Membrane technology has become an important separation

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1 Current Status and Future Prospects of Membrane Bioreactors (MBRs) and Fouling Phenomena: A Systematic Review Hamideh Hamedi, 1,2 * Majid Ehteshami, 1 Seyed Ahmad Mirbagheri, 1 Seyed Abbas Rasouli 2 and Sohrab Zendehboudi 2 * 1. Department of Civil Engineering, K. N. Toosi University of Technology, Tehran, Iran 2. Faculty of Engineering and Applied Science, Memorial University, St. John's, NL, Canada Membrane bioreactors (MBRs) have been widely used for municipal and industrial wastewater treatment around the world due to their advantages, which include higher efficiency, smaller footprint, and lower sludge production over other conventional activated sludge (CAS) processes. However, membrane fouling that results from physicochemical interactions between the membrane and the components of the mixed liquor still remains the most challenging matter preventing the broad application of MBR technology. Recently, a considerable number of experimental and modelling investigations have been conducted concerning MBRs and membrane fouling. Despite the development of low-fouling membrane systems, more research and engineering activities with a focus on surface modification, wastewater specifications, pre-treatment and treatment conditions, and efficient fouling control and remedy strategies are still needed to minimize the probability of the occurrence of fouling. It is vital to investigate important aspects of the characterization and mechanisms of the fouling phenomenon to find reliable and long-term solutions. This review provides a detailed survey of the main aspects of the MBR processes, configurations, advantages and disadvantages, fouling phenomenon, and fouling control strategies in MBRs. Past research and engineering activities in this area are critically reviewed such that pros and cons of recent developments in fouling inhibition and mitigation approaches are also discussed. The main practical and theoretical challenges for the effective utilization of MBRs in various municipal and industrial sectors are then addressed. At the end, we offer useful practical guidelines and recommendations for the better design and operation of MBRs in industrial and public communities. Keywords: wastewater, membrane bioreactor (MBR), membrane fouling, fouling control strategies INTRODUCTION Membrane technology has become an important separation technology during the past few decades. [1,2] The advantages of membrane technology include its high separation efficiency, low energy consumption, and well-arranged operation as well as the absence of chemical additives in this separation strategy. The membrane technology has attracted great interests for water and wastewater treatment. For instance, oily wastewaters are one of the major polluted flow streams in industrial and domestic sewage, which are usually treated by various physical, chemical, and biological techniques. Recently, combined separation technologies have been the focus of numerous investigations in order to improve the efficiency of treatment. Membrane bioreactors (MBRs) technology is a compact and economical system that combines a biological treatment (using activated sludge) and membrane filtration, typically through microfiltration (MF) or/and ultrafiltration (UF). [3 5] The UF membranes are more efficient in some cases, such as with oily wastewater treatment. As Padaki et al. [6] concluded, UF membranes with molecular weight cut-off between and Daltons are able to reject about 96 % of total hydrocarbon concentration, 54 % of benzene, toluene, and xylene (BTX), and 95 % of heavy metals. In another research conducted by Abdollahzadeh Sharghi and Bonakdarpour, [7] the removal efficiency of petroleum substances and chemical oxygen demand (COD) were obtained to be between % and %, respectively, by utilizing the MF membrane. Other researchers also confirmed the high performance of MBRs in real municipal wastewater treatment with around > 95 % and > 80 % removal of pathogens, E. coli, and trace metals, respectively. [8] In MBR systems, the biological treatment has been improved through incorporating physical separation technology such as membranes that exhibits the selective separation potential, in comparison with other separation technologies. [9 11] MBRs are being widely employed for both industrial and municipal wastewater treatment across the globe. An increase in the number and capacity of this type of membranes confirms this claim. For instance, it was reported that the annual growth rate of MBRs in the global market is about 15 %. [12] MBR systems have several advantages compared to conventional wastewater treatment processes such as higher effluent quality with more disinfection capability, larger volumetric organic loading, less sludge generation, and smaller footprint. [13,14] The higher retention time of the biomass in the reactor leads to an improvement in the treatment efficiency and the potential for the removal of nitrogenous compounds. [15] Despite the advantages of MBR systems, a major concern in the operation of this technology is membrane fouling, which results from the undesirable accumulation and deposition of substances on the membrane surface. Membrane fouling leads to a decline in the permeate flux by reducing the membrane permeability, an * Author to whom correspondence may be addressed. address: hamedi.hamideh@gmail.com (H. Hamedi); szendehboudi@mun.ca (S. Zendehboudi) Can. J. Chem. Eng. 97:32 58, Canadian Society for Chemical Engineering DOI /cjce Published online 25 September 2018 in Wiley Online Library (wileyonlinelibrary.com). 32 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

2 increase in the trans-membrane pressure (TMP), and, finally, a considerable reduction in the treatment performance. [14,16,17] Prediction and modelling of membrane fouling have been the focus of numerous investigations. [18 21] For instance, Mannina et al. [18] introduced a new mathematical model to determine the greenhouse gas emissions, including carbon dioxide and nitrous oxide, produced from membrane bioreactors (MBRs). They found that the salinity affects the amount of N 2 O emissions over the nitrification process. There was an agreement between the predictions and measurements. [19] In another research investigation, Mannina et al. [19] developed a comprehensive integrated membrane bioreactor (MBR) model to simulate the main biological and physical processes involved in the wastewater treatment process, where the removal of phosphorus, nitrogen, organic matter, and greenhouse gases was targeted. Based on the research findings, the main producers of N 2 O include heterotrophic denitrification and NH 2 OH oxidation; the membrane fouling can be properly modelled by the developed model. [19] To determine vital aspects/mechanisms, effective/proper strategies in terms of modelling and experimental techniques to further understand the fouling mechanisms and the identification of influential factors of fouling are required. The most important factors affecting membrane fouling can be divided into wastewater (or feed water) characteristics, biomass properties, operating conditions, and membrane characteristics. There are a significant number of research studies with a focus on the characterization and identification of fouling and the influential factors affecting this phenomenon. [22 24] However, the impacts of important operating conditions such as hydraulic retention time (HRT) and flow rate, and sludge properties including sludge viscosity and floc size on the fouling occurrence have not been adequately addressed in previous works. [15,25 27] In addition, review papers prepared by researchers, including Mutamim et al. [28] and Lin et al., [29] briefly describe the fouling mechanisms. An adequate review of fouling mechanisms and control strategies is provided by Meng et al.; [30] however, the vital process and membrane factors related to fouling are not discussed in a systematic way in their work. It conveys the message that a proper parametric sensitivity analysis on the membrane performance over fouling phenomenon needs to be conducted. The current review paper aims to provide clear insights into MBR technology, membrane fouling mechanisms, main factors causing fouling during MBR operation, and new and efficient fouling control strategies in MBRs. We also offer appropriate recommendations in terms of practical and technical aspects for the design and operation of MBR systems that provide guidelines for further research and engineering activities in this area. This paper is structured as follows. The Introduction includes an introduction to MBR technology and fouling phenomenon with a summary of previous studies in this area. In the Membrane Bioreactors section, the MBR systems and their important characters are described. This section involves configurations, advantages, and disadvantages of MBRs. Membrane fouling in terms of classifications, mechanisms, and influential parameters is systematically explained in the Membrane Fouling section. In the Fouling Control Strategies section, a comprehensive literature review on membrane fouling control methods is provided. For clarity and to encourage a better understanding of membrane fouling, numerous previous research studies are discussed and critiqued by the authors from a theoretical and practical perspective. The current status and future prospects of MBRs and fouling events are specifically highlighted in the Technical and Non-Technical Aspects and Challenges section. Finally, a summary of the main points related to MBRs and unfavourable fouling events is given in the Conclusions section. MEMBRANE BIOREACTORS Membrane bioreactors (MBRs) appear in the form of a compact technology that integrates a suspended growth activated sludge bioreactor with a membrane filtration unit. The concept of MBRs was first developed and commercialized by DorrOliver in the late 1960s with an application in ship-board sewage treatment. [31,32] At the same time, more bench-scale MBR systems were in operation for different treatment processes. [33,34] MBR technology has become more popular as a promising wastewater treatment technology for both industrial and municipal wastewater treatment over the last few decades due to its distinct advantages, such as higher effluent quality, smaller footprint, and lower sludge production than conventional activated sludge (CAS) processes. This technology can be employed in areas that have limited space by replacing a large secondary settling tank with a compact membrane module system. [32,35] Wastewater treatment in MBR systems involves two mechanisms, namely, biological treatment in a suspended growth bioreactor for biochemical reactions (e.g., bio-oxidation, nitrification, and denitrification) and the physical membrane filtration process. The membrane applications in MBR systems range from microscopic particles separation to molecules separation, depending on the membrane driving forces (e.g., TMP) and membrane characteristics such as membrane pore size and materials. [32] In general, suitable materials to construct the membranes in MBRs are organic polymers such as polyethene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF). [32] There are four common categories of membranes in water and wastewater treatment processes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), depending on the mean pore size. [36] Two types of membranes, which are typically used in MBRs, are microfiltration and ultrafiltration as low-pressure membranes with less fouling potential due to having larger pores, compared to other types of membranes. In general, lower operational and capital expenditures are needed for these two particular membranes (e.g., MF and UF). [37] Microfiltration is generally utilized for the separation of particles with sizes of about nm. The TMP applied in MF is within the range of 1 2 bar, primarily for overcoming the resistance of the foulants cake layer formed on the membrane. Ultrafiltration can remove macromolecular solids within the size range of nm, where the TMP normally varies from 1 7 bar. [36,38] Advantages and Disadvantages of MBR Numerous advantages of MBRs are listed below. High quality effluent Although contaminant removal efficiency depends on the operation conditions and membrane pore size distribution (PSD), MBRs can produce high quality effluent with an elevated percentage of contaminants removal and large disinfected permeate flow in a single treatment process. This means that MBR technology is utilized in cases where the effluent quality requested by customers and/or downstream units exceeds the CAS capability. [28] Complete control over sludge retention time and hydraulic retention time Sludge production and mixed liquor suspended solid (MLSS) concentration in the bioreactor can be reduced by extending the VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 33

