Advanced Control Strategies of Membrane Fouling in Wastewater Treatment: A Review

Abstract

Reverse osmosis and microfiltration are two membrane-based separation techniques that have gained popularity over the past few decades. A fundamental barrier to improving the overall efficiency of membrane separation is membrane fouling. The accumulation of foulants causes a reduction in permeate flux, a loss of selectivity and permeability, and a reduction in the membrane’s lifespan. Numerous chemical and physical surface changes have been studied to enhance membrane antifouling properties. Additionally, research has concentrated on creating membranes that use cutting-edge materials to improve their antifouling capability. This paper focuses on a wide-ranging and thorough analyses of the different types of fouling during wastewater treatment based on current research results regarding fouling control strategy and the potential of new methods for wastewater treatment. It is a further step forward for the evaluation of mitigation measures for emerging membrane fouling problems.

1. Introduction

Membrane technologies such as ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and other pressure-driven membranes have become popular in recent years for wastewater treatment [1]. Membrane-based technology has shown significant importance because of its low energy consumption, compactness, and easy integration into existing process [2]. However, the membrane fouling problems in the membrane unit hinder more extensive membrane technology applications [3,4]. Membrane fouling is caused by the buildup of materials that the membrane rejects on the surface of a material, lowering the membrane flux and rejection rates [5]. In membrane fouling, the undesirable deposition of particles, colloids, and solutes from mixed liquor suspended solids on the surface of membrane causes the reduction of the membrane’s lifespan and its performance [6,7]. MBR optimization is an important factor; it can create a mitigation plan, lessen fouling effects and improve operation, save costs and make the membrane more viable [5,8]. Liquid streams are recirculated into membrane distillation (MD) modules close to atmospheric pressure, which reduces the impact of fouling, in contrast to traditional pressure-driven membrane filtration processes [9].

The hydrophilic poly ether sulphone (PES) membrane can easily be prepared by UV-polymerization polyvinyl pyrrolidone membrane (PVP) on membrane surfaces that cause the improved antifouling performance of membrane [8].

Some reviews for membrane fouling in wastewater treatment in membrane bioreactors (MBRs) have been published recently, such as research on factor fouling and control techniques for industrial and wastewater treatment in MBRs [10]. Specific information on sludge conditions, such as sludge viscosity, membrane fouling, and mixed liquor suspended solids (MLSS) or bound extracellular polymeric substances (EPS) effects, such as hydrophobicity and flocculation capacity, was not supplied by these previous studies.

The characterization of fouled membranes and their mitigation measures were critically updated by Drews [11]. However, this review paper did not articulate the membrane fouling affected by some parameters, such as hydraulic retention time (HRT) and EPS. The MBR applications and current research trends were discussed by Le-Clech [12]. However, the HRT was not mentioned as one of the significant characteristics of the membrane. Lin et al. [13] discussed MBR technology applications for industrial waste-water treatment, analyzing and examining over 300 scientific papers on industrial waste-water treatment MBRs.

However, their evaluation was incomplete, and fouling variables were only briefly discussed. Their analysis was not exhaustive but supported the claim that MBRs function in the treatment of industrial wastewater with great strength. Operations and fouling mitigation strategies were generally viewed by Mutamin et al. [14]. However, the information regarding biomass, membrane fouling conditions, and membrane fouling mitigation were inadequate. In most current reviews, anti-fouling technologies are utilized to manage membrane fouling but do not examine controlling membrane fouling causes or processes. Porcelli and Judd [15] have mainly examined chemical cleaning approaches for membrane fouling mitigation and assessed the efficacy of several chemical cleaning agents. Xion [16] has been compiling and analyzing biological methods for microbial fixation control and membrane fouling mitigation.

Additionally, we examined the mitigation ways that have been suggested by researchers in many science and engineering disciplines to augment the present knowledge on antifouling technologies. Several review studies on membrane biofouling in MBRs have been published to date. Guo et al. examined an extensive overview of the various fouling phenomena during wastewater treatment [17,18,19], although biofouling during membrane filtration processes was only partially covered.

