Membrane bioreactor and integrated membrane bioreactor systems for micropollutant removal from wastewater: A review

Journal of Water Process Engineering 26 (2018) 314–328

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Membrane bioreactor and integrated membrane bioreactor systems for micropollutant removal from wastewater: A review

T

Lalit Goswamia, R. Vinoth Kumarb, Siddhartha Narayan Boraha, N. Arul Manikandanc, ⁎ ⁎⁎ Kannan Pakshirajana,d, , G. Pugazhenthia,c, a

Center for the Environment, Indian Institute Technology Guwahati, Guwahati, Assam 781039, India Department of Chemical Engineering, National Institute of Technology Andhra Pradesh, Tadepalligudem, Andhra Pradesh 534101, India c Department of Chemical Engineering, Indian Institute Technology Guwahati, Guwahati, Assam 781039, India d Department of Biosciences and Bioengineering, Indian Institute Technology Guwahati, Guwahati, Assam 781039, India b

ARTICLE INFO

ABSTRACT

Keywords: Membrane bioreactor Wastewater treatment Micropollutant removal Hybrid MBR systems

Over the last few decades, the presence of micropollutants in the wastewater stream is imposing a serious concern worldwide. Membrane bioreactor (MBR) technology has been widely employed for the treatment of wastewater contaminated with micropollutants. Hence, the aim of this paper is to provide a consolidate review on the present state of research and advancement in MBR technology along with its augmentation with other wastewater treatment technologies for micropollutant removal, sustainability, and operating cost assessment for its feasibility. The potential application of MBR and hybrid-MBR systems in micropollutant removal along with the fate, removal mechanism, various factors affecting the MBR system efficiency and the operational cost involved were also been discussed in detail. Further, based on the available technologies on MBR, the authors discussed the future research perspective for micropollutant removal from the effluent stream with an objective of the circular economy.

1. Introduction For the past few decades, the available water sources are getting depleted worldwide due to numerous factors including, agricultural and industrial activities, urbanization, climate change, population growth and incessant consumption of water. These have resulted in the need to explore new sources of water worldwide [1]. Agricultural and industrial activities and domestic usage of fresh water generate wastewater containing micropollutants when it is discharged into the aquatic environment without proper treatment, leading to serious environmental and human health concerns [2,3]. Micropollutants are a vast group of chemical substances found in the environment at very low concentrations (ngL−1- μgL−1). These substances include synthetic organic compounds, namely pesticides, personal care/cosmetic products, industrial chemicals, food additives and detergents, and some naturally occurring substances such as estrogens. They create detrimental effects on ecosystems and pollute drinking water resources even when present at trace concentrations [2]. Over 100,000 chemicals have been labeled as micropollutants by the European Union (EU), and out of which 30,000–70,000 chemicals

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are used daily for various activities [3]. Although, some regions/ countries including the EU and Canada, have implemented some regulations for the discharge of water/wastewater containing micropollutants into the environment [4–6], there are no strict standards set until now for the discharge of these pollutants. The EU has recently listed 45 priority substances, enlisting polycyclic aromatic hydrocarbons (PAHs), phthalates, pesticides, metals and endocrine disruptors, that act as a threat to the environment ‒ Directive 2013/39/EU [4,7]. In addition, 17 organic compounds of emerging concern (CEC), out of which five are pharmaceuticals, are enlisted in the Watch List of substances for wide monitoring by the EU‒Decision 2015/495/EU [5]. Pharmaceuticals and personal care products (PPCPs), and steroid hormones are not yet enlisted in the general pollutants. Micropollutants are known to be toxic and bioaccumulate due to their persistent nature. However, it is important to study the biological effects of these compounds in detail to set regulatory limits for such compounds. Furthermore, environmental protection organizations and scientific research communities must take into account the synergistic or antagonistic effects of these pollutants for understanding their actual impact on human health and environment. Moreover, suitable wastewater

Corresponding author at: Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India. Corresponding author at: Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India. E-mail addresses: [email protected] (K. Pakshirajan), [email protected] (G. Pugazhenthi).

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https://doi.org/10.1016/j.jwpe.2018.10.024 Received 3 September 2018; Received in revised form 29 October 2018; Accepted 31 October 2018 2214-7144/ © 2018 Elsevier Ltd. All rights reserved.

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treatment and recycling by appropriate methods can limit their release into the environment. In this context, the conventional activated sludge processes (CASP) employed at wastewater treatment plants (WWTPs) are inefficient in completely eliminating the micropollutants because these are mainly designed to eliminate simple organic matter and nutrients from the wastewater [8]. Further, WWTPs are not well equipped to monitor the levels of micropollutants in the source wastewater [9]. Hence, there is a need to re-design the CASP system in WWTPs for a better handling and control of the micropollutants. Some modifications such as change in the operating conditions, e.g. hydraulic retention time supplementation with inorganic nutrients, vigorous mixing, aeration, addition of surfactants and bioaugumentation have been suggested towards this objective [10], but the outcomes are not that satisfactory for the removal of micropollutants [11]. Post-treatment techniques, mainly physicochemical techniques such as adsorption on activated carbon, nanofiltration (NF), advanced oxidation processes (AOPs) based techniques including photo-catalytic degradation, photo-oxidation and ozonation have also been tested for micropollutant removal [12]. These approaches are unattractive owing to one or other limitations like, high energy demand, high investment cost, use of harsh environmental conditions, secondary sludge disposal problem, requirement of toxic chemicals, etc. Further, micropollutants are present in the various aquatic environments including wastewater, surface water, groundwater and drinking water [13]. Also, based on the nature of micropollutants, conventional vs modern treatment techniques employed in MBR technology and recent developments in MBR and MBR-integrated systems are elaborated. Finally, typical case studies pertaining to MBR for micropollutants removal and cost analysis for operating MBR are also presented.

compounds (EDCs) [7,9]. An incessant discharge of EDCs in the environment leads to reproductive and developmental abnormalities of highly sensitive species even at a low concentration. In addition, augmentation of antibiotic resistant organisms in the environment proves to be an additional risk. The ever increasing and indiscriminant use of antibiotics to improve the health of humans and animals have led to the evolution of antibiotic resistant genes at several environmental matrices. The discharge of micropollutants in the ecosystem is expected to further increase in the future due to population growth and strong reliance on pharmaceutics.

2. Occurrence of micropollutants and their impact on the environment

3.1.1. Coagulation-flocculation In this method, colloidal particles, often called as coagulants are utilized to remove the micropollutants by adsorption which ultimately settles down as flocs. In general, coagulation-flocculation removes suspended solids and organic matter present in wastewater by the addition of metals salts, or metal hydroxides (iron or aluminum based salts). Micropollutants are adsorbed onto metal hydroxide or onto the flocs produced during this process and collected in the form of sludge for further disposal or treatment [18]. The physicochemical properties of a micropollutant determine its adsorption onto metal hydroxide or onto the flocs. In general, hydrophobic pollutants are better treated by the coagulation–flocculation technique. The sorption of hydrophobic micropollutants was predicted by its octanol-water partition coefficient (Kow). Many micropollutants such as musks, polycyclic aromatic compounds and humic acid can be satisfactorily removed by this process. The main disadvantage of this process is that micropollutants with a low sorption potential for coagulants/colloids formed during this process are not removed well [18,19].