3 sludge retention time (SRT) so that the slow-degradable pollutants are completely removed. Generally, the MBR systems can operate at high volumetric loading rates, short hydraulic retention time (HRT), and high MLSS concentrations. [33] Smaller footprint MBR systems can be installed in a smaller space due to the combination of biological treatment and membrane filtration processes in a single tank and the elimination of a settlement tank and secondary clarifier. Operation at low DO concentrations Design of MBRs with longer SRT leads to the possibility of a simultaneous nitrification and denitrification occurrence. [39,40] The important features of MBR technology (compared to conventional processes) are summarized in Table 1. Despite the above advantages of MBRs over conventional alternatives of wastewater treatment, the widespread use of MBRs is mainly limited by elevated operating expenses and fouling phenomenon. [22,24,32,41] Membrane Bioreactor Configurations As is illustrated in Figure 1, there are two key MBR configurations in terms of the membrane module setup. This technology can be presented either as a side-stream process during which the membrane is located outside the bioreactor or in form of a submerged process during which the membrane is immersed in the bioreactor. [42] These two MBR structures have different operation conditions, filtration mechanisms, and membrane types. The first utilization of side-stream MBRs was in the late 1960s when the MBR technology had a high membrane cost and high energy requirement. In the late 1980s, the submerged MBR was presented by Yamamoto et al. [43] as a new configuration of MBR technology to reduce the high energy costs corresponding to side-stream MBRs. Side-stream MBRs are usually equipped with a tube-membrane in the in-to-out filtration mode (named the pressure-driven membrane), while the flat sheet and hollow fibre membranes with the out-to-in filtration structure (called the vacuum-driven membrane) are more common in the submerged MBRs. With the superior advantages of submerged MBRs in comparison with sidestream MBRs in mind, Table 2 compares the characteristics of these two types of MBRs in terms of application and operational conditions. MEMBRANE FOULING Membrane fouling is the greatest challenge in MBR operation, which results from physicochemical interactions between the Table 1. Advantages of MBR systems over CAS processes Advantages MBR CAS High effluent quality High SRT Low sludge production Easy construction Low area requirement (footprint) Low energy consumption Low operational cost Low capital cost membrane and the components of the mixed liquor. Indeed, the fouling phenomenon occurs due to the undesirable attachment and deposition of suspended particles, microorganisms, cell debris, colloids, and solutes in the MBR reactors on the membrane surface and/or inside membrane pores. [24] Membrane fouling results in the reduction of membrane permeability during filtration and, consequently, an increase in its resistance to flux. [44,45] In this case, membrane cleaning would be necessary in order to restore membrane permeability, although this might decrease the lifespan of the membrane, leading to more frequent membrane replacement costs. [15,46,47] The operational expenditures of MBRs are augmented due to control strategies associated with fouling such as air scouring, backwashing, implementation of the cross-flow velocity (CFV), and utilization of chemicals for membrane cleaning. Obviously, the fouling event affects capital expenditures as new membranes need to be purchased in order to replace damaged membranes. [40,48] Mechanisms of Membrane Fouling As is depicted in Figure 2, fouling mechanisms in MBRs include three main stages as follows. Pore narrowing/plugging Slow fouling and pore plugging result from the coverage of the membrane surface by micro-colloidal substances and solid materials with a size smaller than the membrane pore diameter, which leads to a decrease in the permeate flow rate. Pore clogging can be prevented by pretreatment stages; including upstream screening and creating turbulent conditions within the membrane modules. [23,24] Pore clogging/blocking This occurs due to the sorption of soluble microbial products (SMP) and high adsorption of colloids, biomass particles, and organic matters with a size close to the diameter of the membrane pores, resulting in internal fouling. [23,24,40] Cake layer formation Severe fouling is caused by the deposition of substances larger than the membrane pores, leading to the formation of biofilm on the membrane surface, and then external fouling. Cake layer formation is dependent on MLSS concentration, membrane flux, and air scouring. [49] In addition, extracellular polymeric substances (EPS) and soluble microbial products (SMP), which are produced due to the microbial metabolisms in the liquid phase of the MBR operation, are important factors in the mechanisms of cake layer formation. [23,50,51] Membrane Fouling Classifications Fouling can be classified into various categories. Based on the type of foulants, fouling is divided into three categories, namely biofouling, organic, and inorganic fouling. Biofouling is recognized as the major fouling type in MBR systems. [24,41,52] Deposition, growth, and metabolism of the microorganisms, and bacteria cells or flocs, play a vital role in the biofilm or biocake formation on the surface of the membrane. Biofilms cause more flow resistance, leading to the occurrence of biofouling due to the production and release of the bacterial byproducts, EPS, and SMP onto/into the membrane. [24,53] Organic fouling originally involves the deposition of SMP and EPS due to biological activities and the deposition of dissolved and colloidal components on the surface of the membrane. [24,53,54] 34 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

4 Figure 1. Configuration of side-stream and submerged MBRs (adapted from other studies [42] ). Inorganic or mineral fouling refers to the chemical precipitation of inorganic compounds (e.g., Ca 2þ ; Mg 2þ ; Al 3þ ; CO 2 3 ; PO3 4 ; and OH ) due to concentration polarization (which occurs when the inorganic ion concentration exceeds the saturation concentrations) and biological precipitation. It was found that biological precipitation on the membrane surface or into membrane pores is promoted by the bacterial cells and biopolymers, as is depicted in Figure 3. [53,54] Fouling is also classified into three subcategories, namely, removable, irremovable, and irreversible fouling, based on the membrane permanency. [24,41,52] Removable or reversible fouling is caused by the deposition and attachment of a mixture of foulants, which is attributed to the cake layer formation on the membrane surface. This type of fouling can be avoided by lowering the flux and increasing the CFV and air scouring rate. Reversible fouling can be also removed by physical cleaning such as backwashing or/and relaxation. The adsorption of dissolved and colloidal matters, which leads to pore blocking, causes irremovable fouling. Foulants that occur during irremovable fouling might be eliminated by using chemical reagents and substances such as caustics, biocide agents, and enzymatic detergents. Irreversible or permanent fouling is a type of fouling caused by humic substances and proteins that cannot be removed by physical or/and chemical cleaning strategies. It is generally developed over long periods of time and it results in a significant decrease in productivity and, consequently, in the need for membrane replacement. [38,40,48] Figure 4 illustrates cake formation and treatment methods of these three fouling groups in the context of MBR. Factors Affecting Fouling in MBR Fouling in MBR systems is influenced by a variety of parameters that can be categorized into four main groups, including biomass properties, operating conditions, characteristics of feed/wastewater, and membrane properties. Biomass characteristics Membrane fouling is strongly affected by biomass properties such as MLSS concentration, sludge viscosity, the presence of filamentous bacteria, the composition of EPS, SMP concentration, and floc size distribution (FSD). [40,55] The impacts of these factors/ properties are explained as follows. MLSS concentration: Mixed liquor suspended solid (MLSS) concentration has been considered as one of the major factors affecting membrane fouling. Different claims about the effect of MLSS concentration on MBR performance, including a negative effect, [56] a positive impact, [57] and an insignificant effect, [58,59] may imply that there is a threshold/critical concentration for MLSS. Rosenberger et al. [60] revealed that fouling reduction occurs at a low MLSS concentration (<6 g/l), while more fouling happens at a high MLSS concentration (>15 g/l) and there is no considerable influence on the membrane fouling at MLSS concentrations ranging from 8 12 g/l. A study conducted by Cho et al. [61] showed that the MLSS concentration between 4 10 g/l cannot affect the fouling resistance. As can be seen from Figure 5, Wu and Huang [62] concluded that high MLSS concentrations (>10 g/l) exhibit a significant impact on the membrane filterability. Lousada-Ferreira et al. [63] confirmed the possibility of higher filterability at MLSS concentrations equal to or lower than 10 g/l, while the opposite behaviour was encountered at MLSS concentrations above 10 g/l. Hence, the critical concentration of MLSS is about 10 g/l since higher concentrations lead to a rapid flux reduction, lower membrane filterability, higher TMP, and greater fouling resistance. It should be noted that an optimum MLSS concentration depends on the operational and membrane Table 2. Comparison of submerged MBRs and side-stream MBRs Submerged MBRs Side-stream MBRs Application Municipal-scale systems Industrial systems Solids in activated sludge Feed with high temperature Shear provided by Aeration Pump Operation mode Dead-end filtration Cross-flow filtration Pressure Low High Energy consumption Significantly low High Fouling probability Low High VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 35

5 Figure 2. Membrane fouling mechanisms. characteristics. In support of this statement, the critical concentration from g/l has been reported for different operational conditions. [23] A four-fold increase in the membrane fouling can be obtained if the MLSS concentration (> 10 g/l) increases by two times. An increase in the MLSS concentration (> 15 g/l) by three times also results in an increase in the fouling rates by almost nine times. [64,65] Chang and Kim [56] explored the concept that MLSS concentration is directly related to cake resistance (R c ), which is a function of the specific cake resistance (a c ), as follows: R c ¼ a c m c where m c represents the cake area of membrane. This parameter increases as the MLSS concentration increases. Generally, at high concentrations of MLSS, the rapid fouling cake layer is created, while at low MLSS concentrations, the progressive pore blocking occurs by colloids and particles. Creation of the cake layer at high MLSS concentrations leads to a decrease in the permeate flux. [66] Thus, there is a relationship between the permeate flux and MLSS concentration, which considerably affects the membrane fouling. For instance, the MLSS concentration may not have a vital influence on the membrane performance at low fluxes. [23] Since there is no specific relationship between the MLSS concentration and other foulants characteristics, the MLSS concentration seems to be a poor indicator of biomass fouling. [67,68] Hence, it seems logical to consider the effect of MLSS concentration on other ð1þ factors such as sludge viscosity, EPS, and SMP production in the context of the membrane fouling. Sludge viscosity: Sludge viscosity can be increased exponentially as MLSS concentration (> 10 g/l) increases. As a consequence, the positive effect of aeration on the membrane fouling and the movement of membrane fibres will be reduced. [62,69] Increased sludge viscosity also leads to an increase in the net force towards the membrane surface and a decrease in the membrane performance due to the deposition of sludge flocs on the membrane surface. [70,71] Likewise, the efficiency of the oxygen mass transfer declines at a higher sludge viscosity. The possibility of membrane fouling can be then increased at low dissolved oxygen (DO). [72] EPS and SMP: Extracellular polymeric substances (EPS) refer to a complex high molecular weight mixture of polymers such as polysaccharides, proteins, humic acids, nucleic acids, lipids, and other polymeric compounds that can be found outside the cell s surface or inside the intercellular space of microbial aggregates. [73,74] EPS production and accumulation in the MBR systems are affected by several factors such as the substrate composition, mechanical stress, organic loading rate (OLR), SRT, MLSS concentration, food to microorganism ratio (F/M), membrane properties, and temperature. [75,76] EPS are produced by bacterial metabolisms (e.g., bacterial excretion, secretion, shed from the cell surface or cell lysis, and sorption), which consist of insoluble materials. EPS are gel-like and are a highly hydrated matrix for the Figure 3. Schematic illustration of the formation of inorganic fouling in MBRs (adapted from other studies [24] ). 36 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

