REF] Sole | Removal of endocrine disrupting compounds from wastewater by microalgae coimmobilized in alginate beads[END_REF] found that the removal rates of EE2 were significantly higher in immobilized microalgae reactors than in free microalgae reactors.

There are two types of immobilizations: natural or passive immobilization and artificial or active immobilization.

Natural immobilizations are more complex in character than artificial or active immobilization.

The examples of natural immobilization techniques are always mixed-culture, and microorganisms behave symbiotically with each other. Natural immobilization is the innate characteristics of microorganisms to anchor to the different surfaces. The attachments of the cells on solid surfaces generally form biofilms due to the secretion of EPS by metabolically active cells of microalgae and bacteria. The EPS of biofilm protects the immobilized cells from direct contact with extreme conditions (e.g., pH, pollutants or toxic substrates, temperature). Also, extracellular enzymes in EPS assist in the external biodegradation of pollutants [START_REF] Hou | Extracellular polymeric substances from copper-tolerance Sinorhizobium meliloti immobilize Cu 2+[END_REF].

In nature, microalgae-bacteria consortium is present as biofilms, which play an important role in PPCPs removal from surface water. River biofilm (microalgae-bacteria reactors) are found beneficial for degrading ibuprofen but not for clofibric acid [START_REF] Winkler | Selective degradation of ibuprofen and clofibric acid in two model river biofilm systems[END_REF]. Writer et al.

(2011) identified a biofilm useful for the removal of steroidal hormones via biodegradation and sorption mechanisms. Periphyton is a natural consortium of microalgae-bacteria used in lab-scale to remove nutrients and five PPCPs mainly due to biodegradation processes [START_REF] Kang | Removal of nutrients and pharmaceuticals and personal care products from wastewater using periphyton photobioreactors[END_REF].

Among those, nitrogen and phosphorus were removed up to 77% and 68%, respectively, ibuprofen and hydrochlorothiazide were moderately removed up to 48.7%, while gemfibrozil and carbamazepine were poorly removed (i.e., up to 20.6%). Interestingly, these removal trends were similar to those of CWWTPs [START_REF] Luo | A review on the occurrence of micropollutants in the aquatic environment and their fate and removal during wastewater treatment[END_REF].

Artificial immobilization is an induced process to trap the metabolically active cells either by adsorption on a solid surface, entrapment within an emulsion, polymers matrix, or semipermeable membrane [START_REF] Hameed | Biotechnological potential uses of immobilized algae[END_REF][START_REF] Eroglu | Application of various immobilization techniques for algal bioprocesses[END_REF]. The artificial confinement of cells within polymers matrix and adsorption onto solid surfaces are the most common techniques to immobilize cells. The most commonly used matrix is natural polymers such as carrageenan, alginate, chitosan, and agar, which are non-toxic to microalgal cells. The appropriate pore sizes of matrix allow substrates and products to diffuse into and from the matrix of polymers, respectively, during the treatment process. However, immobilizing a microalgae culture into biopolymers is a comparatively expensive technique; thus, restricting the application at large scales. [START_REF] Ismail | Biodegradation of ketoprofen using a microalgalbacterial consortium[END_REF] co-cultured a strain of ketoprofen degrading bacteria with Chlorella sp. for attaining higher biodegradation.

Improved hybrid microalgae culture techniques

Pharmaceuticals are only partially metabolized in animals and humans, while the rest is excreted and end up in municipal WWTPs. The recalcitrant nature of PPCPs causes contaminants to slip through the treatment plant and enter into the environmental subsurface and possibly the drinking water treatment plants [START_REF] Reemtsma | Mind the Gap: Persistent and Mobile Organic Compounds-Water Contaminants That Slip Through[END_REF]. Among all persistent PPCPs, antibiotics have received widespread attention due to their potentiality to induce antibiotic resistance bacteria (ARB) in every compartment of the environment. These ARBs release antibioticresistant genes (ARGs) into the environment, having a fair chance to be acquired by other pathogenic bacteria [START_REF] Berglund | Identification of 76 novel B1 metallo-β-lactamases through large-scale screening of genomic and metagenomics data[END_REF]. On the other hand, microalgal treatment of wastewater has been proven as competent as conventional wastewater treatment processes (U´beda et al., 2017). The microalgae are not the targeted organism of antibiotics and have a higher tolerance to contaminants than bacteria, including antibiotics. However, microalgae show toxicity towards PPCPs, which inhibit their growth. Among all reported PPCPs, tetracycline is the most toxic for microalgae with EC50 lower than 10 µg/L [START_REF] Wang | Removal of pharmaceuticals and personal care products from wastewater using algae-based technologies: a review[END_REF]. The concept of hybrid techniques was developed to enhance the removal capacity of PPCPs from the WWTPs and to reduce the risk impact of PPCPs on the ecosystem (outside the treatment plant), including bacteria and microalgae. However, [START_REF] Hena | Removal of metronidazole from aqueous media by C. vulgaris[END_REF] suggested that the biomass of microalgae produced during the treatment of wastewater containing PPCPs could be used as a valuable feedstock for biofuels production since the treated biomass is not suitable to be used for other purposes such as fertilizers and animal feed. This way, the phycoremediation of wastewater and concomitantly biomass production as feedstock for biofuels are a waste to worth paradigm. In this section, some hybrid techniques are discussed.