3. Micropollutant removal from wastewater Various physico-chemical and biological treatment technologies were investigated for the removal of micropollutant from the water. Several physico-chemical methods viz. coagulation-flocculation, adsorption and advanced oxidation process are reported to perform sufficiently well in treating micropollutants. However, chemical treatment methods are employed mostly where the biological treatment techniques fail. This is because; the biological treatment methods like activated sludge, constructed wetland, membrane bioreactor are cost effective and environmentally benign treatment technologies for the micropollutant removal from wastewater. In recent times, some hybrid techniques combining the biological and physico-chemical methods came into existence and have opened new ways for treating the wastewater containing micropollutant in an exceptionally great way. Therefore, the present section gives an overview of the aforementioned technologies for the treatment of wastewater containing micropollutants. 3.1. Physico-chemical treatment

2.1. Occurrence of micropollutants in wastewater and surface water Table 1 and Fig. 1 present the list of regularly detected micropollutants along with their average concentration in wastewater and surface water. The concentration of these compounds in wastewater may fluctuate because of different factors, including production rate, sales and usage of products, per capita water consumption/day and excretion rate, climatic conditions [14]. The manufacturing and utilization of products consisting of micropollutants also decide the quantity of micropollutants that reaches the WWTPs [15]. 2.2. Occurrence of micropollutants in groundwater and drinking water Contamination of groundwater with micropollutants is mostly caused by sewer systems, seepage of septic tanks, interaction between ground and surface water through soil, landfill leachate and permeation of contaminated water from agricultural lands. Micropollutants levels are found to be low in ground water as compared to that of surface water [16]. Most commonly found micropollutants in the ground water are triclosan, sulfamethoxazole, carbamazepine and non-steroidal antiinflammatory drugs. These compounds are also prevalent in surface water and wastewater, which confirms an association of the occurrence of micropollutants in various water bodies [17].

3.1.2. Pollutant removal by activated carbon adsorption Pollutant removal by activated carbons (ACs) takes place through the process of adsorption, wherein the pollutant gets diffused onto the surface and thereafter onto the pores of the activated carbon. Since the pollutants are removed by diffusion process, hydrophobic pollutants such as toluene and chlorinated solvents show better removal efficiency than the hydrophilic or highly water-soluble pollutants [20]. Various pollutants that are adsorbed by ACs include xenobiotic compounds, pharmaceutically active compounds, endocrine disrupting compounds (EDCs) and other refractory organics. Pollutant removal by ACs depends on various factors such as particle size, pH of the pollutant containing wastewater, concentration and molecular weight of the pollutant, adsorbent dose and presence of other co-contaminants in the wastewater. For instance, Vyrides et al. [20] reported that ACs with an average size less than 0.25 mm adsorbed 98% of micropollutant,

2.3. Impact of micropollutants on the environment Most of the micropollutants are considered as very harmful to the ecosystem including aquatic species (causes genotoxicity, estrogenicity and mutagenicity), wild animals and humans due to their bioaccumulative and non-biodegradable nature. For instance, the feminization of male fish was induced by the exposure to endocrine disrupting 315

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Table 1 Micropollutants commonly found in municipal wastewater and surface water. Categories

Micropollutants

Application

Average concentration in surface water (ng/L)

Average concentration in wastewater (ng/L)

Disinfectants, pharmaceuticals (prescriptions, over the counter drugs, veterinary drugs)

Atenolol Azithromycin Bezafibrate Carbamazepine Clarithromycin Carbamazepin‐10, 11–dihydro‐10, 11‐dihydroxy Diatrizoate (amidotrizoic acid) Diclofenac Erythromycin Ethinylestradiol Ibuprofen Iopamidol Iomeprol Iopromide Mefenamic acids Metformin Metoprolol Naproxen Sotalol Sulfamethoxazole Trimethoprim N4‐Acetylsulfame thoxazole Penicillin V Irbesartan Tramadol Risperidone Trihexyphenidyl Venlafaxine Codeine Fluconazole Diphenhydramine Repaglinide Flecainide Bisoprolol Alfuzosin Bupropion Ciprofloxacin Oxazepam Carbamazepine Diclofenac Orphenadrine Sulfamethoxazole (VITO) Haloperidol Sulfamethoxazole(JRC) Citalopram Fexofenadine Diltiazem Fluoxetine Terbutaline Clindamycin Telmisartan Eprosartan Gemfibrozil Zolpidem Hydroxyzine Ketoprofen Ranitidine Triclosan Levamisole Lincomycin Rosuvastatin Mianserin Clofibric acid Iohexol Memantine Sertraline Clonazepam Tiamulin Alprazolam

β‐blocker Antibiotic Lipid‐lowering drug Anticonvulsant Antibiotic Transformation product

205 12 24 13 30 490

843 175 139 482 276 1551

Contrast medium

206

598

Analgesic Antibiotic Synthetic estrogen Analgesic Contrast medium Contrast medium Contrast medium Analgesic Antidiabetic β‐blocker Analgesic β‐blocker Antibiotic Antibiotic Transformation product Personal care product Antihypertensive Analgesic Neuroleptic Antidementia agent Antidepressant Morphine derivate Antifungal medication Antihistamine Antidiabetic medication Antiarrhythmic β‐blocker Alpha‐blockers Antidepressant Antibiotic Anxiolytic Antiepileptic drug Analgesic Antihistamine Antibiotic

65 25 5 35 92 275 96 7 713 20 37 63 26 13 3

647 42 2 394 377 380 876 870 10347 166 462 435 238 100 67

– – – – – – – – – – – – – – – – – 65 – –

28.7 479.5 255.8 6.9 0.2 118.9 70.6 108.2 11.7 3.1 45.5 41.6 2.8 1.0 96.3 161.7 832.3 647 3.9 280.2

Psychiatric medication Antibiotic Antidepressant Antihistamine Antiarrhythmic agent Antidepressant Antiasthmatic Antibiotic Antihypertensive Antihypertensive Lipid‐lowering drug Hypnotic Antihistamine Analgesic Antihistamine Disinfectant Antihelminthic Antibiotic Statin Antidepressant Lipid‐lowering drug Radiocontrast agent Antidementia agent Antidepressant Anticonvulsant Antibiotics Antidepressant

– – – – – – – – – – – – – – – – – – – – – – – – – – –

32.2 142.3 33.8 165.0 10.7 2.1 1.1 70.4 367.5 226.8 137.7 1.5 1.1 86.0 68 74.8 40.6 31.2 31.0 1.5 5.3 158 22.8 2.1 1.6 3.3 1.3

(continued on next page) 316

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Table 1 (continued) Categories

Micropollutants

Application

Average concentration in surface water (ng/L)