6 Figure 4. Simple schematic of fouling occurrence and removal in MBRs (modified according to other studies [24] ). immobilization of the microorganisms. [76 78] EPS create a threedimensional protective layer around the bacteria against the external stress and toxicity to such an extent that they provide a significant barrier to the permeate flow. EPS have a considerable influence on the sludge flocs cohesion and the biofilms viscoelastic properties. It was reported that EPS are a main part of biofilms and make up about % (w/w) of the total biofilms weight. [73,79] Therefore, EPS facilitate the formation of the cake layer through a co-deposition of EPS and microorganisms in the membrane and filling void spaces between the cells leading to a decrease in membrane permeability. Since the shear modulus of elasticity and shear viscosity are the major factors affecting the Figure 5. Analysis of membrane filterability and MLSS concentration. [62] biofilm and EPS cohesion, EPS play a critical role in the biofouling of MBR systems. [74,75,80] It is worth noting that EPS are largely made of polysaccharides (EPS c ) and proteins (EPS p ). [23,53] EPSc have a hydrophilic tendency due to strong electrostatic interactions and hydrogen bond forces, whereas EPS p are more hydrophobic due to amino groups. Furthermore, the amino groups lead to an increase in zeta potential and a decrease in the net negative sludge flocs surface charges due to carrying positive charges. [81,82] As is depicted in Figure 6, EPS are divided into the following two major subgroups: soluble EPS, which are also called SMP; and bound EPS or extractable EPS (e-eps). Both categories of EPS are decomposed by the bacterial cells in the mixed liquor. Bound EPS also have a double-layer structure that includes inner and outer layers. The inner layer that is tightly bound EPS (TB-EPS) is strongly attached to the cells surface. The outer layer that is loosely bound EPS (LB-EPS) is a slime layer, which is weakly attached to the other layer. [73,78,83] LB-EPS and TB-EPS have a remarkable role in sludge agglomeration and membrane fouling. A number of research studies proved that LB-EPS are mainly responsible for increasing sludge flocculation and, then, membrane fouling, in comparison to TB-EPS. [74,85,86] As can be seen in Figure 7, Liu et al. [85] showed that LB-EPS considerably affect specific resistance to filtration (SRF) (R 2 ¼ ), while TB-EPS have no obvious relationships to membrane fouling rate (R 2 ¼ ). However, Li and Yang [87] claimed that increasing the LB-EPS content decreases cell adhesion, floc structure deterioration, cell erosion, and, consequently, bioflocculation settleability and the dewaterability of sludge flocs. Other research has shown the positive effect of VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 37

7 TB-EPS on the gel-like and sludge structure of MBR systems. [88,89] Also, Azami et al. [90] outlined that higher TB-EPS to LB-EPS ratios induce greater flocculation, which, in turn, creates larger flocs and results in membrane fouling. Some authors found that polysaccharides induce more fouling than proteins. [59,91] It is still being discussed whether the fouling is mostly affected by the protein or polysaccharide component of EPS. In addition, it is not known yet whether or not LB-EPS plays an important role in membrane fouling. [55,76] This uncertainty mainly comes from the differences in chemical characteristics and molecular weight distributions (MWDs) of these two groups of organic compounds as well as the various experimental methods for EPS assessment. [92] Indeed, there are numerous EPS extraction methods, but there is no standard (or generalized) method for extraction. Some of the EPS extraction strategies include cation exchange resin, centrifugal operation with formaldehyde, and heating techniques. [77,93,94] Previous research has demonstrated that formaldehyde extraction is the most effective technique to extract high concentrations of EPS, though the heating method is sometimes preferred due to its simplicity. [82] As was mentioned before, SMP have been identified as soluble EPS. [77,95] SMP consist of different organic compounds such as fulvic and humic acids, proteins, polysaccharides, amino acids, and exocellular enzymes, which are dissolved into the solution. It has been suggested that the SMP production variability is influenced by the environmental stresses imposed on the bacteria such as nutrient deficiency, low ph, low temperature, and the presence of toxic compounds. [96] A high concentration of SMP leads to a decrease in membrane filterability and causes irreversible membrane fouling. [97,98] It was found that SMP have the highest correlation with the membrane fouling rate in comparison to other sludge characteristics, such as MLSS, floc size distribution, and bound EPS. [99,100] During the filtration operation, SMP can be deposited and adsorbed on the membrane surface, block the membrane pores, and then induce the formation of gel layer much easier than the formation of the cake layer. Based on other literature, the gel layer has more SMP and smaller flocs than the cake layer. Also, the gel-specific filtration resistance is about 100 times higher than that of the cake layer. Thus, severe membrane fouling results from a reduction in the porosity of the gel layer as well as the permeate flow rate. [38,101,102] Figure 6. A cartoon of the EPS structure (adapted from other studies [84] ). Figure 7. Relationship between EPS and SRF. [85] The main components of SMP, among the chemical components, are proteins (SMP p ) and polysaccharides (SMP c ). [53] Jermann et al. [103] indicated that the hydrophilic nature of SMP c leads to irreversible fouling due to the membrane pore blocking and the gel layer formation. Furthermore, it was reported that there is a strong correlation between membrane fouling and SMP p concentration due to membrane pore clogging. [104] Yao et al. [105] found that higher SMP p /SMP c ratios lower the chance for the occurrence of irreversible fouling and result in cake layer formation on the membrane surface. Drews et al. [106] claimed that membrane fouling can be affected by the SMP content under certain conditions such as pore size and low sludge age. For example, it was observed that the SMP concentration can be increased at a shorter SRT, but that protein concentrations are higher than polysaccharides in the MLSS, while it is inverse in flocs that are attached to the membrane surface. Through increasing the SRT, SMP content and membrane filtration resistance are normally reduced. [107] Generally, the SMP and EPS contents in the mixed liquor have a significant effect on the floc strength and resistance to shear, floc size distribution, dewaterability, settleability, non-settleable solids fraction, specific sludge volume index (SSVI), and cake filtration properties such as filtration resistance, hydrophobicity, viscosity, and surface charge. [97] Cosenza et al. [108] investigated the EPS influence on reversible fouling (mainly cake deposition) and the SMP impact on irreversible fouling (mainly pore blocking). Various tests such as filamentous abundance, foam rating, scum index, and foam power were conducted to monitor foaming phenomenon in MBR. Based on their research output, there was a relationship between foaming and bound EPS as well as fouling and EPS. It was also found that the foam formation lowers the fouling rate. Floc size distribution (FSD): One of the important factors affecting membrane fouling is the size of sludge flocs. A number of studies indicated that the cake layer formation mainly results from the presence of small sludge flocs rather than bulk sludge due to the 38 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

8 presence of more bound EPS in the small flocs, which facilitate the deposition and adhesion of smaller flocs on the membrane surface. [109,110] It was observed that sludge flocs with a diameter larger than 80 mm have a greater impact on membrane filterability with a lower membrane resistance, while the mixed liquor with a mean floc size lower than 50 mm exhibits poor filterability and a loss in membrane permeation. [62,111] In addition, smaller flocs cause a higher filtration resistance due to the formation of a denser and less porous cake layer, in comparison to the larger flocs. [110, ] However, the higher resistance of the cake layer formed by the small flocs is explained by the Carman-Kozeny equation. [115,116] Generally, the adhesion of sludge flocs on the membrane surface in the MBR systems is governed by two opposite forces, including the permeation drag force induced by the permeate flux and the back-transport force, which consists of Brownian diffusion, inertial lift, and shearinduced diffusion. [117] The flocs can be pushed towards the membrane surface by hydrodynamic forces and adhered to the membrane surface by thermodynamic interactions. [118] From a hydrodynamic point of view, a high shear-induced diffusion, inertial force, and lower Brownian diffusion cause large flocs to be pulled away from the membrane. On the other hand, the movement of small flocs can be controlled by Brownian diffusion. [119] According to some research studies reported in open sources, the desire of small flocs for adhesion can be described by the hydrodynamic forces. It implies that the small flocs with a lower back-transport velocity can be accumulated on the membrane surface more readily due to lift forces, compared to larger flocs. [117] However, Shen et al. [120] claimed that small flocs form less than 3 % of the total biomass in MBRs and that the possibility for larger flocs to be attached to the membrane is higher. [109,111] This means that the stronger adhesion tendency of small flocs cannot be explained by only considering hydrodynamic mechanisms. [120] Thermodynamic analysis indicates that lowering the floc size leads to a slight increase in the specific energy barrier and the growth of attractive interaction energy. Thus, higher interactions may cause a high deposition of small flocs on the membrane, resulting in the formation of less porous cake layers. [38,120] It can be concluded, therefore, that membrane fouling is more influenced by small flocs than large flocs. The effects of all these parameters on the membrane fouling are summarized in Table 3. Operation conditions The operating conditions that have a significant influence on the membrane fouling in MBRs include SRT, HRT, aeration or DO concentrations, CFV, temperature, permeate flux, and sequence and duration of backwashing or relaxation. [40] The effects of the above variables are described below. SRT and HRT: SRT and HRT are considered as critical factors that affect the development of membrane fouling. [23,118,122] SRT not only influences biomass characteristics, but it also has an impact on hydrodynamic conditions, including shear force and permeate drag balance, which can result in the cake layer development and membrane fouling. [123] As Figure 8 demonstrates, a higher SRT causes nitrification capability improvement, a reduction in sludge production due to a low F/M ratio, and an increase in MLSS concentration. [107,124,125] Moreover, a higher MLSS concentration can induce faster membrane fouling by increasing the probability of sludge deposition on the membrane surface, resulting from a higher EPS and SMP concentration. [63] Over a short term of submerged non-woven MBR operation with a larger pore size (e.g., 14 mm) at a constant pressure of 5 kpa, Chuang et al. [126] observed complete reversible fouling at a SRT of 15 d and irreversible fouling at even longer SRTs (e.g., 30 and 60 d). The study of MBR performance at SRTs of 20 and 50 d at a constant F/M ratio of about 0.2 kg COD/kg MLVSS d 1 revealed that there is no notable difference in the membrane fouling between SRTs of 20 and 50 d. Irremovable fouling was observed at an SRT of 50 d (90 % of total fouling) due to high SMP concentrations with polysaccharides as the major component, while 85 % of total fouling occurred at a SRT of 20 d with proteins (the main fraction of SMP). [127] A study on an anaerobic-oxic MBR showed the development of irreversible fouling at a SRT of 90 d due to the pore blocking formation of a cake layer and reversible fouling at a SRT of 10 d. Based on the results, a better filtration efficiency was achieved at a SRT of 30 days, compared to SRTs of 10 and 90 days. [100] It can be also concluded that a very long or short SRT has a negative impact on the membrane fouling as these extreme conditions lead to the damaging of the membrane permeability due to a significant change in sludge properties, pore blocking, and cake formation. [24] Deng et al. [128] recommended that a better optimal performance of MBRs for synthetic wastewater treatment is attained at a SRT of d, whereas this parameter is about d while treating real wastewater streams. As is illustrated in Figure 8, the range of SRTs from d is considered as the moderate SRT for a reliable operation of MBRs. However, this range depends on the membrane materials, type of wastewater, F/M ratio (constant or variant), and influential factors in MBR performance such as the sludge concentration and polysaccharide concentration in EPS and SMP. Nitrogen removal efficiency might be effective in the determination of the moderate SRTs as well. [118] Huang et al. [122] assessed the effect of SRT (30, 60, and infinitive days) on the membrane fouling based on HRT (12, 10, and 8 h). At a shorter HRT (8 10 h), the prolonged SRT enhanced the possibility of biomass growth and SMP accumulation on the membrane surface. The effect of SRT on biomass concentration was negligible at a higher HRT (12 h) due to different SMP compositions. Fallah et al. [129] observed that an increase in the loading rate, EPS levels, and SMP production, and a decrease in the mean floc size (which induces membrane fouling acceleration) are experienced at a shorter HRT (e.g., 18 h) rather than longer HRT (e.g., 24 h). Other studies also confirmed that a higher OLR at shorter HRT results in an increase in biomass concentration and sludge viscosity and a decrease in DO concentration. It also causes a higher EPS production (due to more filamentous bacteria growth), more porous sludge flocs generation, acceleration in the membrane fouling development and cake layer formation, and a reduction in membrane permeability. [80,130,131] It was concluded that too short HRTs and/or too long SRTs lower the filtration efficiency of MBRs. Hence, the optimized magnitudes of HRT and SRT should be employed in order to achieve optimal treatment efficiency with respect to the membrane fouling. Aeration and cross-flow velocity: Air flow rate or DO concentration (as a crucial operating factor), which controls the hydrodynamic conditions, has a remarkable impact on the membrane fouling by changing biomass characteristics such as EPS and SMP. It also causes more energy consumption. [ ] In submerged MBRs, aeration caused by gas bubbling is required in order to achieve the following three goals: providing oxygen for biomass; ensuring that the sludge remains suspended in the bioreactor; and reducing the deposition of foulants by air VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 39