The operating expenses of a membrane system, for example, power consumption, work costs, materials, membrane cleaning, scalable inhibition, membrane life, and replacement, are determined by various factors [20]. The rapid drop in permeate flux due to membrane biofouling over time is a significant barrier to the membrane [21]. RO, NF, UF, and MF-related technologies have significant problems with their membrane efficiency and tramping [22]. The previous studies mainly focused on specific membrane fouling issues and their mitigation measures. Therefore, there is a need for a comprehensive report on novel control strategies of membrane fouling. The objective of the current study is to focus on a wide-ranging and thorough analyses of the many fouling issues in membranes based on current research results. It is a further step forward in the evaluation of mitigation measures for emerging membrane fouling problems. The interactions that are thoroughly discussed include biomass features, feed water features and properties of the membrane, and its impact on the fouling membrane. In addition, advanced membrane surface modification strategies for antifouling performance are also discussed. Furthermore, a new fouling strategy, e.g., cleaning of the membrane, is discussed. Finally, future membrane research topics are recommended.

2. Membrane Fouling

Membrane efficiency in a membrane bioreactor is severely hampered by membrane fouling. The main fouling component in MBRs is membrane fouling, which is caused by sludge cake deposition and pore blockage [10]. One of the predictive method for membrane fouling is a mathematical predictive model that is used for modelling the linkages and various filtration parameters, thus helping to optimize the control strategy [23].

Transmembrane pressure (TMP) and permeate flux rise due to membrane fouling. The following factors contribute to the fouling mechanism in MBRs: (1) Membrane surface is deposited by sludge flocs; (2) The membrane’s surface has a cake-like layer formation; (3) Accumulation of substances onto the membrane surface. Membrane fouling can result in undesired microbial growth and deposition and the deposit of cell debris and colloids on the membrane. Membrane fouling is severely increased by the TMP jump. Due to fouling, TMP increases due to local fluctuation causing local flux to exceed critical flux. The fouling resistance in MBRs is crucial for determining the degree of fouling and optimizing operating parameters.

2.1. Membrane Fouling Phenomenon

Membrane fouling may be observed when the permeate flux decreases and the TMP remains constant. However, the majority of wastewater treatment facilities operate in continuous flux mode. This indicates that fouling phenomena often occur where the observed variation in TMP over time can be discovered [24]. When TMP changes over time, two main patterns usually follow. A two-stage TMP jump makes up the initial pattern. In the first step, particles and microbial flocs adsorb into the membrane pores, but the TMP only causes a slight increase. The local fluxes are still below the threshold value (i.e., the number of open pores will decrease as TMP increases). After a long operation, the second stage significantly increases TMP [25].

The second pattern is the TMP three-stage. When this MBR is first operating, the quick reduction in the pores is primarily caused by sludge particle interaction or membrane compacting as illustrated in Figure 1. However, the duration of this phase is followed by the same processes as previously described. In the second step, the first stage is generally hidden. Thus, the pattern resembles a two-stage jump [26]. Membrane fouling phenomenon is shown in Figure 1.

The literature on the MBR treatment of process waters and forest-industrial effluents has recently been updated. Both the aerobic membrane bioreactor may operate in mesophilic (15–40 °C) or thermophilic (40–70 °C) conditions (AeMBR) [28] and an aerobic membrane bioreactor (AnMBR) was employed [13]. The membrane unit was put in a separate vessel or an isolated tank following biological treatment (side-stream MBR) in the main bioreactor vessel. MBRs demonstrated exceptional results regarding reduced environmental effects, decreased sludge generation, high COD removal, and increased biogas production. The economic viability of MBRs is still impacted by the controlling of membrane fouling due to high maintenance costs, air-scoring energy requirements, and membrane-cleaning chemical supplies. Both thermophilic and mesophilic MBRs have experienced fouling. However, because higher temperatures produce more soluble microbial product (SMP) release, submerged anaerobic membrane bioreactors (SAnMBRs) experience considerable membrane fouling [29].