Fenofibrate Sulfadiazine Cyproheptadine

Lipid‐lowering drugs Antibiotics Chemotherapeutic agent Tilmicosin Methylbenzotriazole Loperamide Gadolinium Buprenorphine Maprotiline Duloxetine Miconazole Chlorpromazine Flutamide DEET, N, N’‐ diethyltoluamide Caffeine Acesulfame Sucralose Diazinon Diethyltoluamide(DEET) Dimethoate MCPA Carbaryl 2, 4‐D Carbendazim Diuron Glyphosate Isoproturon Irgarol (cybutryne) MCPA Mecoprop‐p Triclosan Terbutylazine Atrazine Terbutylazine‐desethyl Bentazone Metolachlor Dichlorprop Simazine Atrazine‐desethyl Chlortoluron Hexazinone Linuron 2, 4, 5‐T Bisphenol A (BPA) Estradiol Estrone Nonylphenol

– – – Antibiotics Personal care Personal care Personal care Personal care Personal care Personal care Personal care Personal care Personal care Personal care

1.1 3.5 3.9 – – – – – – – – – – –

3.1 2900 29.3 115.0 3.9 0.4 0.1 0.2 0.1 0.1 678.1

– 4010 540 15 135 22 – – 67 16 54 373 315 3 40 45 20 – – – – – – – – – – – – 840 2 2 441

191.1 22500 4600 173 593 – 149.9 1.6 13 81 201 – 12 30 25 424 116 90.6 4.2 68.8 9.6 12.4 9.6 26.3 13.8 3.2 0.8 40.1 0.3 331 3 15 267

Detergents, dishwashing liquids, personal care products (fragrances, cosmetics, sunscreens), and food products

Pesticides

Biocides

Hormone active substances (effect on the hormone balance)

product product product product product product product product product product

Food additive Food additive Food additive Insecticide Insecticide Insecticide Insecticide Insecticide Herbicide Fungicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Microbiocide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Herbicide Additive Natural estrogen Natural estrogen Additive

Average concentration in wastewater (ng/L)

*Data were derived from the references [2,27,28].

whereas those with an average size of 0.75 mm yielded only up to 50% COD removal. The high removal efficiency at small size of ACs is due to the large surface area and low diffusion resistance than those at a large particle size.

presence of hydroxyl scavenging species (carbohydrates, proteins or inorganic ions like sulfide, bicarbonate and carbonate) in wastewater, which leads to scarcity of available free oxidants for pollutant degradation, and ultimately reduces the pollutant removal efficiency.

3.2. Advanced oxidation processes (AOPs)

3.3. Biological treatment systems

Complete mineralization or transformation of pollutants to their less toxic forms can be achieved by chemical oxidation processes. Among the different chemical oxidation processes, AOPs are proven to be the most effective in generating hydroxyl radicals (%OH), thereby causing the degradation of organic pollutants present in wastewater. An important feature of AOPs is its non-selective nature in oxidizing the pollutants; for instance, micropollutants ranging from the most complex form to the simplest single ring organic molecule can be degraded at a very high reaction rate, finally leaving inorganic ions, CO2 and H2O as the final products [19,21]. The main disadvantage of AOPs is the

3.3.1. Treating aerobic, anaerobic and facultative bacteria Micropollutant degradation by bacteria depends on various factors such as contact time, microbial community and bioavailability of the pollutants to hydrocarbonoclastic microorganisms, etc [1,2]. Studies on bacterial degradation of diverse micropollutants, such as bisphenol A, nonylphenol, polycyclic aromatic organics and many other pharmaceutical compounds, have been reported by many researchers [7,22]. The removal efficiency of micropollutants by these bacteria varies within the range of 60–100% depending on the concentration of micropollutants and hydraulic retention time of the wastewater. 317

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Fig. 1. Chemical structures of some commonly studied micropollutants.

Wastewater containing Beta blockers, an endocrine disrupting compound was treated with these aerobic and anaerobic treatments and its removal efficiency was less when compared to the other class of micropollutants [1].

3.3.2. Constructed wetland A feasible biological system for treating micropollutant wastewater is based on constructed wetland, which is designed, engineered and operated under controlled environment so as to mimic the natural 318

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wetlands. In a constructed wetland, the pollutant removal takes place by the combination/integration of three main processes such as biodegradation – a biological process, adsorption – a physicochemical process and oxidation – a chemical process. All the three processes occur amid plants, substrate and soil [23]. In this removal process using constructed wetlands, soil plays a major role by providing proper support material and nutrients for the plant growth; it further acts as a matrix for the microbial growth and metabolism [24].

space requirement, environment friendliness. Therefore, it has been recognized as a key technology for water reuse and recycling in many developed and developing countries [28]. MBR is a hybrid process integrating the membrane technique with biological treatment, which enables CASP to be operated as a single step process by avoiding the need for a secondary clarifier. Fig. 2 compares the conventional wastewater treatment process with that employing MBR technology. Also, Table 2 enlists various advantageous and limitations of MBR system for micropollutant removal. In general, MBR system is categorized into two kinds in accordance with the configuration; (1) submerged membrane bioreactor (SMBR) and (2) side stream membrane bioreactor. Fig. 2 shows a brief schematic of MBR system. Earlier the side stream MBR was developed where the membrane module is placed outside the bioreactor for the recirculation pump. Due to its high energy consumption, in 1980s, submerged-MBR system was further developed where the membrane module was submerged within the bioreactor, thus, permitting the effluent to pass through with sludge retention. In a SMBR, aeration maintains the activated sludge in suspended mode, limiting the membrane fouling. Fig. 3 represents different types of membrane fouling mechanism within a MBR system. For determining the fouling mechanism associated with the microfiltration of the membrane system, Hermia’s fouling models, viz., (a) complete, (b) standard, (c) intermediate pore blocking and (d) cake filtration models are applied [44]. In complete pore blocking, the size of the solute particle is larger than the pore size of the filtration membrane, leading to surface pore blocking and not block inside pores of the membrane. Standard pore blocking takes place because of the nonuniform pore paths whereas the intermediate pore blocking occurs when the solute particle size and membrane pore size are nearly same [45]. Cake filtration is related to the state where particles bigger than the average pore size mount up on the surface of the membrane thus forming a cake. The potential features of MBR plants such as high efficiency of separation (solid-liquid) and high concentration of biomass compared to those with CASP tanks affect the cell metabolism and bacterial environment, which restricts the growth and, thereby the sludge production. MBRs have gained more interest in the last decade mainly due to the significant reduction in the cost of membranes and the energy requirements during its development process [46]. MBR and MBR associated systems (e.g. MBR with reverse osmosis (RO)/microfiltration (MF)/ultrafiltration (UF)) are being developed for the removal of micropollutants from wastewater prior to their discharge into the environment (Fig. 4). In MBR plants, separation of solids (biological sludge) from the treated wastewater can be achieved by membrane filtration using UF/MF with pore sizes in the range 0.01–0.1 μm. Moreover, if permeated water through the MBR system contains some viruses and dissolved organic compounds, these can be eliminated using dense membrane processes called NF/RO before reusing the water [47]. Typically, the integrated processes of MBR technology combined with RO/NF system is employed to obtain water suitable for drinking purposes and irrigation applications with reduced salinity [48]. MBRs are adopted for the treatment of almost all classes of micropollutants such as EDCs, pharmaceutical compounds, pesticides, etc. Removal efficiency of 15 endocrine disrupting compounds, each at a concentration in the range 1–5 μg L−1 by MBR technology varied between 92% and 99% for each of these compounds [25]. Personal care products (PCPs) such as salicylic acid and propyl paraben were completely removed using an MBR system. Removal efficiencies of 99%, 97%, and 70–80% were obtained for triclosan, atenolol, and beta blockers, respectively using this system [49]. However, in case of treating wastewater containing pharmaceutical compounds, a mixed performance is observed by using MBR technology. For example, antibiotics (ofloxacin, sulfamethaxazole, and erythromycin), and analgesics (lorazepam, citalopram, ibuprofen, carbamazepine and primidone)