Hybrid microalgae-UV treatment technique

Advanced oxidation processes such as UV, ozone, and H2O2 can efficiently disintegrate the PPCPs from water; however, their by-products, high consumption of energy, and oxidants dose are few drawbacks of these processes. The by-products of cefradine and sulfamethoxazole are more toxic than their parent compounds after treating them with UV and ozone, respectively [START_REF] Du | Understanding the algal contribution in combined UV-algae treatment to remove antibiotic cefradine[END_REF][START_REF] Dantas | Sulfamethoxazole abatement by means of ozonation[END_REF]. To minimize the above problems and to achieve an economical, environmentally sustainable, and efficient process to remove PPCPs, the chemical process has been combined with a biological method (Fig. 6). In association with the biological entities, the intermediates of the recalcitrant compounds produced by chemical processes can biodegrade more efficiently than individual processes. In combined or hybrid treatment, the chemical processes are kept mild to produce biodegradable intermediates but not destined to induce the total degradation of compounds, which save the costs as well as the environment. Microalgae are well documented in the literature for its ability to biodegrade PPCPs and their intermediates [START_REF] Gao | Removal and biodegradation of Nonylphenol by different Chlorella species[END_REF][START_REF] Shao | Biodegradation and enzymatic responses in the marine diatom Skeletonema costatum upon exposure to 2,4-dichlorophenol[END_REF]. [START_REF] Du | Understanding the algal contribution in combined UV-algae treatment to remove antibiotic cefradine[END_REF] reported on the toxicity control of cefradine by a hybrid UV-microalgae treatment system. [START_REF] Yang | An integrated view of the intimate coupling UV irradiation and algal treatment on antibiotic: Compatibility, efficiency and microbic impact assessment[END_REF] demonstrated the removal of antibiotics cefradine and amoxicillin by hybrid UV-green microalgae (Chlorella pyrenoidosa, Selenastrum capricornutum, and Scenedesmus obliquus) system, where microalgae played a major role in the biodegradation while UV triggered the treatment. S. obliquus, along with UVirradiation at 365 nm, showed the best possible removal (99.84%) of cefradine and amoxicillin within 24 h of application. The bacteriostatic test for the bacteria E. coli and S. aureus reflected insignificant toxicity after hybrid treatment. Liu et al. (2017) enhanced the removal efficiency of the hybrid system by incorporating an additional step of microalgal treatment for ceftazidime removal after treating it with UV-microalgae. The process was termed UVmicroalgae/microalgae treatment.

Hybrid microalgae-activated sludge system technique

The exposure of high concentrations of antibiotics to the diversified bacterial population in activated sludge systems offer the most favorable condition for bacteria to develop resistance in WWTPs. Consortiums have always been found more efficient in biodegradation of PPCPs than mono-culture of either microalgae or bacteria. However, the hybrid microalgae-activated sludge system is a relatively new concept, initially reported by [START_REF] Guo | Application of alga-activated sludge combined system (AASCS) as a novel treatment to remove cephalosporins[END_REF], for treating cephalosporins (i.e., one of the most common antibiotics used by human beings, cattle, and aquaculture farming). [START_REF] Kümmerer | Antibiotics in the aquatic environment -a review-Part I[END_REF] stated that almost 60% of the total antibiotics used for therapeutic use belong to the cephalosporins family. The finding of [START_REF] Guo | Application of alga-activated sludge combined system (AASCS) as a novel treatment to remove cephalosporins[END_REF] proved that antibiotics do not come in direct contact with bacteria if they have been pre-treated by microalgae. The proposed technique highly assists in avoiding ARB development.

Simultaneously, the hybrid microalgae-activated sludge system showed an excellent removal capability (up to 97.9%), while the initial concentration of cephalosporins was 100 mg/L. This technique is a promising approach to treat wastewater in two steps (Fig. 7) to protect the bacterial community from acquiring resistance and developing ARB or releasing ARGs in the environment.

Hybrid microalgae-membrane photo-bioreactor technique

Several studies have focussed on membrane bioreactor (MBR) combined with activated sludge for PPCPs removal from wastewater [START_REF] Kovalova | Hospital wastewater treatment by membrane bioreactor: performance and efficiency for organic micropollutant elimination[END_REF][START_REF] Tadkaew | Removal of trace organics by MBR treatment: the role of molecular properties[END_REF]Radjenovic et al., 2009;[START_REF] Alvarino | A UASB reactor coupled to a hybrid aerobic MBR as innovative plant configuration to enhance the removal of organic micropollutants[END_REF]. [START_REF] Spring | Removal of endocrine disrupting compounds using a membrane bioreactor and disinfection[END_REF] suggested that an MBR is more efficient in removing PPCPs than conventional treatment due to following reasons:

1. Many compounds adhered to the surface of the sludge during treatment are also retained in the sludge by membrane filtration.

2. In many cases, the membrane surfaces also capture the compounds.

Longer sludge retention time enhances the biodegradation of recalcitrant compounds in MBR.

Similar to MBR, the membrane photo-bioreactor technology (MPBR) is a bioreactor equipped with light and membrane units, which assist microalgae to grow and harvest complete biomass (Fig. 8). Currently, comprehensive studies have been conducted on MPBR; nevertheless, most of these studies revolved around the removal of nitrogen and phosphorus from wastewater and synthetic culture media [START_REF] Honda | Carbon dioxide capture and nutrients removal utilizing treated sewage by concentrated microalgae cultivation in a membrane photobioreactor[END_REF][START_REF] Bilad | Coupled cultivation and pre-harvesting of microalgae in a membrane photobioreactor (mpbr)[END_REF][START_REF] Marbelia | Membrane photobioreactors for integrated microalgae cultivation and nutrient remediation of membrane bioreactors effluent[END_REF][START_REF] Gao | A novel algal biofilm membrane Homeffluent[END_REF][START_REF] Boonchai | Microalgae membrane photobioreactor for further removal of nitrogen and phosphorus from secondary sewage effluent[END_REF][START_REF] Gao | Continuous microalgae cultivation in aquaculture wastewater by a membrane photobioreactor for biomass production and nutrients removal[END_REF][START_REF] Lee | Semi-continuous operation and fouling characteristics of submerged membrane photobioreactor (SMPBR) for tertiary treatment of livestock wastewater[END_REF]. Few studies are available on the MPBR system for PPCPs removal [START_REF] Derakhshan | Studies on influence of process parameters on simultaneous biodegradation of atrazine and nutrients in aquatic environments by a membrane photobioreactor[END_REF].