Thermophilic bacteria produce more EPS and are less resistant to bio-flocculation than mesophilic bacteria [18]. The concentrations of EPS were found to be 2.5 times higher in thermophilic activities compared to mesophilic activities. Due to its heterogeneous nature, the EPS are deposited on the membrane’s surface, forming a thin gel layer as a crucial barrier to permeability flow. According to many investigations, the formation of EPS in MBRs is correlated with variables like time of restraint, temperature, and organic loading rate.

Generally, EPS are composed of nucleic acid, proteins, and humic acid compounds. EPS are divided into two categories: binding EPS and soluble EPSs. Irreversible fouling and soluble EPS are frequently linked. Filtering at a high flux reveals that fouling in MBRs can result in a thin gel layer and a sizeable multi-layered cake [30]. Higher pressure and flow rate make it easier for large, suspended particles, microscopic colloidal particles, macromolecules, and proteins to stick to the membrane surface. Additionally, a key element in determining the kind of cake surface layer is particle size. When operating at thermophilic temperatures, the particle sizes often decrease to mesophilic temperatures [28]. Typically, the suspended solids create the cake layer while the soluble colloidal material clogs pores. However, a cake layer could form on the surface membrane if the foulants are too large for the holes in the membrane. Foulants often adsorb and block membrane pores when they are smaller than the membrane pores [10]. Lin et al. [31] investigated bacterial cluster and small block attachment to the membrane surface as the earliest stages of the development of the cake layer. According to Gao et al. [32], the fouling of an MBR by the build-out of colloidal materials could be significantly impacted by a sludge floc split brought on by high pH shocks. Simstich et al. [28] examined the cake layer’s constituent parts. They discovered that calcium, aluminum, barium, and iron had a higher propensity to settle on the membrane surface. Sodium, potassium, and silicon, on the other hand, are less likely to contribute to the fouling layer.

2.2. Classification of Membrane Fouling

2.2.1. Reversible and Irreversible Fouling

Reversible and irreversible membrane fouling can be caused by various complex and interconnected factors, including colloids, solutes, and sludge particle size in mixed liquor. Assume that pore blocking has reduced or equalized the size of the foulants and membrane pores. The cake layer forms on the membrane surface regardless of if the foulants are more severe. In general, irreversible fouling, including chemical cleaning, cannot be removed from the membrane. However, several researchers have demonstrated that irreversible fouling might be removed using chemical cleaning alone, excluding any manual cleaning [33].

2.2.2. Bio Fouling

The term “biofouling” refers to the development, accumulation, and flocs or cells of bacteria on the surface of membrane that interferes with the function of the membrane [34,35]. Due to fewer contaminants than membrane holes, biofouling poses severe issues in low-filtration membranes like microfiltration and ultrafiltration. On the membrane surface, the individual bacterial cells cluster and collect again to create cakes. According to many researchers, the EPS and SMP are the key ingredients that cause the production of the cake layer on the membrane surface [18,19,36]. Biofilms may or may not cover the medium uniformly and may or may not have one or more layers of both live and dead microorganisms and the extracellular products that correspond to them. The two ways in which bacteria agglomerate on membranes are adhesion (bioadhesive and bioabsorption) and growth (multiplication) [37,38]. The features, solution, and surface of the microorganisms are critical factors in the bioadhesion phase. According to Nielsen and Jahn, cell biomass and other extracellular polymeric compounds are the suggested biofilm components [39]. Most foulants on proteins are bacterial cells comprising 50–80% of EPS on the membrane surface. Activated sludge flocs and EPS are the building blocks for biofilms and microbial aggregates. EPS are polar and contain various groups, including carboxylic, phosphoric, sulphuryl, phenolic, and hydroxyl. Since EPS are amphoteric molecules, as shown by their hydrophilic and hydrophobic groups, a hydrophobic membrane is likely to interact with bacteria, resulting in membrane biofouling. Greater exposed surface regions on rough-surfaced membranes as opposed to those on smooth-surfaced membranes increase the chances of biofouling [40]. The common bacteria where buildup on membrane surface are shown are discussed in

Table 1. Identifying common bacteria in biofilms (adapted from [40]).