3.3.3. Membrane bioreactor (MBR) Membrane bioreactors are considered as the most recent technology for the treatment of wastewater containing micropollutants and they can be made compact based on the volume of water that needs to be treated. Furthermore, treatment efficiency using a MBR is higher than the other biological systems due to a very high microbial population in the immediate vicinity of the membrane surface ensuring the complete removal of pollutant before the wastewater gets filtered through the membrane. In addition, due to the membrane sieving effect, pollutants with a molecular weight greater than the molecular weight cut-off of the membranes are retained, thereby, bringing it in contact with the degrading microorganisms inside the MBR for its complete degradation [19]. Details of the different membrane bioreactors, strategies to improve this upcoming technology and operating-cost assessment are provided in a separate section of this paper. 3.3.4. Hybrid reactor system Hybrid reactor systems are very well suited for treating wastewater containing micropollutants of different nature. In this system, biological treatment is followed by chemical processes or vice-versa based on the treatment requirements. Biological system comprising of either aerobic or anaerobic systems are found to be suitable for treating EDCs and pharmaceutical compounds. Activated carbon based adsorption method is effective in removing micropollutants such as pesticides, antibiotics and analgesics. MBR process is efficient to treat wastewater with micropollutants even at an elevated concentration, wherein the other treatment techniques, particularly the constructed wetland is found to be ineffective [19,25]. Thus, a combination of these methods called hybrid treatment systems can be applied for treating wastewater containing micropollutants. For e.g., a hybrid system using MBR followed by oxidation process is beneficial for the complete removal of EDCs and pharmaceutical compounds from wastewater. Similarly, oxidation followed by a biological treatment system is found to be effective yielding nearly 100% removal efficiency in treating wastewater containing Beta blockers and pesticides [19,21,25]. 4. MBRs for micropollutant removal 4.1. Micropollutant removal using MBR system Over the past two decades, membrane bioreactors (MBRs) have been designed and operated for treatment of a variety of pollutants, such as particulates, carbonaceous substances, nutrients and pathogenic microorganisms [26]. Compared with these pollutants, which can be removed easily by conventional methods, the removal of certain other pollutants, particularly the micropollutants (pharmaceuticals, personal care products, steroid hormones, surfactants, industrially generated chemicals, etc.) is often very different. Therefore, examination of the fate and removal of micropollutants during wastewater treatment is very much crucial for any of the treatment process to avoid their discharge into the environment [27]. In this section, characterization and different removal processes utilizing MBRs with a focus on the fate and mechanism of micropollutant removal are discussed. Membrane bioreactor (MBR) systems seem to be promising under this scenario, due to their several advantages, including high performance efficiency compared to conventional activated sludge treatment plant (CASP), less 319

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Fig. 2. Scheme of the (a) conventional wastewater treatment process, (b) Side stream MBR (c) submerged MBR [1].

were removed with an efficiency in the range 75%–95% using this technology [1]. However, in the case of many other pharmaceutical compounds, the removal efficiency is not satisfactory. Overall, the removal efficiency of different micropollutants using MBR system follows the order: endocrine disrupting compounds > beta blockers > pharmaceutical compounds > pesticides [19,25,34]. The MBR processes and integrated MBR systems for the removal of micropollutants have been described in the literature by several authors. The micropollutants removal using MBR and MBR integrated systems is influenced by various factors which have not been elaborated in the available literature. These factors include microbial community structure, its activities, and effect on MBR performance and fouling,

effect of operating conditions on removal efficiency, integrated MBR systems and hybrid processes for micropollutant removal. Therefore, this review aims to summarize and evaluate all those factors affecting micropollutants removal using MBR processes and integrated MBR system. 4.2. Different integrated MBR systems and hybrid processes for micropollutant removal Integrated approaches involving MBRs with some other processes such as advanced oxidation process (AOPs), membrane distillation process, bio-entrapped membrane bioreactor, granular MBRs, etc. are

Table 2 Advantages and limitations of MBR system for wastewater treatment. S. No.

Advantages

References

1. 2. 3. 4. 5. 6. 7. 8. 9.

Micropollutants removal can be achieved up to the discharge limits Low working space is required and lower foot print Utilized as a pre-treatment technique for RO and NF with excellent effluent quality Full retention of bacterial flocs with the membrane Membrane perform the biomass retention Perform at elevated solid retention time (SRT) Faster removal of persistent micropollutants High MLSS (10–15 g L−1) and high SRT depict low sludge yield Low feed to microorganism ratio (F/M) Limitations Membrane fouling No significant removal of micropollutant when activated sludge process and MBR operate at comparable SRT. Enhanced removal efficiency could be achieved with integrated approach Requires high energy input to aerate MLSS and to reduce the membrane fouling Very less removal efficiency is achieved for some recalcitrant micropollutants, e.g., carbamazepine and diclofenac (5–10%)

[29] [29,30] [31,32,33,34] [35,36] [31] [31,34] [37,38] [31] [34,39]

1. 2. 3. 4.

320

[1,40] [38,41] [39] [42,43]

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Fig. 3. Different types of Fouling mechanisms due to (a) complete pore blocking, (b) intermediate blocking, (c) standard blocking, and (d) cake layer formation [45].