9 Table 3. The effect of biomass characteristics on membrane fouling Biomass characteristics Impact on membrane fouling References MLSS"! cake resistance", specific cake resistance# Chang and Kim [56] MLSS MLSS"! flux reduction", membrane filterability# Wu and Huang [62] MLSS"! TMP", fouling potential" Lousada-Ferreira et al. [63] MLSS"! Viscosity", membrane permeability# Trussell et al. [121] Viscosity Viscosity"! oxygen mass transfer#, fouling possibility" Lay et al. and Germain et al. [69,72] Viscosity"! net force towards membrane", floc deposition", membrane Kornboonraksa and Lee and Meng et al. [70,71] permeability# LB-EPS"! sludge flocculation", fouling potential" Wang et al. and Ramesh [74,86] EPS LB-EPS"! considerably affect specific resistance Liu et al. [85] TB-EPS/LB-EPS ratio"! flocculation", fouling potential" Azami et al. [90] SMP"! membrane filterability#, irreversible fouling" Deng and Janus [38,40] SMP SMP"! membrane pore blocking", gel layer formation" Benyahia et al. and Hong et al. [101,102] SMP is more important that MLSS, EPS, and FSD Drews et al. and Zhang et al. [99,100] SMP p /SMP c ratio"! irreversible fouling#, cake layer formation" Yao et al. [105] Smaller flocs develop denser and less porous cake layer Mahendran et al. and van den Brink et al. [112,113] FSD Amount of small flocs"! filtration resistance", membrane permeability# Lin et al. and Wang et al. [110,114] Sludge flocs diameter"! membrane resistance# Wu and Huang and Meng et al. [62,111] scouring during filtration, which leads to the back-transport of foulants and shear stress on the membrane surface. [135] However, in side-stream MBRs, cross-flow velocity generated by the recirculation pump has a key role in the detachment of foulants from the membrane surface. Indeed, fouling can be linearly reduced by increasing the CFV up to 4.5 m/s due to a decrease in the deposition tendency of sludge particles. [23] However, as the CFV should be generated by the recirculation pump, a considerable amount of energy for pump operation is needed. During the aeration operation, the air bubbles generate liquid flow fluctuations near the membrane, which cause membrane movements, enhance the back-transport of particles, and Figure 8. Effect of SRTs on (a) MLSS and F/M, (b) EPS, and (c) SMP concentrations. [118] 40 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

10 physically scour the membrane surface by creating shear stress which prevents large particle deposition on the membrane surface. Hence, aeration has a positive effect on fouling reduction and leads to an improvement of the MBR filtration performance. [23,38] However, Martinelli et al. [136] illustrated that a high intensity of aeration can deteriorate the floc structure by breaking them up and decreasing their size, which leads to higher fouling (see Figure 9). This finding was confirmed by other researchers. [38,134,137] On the other hand, at low air flow rates, the biological process is negatively affected, resulting in bioflocculation and cake layer formation through increasing EPS, SMP, and MLSS deposition on the membrane surface. [133,138] Generally, the determination of an optimal air flow rate depends on the operating parameters, MBR configuration, and cyclic of aeration. [38,139,140] As is shown in Table 4, the amount of aeration demand is different in various MBR systems. Aeration is significantly energy consuming during MBR operation. [141] To save energy, MBRs can be operated without air scouring that leads to a reduction in the back-transport (due to the lack of shear stress) and, consequently, deterioration of membrane filtration. In this case, the operation should be implemented at high fluxes to control membrane fouling by physical cleaning and increasing the backwashing efficiency. [142,143] There are studies conducted by researchers in other literature that propose effective strategies to reduce the aeration demand during filtration processes. [144,145] Recently, Dıaz et al. [132] obtained the optimal operating conditions by reducing the DO concentration to 0.38 mg/l at a high permeate flux of 42.2 L/m 2 h to reduce the energy required in the process. Permeate flux: The permeate flux is an important factor to reduce the membrane fouling potential. [151,152] Higher flux results in a higher fouling rate as it increases the sludge deposition on the membrane surface through the generation of higher concentrations of MLSS and EPS. [153] Hence, MBRs should be operated at a low flux. As the TMP slope is smaller at the lower flux, a flux that causes a sudden change in the TMP slope is proposed as a critical flux. [154] The concept of critical flux was first proposed by Field et al., [155] who define that it is a permeate flux value below which no reduction in the flux versus time, or increase in TMP, and no Figure 9. Influence of air flow rate on membrane resistance. [136] fouling occur (due to a sufficient back-transport rate) to eliminate particle deposition on the membrane surface. [40,152] The critical flux depends on the back-transport of biomass particles, which is affected by the cross-flow velocity and the solute-membrane interactions due to hydrophobicity. This means that MBRs can be operated at higher fluxes with a constant TMP if the back-transport is increased sufficiently by enhancing cross-flow velocities. [23] However, above the critical flux, fouling occurs due to particle and colloid deposition on the membrane surface. At this time, TMP increases quickly during the filtration with a constant flux and the permeate flux declines rapidly at a constant TMP. [40,151,154] The critical flux is determined by various strategies including flux stepping, hysteresis, square-wave, and pressure-cycling methods. [ ] For example, Sarioglu et al. [160] applied the flux stepping method to determine the critical flux, as is shown in Figure 10. The results demonstrated that the critical flux is within the range of L/m 2 h for an aeration intensity of 1 m 3 / m 2 h. Janus [40] criticized both the flux stepping and hysteresis methods as it is not possible to forecast the permeability data throughout long-time operation of complex influents. Diez et al. [154] believed that the flux stepping method is the most widely accepted approach among the critical flux determination methods. Consequently, the threshold flux concept was introduced, which separates a high fouling region and a low fouling region. In other words, at or below the threshold flux, a low or relatively constant rate of fouling occurs, while the rate of fouling increases significantly above the threshold flux. [161] Thus, the main focus is below the threshold flux, which results in an acceptable fouling rate, compared to a non-fouling condition below the critical flux, and a long-time operation with a sufficient increase in the flux is possible. [151] The threshold flux can be applied to the dead-end and cross-flow systems, while the applicability of critical flux in deadend systems is limited due to the absence of back-transport velocity in these systems. [40] Field and Pearce [161] defined the sustainable flux concept as the net flux that provides an optimal balance between the operation and capital expenditures while maintaining the required productivity level. The sustainable flux mainly focuses on the operating costs of the membrane as opposed to the critical and threshold fluxes which consider the rate of fouling development on the membrane surface. The limiting flux is another conceptual flux in fouling control that is defined as the maximum permeate flux, which is attainable by increasing the TMP. At the limiting flux, further increase in TMP cannot affect the flux variations. [151] It was found that the limiting flux is highly affected by the feed water composition, and it can be equal to, greater, or less than the critical flux. [162,163] In spite of numerous studies that have attempted to determine the optimum permeate flux for fouling control, an accurate and reliable strategy has not yet been introduced. The main reason for this drawback is that this vital factor is affected by many operating conditions such as membrane material, cross-flow velocity, aeration intensity, and sludge concentration and characteristics. Temperature: Temperature is known as an effective factor in the biological processes of MBRs because they alter the microbial composition and rate of treatment. Temperature impacts sludge flocs characteristics (especially EPS and SMP production), permeate viscosity, membrane filtration, and membrane fouling, all of which have been studied by several researchers. [164,165] Previous studies have revealed that low temperature can lower MBRs operational efficiency by reducing biodegradation, VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 41

11 Table 4. Summary of aeration demand with different MBRs systems Membrane type Pore size (mm) Surface area (m 2 ) Flux (L/m 2 h) HRT (h) SRT (d) Aeration rate (L/h) References Hollow fibre Park et al. [146] Hollow fibre (PVDF) Deng et al. [128] (UF) PES Deowan et al. [147] Flat sheet Xie et al. [148] Flat sheet Ding et al. [149] Flat sheet Dalmau et al. [145] Ceramic flat sheet Chen et al. [139] Tubular Ceron-Vivas et al. [150] increasing sludge viscosity rather than permeate viscosity, decreasing flocculation tendency of activated sludge and floc sizes, improving EPS and SMP production, and lowering the deposition of particles on the membrane due to lack of shear stress and a reduction of the back-transport velocity of particles. [23,38] All of these changes have adverse effects on the membrane performance and cause the membrane to foul severely. As is illustrated in Figure 11, van den Brink et al. [113] also confirmed poor membrane permeability at lower temperatures by conducting a flux step experiment at three different temperatures. The effect of temperature on membrane fouling has been investigated in several studies. Wang et al. [166] observed a higher concentration of EPS, polysaccharides, and proteins at lower temperatures, which led to severe membrane fouling since biopolymers were attached to the membrane surface. After several experiments, van den Brink et al. [113] found a high polysaccharide concentration at a lower temperature and a reduction of particles size in mixed liquor, resulting in pore blocking and membrane fouling. Similar results were obtained in other studies. [95,99,113] Miyoshi et al. [167] concluded that at lower temperatures, reversible fouling (eliminable by physical membrane cleaning) occurs due to a concentration of dissolved organic matter in the mixed liquor suspension with high amounts of polysaccharide and humic acid substances. According to Krzeminski et al., [141] sludge settleability and filterability become weak at low temperature conditions due to a lower biomass concentration, higher volatile suspended solids/total suspended solids ratio, and less biodegradation of feed water. The higher temperature can affect microbial communication and cause poor sludge settleability and filterability, which facilitate membrane fouling. [168] Highlighting the importance of temperature, Deng [38] suggested that the optimal range of temperature is between C. Generally, the temperature is an important parameter in the membrane design and fouling mitigation strategies. Table 5 demonstrates the summary of the operating conditions affecting membrane fouling. Feed water characteristics Knowing the vital influence of biomass characteristics on membrane performance, it is also important to note that feed water properties also affect the physiochemical characters of liquid suspension. [23] The most common parameters include OLR or/and F/M ratio, nutrients, and salinity of influent, which are discussed below. OLR or F/M ratio: The biological process in the MBRs can be controlled by a F/M ratio corresponding to the presence and extent of organic substrate such as COD and BOD for biomass concentration in the reactor. The differences in the OLR alter Figure 10. Variations of: (a) TMP; and (b) fouling rate with flux for different aeration intensities. [160] Figure 11. Membrane resistance at three different temperatures. [113] 42 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