Microorganism	Examples
Bacteria	Corynebacterium, Bacillus, Mycobacterium, Flavobacterium, Pseudomonas, Cytophaga, Moraxella, Aeromonas, Lactobacillus, Serratia, and Micrococcus.
Fungi	Trichoderma, Mucor, Fusarium, Penicillium, and Aspergillus

and important factors which affect membrane fouling are shown in

Table 2. Important factors relating to membrane biofouling.

Factors Affecting Fouling	Effects of Fouling	References
MLSS	With an increase in MLSS concentration, EPS and SMP both have higher protein and carbohydrate contents.	[41]
OLR or F/M ratio	Low sludge filterability and filtration index are caused by the formation of EPS and SMP when the F/M or OLR ratio is high.	[42]
Dissolved oxygen concentration	Excessive aeration breaks up sludge flocs and raises SMP concentrations, causing poor filterability.	[17]
HRT	Reduced HRT causes the production of EPS by bacterial cells, which raises SMP and results in sludge deflocculation. Additionally, increased filamentous bacterial growth and the formation of enormous, atypical flocs are caused by reduced HRT. On the other hand, excessive HRT leads to the accumulation of foulants.	[43]
Temperature	Increased temperature results in the degradation of biomass, an increase in SMP and turbidity, and a reduction in the protein content of EPS. In contrast, filamentous bacteria grow at low temperatures, increasing the SMP they create in the mixed liquid.	[44]
Nutrients	Protein concentrations in EPS rise due to nitrogen deficiency resulting in negatively charged floc surfaces. Phosphorus deficit in activated sludge causes flocs to have a reduced surface charge, lower protein content in EPS, and to increase.	[45]
Salinity	The physical and biochemical characteristics of activated sludge are significantly affected by higher salt concentrations, which result in higher concentrations of SMP and EPS and poorer membrane permeability.	[39]

.

Organic macromolecules consist of many groups, including polysaccharides, proteins, humic substances, and others, and were discovered both on and outside the cell surface and in the microbial intercellular space. EPS can be discharged into the aqueous phase from microbial aggregates, thus called soluble microbial products (SMP) [39]. Usually, the essential components contributing to the fouling are polysaccharides (as carbs) and proteins.

The techniques involved in bacteria visualization are atomic force microscopy (AFM) and scanning electron microscopy (SEM) which are extensively used to characterize membrane fouling [46,47]. Membrane biofoulings are characterized by a powerful tool known as confocal laser scanning microscopy (CLSM) through which the bacteria are visualized on the membrane surface [48].

2.2.3. Organic Fouling

In MBRs, organic fouling (Figure 2) deposits biopolymers onto the membrane, such as proteins and polysaccharides. Alongside the permeate flow, the small size of the biopolymers makes it easier for them to be deposited on the membranes. However, due to the lifting forces being smaller than those of large particles, the return transport speed is slowed (e.g., colloids and sludge flocs). A more in-depth investigation of the characteristics of deposited biopolymers in MBRs was recently carried out by Metzger [49]. After membrane filtration, the top, intermediate, and bottom layers were separated using washing, backwashing, and chemical purification procedures. The findings pointed to a porous, weakly connected cake layer with sludge flows made of the upper fouling layer as its composition. The intermediate fouling layer also contained SMP and bacterial aggregates, highly concentrated in polysaccharides.