Hybrid microalgae-constructed wetland technique

Constructed wetlands (CW) have been proven as an operational technique for PPCPs removal [START_REF] Dordio | Removal of pharmaceuticals in microcosms constructed wetlands using Typha spp. and LECA[END_REF][START_REF] Matamoros | Behaviour of pharmaceutical products and biodegradation intermediates in horizontal subsurface flow constructed wetland. A microcosm experiment[END_REF][START_REF] Zhang | Removal of pharmaceutical compounds in tropical constructed wetlands[END_REF][START_REF] Hijosa-Valsero | Removal of antibiotics from urban wastewater by constructed wetland optimization[END_REF][START_REF] Hussain | Removal of ionophoric antibiotics in free water surface constructed wetlands[END_REF]. The shallow bed of CW contains free-floating plants or plants with rudimentary kinds of roots [START_REF] Cole | The emergence of treatment wetlands[END_REF], which are capable of producing water of quality enough for disposal into natural water bodies [START_REF] Garcia-Rodríguez | The influence of light exposure, water quality and vegetation on the removal of sulfonamides and tetracyclines: A laboratory-scale study[END_REF]. Regarding nutrients removal, the nitrification process in nitrogen removal is a rate-limiting step due to oxygen deficiency (Xiong et al., 2018b). This drawback can be overcome by incorporating microalgae into the system, which produces oxygen during photosynthesis. The configuration of the expected hybrid microalgae-constructed wetland techniques is illustrated in Fig. 9. [START_REF] Zhang | Degradation of paracetamol by pure bacterial cultures and their microbial consortium[END_REF] reported 74% removal of carbamazepine and almost complete removal of naproxen in a wetland system containing Scirpus validus. On the other hand, carbamazepine could only be removed by 32.7% by activated sludge in conventional wastewater treatment [START_REF] Postigo | Analysis and occurrence of selected medium to highly polar pesticides in groundwater of Catalonia (NE Spain): an approach based on on-line solid phase extraction-liquid chromatography-electrospray-tandem mass spectrometry detection[END_REF].

Comparative removal efficiency of PPCPs between microalgae and conventional wastewater treatment

The removal efficiency of PPCPs depends upon the physicochemical properties (Table 1) of the compounds as follows;

1. The higher the Kow, the higher is the removal efficiency of the compound from WWTPs via sorption processes.

2. The higher the tendency of the compound towards the sludge phase, the higher is the solidaqueous distribution coefficient; therefore, the better is the removal of the compound via sorption.

3. The negative charge of the functional group of the compound repels the negative charge of the microbial cell membrane; thus, hindering the removal of compounds via external biodegradation.

4. The complexity of the structure also regulates the biodegradability of compounds. A compound with linear, unsaturated, and electron-donating groups was found as more biodegradable than compounds possessing cyclic structure, saturated long linear carbon chain, and electron-withdrawing groups in their structure.

Wastewater is extremely complex, and for many PPCPs, the removal efficiencies do not follow the above-discussed rules. Also, in most cases, the removal efficiency is lower than expected. In brief, conventional wastewater treatment systems are not completely able to remove PPCPs from urban wastewater, and hence, more efficient and specific treatments are required. However, no available specific treatment is capable of removing all PPCPs from wastewater due to their wide varieties. A conclusion on comparative efficiency between conventional and microalgaemediated treatment systems is a difficult task since most of the studies have been conducted on different influent concentrations of PPCPs. Nevertheless, Zhou et al. (2014) compared activated sludge systems with microalgae-mediated systems for a broad range of PPCPs. They found that microalgal systems can remove, in general, antibiotics and antimicrobial agents (e.g., sulfamethazine, clarithromycin, sulfamethoxazole, sulfapyridine, roxithromycin, and triclocarban) more efficiently than activated sludge systems. However, it is difficult to draw conclusions on the comparative efficiency of the systems without treating the same wastewater. In this section, the results from different CWWTPs and microalgae-mediated wastewater treatment plants were collected, compared, and shown in Table 8.

Microalgae-based wastewater treatment systems can competently remove many types of PPCPs, such as NSAIDs, antibiotics, and stimulants. The lower retention time of conventional activated sludge processes is one of the major reasons for the low removal capacity of highly polar pharmaceuticals [START_REF] Buttiglieri | Removal of emerging contaminants in wastewatertreatment: conventional activated sludge treatment[END_REF][START_REF] Le-Minh | Fate of antibiotics during municipalwater recycling treatment processes[END_REF]. [START_REF] Ahmed | Progress in the biological and chemical treatment technologies foremerging contaminant removal from wastewater: A critical review[END_REF] claimed that the general pattern of PPCPs removal in microalgae-based wastewater treatment systems and activated sludge system follow the next trend: pharmaceuticals > PCPs > EDCs and

EDCs > PCPs > pharmaceuticals, respectively.

As long as phycoremediation is concerned, microalgae and cyanobacteria have more advantages over bacterial treatment since they can grow in various modes of cultivation, i.e., phototrophic, heterotrophic, and mixotrophic [START_REF] Delrue | The Environmental Biorefinery: Using Microalgae to Remediate Wastewater, a Win-Win Paradigm[END_REF]. Microalgae have numerous potential applications. Remarkably, the most promising future objective on a large scale is their use for bioremediation of wastewater. Microalgae are being used to bioremediate numerous pollutants of different characteristics and properties released from the domestic, industrial, agricultural, and farm sectors [START_REF] Hena | Dairy farm wastewater treatment and lipid accumulation by Arthrospira platensis[END_REF][START_REF] Chinnasamy | Microalgae cultivation in a wastewater dominated by carpet mill effluents for biofuel applications[END_REF]. Additionally, CO2 mitigation during culture and the use of biomass as feedstock for biodiesel or biofuel production are other benefits of microalgae-mediated treatment over CWWTPs [START_REF] Ga´lvez | Microalgae cultivation in urban wastewater: Coelastrum cf. Pseudomicroporum as a novel carotenoid source and a potential microalgae harvesting tool[END_REF][START_REF] Cuellar-Bermudez | Nutrients utilization and contaminants removal. A review of two approaches of algae and cyanobacteria in wastewater[END_REF].

Concluding remarks and perspectives

Pharmaceuticals and personal care products are micro-pollutants that, even at low doses, can induce physiological effects in humans and other organisms in the environment. Their stable structure resists disintegration and consequently induce longer persistence in the environment; thus, remaining biologically active. The main sources of PPCPs into the aquatic environment are the effluents of WWTPs, runoff water from agriculture and farmlands, and the sipping of landfill leachate and their continuous exposure to microalgal life; consequently, exerting harmful effects.