12 the F/M ratio or DO levels, which are considered as two important factors for microbial community. The variation in the F/M ratio can change the microbial properties and biomass characteristics in mixed liquor (e.g., particle size, viscosity, floc structure, and EPS and SMP production), resulting in membrane fouling. [130,172,173] A higher value of organic substances, which increases the F/M ratio, disturbs the balance between the food supply and mass of microorganisms in the bioreactor, causing higher metabolic activity and microbial growth. Hence, more EPS production (the bound EPS and protein contents) can facilitate the cake layer formation on the membrane surface due to sludge flocculation. In addition, a very low value of F/M ratio restricts the cell growth and causes sludge deflocculation. However, a moderate range of organic loading and F/M ratio might increase filtration and organic removal efficiency through fouling control and an indirect decrease in operational expenditures. [7, ] There are several studies in open sources that have focused on the determination of the optimal value for the F/M ratio; however, this parameter depends on the other variables such as the type of wastewater. For example, the best performance of anaerobic MBRs for the treatment of low strength wastewater streams (with no complex contaminants) was obtained at a F/M ratio of g COD/g MLSS.d with long HRTs 8 h to 2 d, [122,169,177] while for industrial treatment, a higher F/M ratio (3.75 g COD/g MLSS.d) was suggested as an optimal value. [178] It was found that there is a strong correlation between the SMP concentration and the steady state membrane fouling rate. Trussell et al. [179] reported an increase in steady state membrane fouling rate by 20 times due to a four-fold increase in the F/M ratio (from g COD/ g VSS d), which contributed to a higher SMP production. Domınguez et al. [174] concluded that an increase by two times in the loading rate ( to kg COD/m 3 d) leads to an 8 and 5 times increase in the biomass growth rate for UF-MBR and MF-MBR, respectively. Moreover, they found a two-fold increase in the biomass growth rate at a higher volumetric loading rate (1.3 kg COD/m 3 d). Domınguez et al. [174] recommended to operate MBRs with higher loading rates until achieving the steady state conditions and the required MLSS concentration. For the operation of sustainable MBRs, a reasonable range of F/M ratio or OLR should be determined based on operating conditions, such as SRT, HRT, sludge characteristics (MLSS and EPS concentration), and the type of membrane and feed water. C/N or C/P: The carbon to nitrogen ratio (C/N) or/and carbon to phosphorus ratio (C/P), as the nutrient parameters of influent wastewater, have a critical impact on the removal of nutrients by affecting the activity of specific microorganism populations (e.g., autotrophic ammonium, nitrite oxidizing bacteria, and heterotrophic denitrifying bacteria) and biomass characteristics, including particle size, flocculation, settleability, and EPS and SMP production, which lead to the membrane fouling. [38,180,181] It was demonstrated that decreasing the C/N ratio (from 20 to 4) has more influence on the flocculation, settleability, and dewaterability of sludge flocs than increasing the C/N ratio (from 20 to 100). [182] Choi et al. [180] concluded that the highest removal efficiencies of COD, nitrogen, and phosphorus are attained at higher C/N ratios (among the C/N ratios of 4.5, 7, and 10). The results showed that the required C/N ratios for nitrogen and phosphorus removal are over 7 and 10, respectively, at a relatively high SRT value. [180] Fu et al. [181] found that a reduction in the C/N ratio (from 9.3 to 5.3) results in an increase in both the nitrification and denitrification rate, while it decreases the total efficiency of nitrogen and phosphorus removal. Wu et al. [183] investigated the effect of the nitrogen or phosphorus loading on the efficiency of MBRs at steady state conditions. As is shown in Figure 12, a low C/N or C/P ratio (increasing nitrogen or phosphorus loading one-fold) can facilitate the lowering of the fouling rate since the low C/N or C/P MBRs have a greater floc size and less EPS content. Therefore, a very high or low value of nitrogen or phosphorus might change biomass characteristics and deteriorate the membrane performance. Salinity: Among the biological treatment process, MBR systems have emerged as an efficient option for saline wastewater treatment. Various aspects of salinity and their effect on the membrane performance of MBRs have been studied by a number of researchers. For instance, a higher density of saline water causes more resistance to decantation due to a greater buoyant force. Increasing the osmotic pressure at higher salt concentrations also results in cell plasmolysis and the death of microbes Table 5. The effects of operation conditions on membrane fouling Operation condition Impact on membrane fouling References SRT HRT Aeration CFV Flux Temperature SRT"! nitrification capability", F/M ratio#, sludge production# Canziani et al. and Yoon et al. [124,125] SRT"! EPS, SMP, and MLSS deposition", fouling potential" Lousada-Ferreira et al. [63] HRT# and SRT"! biomass growth and SMP accumulation" Huang et al. [122] HRT"! the effect of SRT on biomass concentration is negligible Fallah et al., Meng et al., and Shariati et al. [ ] HRT#! organic loading rate, EPS, and SMP production", floc size#, membrane permeability# Aeration intensity"! deteriorate flocs structures, floc size#, pore blocking" Deng and Park et al. [38,137] Aeration intensity#! EPS, SMP, and MLSS deposition", bioflocculation" Faust et al. and Gao et al. [133,138] CFV"! sludge deposition#, fouling rate# Le-Clech et al. [23] CFV"! shear force", floc size#, flux # Ho and Sung and Jeison and van Lier [169,170] Flux"! EPS and MLSS concentration", sludge deposition", fouling rate" Ng and Ng [153] Flux#! TMP slope#, fouling potential# Diez et al. [154] sub-critical flux! fouling potential# Guo et al. [171] Temperature#! biodegradation#, sludge viscosity", membrane permeability# Le-Clech et al. and Deng [23,38] Temperature#! polysaccharide concentration", particle size#, pore blocking" ven den Brink et al. [113] Temperature#! sludge settleability#, biomass concentration#, VSS/TSS ratio" Krzeminski et al. [141] VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 43

13 and, thereby, decreases particle size and density. The reduction of filamentous bacteria and protozoans in hypersaline wastewater leads to the deterioration of the mechanical integrity and structure of the flocs, sludge deflocculation, poor sludge quality, and a reduction of membrane permeability. [184] In addition, high salt concentration causes more EPS and SMP production due to the incomplete degradation of organic substances and the release of cell contents to protect microbial cells from salt toxicity. [140,185] All these changes eventually result in membrane fouling due to increasing pore blocking resistance. [184,186] In addition, salinity variation may also affect the nutrient and COD removal efficiency. Sharrer et al. [187] found that the removal efficiency of phosphate can also be reduced (from 96.1 to 65.2 %) due to an increase in the NaCl concentration (from 0 to 32 g/l). Yogalakshmi and Joseph [188] revealed that the NaCl shock load change from 5 to 60 g/l results in lowering the COD and total Kjehldahl nitrogen (TKN) removal efficiency from 95 to 64 and 23 %, respectively. It also caused the ammonia removal efficiency to vary from 84 to 13 %. Johir et al. [189] observed a reduction of dissolved organic carbon and ammonia removal efficiency from 77 and 93 to 10 and 0 % by increasing the NaCl concentration to 35 g/ L, respectively. Thus, the research outcomes imply that carbon removal is not appreciably affected by the wastewater features/ characteristics; conversely, nitrification is strongly affected by salinity. The same conclusions were made by Mannina et al. [190,191] The effects of salinity on biomass kinetics, membrane fouling, and carbon and nutrient removal were studied by Mannina et al. [190] by employing a sequential batch membrane bioreactor pilot plant. Their results showed that COD removal efficiency is affected by salinity, however, an increase in salinity does not considerably affect the heterotrophic bacteria. [190] Another experimental work was also conducted by Mannina et al. [191] to perform treatment on synthetic shipboard slops using a membrane bioreactor pilot plant. A high extent of sludge viscosity was experienced during the experiments, leading to a decrease in the sludge filterability; irreversible cake deposition seems to be the key mechanism for fouling occurrence. [191] Jang et al. [184] noted the limitations of the nitrification process at high salt concentrations due to a decrease in microbial communities (where Nitrosomonas bacteria is still present). This finding was confirmed by Artiga et al. [192] Nevertheless, Sun et al. [186] reported that a better membrane performance with a lower membrane fouling rate can be achieved through longer acclimated times at high salt concentrations. Microorganisms named halophilic microorganisms or halophiles can biodegrade organic matters from saline water since they require a certain amount of salt to grow. In this case, Figure 12. Effect of C/N/P ratio on membrane fouling rate. [183] microorganisms can be classified in terms of the salt concentration, including nonhalophilic (10 g/l NaCl), slightly halophilic (10 30 g/l NaCl), moderately halophilic ( g/l NaCl), and extremely halophilic (> 150 g/l NaCl). [69] However, Abdollahzadeh Sharghi et al. [185] observed no significant fouling over the treatment process of hypersaline synthetic produced water ( g/l NaCl) with moderately halophilic bacteria. Overall, the biological treatment at high salt concentrations due to the proper adaptation of halophilic microorganisms or sufficient time for acclimation is feasible. [189] The effects of feed water characteristics on the membrane fouling rate are summarized in Table 6. Membrane properties The most common membrane characteristics affecting fouling can be categorized into physical properties (e.g., pore size distribution, porosity, thickness, and membrane configuration) and chemical properties such as membrane materials, which have significant effects on the hydrophobicity/hydrophilicity, zeta potential, mechanical and chemical resistance, and biofouling tendency. [22,38,196] Pore size: The effect of pore size on membrane fouling is a controversial issue. Researchers have explored that a membrane with a larger pore size causes more fouling due to the deposition of organic and inorganic materials on the membrane surface over a long period of operation, while smaller pore membranes experience cake layer formation as a result of rejecting a wide range of materials, which leads to pore clogging and reversible fouling. [23,197] However, the contradictory results were reported in other literature. [198,199] For instance, it was concluded that the effect of pore size on membrane fouling strongly depends on the feed water characteristics, especially floc size distribution and operating conditions such as duration of experiments, cross-flow velocity, and constant flux. [23] Miyoshi et al. [200] showed that the effect of membrane pore size on fouling development is related to the membrane polymeric material. In this study, as is depicted in Figure 13, the larger pores diminished fouling development for PVDF membranes, while the smaller pores resulted in lower fouling for the cellulose acetate butyrate (CAB) membranes. It was demonstrated that this difference is related to different dominant foulants in each membrane, since the dominant foulants were organic compounds and microbial flocs in the PVDF and CAB membranes, respectively. [200] Van der Marel et al. [199] indicated that the influence of membrane pore size on fouling potential is attributed to the membrane porosity and pore morphology. Porosity and roughness: It is expected that membranes with a high porosity have more permeability. Also, the smooth surface morphology of a membrane can prevent fouling development. [197,201] In another study, Choi et al. [202] revealed that the impact of pore structure on fouling development is greater than the roughness of membrane surfaces. [200] Based on the study conducted by He et al., [178] although there is a belief that the larger crevices on rougher membranes lead to more fouling than smaller crevices on smoother membranes, the effect of roughness on membrane fouling has not yet been sufficiently explored. Membrane configuration: As was mentioned before, different types of membranes in MBR processes include the vertical flat sheets, vertical or horizontal hollow fibres, and rarely tubes (generally preferred for side-stream configuration). [23] Cui et al. [203] suggested that hollow fibre membranes are more susceptible to fouling and require more frequent cleaning; however, they are generally cheaper in terms of manufacturing costs. Hai et al. [204] 44 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