Irreversible fouling mainly comprises SMP components and contains a more significant concentration of proteins. This investigation demonstrated how biopolymers were distributed spatially throughout the membrane’s surface. To aquire in-depth information on the biopolymers deposited, these problems must be identified. The main components of biopolymers, such as polysaccharides and proteins, have been determined using Fourier transform infrared spectroscopy [50]. The 13C-NMR investigations by Kimura et al. [51] also revealed that while foulants contain significant amounts of proteins and polysaccharides, their composition might vary depending on the F/M ratio. It may be deduced with the help of HP-SEC and fluorescence tests that organic colloids, humic materials, and particles that resemble membrane fouling appear to form in membrane fouling layers. Teychene et al. [52] evaluated the soluble composites’ deposition in the fouling layer, where the colloidal fractions influence was lessened (present in the supernatant). According to the outcomes of HP-SEC and fluorescence analyses, polypeptides, similar to proteins, are crucial for MBRs. However, in early research by Rosenberger et al. [53], with a weight of more than 120,000 Da, polysaccharides and other unsettleable organic compounds were found to affect membrane fouling.

Furthermore, high levels of fouling were equivalent to high quantities of polysaccharides in supernatant sludge. According to these results, organic fouling contributes significantly to MBR fouling and is brought on by soluble microbial product (SMP) or extracellular polymeric substances (EPS). The membranes affinity and molecular size considerably impact the deposition of SMP or EPS on them; Figure 2 shows the schematic representation of organic fouling.

Figure 2. Schematic representation of organic fouling [54].

Figure 2. Schematic representation of organic fouling [54].

2.2.4. Inorganic Fouling

Biofouling and organic fouling are more significant components of MBRs than inorganic fouling but during activated sludge membrane filtration, all three-take place at once. Inorganic fouling in MBRs has only been detected in a few investigations; the majority of the research has connected bacterial cell and biopolymer buildup to membrane fouling [19].

According to Ognier et al. [55], in the pilot scale where MBRs using the ceramic ultrafiltration membrane module were used, significant CaCO₃ fouling was observed. The highly alkaline activating sludge may encourage CaCO₃ precipitation. According to the study of Kang et al. [56], inorganic fouling may be easier to spot on inorganic membranes. An inorganic cake may be impossible to remove due to its cohesive properties.

Table 3. Contribution of inorganic compounds for scaling in the membrane system [17].

Cations	Anions
Al3+	F−	OH−	CO32−	SO42−	PO43−
	AlF3	Al(OH)3	-	-	AlPO4
Ca2+	CaF2	Ca(OH)2	CaCO3	CaSO4	Ca3(PO4)2
Fe3+	FeF3	Fe(OH)3	-	-	FePO4
Mg2+	MgF2	Mg(OH)2	MgCO3	MgSO4	Mg3(PO4)2

shows the inorganic compound causes the membrane scaling.

3. Membrane Fouling Mitigation

The mitigation of membrane fouling, restoration of water flux, and reduction in transmembrane pressure may be affected by membrane cleaning [56]. In MBR processes, physical and chemical cleaning is also an important phenomenon for removal of fouling. A single cleaning is not feasible for achieving optimum outcomes during accurate operation. Different techniques are used to clean the membranes for maximum output [55].

3.1. Surface Modification of Membranes

Surface modification and membrane function are frequently used to decrease fouling. Two of the most often modified surface characteristics are the electric load and the smoothness of the membrane surface. Smoother membranes are less likely to foul because the surface prevents foulants from adhering or depositing in groves or valleys. Electrostatic interactions will cause additional fouling events if the foulants are put on top of the membrane. Noble materials must be used to coat and lubricate the membrane to stop these undesirable interactions [57].

3.2. Physical Surface Modifications

Coating the surface of a RO membrane with a sacrificial layer, which serves as a barrier to stop foulant growth and adsorption onto the membrane surface, is a straightforward method for enhancing the anti-fouling capabilities of the membrane. Whether it is coated-to or coated-from, it can be achieved in one of two ways [58]. Many coatings have been investigated. For instance, Son et al. [59] inserted two protective poly-electrolyte layers—one poly positive (diallyl dimethylammonium chloride) (PDDA) and the other poly negative (sodium 4-styrene)—to the commercial RO membrane (PSS). The unbound electrolytes on the membrane surface were removed using an intermediate deionized water wash and five-layer coatings.