Globally, WWTPs are unable to completely remove PPCPs from wastewater. Therefore, there is an urgent need for more effective and advanced technologies to remove this wide range of emerging pollutants. There are numerous systems currently used to remove bio-refractory organic and inorganic pollutants from aqueous media (e.g., advance oxidation processes, adsorption, and liquid-liquid extraction); nevertheless, these systems have yet to be improved to be applied on an industrial scale. Besides recent breakthroughs in physical and chemical technologies, the development and further exploration of the potentiality of microalgae to efficiently bio-transform or bio-adsorb micro-pollutants is a promising approach. This review highlights the feasibility of microalgae in removing PPCPs from wastewater and toxicological effect of PPCPs on microalgae. However acclimatization and the improved hybrid microalgae culture techniques are the keys to increase the removal efficiency of PPCPs from wastewater and reduce the toxicological effect of PPCPs on bacteria and microalgae present in ecosystem Additionally, the recovered biomass would be used in other sectors. Despite comprehensive studies, there are still many WWTPs around the world where PPCPs have not been thoroughly

analyzed. Therefore, it is highly required to study treatment performance, which could expand the current understanding of the fate of PPCPs and might breakthrough some strains for bioremediation. This combination reduces the oxygenation and CO2 supply cost for bacteria and microalgal growth, respectively.

Feedstock

The harvested biomass can be used for fertilizers, biofuel production, and land rehabilitation.

The harvested biomass is not fit for biofuel production but can be used in land rehabilitation more efficiently than microalgae consortium.

  1 (i)) is used for absorption or bioaccumulation analysis. It is necessary to break the cell walls of microalgae to release the accumulated or absorbed micropollutants. Correa-Reyes et al. (2007) proposed to freeze the algal cell pellet at -20ºC, causing the cell wall to break due to the expansion of the aqueous portion of the cytoplasm into ice. Peng et al. (2014) used this method to successfully disrupt the cell of C. pyrenoidosa and S. obliquus and release bio-accumulated progesterone and norgestrel.

  photodegradation increased from 35.9% to 40.9%, with increasing cell counts from 1.7× 10 6 /L to 1.7×10 7 /L within 60 mins. Clearly, more cells produce more hydroxyl radicals as photosensitizers from dissolved organic matter excreted by microalgae under illumination.However, a further increase in cell counts caused self-shading effects of algal cells, preventing light from penetrating and hence decreased photodegradation. Conversely, microalgae have only shown a 5% norfloxacin removal under dark conditions, possibly due to hydrolysis and adsorption. However, direct photolysis in the presence of light and the absence of microalgae attained a maximum removal of 9.1%.[START_REF] Hena | Dairy farm wastewater treatment and lipid accumulation by Arthrospira platensis[END_REF] increased the intensity of light from 160 to 300 µmol photon/m 2 /s during the late exponential phase of Arthrospira platensis for treating dairy farm wastewater to avoid the shading effect of cells.[START_REF] Garcia-Rodríguez | The influence of light exposure, water quality and vegetation on the removal of sulfonamides and tetracyclines: A laboratory-scale study[END_REF] studied the role of microalga Spyrogira sp. and aquatic plant Zannichellia palustris in photolysis degradation and the contribution of light during the total removal of six antibiotics from sulfonamides and tetracyclines families. The difference in antibiotic removal between covered and uncovered conditions of wastewater showed a minimal decrease in the concentration of all antibiotics studied, except for oxytetracycline. Oxytetracycline significantly degraded even in the dark, possibly due to hydrolysis. The rest of the tested antibiotics showed better removal in the presence of light and even higher removal in the presence of light and microalga. Out of the 20 days of the inoculation of microalgae and aquatic plants, the degradation rate remained higher than under controlled conditions until the first 5 days (i.e., absence of microalga and aquatic plants), followed by either decrease in degradation rate or remains constant in the presence of vegetation. During the initial days of inoculation, both direct and indirect photolysis occurred. However, with time, an increase in biomass caused hindrances in photodegradation, and

  3. Mendes and Vermelho (2013) postulated the concept of metabolites exchange between microorganisms for cooperative interaction, enhancing the biomass and, consequently, the nutrients and micropollutants removal efficiency.Xiong et al. (2017b) demonstrated the removal efficiency of monocultures of Chlorella vulgaris, Micractinium resseri, Scenedesmus obliquus, Ourococcus multisporus, and Chlamydomonas mexicana and their consortium for antibiotic enrofloxacin in aqueous media.The removal efficiency of the consortium was comparable to the most efficient strains, although the consortium showed the highest growth inhibition in the presence of enrofloxacin than an individual culture of microalgae. The co-cultivation of microorganisms or microalgae produces secondary metabolites known as allelochemicals, which might inhibit the growth of other microorganisms[START_REF] Cembella | Chemical ecology of eukaryotic microalgae in marine ecosystems[END_REF][START_REF] Gross | Allelopathy of aquatic autotrophs[END_REF]. According toXiong et al. (2017b), the growth inhibition would be due to allelochemicals excreted by microalgae or degraded products of enrofloxacin performing as allelochemicals. Raphidocelis subcapitata showed inhibition in cocultivation with C. vulgaris caused by the production of allelochemicals chlorellin (fatty acid) by the latter. The production of allelochemicals is controlled by abiotic and biotic factors.[START_REF] Ishika | Sustainable saline microalgae co-cultivation for biofuel production: a critical review[END_REF] used a co-cultivation technique to culture marine, halotolerant, and halophilic microalgae to increase biodiesel production and nutrients removal from wastewater. Although the reason was not clear, it was referred to as an example of cooperative interactions between different strains of microalgae.

6 .