Fig. 4. An overview ofMBR and integrated MBR systems.

of recent focus as these combined processes improve permeate quality, mitigates membrane fouling problem and enhance the treatment efficiency. Fig. 5 presents a brief schematic of different types of integrated MBR systems that were already been utilized for wastewater treatment.

integrating it with ozonation completely removed all the intermediate metabolites during the chemical oxidation. In a similar study by Mascolo et al. [52], the degradation of acyclovir from pharmaceutical wastewater was achieved by integrating MBR with ozonation process. The ozonation reactor was kept in the recycling mode of the MBR effluent and showed a removal efficiency of 99% in the final effluent. Qu et al. [53] reported a high quality effluent by integrating thermophilic submerged aerobic membrane bioreactor and electrochemical oxidation technology for treating paper and pulp effluent. Giacobbo et al. [54] evaluated an integrated approach involving MBR-photoelectrooxidation for tannery wastewater treatment, and using this approach, biological oxygen demand (BOD) present in the wastewater was completely removed by the MBR whereas chemical oxygen demand (COD) was removed by photoelectrooxidation process. In another study, Lopez et al. [55] reported the permeate (effluent) from a MBR system as ready-to-use (high quality) water integrating it with solar photo catalysis with MBR system for treating pesticide containing wastewater. Integrating the electrocoagulation technique with MBR system is also shown to be useful for the removal of organic pollutants and inorganics from wastewater stream (Fig. 5). During electrocoagulation, metal ions are generated at the anode and hydrogen gas at the cathode which further assists in easy removal of the flocculated particles by

4.2.1. Advanced oxidation processes (AOPs)-MBR AOPs are generally recognized for their capability to remove various organic pollutants from wastewater by converting it to easily biodegradable intermediates. However, suspended solids present in waste stream acts as a scavenger of hydroxyl radicals is a major problem [50]. In this, MBR plays a vital role in removing the suspended solids from effluents for improving the efficiency of degradation process. The integrated AOP-MBR process totally depends upon the amount of biodegradable organics and recalcitrant pollutants present in the wastewater. If biodegradable organics are less in comparison with recalcitrant pollutants, AOPs are used prior to the biological treatment step and viceversa in case of more biodegradable organics than recalcitrant organics in the wastewater [51]. Laera et al. [51] examined the removal of simulated wastewater containing nalidixic acid (utilized in treating urinary tract infection) by integrating a MBR system with either UV/H2O2 or ozonation process and by including chemical oxidation in the recirculation stream of the MBR. Since, MBR system alone was inefficient to treat the wastewater, 321

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Fig. 5. Schematic diagram of some hybrid MBR systems (a) electrocoagulation augmented an external side stream membrane bioreactor system (b) MDBR with an external side stream membrane (c) Biofilm-Membrane Bioreactor (BF-MBR) and (d) Bio-entrapped-Membrane Bioreactor (BE-MBR) [50].

examined the effect of silver nanoparticles on the performance of an osmotic MBR comprising of cellulose triacetate FO membrane for treatment of synthetic municipal wastewater. The results demonstrated that the inclusion of silver nanoparticle reduced the nitrifying efficiency with the production of extracellular polymeric substances (EPS), which led to foul the membrane [63]. Both the sludge quality and membrane fouling played a vital role on the performance of OMBR system. In addition, low water flux and accumulation of salts are two major disadvantages of the OMBR system [50]. Praveen et al. [64] utilized the forward osmotic hollow fiber MBR (FOHFMBR) with Pseudomonas putida for treating high strength saline phenolic wastewater. In this study, the membrane was successfully regenerated through osmotic backwashing, which reduces the membrane fouling problem. Ding et al. [65] reported an enhancement in the removal of nitrogen and phosphorus from wastewater by using integrated anaerobic membrane bioreactor (AnMBR) with forward osmosis (FO) systems. The permeate flux was declined with an increase in the temperature from 25 °C to 35 °C due to reduced microbial growth. This resulted in accumulation of membrane foulants. Cornelissen et al. [66] integrated RO and FO systems for wastewater treatment, in which the FO was utilized for pumping the wastewater while RO was used for the separation and recycling the draw water. It was proposed that RO membrane is costeffective in comparison with FO membrane although the latter achieved better recovery as it is not limited by osmotic gradient; moreover, FO system requires less energy in comparison with RO. Furthermore, for enhanced treatment efficiency over prolonged treatment time, it is recommended to bioaugment the system with salt-tolerant microbes or follows shorter SRT.

skimming from the surface [56]. Vijaykumar and Balasubramanian [57] used aluminum as anode and stainless steel as cathode in an electrocoagulation setup integrated with MBR for heavy metal removal (chromium, copper and zinc) with a removal efficiency of 90%. The integrated system improved both the permeate flux and the membrane life. Fard et al. [58] integrated electrocoagulation, electrofenton and electron-Fenton processes with MBR system and reported a zero excess sludge generation. The AOPs aided in overcoming the membrane fouling problem, thereby a longer membrane life and its performance were achieved. 4.2.2. Reverse osmosis and forward osmosis membrane system Membrane bioreactor in combination with osmotic techniques is found to be better in producing quality water with low energy consumption [50]. In the conventional MBR system, the removal of some trace recalcitrant organic micropollutants that are hydrophobic by adsorption onto suspended solids in wastewater, which retain the contaminants inside the bioreactor for their biodegradation, is difficult [59]. A study performed by Ogawa et al. [14], reported that no major variation in the permeate water quality obtained from the nanofiltration and RO membranes in MBR for treating municipal wastewater since very less fouling occurred during its operation [50]. De Jager et al. [60] achieved 75% COD removal and 94% turbidity removal (∼12 ADMI (American Dye Manufacturing Index), lower limit of potable water ∼17 ADMI) from textile effluent by employing a pilot-scale dual stage MBR system incorporated with an ultrafiltration membrane. Farias et al. [61] found that an increase in the sludge retention time (SRT) resulted in an enhanced removal of carbohydrates from municipal wastewater, which however, increased the RO membrane fouling. Moreover, improved removal efficiency was achieved by operating forward osmosis (FO) at a low hydraulic pressure [50]. Tan et al. [62]

4.2.3. Granular MBR Aerobic granular sludge systems are currently utilized for treating wastewater with high content of organic [67,68]. These systems are 322

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study, MDBR system with Rubrobacter and Caldalkalibacillus sp. employed was used to treat synthetic wastewater [75]. Thus, MDBR system can be utilized for the efficient removal of persistent organic compounds from high temperature wastewater.

efficient to perform simultaneous nitrification and denitrification inside the granules due to its compact spherical structure [50]. Vijayalayan et al. [69] reported the treatment of synthetic wastewater by combining sequencing batch airlift reactor (SBAR) with membrane airlift bioreactor (MABR). In SBAR, the aerobic granules were cultivated for handling both nitrification (high aeration) and denitrification (low aeration) whereas in MABR, both aerobic and anoxic zones exist. A reduction in the membrane fouling was observed due to the granular sludge cultivated in the integrated system. Zhao et al. [11] reported the treatment of pharmaceutical and personal care products (PPCPs) using an aerobic granular sludge membrane bioreactor. The integrated system provides good removal of COD, ammonium nitrogen and total phosphorus along with rapid but unstable sludge granules. In another report, Li et al. [70] examined Anammox (anaerobic ammonia oxidation) granules formation and evaluated its performance using a submerged anaerobic MBR. Under complete mixing and continuous feeding condition, removal efficiency upto 88% of total nitrogen content was achieved in the system and the granules showed excellent activity, high resistance to shock loadings, rapid growth rate. Besides, the system was found to operate with minimum fouling for wastewater.