14 Table 6. The effects of feed water characteristics on membrane fouling Feed water characteristics Impact on membrane fouling References F/M ratio C/N or C/P ratio Salinity F/M "! metabolic activity", microbial growth", EPS production" Khan et al. and Liu et al. [172,175] F/M #! cell growth#, sludge deflocculation four-fold rise in the F/M ratio!20 times increase in the steady state membrane fouling Trussell et al. [179] rate C/N or C/P ratio #! membrane fouling rate# Wu et al. [183] C/N ratio#! organic matter and nutrients removal efficiencies# Mannina et al. [193,194] C/N ratio"! protein content", EPS", cake layer" Di Trapani et al. [195] decreasing C/N ratio has more influence on the flocculation, settleability, and Ye et al. [182] dewaterability of sludge flocs, compared to increasing C/N ratio Salinity "! osmotic pressure", cell plasmolysis and death of microbes, floc size#, pore Jang et al. and Abdollahzadeh blocking" Sharghi et al. [184,185] Salinity "! incomplete degradation of organic substances, EPS and SMP production" De Temmerman et al. and Abdollahzadeh Sharghi et al. [140,185] investigated the fouling phenomenon using hollow fibre and flat sheet membranes (with the similar pore size of 0.4 m at the same flux rates). They observed that slightly more fouling occurs in the flat sheet membranes due to pore blocking, which can only be removed by chemical cleaning. According to Judd, [205] the price of flat sheet membranes is estimated to be % higher than that of hollow fibre membranes, however, their fouling rate and maintenance operations are less than hollow fibre membranes. This might be attributed to higher fluxes in hollow fibre membrane operations (23 33 l/m 2 h). Hence, the main differences between these two types of membranes are related to the different maintenance and operating conditions rather than the membrane configuration. [205] Membrane materials: Parameters such as strength, integrity, foulant-resistant ability, temperature resistance, and lifespan should be considered when choosing membrane materials. The most common membranes used in MBRs are polymeric membranes such as polyethersulfone (PES), polyethylene (PE), polyvinylidene fluoride (PVDF), polypropylene (PP), polyacrylonitrile (PAN), and polysulfone (PSF). [201] According to other literature, PES and PVDF membranes have high strength and permeability, while PVDF membranes have a longer life due to higher elongation and more chemical and chlorine resistance than PES membranes. It was found that PE and PP membranes can be dried out easily in air based integrity tests and they are chemical and biodegradable resistant. [206] Currently, ceramic membranes, textile materials, and non-woven meshes are being successfully used for several MBRs due to better chemical, physical, thermal and hydraulic resistances, higher integrity, longer lifespan, and effective fouling control, in comparison to polymeric membranes. However, they cannot be widely used due to their higher costs. [139,186] Hydrophobicity/Hydrophilicity: Whether the hydrophilicity affects the membrane fouling or not has been a commonly asked question. It is generally believed that hydrophobic membranes exhibit higher membrane fouling potential than hydrophilic membranes due to the interactions between feed water, microbial cells, and membrane materials. [207] van der Marel et al. [199] also indicated that lower fouling occurs in the higher hydrophilic membrane, complete asymmetric structure, and highly porous membrane (with large pore size). Maximous et al. [208] also observed that more pore plugging occurs and higher amounts of polysaccharide and protein in SMP are rejected in hydrophobic MBRs due to cake layer formation on the membrane surface, which results in a lower permeability. Fang and Shi [209] reported that a membrane with higher hydrophilicity is more vulnerable to the foulant deposition due to having more open pores and a notable extent of hydrophilic EPS, which is an important foulant. Despite the importance of the surface hydrophilicity of membranes for interactions between membrane and foulants and considerable effect on the membrane fouling reduction, this parameter has a minor role in membrane development, compared to other membrane properties. [23,200] It was reported that the membrane surface zeta potential and roughness have more influence on the interactions between the membranes and foulants than membrane hydrophilicity/hydrophobicity. Higher zeta potential and higher roughness might mitigate fouling by inducing stronger electrostatic double layer interactions and repulsive interactions. [210] Contradictory results of the effect of membrane properties on fouling development imply the difficulty that researchers face when attempting to evaluate the effects of membrane properties on the evolution of membrane fouling. [200] The influences of membrane properties on membrane fouling in different MBRs are tabulated in Table 7. Figure 13. Relationship between membrane pore size and fouling rate. [200] FOULING CONTROL STRATEGIES As membrane fouling mitigates the performance of MBRs and membrane lifespan, it should be controlled for economic reasons VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 45

15 and to ensure long-term operation during which the operational and maintenance costs are minimized. The adjustment of operating conditions during the optimum and influent pretreatment can reduce the potential for membrane fouling. Fouling control strategies that affect fouling mitigation (not complete elimination) include physical, chemical, biological and mechanical cleaning, the addition of adsorbents and coagulants, and the modification of membrane surface properties. [40,47] Membrane Cleaning Periodic membrane cleaning is employed to remove foulants and to restore membrane productivity. [211] Membrane cleaning operations need to be carried out before the TMP exceeds the critical values defined by the membrane manufacturer. This implies that the membrane cleaning protocols provided by manufacturers might not be enough to clean fouled membranes. There are various process and membrane parameters that affect the effectiveness of membrane cleaning, therefore, the mechanisms involved in the membrane cleaning process still need further experimental and modelling investigations. [212] The cleaning processes are categorized into the physical, chemical, and biological cleaning operations. Depending on the type of foulants, an appropriate cleaning operation is chosen. As was discussed in the Membrane Fouling Classifications section, physical cleaning can be applied in the case of removable (reversible) fouling due to the deposition of a mixture of foulants that causes cake layer formation. Chemical cleaning can be employed for irremovable fouling cases, which result from the adsorption of dissolved and colloidal matters, leading to pore blocking. A further discussion is given in the following sections. Physical cleaning Physical cleaning, which is required to control reversible fouling by removing deposited particles and cake layer, includes air scouring, backwashing, and relaxation. Recent studies investigated other methods such as dynamic shear-enhanced filtration by vibration, rotation, reciprocation membrane movement, and ultrasonic cleaning. [38,42] Various physical cleaning methods are briefly explained below. Aeration: Air scouring, or aeration is accomplished using coarse bubble to remove reversible fouling and concentration polarization (which results from an accumulation of rejected materials next to the membrane, forming a boundary layer) through crossflow velocity and shear stress. Furthermore, aeration is required for maintaining the sludge flocs that are suspended in the reactor and for providing DO for microorganisms. [50] Sun et al. [213] revealed that the required energy for aeration is about 50 % of the total energy consumption in MBRs. Therefore, several studies have focused on the optimizing of aeration in terms of the aeration rate, bubble size, and aeration mode (intensity and duration) by intermittent or cyclic aeration. [50,214] Intermittent aeration at s intervals lowers the fouling rates more efficiently than continuous aeration at the same aeration rates. [215] It also saves approximately 50 % of aeration energy consumption. [216] Fouling reduction and 50 % energy saving were also reported by Fan and Zhou [217] who employed a cyclic aeration of 10 s on and 10 s off. In addition, intermittent aeration can significantly increase nutrient removal in MBRs by enriching microorganisms that are responsible for nitrogen and phosphorus reduction. [ ] On the other hand, it was found that cyclic aeration induces sludge deflocculation and thereby changes the fouling rates. [214,221] Hence, the parameters affecting intermittent aeration such as aeration rates and cycle intervals should be optimized to effectively control fouling for the sake of better MBR operation efficiency. Backwashing and relaxation: Backwashing and relaxation can mitigate reversible fouling by removing cake layers. In backwashing, the permeate flow is reversed to detach particles deposited on the membrane surface and relaxation is achieved by stopping the filtration process (no permeate flow) to relieve the membrane from the produced pressure. [38,47] Rotating speed: Increasing the rotation speed leads to a higher membrane filtration efficiency by encouraging fouling control. [222] Jiang et al. [223] confirmed that rotating flat sheet MBRs result in lower fouling potential than with conventional MBRs with the same energy consumption. A critical speed of 60 rpm was suggested by Wu et al. [224] In another study, Paul and Anderson Jones [225] observed that the contribution of rotation in fouling reduction is only 12 %, while air scouring has more influence on fouling. Vibrating: Different movements and mechanical forces generate various shears in vibrating MBRs to avoid the fouling phenomenon. Different designs of vibrating MBRs have been suggested, including a novel magnetically induced membrane vibrating module (MMV), [226] transverse vibration system, [227] and highfrequency power vibration (HFPV). [228] Vibrating MBRs with the implementation of cyclic vibration lead to a low air scouring operation due to the enhanced critical flux and, thereby, to the improvement of membrane performance by reducing the fouling rate. [ ] It should be noted that this finding has been obtained based on small-scale tests. Reciprocation MBR: The reciprocation MBR can affect fouling mitigation due to the inertial force on the membrane by the Table 7. Summary of membrane properties in different MBR systems Membrane type Membrane material Pore size (mm) Surface area (m 2 ) Flux (L/m 2 h) Fouling resistance (10 12 /m) Type of Wastewater References Flat sheet PES Municipal Wang et al. [74] Flat sheet PE Municipal van den Brink et al. [113] Flat sheet PVDF Synthetic domestic Xie et al. [148] Flat sheet (NF) PES 15 nm Synthetic grey water Ding et al. [149] Flat sheet (UF) Ceramic Real domestic Chen et al. [139] Flat sheet (UF) PES Industrial Deowan et al. [147] Hollow fibre PVDF Synthetic domestic Deng et al. [128] Hollow fibre PVDF Municipal Ma et al. [164] Hollow fibre PVDF Municipal Dıaz et al. [132] 46 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