To assess the effectiveness of coating in reducing fouling and preventing cases of flux degradation, a series of dead-end filtering experiments using alginate as a foulant in synthetic brackish water were conducted. The details of surface coating with base polymers are shown in

Table 4. Physical surface modification of membranes.

Surface Coatings	Base Polymers	References
Chitosan coatings	Polyamide membrane	[60]
Macro initiators photo reactor coatings from PEG based hydrogel	Polyamide on PSF membrane	[61]
Silver pegylated dendrimer nanocomposite coatings	TFC membrane	[62]
PDMS/PMMA copolymers	PES membranes	[63]
Zwitterionic coating	PDMS coatings	[64]
Ylene glycol diacrylate	TFC-HR, flat sheet Koch	[65]

.

The most successful modification was the uncoated membrane flux, which was around 13 L.m⁻².h⁻¹, whereas the coated membrane’s average penetration was almost 16 L.m⁻².h⁻¹. Layer-by-layer analysis was utilized by Li et al. [66] to assess the performance of a single bilayer polymeric polyelectrolyte membrane. During the separation test, BSA was used as a foulant in a brine feed. The study discovered that using the modified membrane increased the effectiveness of salt rejection by about 2% and demonstrated enhanced anti-fouling capabilities. Similar to this, the sprayers developed by Halakoo [67] used the LbL to introduce anionic graph oxide (GO) and cationic polyethylene (PEI) particles into the membrane matrix, changing the TFC polyamide membranes’ surface (PA).

The modified membrane was tested for the desalination of aqueous solutions comprising the salts NaCl, Na₂SO₄, MgSO₄, and MgCl₂. The highest salt rejection for all salts at different feed rates and temperatures was 99.9%. Several tests were conducted to characterize AFM, SEM, and prospective measurements for streaming. The modular membrane improved antifouling performance in a model screening BSA and achieved a 99.17% salt rejection rate. Gum Arabic-related PVA-RO synthetic membranes were used. This experiment showed that PVA-GA-5 with 0.9 wt% GA had a 98% increase in salt reject, anti-bacterial properties, and an 83% increase in chlorine resistance.

The cellulose membranes in PVA-networked RO also contain nano zeolite-Y at loading ranging from 0.05 to 1.0 wt. The best formulation, which had a 0.5% weight of nano zeolite, resulted in a significant improvement in membrane performance, with flux improvement of roughly 34% and a salt rejection rate of 99.5%. Shao et al. [68] used spin-coating on the surface of the PA-TFC-RO membrane to create assembled GO layers. Chlorine resistance improved with the amount of GO layers. After a 16 h exposure to chlorine, the modified membrane resulted in a rejection of salt of 75% in an ethanol solution. In their study on the modification of GO nanosheets [69], they used membranes made of cellulose acetate and polyethylene glycol (CA/PEG) that ranged in wt% from 0.0025 to 0.0125. Membranes were examined using SEM, touch angle analytics, mechanical analysis (DMA), and Fourier spectroscopy. The best membrane had a 0.01 wt% GA addition and a salt rejection rate of 37%. In a different study, Zhang et al. [70] chelated PEG onto the surface of the PA RO membrane. They found that the modified membrane had a more significant % of salt rejection than the naked membrane, 99.04%, as opposed to 97.8%. The PEG-modified membrane additionally displayed enhanced resistance to organic fouling. The uncoated membrane lost about 40% of its original flow after twelve hours of BSA fouling, whereas the more modern membrane lost around 19%. The predicted decline in the antifouling properties of the modified RO membranes caused by the insufficient hydrogen bonds and van der Waals forces holding the coatings to the membrane surface is a significant barrier to the use of physical modifications.