4.2.5. Biofilm/ bio-entrapped membrane bioreactor In biofilm-MBR, the addition of biosupport inside the MBR minimizes suspended solid concentration, which in turn reduces the fouling of the membrane. Leyva-Diaz et al. [76] examined the performance of biofilm-MBR containing carriers under two different conditions (anoxic and aerobic) for nitrification and denitrification of wastewater, respectively. In both the cases, the system yielded a better efficiency than MBR without carrier. Subtil et al. [77] reported the enhanced removal efficiency of ammonia and total nitrogen and reduction in the membrane fouling rate using a bio-film-MBR system. Yang et al. [38] compared the performance of biofilm-MBR with that of a conventional MBR and reported an enhanced removal of ammonical nitrogen and total nitrogen from wastewater. Specific oxygen utilization rate (SOUR) in the system showed a greater microbial activity in the biofilm-MBR. Ng et al. [78] compared two different integrated process i.e., bioentrapped membrane reactor (BEMR) and salt marsh sediment membrane bioreactor (MSMBR) for pharmaceutical wastewater treatment. In BEMR, activated sludge entrapped in its bio-carriers was used; however, it could not tolerate the hypersaline conditions, whereas the MSMBR was able to efficiently degrade persistent compounds with reduced membrane fouling. Table 3 represents the different types of MBR and hybrid MBR systems utilized for micropollutant removal.

4.2.4. Membrane distillation bioreactor (MDBR) This type of integrated system uses a combination of thermophilic bioprocess along with membrane distillation process [50]. Both, hydrophobic and microporous membrane produces water vapor across the thermal gradient, generating the quality water. In comparison with the MBR system, MDBR is capable of achieving excellent organic removal efficiency for municipal wastewater recuperation along with reduced sludge generation [71]. Moreover, it is cost-effective, mitigates membrane fouling and achieves high performance [72]. Wijekoon et al. [73] examined the performance of MDBR for the removal of trace organic contaminants (TrOCs), representing pharmaceutical and personal care products, and achieved a very high removal of the pollutants. The removal of TrOCs was attributed to biodegradation, rejection by membrane distillation and adsorption onto the sludge. Also, removal efficiency of total organic carbon and total nitrogen was > 99% and > 96%, respectively. By utilizing only the MBR system, compounds such as carbamazepine and diclofenac could not be removed efficiently. Phattaranawik et al. [74] reported very good removal efficiency of micropollutants from organic wastewater using MDBR along with a reduction in TOC, reduced greenhouse gas emissions, and salt concentration as compared with MBR-RO integrated system. In another

4.3. Micropollutant removal mechanism using MBR system The removal of micropollutants from wastewater by a MBR system is due to the physical retention by membrane, biodegradation, air stripping, sorption, and photo-transformation [79,80]. For apolar pollutants, sorption followed by membrane retention of the solids is the main mechanism involved for its removal whereas for polar pollutants, biodegradation is the prime mechanism and sorption is very limited [1]. When the size of micropollutants is much lower than the pore size of microfiltration membrane, limited area for sorption is available. However, the sorption of micropolluatants occurs due to the formation of an additional secondary layer by the deposition of micropollutants [45,81]. While the highly volatile trace organic from the wastewater is removed by air stripping/volatilization and is considered insignificant

Table 3 Removal efficiency (%) of micropollutants utilizing MBR and integrated-MBR system [1]. S.No.

Micropollutants

1

Removal of analgesics non-steroidal anti-inflammatory and anti-pyretics (NSAID) (%)

2

Removal of anti-epileptics and anti-depressant (%)

3

Removal of hormones and endocrine disrupter compounds (EDCs) (%)

4

Removal of lipid regulator and cholesterol lowering drugs (%)

5

Removal of antibiotics (%)

6

Removal of beta blockers (%)

7

Removal of musk fragrances (%)

323

MBR

MBR-NF

MBR-RO

MBR-PAC

MBR-GAC

Ibuprofen

73.0– 99.8

99.4–99.8

99.4–99.8

–

–

Ketoprofen Naproxen Diclofenac Acetaminophen Carbamazepine Diazepam Estrone 17β-estradiol 17α-ethynylestradiol Bisphenol A Bezafibrate Clofibric acid Gemfibrozil Sulfamethoxazole Erythromycin Atenolol Metoprolol Galaxolide Tonalide

3.7–91.9 40.1–99.3 15.0–87.4 95.1–99.9 – 42 - 51.0 67.0 76.9–99.4 > 99.4 0–93.5 88.2–97.0 88.2–95.8 25.0–71.0 32.5–85 20.0–91.9 25.2–90.4 5–96.9 29.5–58.7 – –

78.0 87.5–97.0 91.0–99.9 81.0–93.0 > 76.0 > 71.0 > 71.0 95.0

90.0 85.0 71.2 – –

88.3–95.9 99.6–99.9 84.8–99.0 > 99 > 76.0 > 71.0 96.0

> > > > – –

99.0 99.0 99.0 99.0

87.3 > 98.0 80.0–99.0 80.0–90.0

> 98.0 > 98.0 > 98.0

92.4 86.7

82.0 > 88.0 > 99.0 – –

– –

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as the majority of the pollutants have a Henry constant < 0.005 [82]. For hydrophobic pollutants, the biosorption occurs on to the activated sludge and for hydrophilic pollutants; it is primarily followed by the biodegradation [83]. Further, the participation of numerous degraded intermediate byproducts/metabolites imposes its negative impact on the removal mechanism by biodegradation and sorption [84].

indomethacin, were reported as a viable option for such kind of cases [90]. According to Kbiol value for musk fragrances, both sorption and biodegradation were responsible for their removal [91]. In addition, the difference in the biodegradation removal rate is totally dependent on the sludge origin, sludge age, wastewater composition, microbial consortium and aeration [92].

4.3.1. Sorption In this phenomenon, the pollutants make association with the solid phases (activated sludge). Few pollutants get sorbed into the sludge while most of them remain as it is. For quantifying the sorption of micropollutants, the solid water distribution coefficient (Kd) plays an vital role and is defined as the pollutant concentration present in the solid to concentration present in the aqueous phase (at equilibrium) [37]. Kd (L kg−1) value at equilibrium can be expressed by the equation (1) [85].