16 horizontal reciprocating movements without the use of air scouring. [230,231] Moreover, the generated turbulence can be effective in eliminating deposited solids from the membrane surface. Ho et al. [230] reported that the energy consumption in the reciprocation MBR is about 75 % less than the conventional coarse air scouring in an MBR system. Reciprocation MBRs are not only free of the hydrodynamic limitations of air scouring or crossflow membrane systems, but also exhibit less energy consumption and higher energy efficiency. [230] Chemical cleaning There are some chemical cleaning protocols such as chemically enhanced backflush (CEB), cleaning in place (CIP), recovery cleaning, and ex-situ cleaning. CEB and CIP are carried out frequently on a daily/weekly and monthly basis, respectively. [232] Chemical cleaning is conducted by using chemical reagents (as is described in Table 8) such as acids, bases, disinfectants or oxidants, and other in-situ and/or ex-situ chemicals to eliminate the organic, inorganic, and biological foulants from the membrane surface, leading to the prevention or treatment of irremovable and irreversible fouling. [41,47] Choosing a suitable reagent to be compatible with the membrane is necessary because some reagents damage the membrane surface. In addition to the reagent type and concentration, other parameters, including temperature, ph, flow rate, and cleaning time should be considered. [211] Chemical reactions between reagents and deposited foulants affect filtration efficiency recovery. For example, variations of foulant characteristics can occur by acids and bases through hydrolysis and solubilization, while the deterioration of the structure and components of foulants are caused by oxidants due to the oxidation of functional groups of foulants. [233,234] The insitu chemical cleaning such as CEB, CIP, and maintenance cleaning can be carried out during the normal MBR operations without removing the membrane module and conducting the intensive cleaning frequency. In contrast, the ex-situ chemical cleaning is required at the severe fouling conditions through transferring out the membrane module and immersing it into the cleaning tank with chemical reagents. [30,47] Although, the polymeric membranes have a certain thermal, mechanical, and chemical resistance, the frequent exposure to chemical reagents results in adverse effects caused by the deterioration of membrane properties such as pore size, surface porosity, surface hydrophilicity, integrity, mechanical resistance, and life span. [235,236] It was reported that NaOCl, NaOH, and acids increase the pore size and surface porosity of PVDF, PES/PVP, and PE membranes, respectively. [35,237] Furthermore, Ettori et al. [238] noticed an enhancement of the surface hydrophobicity of the polyamide membranes after membrane chlorination. Obviously, these changes significantly affect the membrane performance and filtration efficiency. Adding chemical reagents also results in the production of some byproducts, foaming, cell lysis, and interruption of bioprocess through affecting the microbial community. [48,233] For instance, the addition of NaOCl can lead to extra exopolysaccharides production, which causes more fouling. [239,240] Piasecka et al. [241] investigated the impact of NaOCl on the mitigation of microbial enrichment in the cake layer. In another study, Wang et al. [35] suggested that the main reason for membrane fouling reduction is a considerable decrease in filamentous bacteria at lower NaOCl dosage. Therefore, the response of bacterial communities to the chemical reagents such as NaOCl can be different such that very high or low values of NaOCl might lead to severe membrane fouling. [35,64] In addition to the adverse effect of chemical cleaning, Cote et al. [242] highlighted the high operational costs of more frequent chemical cleaning processes. Hence, membrane replacement as an alternative solution is suggested in some severe cases. Generally, it is concluded that the selection and optimal operation of cleaning techniques for membrane fouling reduction are based on membrane integrity, permeability, lifespan, and economic considerations. Biological cleaning Biological cleaning with less adverse effects on the microbial community and membrane characteristics is an efficient and sustainable technique to eliminate membrane fouling. Generally, biological cleaning techniques can reduce the biofilm formation potential by affecting cell-cell or cell-membrane interactions and decreasing microbial activity without killing the deposited cells. [41,243,244] The biological cleaning methods control the fouling mainly through enzymatic and bacterial degradation of biopolymers and the optimization of the cake layer structure such as quorum quenching, D-amino acids, protozoan, and metazoan. [111] As mentioned earlier, biopolymers such as EPS and SMP have been considered as the important foulants. Hence, their biodegradation is crucial for the membrane fouling control. The enzymes existing in the cake layer, sludge supernatant, and the enzymes that are released from dead cells can biodegrade the proteins and polysaccharides of EPS and SMP. [245] Nevertheless, the utilization of enzymes in fouling control in MBRs is limited due to their low stability in the mixed liquor as well as their high cost. [244,246] In this case, using bacteria is suggested as an alternative method. [247,248] Zhou et al. [248] observed that polysaccharides are much less biodegradable than proteins. They discussed the importance of isolation and enrichment of polysaccharide-degrading bacteria in the mixed liquor to increase the biodegradation rates through the over-expression of polysaccharide-degrading enzymes by gene editing. [30] Quorum quenching (QQ) MBRs led to less fouling without the alteration of mixed liquor properties through the degradation and inhibition of N-acyl homoserine lactones (AHLs). They also involve biochemical molecules, which can control bacteria population densities and thereby biofilm formation. [243] Kim et al. [44] entrapped QQ bacteria (Rhodococcus sp.) to prepare cell entrapping beads (CEBs). Less EPS production and biofilm detachment from the membrane surface was observed due to the scouring and biological effects of CEBs. Lee et al. [249] immobilized QQ enzyme (acylase) into magnetically separable mesoporous silica by enzyme adsorption and subsequent crosslinking processes. Their results showed a higher stability, separation magnetically, and antifouling properties of the system for a long operation time. In addition to QQ enzymes, a trace concentration of D-amino acids can be effective in the reduction of membrane fouling by controlling biofilm through the inhibition of protein synthesis. [250] This is a low-cost technique since it can be synthesized and released by different bacterial specious. However, a few studies indicated that some bacteria are not susceptible to the D-amino acids. [251,252] Therefore, the inhibitory effects of D-amino acids on biofilm formation are strongly dependent on the bacteria species. It was found that the D-amino acids are not effective on the inactive and dead bacteria in the bottom of the cake layer. [30] The addition of protozoan or metazoan (as the major consumers of bacteria) can mitigate the fouling rate by decreasing the biofilm formation that results from predation. [ ] The protozoan or metazoan species, such as worms, can influence the amount or VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 47

17 Table 8. Different chemical reagents for membrane cleaning and corresponding mechanisms Acids Chemicals Reagents Foulants Mechanisms Citric acid, HCl, H 2 SO 4, HNO 3,H 3 PO 4 - Inorganic foulants (e.g., multivalent cations) - Solubilization, neutralization - Biological-induced precipitants - Chelation Bases NaOH - Large organic particles (e.g., colloids and microbes) - Hydrolysis, Solubilization, Saponification - Organic matters (e.g., protein and carbohydrate related foulants) - Fat and oil Oxidants and disinfectants NaOCl, H 2 O 2 - Organic foulants - Oxidation, Disinfection - Biological foulants - Colloids and microbe cells Other chemicals EDTA, STP, DTPA, - Biopolymers associated with metal ions - Chelation/ligand exchange reaction Surfactants and related detergents - Macromolecules (e.g., proteins) - Organic foulants - Hydrophilic/hydrophobic interactions - Bacteria Emerging chemicals FNA, - Inorganic and biological foulants - Solubilization and disinfection Rhamnolipid - Organic and biological foulants - Hydrophilic/hydrophobic interactions composition of EPS and SMP, and cake layer structure. [ ] Derlon et al. [256] noticed more heterogeneous and porous structure of the cake layer due to the addition and movement of protozoan and metazoan in the cake layer. Addition of Coagulants, Flocculants, and Adsorbents Incorporating additives such as coagulants and adsorbents in sludge suspension is another effective technique to reduce the potential for fouling by enhancing flocculation through neutralizing the negative charges of biomass and adsorbing colloidal and soluble substances. [258] Wu and Huang [259] and Yang et al. [260] investigated the influence of coagulants such as polymeric ferric sulfate (PFS) and polymeric ferric chloride (PFC) on membrane performance. It was demonstrated that PFS can be effective for the reduction of membrane fouling by increasing the sludge floc size by enhancing the charge neutralization of organic particles. Also, PFC can improve phosphorus removal and reduce membrane fouling. Severe membrane fouling at higher concentrations of PFC is experienced due to the formation of a compact gel layer of organic substances. Natural zeolites and powdered activated carbon (PAC) have been introduced as adsorbents to decrease biopolymer concentration and specific resistance of the cake layer. [261] According to other literature, fresh PAC eliminates biopolymers and fine colloids in mixed liquor through various mechanisms such as adsorption, decomposition, and biodegradation. Hence, membrane pore blocking decreases through the development of a more porous and less compact cake layer with more stability. [ ] It was reported that the PAC-MBR can be effective in fouling control under stressful conditions including salt shock. [265] Recently, researchers introduced granular activated carbon (GAC) as a fluidized medium to provide additional scouring on the membrane surface and to adsorb SMP in the mixed liquor. [ ] In addition to coagulants and adsorbents, adding flocculants can mitigate the fouling rate by decreasing gel layer formation and membrane pore blocking due to the accumulation of destabilized particles and the reduction of their concentration and molecular weight distribution in the supernatant. [269] A research investigation performed by Guo et al. [270] showed that adding natural organic flocculants (e.g., chitosan) causes more stable sludge volume indexes (SVI) and specific oxygen uptake rates, while the addition of inorganic flocculants (e.g., FeCl 3 and polyaluminium chloride) leads to lowering SMP concentration and membrane fouling. There are several research studies in the literature that focus on the reduction of membrane fouling by adding flocculants including cationic polymers and starch, [271] organic flocculent, [272] and modified starch and its polyacrylamide-starch composite flocculent. [273] It was concluded that all flocculants have a significant impact on fouling mitigation. Moreover, Nguyen et al. [274] investigated the combination of inorganic-organic flocculent (e.g., FeCl 3 and MPE50). A stable SVI and low TMP were observed after adding FeCl 3 and MPE50, resulting in decreasing membrane fouling. Since flocculants can generate secondary pollutants, a natural starch-based cationic flocculent (e.g., HYDRA Ltd., Hungary) was suggested by Ngo and Guo [275] by using a new green bioflocculent as the safe biodegradable natural flocculent. Although, using additives (instead of chemical membrane cleaning) leads to less energy consumption and low maintenance costs, a high dosage of them may deteriorate membrane permeability because of increasing the sludge viscosity and EPS levels. It also causes the adhesion of more organic substances and biopolymers to the membrane surface and the formation of a nonporous and dense cake layer, resulting in more pore blocking. [263,276,277] In addition, the utilization of additives in full-scale MBRs is generally limited due to the high cost of chemicals for long-term implication. Addition of Media The addition of media and hard particles (carriers) into the mixed liquor plays a crucial role in fouling reduction through the following mechanisms: (i) providing more mechanical scouring on the surface of membranes; (ii) adsorbing the biopolymers (e.g., EPS and SMP); and (iii) developing more porous cake layers. [278] There are three kinds of media, namely plastic media, sponge, and biofilm carriers. The fouling controlling mechanisms of these various media are different. The addition of plastic media in the supernatant of mixed liquor can develop a stable fluidized bed and enhance the scouring and cutting effect on the existing foulants. [ ] Another important medium is a sponge, and its advantages are high porosity and specific surface area, stability to hydrolysis, and low cost, when compared to plastic media. It was shown that sponges can enhance permeate flux, increase biofilm and bacterial development (by 48 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