 6 photoautotrophic mode to heterotrophic mode may be fatal for some facultative strains of microalgae.[START_REF] Azma | Improved protocol for the preparation of Tetraselmis suecica axenic culture and adaptation to heterotrophic cultivation[END_REF] confirmed a long lag phase for Tetraselmis suecica when it was transferred to heterotrophic mode after a photoautotrophic mode of cultivation for cell enrichment. This strain was finally acclimated through a gradual photoperiod adaptation.Hena et al. (2015b) introduced a photoheterotrophic mode of cultivation between photoautotrophic and heterotrophic systems as a novel procedure of adaptation for Chlorella sorokiniana without compromising the extended lag phase of cell adaptation.[START_REF] Yang | An integrated view of the intimate coupling UV irradiation and algal treatment on antibiotic: Compatibility, efficiency and microbic impact assessment[END_REF] investigated the compensatory response of green microalgae Chlorella pyrenoidosa towards the removal of antibiotic ceftazidime after treating it under harsh or limiting conditions. C. pyrenoidosa was cultured under limited light, nitrogen, and phosphorus for 6 h before exposure to ceftazidime; consequently, the removal efficiencies were improved up to 99.15%, 98.57%, and 99.9%, respectively.Hena et al. (2015c) observed that the bio-volume of microalgae in the consortium varied during acclimatization in harsh conditions, while the fittest showed the highest % of biovolume after acclimatization. Hence, the other aspect of acclimatization is to maintain the biovolume of microalgal strains before treatment. Alteration in salinity of culture medium can easily acclimatize microalgal cells through genetic and physiological adaptation.Xiong et al. (2017a) acclimated wild strains of Chlorella vulgaris with several exposures from low to high

1 .

 1 Conventional microalgae culture techniques Phycoremediation is considered a promising substitute for conventional biological treatments to remove micropollutants and nutrients from wastewater, along with several other benefits of biotechnological applications (Ruiz-Marin et al., 2010). Microalgae are considerably flexible in adapting to different techniques of culture. The culture techniques are mainly discussed on the basis of the motility of the cells in culture media and the degree of control of the abiotic factors affecting the removal of PPCPs from wastewater. On the basis of the degree of control of the abiotic factors, the microalgal culture techniques are broadly divided into two categories: closed or open culture techniques. The closed and open culture techniques are further divided into two subcategories: suspended or immobilized culture technique depending upon the motility of the cells in culture media (Fig. 4).

  Suspended culture technique is more popular than immobilized culture in WWTPs due to its cost-effectiveness. Nevertheless, harvesting biomass of microalgae at the end of the process is one of the main limitations for the development of this type of wastewater treatment system worldwide. Suspended microalgae culture is being used in closed and open bioreactors for different operational modes (batch, semi-batch, and continuous operation modes). Ciprofloxacin (Hom-Diaz et al., 2017a) and tetracycline (de Godos et al., 2012) were efficiently removed from HRAP by suspended algal cells. Escapa et al. (2017) cultivated suspended cells of Chlorella sorokiniana for removal of paracetamol and salicylic acid in batch and semi-continuous mode of operations, where semi-continuous showed better removal efficiency for both pharmaceuticals.

  Immobilized cultures are capable of accomplishing two steps in a single stage: bioprocessing and harvesting.[START_REF] Gonçalves | A review on the use of microalgal consortia for wastewater treatment[END_REF] suggested a better microalgal growth in biofilm than suspended culture.[START_REF] Xie | Biological sulfamethoxazole degradation along with anaerobically digested centrate treatment by immobilized microalgalbacterial consortium: Performance mechanism and shifts in bacterial and microalgal communities[END_REF] found that immobilized cell culture techniques are more appropriate for treating antibiotic-containing water than suspended cell culture technique because the former technique can inhibit the toxic effect of antibiotics on microalgae.[START_REF] Sole | Removal of endocrine disrupting compounds from wastewater by microalgae coimmobilized in alginate beads[END_REF] suggested a light-blocking effect of free microalgae in the suspended cell culture technique, which consequently prevents the photo-biodegradation mechanism of PPCPs.

Figure 1 :

 1 Figure 1: Mechanisms involved in the removal of PPCPs by microalgae

Figure 2 :Figure 3 :

 23 Figure 2: Mechanism of microalgal photo-degradation of pollutants in water

Figure 4 :Figure 5 :Figure 6 :

 456 Figure 4: Microalgae culture techniques and examples

Figure 7 :Figure 8 :

 78 Figure 7: Hybrid microalgae-activated sludge system

  Successful cultures of microalgae in varieties of wastewaters, including industrial, domestic, and

farming, all containing a considerable load of PPCPs, is clear evidence of algal tolerance towards a range of pharmaceuticals. However, all microalgae cannot remove every kind of PPCPs from wastewater. The selection of microalgal strain is an important step of wastewater treatment, depending upon the type of PPCPs present. The removal of some PPCPs by microalgae is compound-specific, while others are species-specific. Most of the antibiotics are compoundspecific; thus, showing a constant removal regardless of any microalgae used for phycoremediation. For instance, 7-amino cephalosporanic acid can be completely removed by different microalgae, such as Chlorella sp., Chlamydomonas sp., and Mychonastes sp.

Table 1 :

 1 Physicochemical properties of PPCPs

	PPCPs	Log Kow	pKa	Kd	Reference
	17α-Ethynylestradiol 3.67	10.4	13820	Xue et al., 2010
	17β-Estradiol	4.01	10.4	24350	Xue et al., 2010
	Acetaminophen	0.27	9.4	6.7±6.2	Radjenović et al.,
	Atenolol	0.1-0.57	9.4	95±60	Radjenović et al.,
	Carbamazepine	2.25	7	314±205	Radjenović et al.,
	Clofibric acid	2.57	3.18	4.8±2.5	Ternes et al., 2004
	Diazepam	2.83	3.3	44±26	Ternes et al., 2004
	Diclofenac	4.02	4.51	459±32	Ternes et al., 2004
	Erythromycin	2.48	8.88	309±271	Radjenović et al.,
	Estrone	2.25	10.4	20240	Xue et al., 2010
	Galaxolide	5.9	-	4920±2080	Ternes et al., 2004
	Gemfibrozil	4.77	4.75	23±23	Radjenović et al.,
	Ibuprofen	3.79	4.91	9.5±3.1	Radjenović et al.,
	Ketoprofen	3	4.45	226±180	Radjenović et al.,
	Naproxen	3.1	4.15	35.5±18.4	Fernandez-Fontaina et al., 2013
	Sulfamethoxazole	0.48	5.5	3.2±4.5	Radjenović et al.,
	Tonalide	5.7	-	5300±1900	Ternes et al., 2004