4.3.3. Stripping/volatilization Stripping involves the gaseous pollutant removal from the aerobic WWTPs containing micropollutants. The stripping is mainly dependent on vapor pressure of wastewater (i.e., Henry’s constant (H)) and hydrophobicity [1]. As majority of the micropollutants have their very low value of H/log Kow value (Kow: octanol water partition coefficient), practically the phenomenon is insignificant. Volatilization is significant when H value is high [39,50,93] and the same is calculated by the below expression [94]:

Kd =

(csorbed ) (xss ) (ssoluble )

−1

where Bi,stripped: amount of pollutant ‘i’ removed (μg d ), Ci,air : concentration of pollutant i in gas phase (μg Lair−1), qair : aeration applied per unit of wastewater treated (Lair L−1 wastewater), Q : flow rate treated in the pilot plant (L d−1), H : Henry’s constant and Ci,dissAer: (μg Lair−1/ μg Lwastewater−1). Volatilization is insignificant for pharmaceuticals [82] and estrogens [27] because of low H-value and hydrophobicity; though, celestolide is considerably removed by air stripping [27].

where, Csorbed: sorbed compound (μg L−1), xss: concentration of the suspended solid in wastewater (kg L−1) and sSoluble: concentration of the soluble part of the compound (μg L−1). The two major mechanisms resulting in the sorption of a pollutant into the primary and secondary activated sludge are adsorption (electrostatic interaction of positively charged groups of micropollutants with the negatively charged surface of microbes) [37] and absorption (hydrophobic interactions of pollutants with the lipophilic cell membrane of the microorganisms and the lipid fractions of the sludge) [39]. Absorption is characterized by the octanol water partition coefficient (Kow) value [39]. Micropollutants other than hormones and musk fragrances (e.g., galaxolide and tanolide), have a Kd value less than 500 L kg−1 and hence, sorption is minimum [86]. A low log Kow value also infers about the sorption phenomena, indicating the hydrophobicity of the pollutants and hence, is first sorbed in the activated sludge, followed by biodegradation [87]. Normally the Kd value is a bit higher in the primary sludge in comparison with activated sludge, clearly depicting the inhibition in the primary sludge.

4.4. Factors affecting MBR efficiency for micropollutants removal Various aspects are responsible for the effectiveness of a MBR system for micropollutants removal from wastewater. Biological factors that affect the operation of a MBR are as follows: age of the sludge and its concentration, aerobic, anaerobic, or anoxic environment prevailing in the membrane compartments, wastewater constituents and other physical parameters such as temperature, pH and conductivity. Physicochemical properties of the micropollutants, operating conditions of MBR, characteristics of the wastewater (pH, organic matter concentration and ionic strength), and membrane characteristics are also among the prime factors responsible for affecting the MBR wastewater treatment efficiency.

4.3.2. Biodegradation Biodegradation is a well-known biological phenomenon where microbes are capable of degrading the micropollutants and are dependent on the redox conditions prevailing in the contaminated aqueous system [39,88,89]. It is the most important phenomena for the micropollutant removal in a MBR system and follows the pseudo-first order degradation kinetics (Eq. (2)) [3]:

dC = Kbiol Xss Ssoluble dt

(3)

Bi, stripped = Ci, air qair Q = H Ci, dissAer q Q

(1)

4.4.1. Molecular weight and complexity of micropollutants Physicochemical properties of the micropollutants highly affect the performance of a MBR system [83]. Micropollutants, such as estrone, and 17α-ethynylestradiol were removed upto 80% and 99%, respectively, in a bench top MBR system [83,95,96]. Bo et al. [86] showed a removal efficiency of 8%–38% for diclofenac with SRT in between 20–48 days, whereas clofibric acid was removed considerably after acclimatization of microbes to the activated sludge [86]. With an increase in the SRT, micropollutants, such as ketoprofen and naproxen, possessing double aromatic rings, depicted an enhanced removal using MBR system due to the presence of diverse microbial population, which is acclimatization and able to degrade the aromatic rings. Although it is difficult to correlate the removal efficiency and compound complexity, but usually it can be accomplished that aliphatic monocyclic aromatic compounds having electron donating group, are readily biodegraded whereas polycyclic compounds having electron withdrawing group, are difficult to degrade, in comparison [27,97].

(2) −1

where, C : concentration of the micropollutant (μg L ), Ssoluble : concentration of the soluble part of the micropollutant (μg L−1), Kbiol : pseudo first order reaction rate constant (L gss−1 day−1) and Xss : concentration of the suspended solid (gss L−1) and t : time (day). Since, the aerobic conditions are much favorable to microbes/microbial consortia present inside the MBR system; it is also favorable conditions for biodegradation. The removal efficiency of ibuprofen, acetaminophen, fenoprofen, 17α-ethynylestradiol, fenofibric acid, roxithromycine, and tonalide was reported to be greater than 80% in the MBR sludge [98,90]. A similar negative charge between anti-inflammatory drugs and activated sludge makes the sorption limited and making the biodegradation as the chief removal phenomenon [85]. Furthermore, micropollutants, such as carbamazepine, diazepam, indomethacin, are few lipid regulators and Beta blockers are persistent compounds which showed a very least biodegradation (< 20%) and hence, the washout of such pollutants will also be a menace during continuous MBR operation [43,91]. Henceforth, MBR systems having a high SRT for naproxen, diclofenac, mefenamic acid, gemofibrozil and

4.4.2. Operating conditions Various literatures, reported the effect of different operating conditions that lead to increase/decrease MBR efficiency in relation with the micropollutant removal, effluent quality, membrane fouling, energy utilization etc. These conditions are discussed in detail below. 4.4.2.1. Sludge retention time. Amongst the most significant parameters 324

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for MBR operation, SRT is an important factor during wastewater filtration as it controls membrane fouling. SRT may be defined as the average time period for which the activated solid sludge is retained in the MBR system [39]. In MBR, treatment can be performed with high SRT. Various studies have reported the enhanced micropollutants removal with an increase in the SRT [81,94]. Kimura et al. [98] reported an enhancement in acidic pharmaceutical wastewater treatment by increasing SRT from 15 days to 65 days. A boost in the removal from 83%–99% and 50%–82% for ketoprofen and clofibric acid, respectively, was reported. Similarly, for 17α- ethynylestradiol, 11% increase in the removal efficiency was achieved by increasing SRT more than 20 days [94]. High SRT enhances the removal efficiency of micropollutants, lowers the sludge yield and improves the Mixed Liquor Suspended Solids (MLSS) concentration. Though, for the majority of micropollutants, SRT between 20 and 30 days is sufficient for their removal in MBR system [1]. However, a high energy input is needed for the viscous MLSS in order to decrease the membrane fouling. Furthermore, a high SRT (beyond critical point) leads to severe fouling and ageing of the membrane. Also, for pharmaceutical wastewater, the effect of hydraulic retention time (HRT) was evaluated using a bench top scale MBR system [86].