18 providing both attached and suspended growth), and postpone cake layer formation and pore blockage. [171,282,283] Tien Thanh [284] observed TMP reduction, microbial activity improvement, and stable SVI maintenance by adding a sponge to the mixed liquor, when compared to conventional MBRs. Moreover, sponges can improve phosphorus and nitrogen removal by boosting the bacterial activities through alternating anoxic/aerobic processes in their pores. [285] Deng et al. [286] suggested that sponge-modified plastic carriers provide better membrane performance than conventional plastic media by lowering the concentration of SMP in mixed liquor and EPS in activated sludge and, ultimately, reducing the cake layer and the pore blocking resistance of the membrane, as is schematically depicted in Figure 14. In contrast, Yang et al. [4] observed more membrane fouling by using nonwoven as carriers in the reactor. It is concluded that the mechanical and cutting effects of sponges and nonwoven carriers on the membrane surface are weak since they are too soft to provide strong scouring. [287] Thus, the effect of media on fouling reduction strongly depends on their nature or properties. Recently, it was found that biofilm carriers (biocarriers) can exhibit positive effects on fouling control where carriers that are denser than water are used as they can move freely inside the reactor and provide larger surface areas for biomass growth. [11,139,286] Moreover, the complete nitrification of influent ammonia was reported by Mannina et al. [288] in the presence of biofilm carriers. In their work, the performance of an integrated fixed film activated sludge MBR system was examined, where variations of solid retention time (SRT) and hydraulic retention time (HRT) were considered. It was found that the system performance is improved by decreasing the SRT/HRT. They also concluded that shorter SRT/HRT results in higher heterotrophic biomass activity. [288] Important factors in choosing biocarriers include specific surface area, porosity, and density to promote biofilm attachment and growth, non-toxicity, and chemical and mechanical stability. [46] It was reported that adding biocarriers to MBRs leads to an increase in membrane filtration and alleviate membrane fouling because of improving mixed liquor properties, decreasing biopolymers concentration, and providing a strong mechanical scouring effect on the membrane surface. [46,139,289,290] Membrane Surface Modification Membrane modification is taken into account as an effective strategy to alleviate membrane fouling by changing membrane surface properties, especially hydrophilic modification. [291] Since most polymeric membranes are weakly hydrophilic, efforts have been made to enhance the hydrophilicity of membrane through physical and chemical methods. [292,293] The techniques commonly used for membrane modification include blending, [294] grafting, [295] coating, [147,296] plasma treatment, [297,298] surface chemical reaction, [294,299] and nanoparticle incorporation. [292,300,301] Among these techniques, the incorporation of nanoparticles into/onto polymeric membranes has attracted great attention in the recent years. It was reported that the nanoparticles can improve surface hydrophilicity and increase the antifouling properties by enhancing membrane porosity, permeability, selectivity, and strength as well as decreasing membrane surface roughness and altering some mechanical, thermal, and magnetic surface characteristics. [298,30,303] The main characteristic of this technology corresponds to the enhanced interactions of well-distributed smaller particles in order to use nanoparticle sites more efficiently for flux enhancement and fouling reduction. [292] Figure 15 schematically illustrates the effect of nanoparticle presence on membrane roughness and biofilm formation. As depicted in Figure 15, membrane surface roughness and biofilm formation can be reduced by increasing the nanoparticle loading rate, which results in enhancement of the permeate flux. Figure 16 shows a cross-sectional view of a PES membrane in the presence of different percentages of nanoparticles. As is clear from Figure 16, nanoparticles provide an asymmetric structure with finger-like macro pores and a dense layer formation at the top layer of membrane, leading to a better membrane performance. Several research works have studied the influences of various types of nanoparticles. Ag nanoparticles have been successfully implemented in PSF ultrafiltration membranes, [304,305] PVDF membranes, [306] and thin film composite (TFC)-RO membranes. [307] The main drawback of Ag nanoparticles used in the modified membrane is the depletion of Ag over time. There are numerous studies on the use of ZnO nanoparticles for the modification of membrane surfaces. [308,309] The results showed that the incorporation of ZnO nanoparticles leads to an increase in the permeability of the PVDF membrane around five times [310] and the permeability of the PES membrane by %, [311] compared to the neat membranes. Another important nanoparticle is TiO 2 with the superior photocatalytic oxidation property, which can improve the antifouling characteristic of the membrane through synergetic effects in combination with hydrophilicity. [9,312,313] It was shown that the TiO 2 nanoparticles utilized in the membranes have a selfcleaning ability and that no membrane cleaning is required during MBR performance. [314,315] The TiO 2 nanoparticle membranes can be performed for long-term tests since TiO 2 nanoparticles, as catalysts, are not consumed over time. However, TiO 2 nanoparticles are more expensive than ZnO nanoparticles. Carbon nanotube (CNT) incorporated membranes have attracted great interest as an important technique for membrane surface hydrophilicity modification since they have functional groups containing oxygen that improve membrane permeability. [196,316,317] A number of research studies have discussed the antifouling effect of CNTs and have shown their excellent antimicrobial ability, hydrophilicity and anti-biofouling properties, and their considerable impact on improving permeability. [303,318] Moreover, CNTs last longer than traditional nanoparticles. [319] Electrically Assisted Filtration Recently, electro-mbrs have been recognized as an effective method for membrane permeability improvement, which can be performed externally (e.g., electrocoagulation and electrophoresis) and internally (e.g., microbial fuel cell). The electrocoagulation-integrated MBRs, which are composed of one anode and one cathode powered with direct current, can provide the interactions of electric currents with bacteria and membranes. [320,321] The electrocoagulation reduces the fouling rate by affecting soluble and bound biopolymers, respectively, due to charge neutralization and sorption and chemical oxidation. [322] Indeed, using a metal (e.g., iron) as an anode can mitigate the deposition of foulants on the membrane surface. Moreover, electrocoagulation changes the mixed liquor characteristics by increasing the elimination of biopolymers, controlling the overgrowth of filamentous bacteria, enhancing sludge settleability and filterability, and improving the removal of heavy metals. [ ] In electrophoresis-integrated MBRs, the cathode is usually employed as a part of the membrane module to remove the foulants from the membrane, and the anode is submerged into the VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 49

19 Figure 14. Experimental set-up of sponge modified plastic carrier (modified according to other studies [286] ). mixed liquor. This technique affects membrane fouling by creating an electrically repulsive force between the foulants and membranes. In addition, the electro-generated H 2 O 2 can provide in-situ membrane cleaning. [326,327] Liu et al. [328] concluded that electrophoresis-integrated MBRs are more effective in reducing irreversible fouling than total fouling. The microbial fuel cell (MFC) can be integrated with MBRs since wastewater streams contain a significant amount of energy to be converted into electricity. [329,330] In this method, the conductive membranes are often used as a cathode so MFCs and MBRs can obtain the benefits from each other. [331] Thus, membrane filtration improves the effluent quality of MFC, and MFC reduces the polysaccharides of bound EPS, [332] prolongs the membrane filtration by increasing the TMP rates, [81] and decreases the total energy consumption of MBRs. [333,334] TECHNICAL AND NON-TECHNICAL ASPECTS AND CHALLENGES Industrial applications of MBR technology are growing fast, and membrane bioreactors are being widely used for wastewater Figure 15. Membrane fouling and surface roughness in terms of nanoparticle loading rate (modified according to other studies [302] ). 50 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 97, JANUARY 2019

20 Figure 16. Cross-section SEM images of PES membrane: (a) unfilled PES; (b) 0.05; (c) 0.1; and (d) 1 wt% NH 2 -multi-walled CNTs (adapted from other studies [303] ). treatment in textile, food, petrochemical, and pharmaceutical sectors/industries. [5] Adequate knowledge and data on the characteristics of wastewater components or pollutants, including type, colour, ph, suspended solids, biochemical oxygen demand, chemical oxygen demand, concentration of heavy metals, oily components concentration, and turbidity, are needed to choose an appropriate type of MBR and to efficiently design the important parts of MBR systems. For instance, hybrid MBRs and submerged MBRs are generally employed for the treatment of mixed wastewater and oily wastewater streams, respectively. [8,15] Focal challenges such as high energy consumption, complex biological phenomena, and relatively high transmembrane pressure limit the utilization of full-scale MBRs. The total energy required for the operation of an MBR is a vital factor in whether or not the technology is viable for use in industrial and municipal wastewater systems. It has been confirmed that the aeration stage has a contribution of over 50 % to the total required energy. [32,43] Hence, sludge retention time and membrane aeration time can be the most influential parameters that affect the total costs corresponding to the energy consumption. Based on the real data for municipal and industrial wastewater treatments, the rate of energy consumption varies between kwh/m 3 for largescale plants, and about 80 and 5 % of the total costs are attributed to the aeration and permeate suction/permeate back-flush phases, respectively. [5,43] Currently, a number of various MBR configurations including external, submerged, and airlift are generally used in treatment plants to overcome these critical issues. Membrane feed characteristics as well as design and operational parameters affect membrane performance and fouling. In addition, it is important to comprehensively characterize the complicated nature/structure of membrane foulants and activated sludge to adequately understand fouling behaviours in MBRs. These complexities deter the timely industrialization of MBRs in particular industries where wastewater streams contain complex chemical components and oil. Sludge is characterized by important parameters such as ratio of food to micro-organism (F/M), sludge retention time (SRT), hydraulic retention time (HRT), and dissolved oxygen (DO). [8,43] Thus, the optimization of these parameters can lead to positive modifications of activated sludge, resulting in fouling reduction. Numerous research studies have been conducted to avoid fouling in MBR systems. [5,8,15 25] There are two logical and feasible ways to combat this problem. The first strategy is to fully understand the treatment process and to optimize the central operating and design parameters, such as aeration intensity, filtration mode, temperature, pressure, membrane pore size distribution, organic loading rate, membrane surface modification, and suction time to non-suction time ratio. The second efficient approach is to enhance the filterability of mixed liquor by adding proper chemical and biological components such as zeolite, activated carbon, and diatomite to MBRs. [43] Biofilm formation eventually results in blocking the membrane surface or/and pores. In practice, mechanical and chemical cleaning procedures are normally followed to maintain membrane rejection performance and stabilize water flux. The main engineering steps for fouling control involve air scrubbing, backwash by water, initial chemical cleaning process, and restorative chemical cleaning strategy (both ex-situ and subsitu). [5,8] Lab-scale and pilot-scale tests of MBR technology have led to promising outcomes. It is expected that the implementation of MBRs can occur in many countries across the world in the near future due to the global water concern/crisis, stricter environmental regulations, and predominant market situations. The presence of grease, oil, and fats accelerates membrane fouling as the membranes are normally hydrophobic. [7 10] Hence, surface wetness alteration, the identification and characterization VOLUME 97, JANUARY 2019 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 51