Table 2 :

 2 Microalgal removal of PPCPs from different matrixes of water and mechanisms involved

	Contaminant	Microalgae	Water matrix and technique	Mechanisms involved Removal	Reference
	(PPCPs)		specifications		(%)
	Metronidazole	C. vulgaris	Culture medium (BG 11) was	Adsorption	100	Hena et
			spiked with 5µM metronidazole for			al., 2020
			18 days
	Florfenicol	Chlorella sp.	Culture medium (BG 11) was	Bioaccumulation,	97	Song et
			spiked with 46 mg/L of Florfenicol	Biodegradation and		al., 2019
				adsorption
	Ciprofloxacin	Algae-bacterial	Wastewater spiked with 2 mg/L of	Photodegradation and	97.46	Hom-Diaz
		consortium	ciprofloxacin. Experiments were	sorption		et al.,
			conducted for 7 days at HRAP with			2017a
			a suspended culture of an algae-
			bacterial consortium
	Acetaminophen	Microalgae	Experiments were conducted with	Biodegradation,	>99	Hom-Diaz
	Ibuprofen		toilet wastewater for total 20 days.	photo-biodegradation	>98	et al.,
	Naproxen				69	2017b
	Salicylic acid				72
	Enrofloxacin	Scenedesmus	Experiments were conducted with	Bioaccumulation,	23	Xiong et
		obliquus	five pure strains and consortium of	bioadsorption,		al., 2017b
		Chlamydomon	them separately in BBM culture	and/or biodegradation	25
		as mexicana	medium spiked with 1mg/L
		Chlorella	enrofloxacin for 11 days incubation,		26
		vulgaris	while the consortium was prepared
		Ourococcus	with an equal amount of all		18
		multisporus	microalgae with final OD680 of ~1.0
		Micractinium			20
		resseri
		Microalgal			26
		consortium
	Tetracycline	Microalgae-	Artificial wastewater was treated	Indirect	99	Norvill et
		bacterial	with a microalgae-bacterial	photodegradation and		al., 2017
		consortium	consortium in HRAP for 7 days	sorption
	Methyl parathion Fischerella sp. Culture medium (BG 11) was	Sorption and	>80	Tiwari et
			spiked with 20 mg/L methyl	biodegradation		al., 2017
			parathion for 4 days
	Trimethoprim	Nannochloris	F/2 algal culture media was spiked	Non-degradable	0	Bai and
	Sulfamethoxazole	sp.	with target compound and treated	Photodegradation	32	Acharya
	Triclosan		with Nannochloris sp. for 14 days.	Sorption and	100	2017
				photodegradation
	Diclofenac	Chlorella	Anaerobically treated wastewater	Biodegradation,	45	De Wilt et
	Ibuprofen	sorokiniana	spiked with	photolysis, and	99	al., 2016
	Paracetamol			sorption	99
	Metoprolol				38
	Carbamazepine				30
	Trimethoprim				55
	7-amino	Chlorella sp.	The experiments were conducted in	Hydrolysis,	76.2	Guo et al.,
	cephalosporanic	Chlamydomon	a synthetic medium; BM, BBM, and	photolysis, and	69.3	2016
	acid	as sp.	BG-11, respectively, for 24h spiked	biosorption
		Mychonastes	with 100mg/L 7-amino		64.9
		sp.	cephalosporanic acid.
	Cefradine	Chlorella	Water was spiked with 100 mg/L of	Sorption, hydrolysis,	89.9	Guo and
	Cefalexin	pyrenoidosa	antibiotics in separate flasks and	and photodegradation	94.9	Chen 2015
	Ceftazidime	and activated	incubated with microalgae later		89.7
	Cefixime	sludge	combined with activated sludge		100
			from wastewater treatment plant.
			Total time span for experiments

Table 3 :

 3 Comparison between microalgal strains for removing PPCPs

	PPCPs	Microalgae	Specific conditions	Removal	References
				(%)
	Enrofloxacin	Scenedesmus obliquus Cultures were developed in sterilized	23	Xiong et al.,
		Chlamydomonas	Bold's Basal Medium for 11 days	25	2017b
		mexicana	under 40-50 µmol photon/m 2 .s light
		Chlorella vulgaris	intensity, at 27°C temperature,	26
		Ourococcus	shaking with 150 rpm and light/dark	18
		multisporus	cycle of 16:8h. The initial
		Micractinium resseri	concentration of enrofloxacin was 1	20
		Consortium	mg/L. The consortium was prepared with an equal amount of all	26
			microalgae with final OD680 of ~1.0
	Naproxen	Cymbella sp.	The freshwater algae Cymbella sp.	97.1	Ding et al., 2017
		Scenedesmus	and Scenedesmus quadricauda were	58.8
		quadricauda	cultured in sterile D1 and BG11
			medium and spiked with 1mg/L
			naproxen. The inoculum size was
			maintained with 1 × 10 5 cells mL.
			The culture was maintained for 30-
			day incubation.
	Salicylic acid	Chlorella vulgaris	Microalgal strains were cultured in	>25	Escapa et al.,
		Tetradesmus obliquus	Mann and Mayers medium with an	>93	2017
	Paracetamol	Chlorella vulgaris	initial inoculum size of about 3 × 10 6	>21
		Tetradesmus obliquus	cells mL, for 24h under 175 µmol photon/m 2 .s light intensity, at 25°C	>40
			temperature, with 7% CO2 and 250
			rpm with 12:12 photoperiods. The
			initial concentration of each targeted
			compound was 25 mg/L.
	7-amino	Chlorella sp.	Chlorella sp., Chlamydomonas sp.,	76.2	Guo et al., 2016
	cephalosporanic	Chlamydomonas sp.	and Mychonastes sp., were	69.3
	acid	Mychonastes sp.	developed in a synthetic medium;	64.9
			BM, BBM and BG-11 respectively
			for 24h under 200µmol photon/m 2 .s
			light intensity, at 26°C temperature,
			with 2.5% CO2 aeration. The initial
			OD680 of inoculum was 0.1, and the
			initial concentration of 7-amino
			cephalosporanic acid was 100 mg/L.
	Carbamazepine	Chlamydomonas	The microalgae were cultured in	37	Xiong et al.,
		mexicana	Bold Basal Media in two different		2016
		Scenedesmus obliquus	flasks at 27°C and 150 rpm, under	30
			45-50 mmol photon/m 2 s for 10 days.
			The initial OD680 of inoculum was 1,
			and the initial concentration of
			carbamazepine was 1mg/L.
	Diclofenac	Chlorella sorokiniana	The cultures were cultivated in	70.01	Escapa et al.,
		Chlorella vulgaris	synthetic media at 25°C, under 175	78.42	2016
		Scenedesmus	µmol photon/m 2 s, shaking with 250	20.9
		obliquus	rpm and light/dark cycle of 12:12h.
			The inoculum was maintained as 3 ×
			10 6 cells/mL, and the initial
			concentration of diclofenac was 25
			mg/L.
	Nonylphenol	Chlorella vulgaris	The cultures were cultivated in	99	Gao et al., 2011
		Chlorella sp. (1uoai)	Bristol medium at 25°C, under 40	92
		Chlorella sp. (2f5aia)	µmol photon/m 2 s, shaking with 160	94