bezafibrate and naproxen [42]. Insignificant variation in the removal efficiency was observed for both carbamazepine and diclofenac at room temperature [104]. The removal of most hydrophobic micropollutants in a MBR system (log Dow > 3.2) was stable at pH 8 with working temperature of 20 °C to 35 °C. While raising the temperature upto 45 °C, a clear decline in the removal rate was observed. This decline in the removal efficiency can be attributed to the decrease in the metabolic activity and sludge disintegration. In addition, an enhancement in the soluble microbial products (SMP) was noticed with increasing the temperature from 20 °C to 45 °C, which further leads to membrane fouling [33]. In some cases, the temperature in the range of 18–23 °C had an encouraging outcome for antibiotics (sulfamethoxazole and erythromycin, atenolol, ibuprofen, and sulfamethoxazole) removal [94,105]. 4.5. Operating cost of MBR system Despite the fact that MBR system involves the energy operation, yet not any substantial study was involved for its energy consumption [37], although cost of membrane decreased considerably [106]. Globally, the market for MBR system valued around $ 425.7 million (2014) and is expected to touch $ 777.7 million (2019), with an annual growth rate of 12.8% [1]. Usually the MBR system depends on different factors, but its operating cost is usually higher than the conventional activated sludge process because much energy is required for the aeration (around 60–70% of the overall price) to reduce the membrane fouling [30,107]. The total energy required for 1 m3 of the treated water from MBR system is around 1 kW h [82] whereas for a RO system, the total energy needed is 3–4 kW h [108]. This cost is low in case of CASP [109]. In An-MBR system, operating cost may absolutely be compensated by biogas recuperation [110]. For operating a plant having a capacity of 20,851 m3 d−1, €618,602 per year is required, whereas for an activated sludge process having SRT of 15 days, $241,000 per year is needed [1]. The expenditure for sludge treatment is inversely proportional to the aeration cost [111]. Further, Kim et al. [112] reported that for An-MBR augmented with a conventional anaerobic reactor, the energy required for fluidization of both the reactors was approximately 0.058 kW h m-3. It is equivalent to the energy that could be recovered only by 30% of the methane generated via An-MBR operated by using 0.028 kW h m-3 [112]. Also, a decade ago, Jeison and van Lier [113] reported that the membrane prices were on a higher note in comparison to the energy consumption cost for an An-MBR system, estimating 0.5 € m-3 and 0.046 € m-3 of the effluent, respectively. Though the elevated degree of automation process reduces the labor cost, the cost for a membrane to replace is estimated to be 10–14% of the overall operation cost [1]. Henceforth, to diminish the energy needed for aeration, (i) course bubbling aeration for constant membrane cleaning [107], (ii) better aeration regime [114] and (iii) air cycling or intermittent bubbling [30] are few areas by which the operational cost can be reduced.

4.4.2.2. Effect of pH. For a MBR system containing ionisable micropollutants, pH plays a significant part in the removal efficiency [99]. pH ranging between 6–7 is generally suggested for the biological treatment. Tadkaew et al. [99] reported the effect of mixed liquor pH in between 5–9 for selected micropollutants. It was found that sulfamethoxazole, and diclofenac were ionisable pH dependent. Maximum removal was obtained at pH 5 when compared to less acidic conditions, whereas for pharmaceutical wastewater containing ibuprofen, clofibric acid and diclofenac, the removal was efficient at acidic pH [100,101] due to the hydrophobicity of these compounds at pH 5. The removal of Bisphenol A and carbamazepine was comparatively independent and not affected by the mixed liquor pH [99], although bisphenol A showed a better removal than carbamazepine due to its hydrophobic nature. Further, the alteration of pH and HRT, did not improve the removal efficiency [86], whereas, ibuprofen was degraded under neutral and acidic pH. At low pH conditions, the biodegradation of clofibric acid was reduced due to decline in the microbial activity. In addition, the functional groups of the compounds play a major role in determining the removal efficiency [27]. For example, hormone possessing phenolic and hydroxyl groups dissociates at a pH above their pka value, resulting in enhanced charge repulsion between the microbial surface/sludge and the membrane, thereby reduces the adsorption and removal efficiency. 4.4.2.3. Effect of redox conditions. By performing the MBR at diverse redox conditions results in high microbial variety and activity. Suarez et al. [94] reported a significant amount of biodegradation of certain micropollutants (naproxen, 17α-ethynylestradiol and ibuprofen) under aerobic conditions whereas some compounds (galaxolide and tonalide) can be degraded under both aerobic and anoxic conditions. Many literatures have reported that anoxic conditions are capable enough for removal of micropollutants from wastewater [102]. For example, under anoxic conditions, 95% of diuron biodegradation was achieved in comparison with only 60% using aerobic conditions [94]. Nitrification showed a partial removal of micropollutants, such as gemfibrozil, diclofenac, bezafibrate and ketoprofen. A high SRT in MBR system also enables the enrichment of nitrifying bacteria resulting in removal of certain trace micropollutants [103].

5. Future perspective For the proper advancement and wide scale implementation of MBR for micropollutant removal from wastewater, a proactive approach to redress the aforementioned concerns is very essential. MBR system has proven to be very efficient for removal of many persistent organic micropollutants. The integrated approach can augment this efficiency to further enhance the removal efficiency of the conventional MBR. However, the major roadblocks to the augmented system are its high energy consumption and cost of operation. Micropollutants generated from the pharmaceutical industries usually have low removal efficiency with MBR. This problem might be resolved by the addition of specific enzymes in MBR system for the removal of these persistent organic micropollutants. Besides this, utilization of enzymes instead of microbes will lower the menace of emerging resistance to micropollutants. However, the production of hazardous transformed metabolites or by-

4.4.2.4. Effect of temperature. It is well established that microorganism growth and activity are affected by the change in temperature [33], although most of the micropollutant removal using MBR system takes place at room temperature, such as ibuprofen, acetaminophen, 325

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products during enzymatic treatment reduces its efficiency. Enzyme immobilization and mediator cost reduction through the available natural resources can play a vital role to mitigate this issue. Furthermore, membrane fouling is among the foremost contrivances in the MBR system. The extracellular polymeric substances (EPS) are the major contributor in the MBR fouling mechanism. The level of EPS in the wastewater stream, among other factors, can also rise due to seasonal variations and the improper discharge of un-utilized pharmaceuticals from residence and hospitals. The pharmaceutical companies additionally contribute a huge amount of their own micropollutants into the wastewater stream. These micropollutants in combination with the municipal wastewater make the membrane fouling an even more difficult issue to deal with. Public awareness is imperative in this regard and the proper labeling of the respective drugs will encourage patients/ persons to discard them properly. Also, very commonly utilized technique to prevent membrane fouling is aeration, and aggressive cleaning is not effective in MBR system. Involvement of various pretreatment methods, operating conditions, membrane categories will play a vital role for enhancing the removal efficiency as well as in reducing the membrane fouling. The newly developed MBR configuration may assure low energy consumption along with the reduced cost for fouling mitigation. For example, biocatalytic membrane reactor (BMR) with proper biocatalyst can be used for in-situ degradation of foulant. The emphasis should be given to the development of innovative processes to make the aeration in a MBR system more efficient while consuming less energy at the same time. New MBR designs such as electrochemical membrane bioreactor could be used as an energy efficient wastewater technology. Also, evaluation of biofilm thickness, nutrient gradient and simulated dissolved oxygen may save energy, time and cost. An in-depth study on the effect of various additives viz. alum, zeolites and activated carbon on membrane fouling is much needed. A balanced modeling of the degree of fouling and operating conditions is very much essential. Similarly, the potential application of promising membrane operations e.g. forward osmosis, membrane distillation, membrane emulsification should be explored. Although FO and MD have a good potential in low membrane fouling along with high removal efficiency for small molecular weight recalcitrant organic pollutants present in the wastewater stream.

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