3.3. Chemical Surface Modifications

Chemical reagents and interactions with the necessary functionality are used in chemical surface modification approaches to modify membrane surfaces [71]. The following is the most common chemical coating technique that has been studied in research: with the aid of protic acids like hydrofluoric acid, sulfuric acid, hydrochloric acid, nitric acid, ethanol, or 2-propanol, the anti-fouling lining is hydrophilized at the membrane’s surface. Miyamoto et al. investigated [71] the mixing of polyvinylpyrrolidone (PVP) into a polysulfone membrane. According to their findings, hydrophobic PSF membranes are prone to NOM fouling. As a result, the hydrophilization process involves a non-solver-induced phase separation in which PV P is added to a PSF membrane.

In free radical grafting, monomers react with free radicals to modify the membrane’s surface. Wei et al.’s research on radical grafting used the grafting monomer 3-all-5,5-dimethyl hydantoin [72]. The unmodified membrane’s biofouling resistance was compared to the modified membrane in a microbial cell solution that had more notable fluctuations and microbe adsorption. In addition, there were ZnO nanoparticles RO grafting membranes (NPs) [73]. Chemical modification has played a vital role and is considered an attractive method to achieve the desirable surface properties of the membrane, such as chemical resistance, mechanical strength, and morphology of the membrane. The details of the chemical membrane modification techniques are shown in

Table 5. A summary of specific studies that used chemical membrane modification techniques.

Method	Modifier	Test Conditions	Permeate Flux (Lm−2 h−1)	Salt Rejection (%)	References
Surface coating	PDDA and PSS	2000 ppm NaCl solution at 4.1 MPa	15.5	99	[59]
LbL surface coating	Pluronic F127 amphiphilic triblock copolymer	2 gL−1 NaCl solution at 4 MPa	30	94	[66]
LbL surface coating	PEI and GO	200 g L−1 NaCl at 65 °C	8.4 kg m−2 h−1	99.99	[67]
Surface coating	SPVA	2000 ppm NaCl solution at 1.55 MPa	42.6	99.18	[74]
Surface coating	Pluronic F127 and Gum arabic	2000 gL−1 NaCl solution at 55.2 bar	-	98	[75]
Slip casting	Nano zeolite-Y	25,000 mgL−1 NaCl solution at 25 bar	5.1	99.52	[76]
Hydrophilization treatment	PVP	-	-	-	[77]
Hydrophilization treatment	Chromic acid	60 °C	61	-	[78]
Free radical grafting	ADMH	200 PPM NaCl solution at 1.5 Mpa and 25 °C	184.5	95.8	[79]
Free radical grafting	ZnO NPs	200 mgL−1 NaCl solution at 15 bar and 25 °C	35	97	[73]
Chemical coupling	PVA	500 mg L−1 NaCl solution at 5 bar and 25 °C	27	98.46	[80]
Chemical coupling	Aldehydes	2000 ppm NaCl solution at 1.6 MPa and 25 °C	37.5	98.6	[81]
Glow discharge plasma treatment	Clinoptilolite	16,000 ppm NaCl solution at 1.5 MPa and 25 °C	-	97.12	[82]
Dielectric barrier discharge plasma treatment	-	-	-	-	[83]
Plasma polymerization and si-ATRP	HEMA, MPC, and SBMA	-	6042	99	[84]

.