Table 4 :

 4 PPCPs removal by bacterial strains and mechanisms involved

	Bacterial strain	PPCPs	Mechanisms involved	Reference
	Alcaligenes xylosoxidans	Triclosan	Metabolic degradation	Meade et al. (2001)
	subsp. denitrificans TR1
	Pseudomonas putida	Triclosan	Metabolic degradation	Meade et al. (2001)
	TriRY
	Pseudomonas aeruginosa	17b-estradiol	Metabolic degradation	Zeng et al. (2009)
	TJ1
	Delftia tsuruhatensis	Acetaminophen	Metabolic degradation	De Gusseme et al. (2011)
	Pseudomonas aeruginosa Acetaminophen	Metabolic degradation	De Gusseme et al. (2011)
	Stenotrophomonas sp. f1	Acetaminophen	Metabolic degradation	Zhang et al. (2013)
	Pseudomonas sp. f2 and	Acetaminophen	Metabolic degradation	Zhang et al. (2013)
	fg-2
	Pseudomonas aeruginosa	Acetaminophen	Metabolic degradation	Hu et al. (2013)
	HJ1012
	Pseudomonas sp. CBZ-4	Carbamazepine	Metabolic degradation	Li et al. (2013)
	Ammonia-oxidizing	Atnolol	Co-metabolic degradation Sathyamoorthy et al.,
	bacteria			2013
	Stenotrophomonas	Naproxen	Metabolic degradation	Wojcieszyn´ska et al.
	maltophilia KB2			(2014)
	Pseudomonas sp. I-24	Iopromide	Co-metabolic degradation Xu et al. (2014)
	Pseudomonas	Sulfamethoxazole	Metabolic degradation	Jiang et al. (2014)
	psychrophila
	HA-4
	Variovorax Ibu-1	Ibuprofen	Metabolic degradation	Murdoch and Hay (2015)
	Bacillus thuringiensis B1 Ibuprofen	Co-metabolic degradation Marchlewicz et al. (2016)
	Bacillus thuringiensis B1 Naproxen	Co-metabolic degradation Marchlewicz et al. (2016)
	Nitrososphaera gargensis Mianserin	Co-metabolic degradation Men et al., 2016
	Nitrososphaera gargensis Ranitidine	Co-metabolic degradation Men et al., 2016
	Ochrobactrum sp. MC22 Triclocarban	Metabolic degradation	Sipahutar and Vangnai
				(2017)
	Ammonia-oxidizing	Acyclovir	Co-metabolic and	Xu et al., 2017
	bacteria		Metabolic degradation
	Ammonia-oxidizing	Atenolol	Co-metabolic degradation Xu et al., 2018
	bacteria
	Ammonia-oxidizing	Sulfadiazine	Co-metabolic degradation Wang et al., 2019
	bacteria
	Ammonia-oxidizing	Tetracycline	Bioadsorption on	Lengbehn et al., 2020
	bacteria		bacterial EPS
	Ammonia-oxidizing	Sulfadiazine	Co-metabolic degradation Wang et al., 2020
	bacteria
	Ammonia-oxidizing	Cephalexin	Co-metabolic degradation Wang et al., 2020
	bacteria

Table 5 :

 5 Comparison between microalgal and microalgae-bacterial consortiums

		Microalgal consortium	Microalgae-bacterial consortium
	Interactions	Cooperative and competitive interactions	Cooperative and competitive interactions
		occur between different strains of	occur between different strains of
		microalgae	microalgae as well as bacteria
	Robustness	It shows more resistance to environmental	It shows more resistance to
		fluctuations and invasion of foreign	environmental fluctuations and invasion
		species than monoculture but less than	of foreign species than microalgal
		microalgae-bacterial consortiums.	consortiums.
	Specificity	It shows a broader spectrum of	It shows better and broader removal
		contaminants removal efficiency than	efficiency of PPCPs than microalgae
		mono-algal culture, but less efficient than	consortium
		microalgae-bacterial consortiums.
	System substitution	It can substitute the tertiary treatment step	It can substitute secondary and tertiary
		of a wastewater treatment plant	steps of a wastewater treatment plant
	Space requirement	It is generally less compact than	Microalgae-bacterial consortiums are
		microalgae-bacterial consortiums since it	more compact than conventional mono-
		substitutes tertiary step of wastewater	algal and microalgal consortium
		treatment plant with microalgae mediated	mediated wastewater treatment plant
		reactor.	since it replaces two steps by one
			component.
	Cost	Although it can mitigate the atmospheric
		CO2, aeration needs an extra cost.

Table 6 :

 6 Toxicity of PPCPs on microalgae

	PPCPs	Microalgae	EC50	Observation	Reference
	Sulfadiazine	Selenastrum	2.19 mg /L	-	Eguchi et
		capricornutum			al., 2004
	Chloramphenicol	Isochrysis galbana	-	Isochrysis galbana was not	Campa-
	Erythromycin	Isochrysis galbana	-	remarkably affected by	Córdova et
	Furazolidone	Isochrysis galbana	<0.5 mg/L	erythromycin and	al., 2006
				chloramphenicol up to 12 mg/L;
				however, furazolidone inhibited
				the growth even at 0.5 mg/L
	Chloramphenicol	Chaetoceros	>3 mg/L	Chaetoceros gracilis was found
		gracilis		more sensitive towards
	Erythromycin	Chaetoceros	>0.5 mg/L	chloramphenicol and
		gracilis		erythromycin than Isochrysis
	Furazolidone	Chaetoceros	<0.5 mg/L	galbana.
		gracilis
	Ibuprofen	Navicula sp.	>10 mg/L	0.1-1 mg/L cause an increase in	Ding et al.,
				growth rate, chlorophyll a, and	2017
				carotenoids; however, as high
				dose as 10 mg/L ibuprofen causes
				a decrease in growth, chlorophyll,
				and carotenoids.
	Ibuprofen	Chlorella vulgaris	82.25 mg/L	EC50value was tested for 48 h;	Geiger et
				however, the EC50 value	al., 2016
				increased with an increase in
				exposure time.
	Carbamazepine	Dunaliella	53.2 mg/L	EC50value was observed for 24 h.	Tsiaka et
		tertiolecta		Concentrations of more than 50	al., 2013
				mg/L caused a significant increase
				of MDA content after exposure of
				24 h. 10 mg/L and more
				carbamazepine after 24 h of
				exposure caused increased in
				carotenoids through a decrease in
				chlorophyll a.
	Sulfadiazine	Scenedesmus	81.6 mg/L	At EC50 concentration SOD,	Wang et
		obliquus		CAT and MDA increased by 1.2,	al., 2017
				3.8, and 5 fold respectively to its
				control culture.
	Sulfamethoxazole	Isochrysis galbana,		Sulfamethoxazole was noticed	Teixeira
	Sulfamethoxazole	Chaetoceros		more toxic than trimethoprim for	and Granek
		neogracile		all three marine algae, while	(2017)
	Sulfamethoxazole	Nannochloropsis		Nannochloropsis oculata was the
		oculata		most sensitive species of three.
	Trimethoprim	Isochrysis galbana,		Photo-degradation products of
	Trimethoprim	Chaetoceros		sulfamethoxazole are more toxic
		neogracile		and less biodegradable in
	Trimethoprim	Nannochloropsis oculata		seawater than the products of trimethoprim.
	Carbamazepine	Trebouxia TR9		Trebouxia TR9 and Asterochloris	Domínguez
	Carbamazepine	Asterochloris erici		erici were isolated from lichen.	-Morueco
	Diclofenac	Trebouxia TR9		The toxicity of tested analgesics	et al.,
	Diclofenac	Asterochloris erici		was analyzed by cell aggregation	(2014)
				and optical dispersion. Trebouxia
				TR9 shows large aggregation,
				while Asterochloris erici
				exhibited either no or two cell

Table 7 :

 7 Advantages and limitations of techniques used for suspended and immobilized microalgal mediated wastewater treatment

		Suspended culture technique		Immobilised culture technique
		Advantages	Limitations		Advantages	Limitations
	High rate	* It costs less in	* Light can only be	Algal turf	* It costs less in	* Only surface
	algal pond	installation,	utilized from top of	scrubber	installation, scaling-	cells can utilize
		scaling-up, as	the surface; to		up, as well as	light efficiently,
		well as	increase the light		maintenance with	and those which
		maintenance	utilization efficiency		simple operational	are embedded
		with simple	pond should be		steps.	inside, start losing
		operational	shallow and large in			the capability to
		steps.	area, which uses		* High evaporation	take part in
			large land area and		favors the	indirect light
		* Volatilization	paddlewheel to		volatilization	photodegradation.
		mechanism of	circulate the culture.		mechanism of
		PPCPs removal			PPCPs removal.	* To increase the
		is high.	* Since it is an open			efficiency of the
			system, it has a high		* Harvesting is easy.	technique, the turf
			risk of contamination,			is supposed to be
			high evaporation, and			thin and large in
			culture parameter			area, which
			cannot be controlled.			consequently
						covers a large area
			* Harvesting is the			of land.
			most energy-
			consuming step.
	Stirred-	* Optimization	* It costs more in	Rotating	* It requires small	* It consumes high
	tank Algal	and	installation, scaling-	algae	space and can work	energy in rotating
	Photo-	maintenance of	up, and maintenance.	biofilm	with a deep tank	disk.
	bioreactor	culture		photo-	since every
		parameter is	* Operation is	reactor	rotational disk gets a	* Poor in
		easy.	complicated.		chance to be	controlling
					exposed in light	species; since the
		* Since	* Poor light		periodically.	system is open
		contamination	utilization efficiency.			hence
		is low so			* Harvesting is easy.	contamination is
		biomass	* Harvesting is not			high.
		productivity is	cost-effective.		* It has high gas
		high.			transfer efficiency.	* Gradual increase
						in biomass causes
		* It requires			* Scaling-up and	complexity in the
		less space than			maintenance are	rotational pattern,
		high rate algal			easy.	which needs to
		pond.				supervise
						constantly.
	Plane	* The closed	* Expensive	Plane	* It possesses high	* Expensive
	algal	system of plane	installation	algal-	light utilization	installation.
	Photo-	algal photo-		biofilm	efficiency.
	bioreactor	bioreactor	* Required	photo-		* Required
		facilitates high	optimization of	reactor	* It shows better	optimization of
		light utilization	parameters during		control of culture	parameters during
		efficiency,	scaling-up causes		conditions.	scaling-up causes
			complication in			complication in
		* Better control	operation and makes		* Low risk of	operation and
		on culture	it unsuitable for		contamination.	makes it
		conditions,	wastewater treatment.			unsuitable for
					* High biomass	wastewater
		* Low risk of	* Harvesting is not		production.	treatment.