3.4. Treatment by Plasma

Plasma polymerization is another name for this method (Figure 3). First, plasma is used to encourage the formation of a polymer layer on the TFC and PA-RO membrane surfaces. In the latter, plasma activates the membrane’s surface oxides and hydroxides, which later participate in different kinds of polymerization [85]. In Safarpour et al.’s work [82], TFC RO membranes were produced and modified using interface polymerization by adding dimethyl sulfoxide and glycerol. SEM, FTIR, and contact angle measurements have been used to define the alteration of the membranes. While maintaining the same performance in salt refusion, dimethyl sulfoxide and glycerol have improved permeate flux, surface roughness, and membrane hydrophilicity. The dielectric barrier dissipation (DBD) plasma approach was used to improve the antifouling capabilities of the PA-TFC-RO membrane [83].The novel membrane has been described using SEM, the total attenuation of the FITR reflex (ATR-FTIR), and contact angle measurements, which show changes in surface shape. The majority of surface modifications must be restricted to membrane fouling control. To ensure effective internal mitigation of the fouling, the modification should include a membrane pore support layer for some membrane processes, such as forward osmosis (FO). However, a very hydrophobic surface membrane is a significant problem for membrane fouling. Therefore, it is essential to thoroughly evaluate the modification technique’s viability in light of several factors [86]. Figure 3 illustrates the schematic representation of plasma treatment.

3.5. Fouling-Resistant Coatings

The fabrications of membranes with antifouling and anti-scaling become more interesting to researchers through the incorporation of nanomaterials or by the use of chemical modifications [87,88,89,90,91,92,93,94]. Proteins, algae, and bacteria cannot settle on fouling-resistant coatings [95]. This coating primarily depends on surface chemical changes to avoid unwanted adherence. It frequently has highly hydrated surfaces with high interfacial energy to prevent any chance of foulants attaching or replacing themselves. Flexible tools for manipulating or altering the surface interface energy are polymer brushes. It can be described as a dense arrangement of polymer chains that are extended into solutions and connected end to end to an interface. They make it possible for function groups that combat adhesion, germs, and corrosion to be easily integrated. Additionally, their high polymer density is a physical and free energy barrier to keep particle fouling at a distance. This barrier is often connected to a firmly bound water surface. Thus, polymer brushes are the main topic of this section and the most remarkable anti-fouling brushes from the previous five years are highlightable [96]. The most cutting-edge antifouling brush has developed from this finding. However, polyethylene glycol, the gold standard, must first be introduced.

Table 6. Surface modification of fouling-resistant coating.

Membrane	Coated Material	Application	References
Polyether sulphone	TiO2 nano-particles	Ultra-Filtration	[97]
Poly-propylene	Polyhydroxylated	Fltration	[98]
PVDF composite membrane	Polydimethylsiloxane	Separation of VOCs	[99]
Utem/P84 co-polyamide	Al2O3	Gas separation	[100]
Polypropylene	Fluorosilicone	Polydimethylsiloxane	[101]

Shows the details of the surface modification of fouling-resistant coatings.

3.5.1. Golden Standard-Linear PEG Brushes Coatings

The primary way of resistance against adsorptions in many protein molecules is still surface grafting of PEG glycol (PEG), which results in linear PEG brushes [102]. PEG has low immunogenicity, is approved for use internally, is extraordinarily big, water-soluble, non-toxic, very flexible, and biocompatible [102]. The considerable hydration level (and hence a substantially excluded volume impact) has been linked to quick conformational changes and steric rejections because of its effective rejection of foulants [103]. By coating substrates with an Oligo SAM, Prime and White sides were the first to demonstrate PEG derivatives’ increased protein-repelling capabilities (ethylene glycol). It was also shown that their addition to the chains (n = 1 to n = 17) resulted in more excellent adhesion resistance since the surface was effectively covered. The surface was instead more thoroughly coated [104]. Following the discovery, several other organizations covered PEG with various substrates with diverse patterns and used different grafting procedures. PEG-tethered surfaces on gold [104], glass-adsorbed poly(2-amino-ethyl hydrochloride)-PEG, and glass-adsorbed PEG monomethyl ether grafted onto (3-amino-propyl) dimethyl and PAA treatments [105], as well as polyvinyl nanofiber PEGylates [106], are the surfaces that were produced. All of these surfaces exhibit excellent protein and cell resistance.

Furthermore, the antifouling capabilities of PEG were improved by adding zwitterionic polymer chains. PEG chains from zwitterionic chains experienced less electrostatic repellence and their grafting densities on gold surfaces rose. Zwitterionic polymers successfully enhanced the antifouling performance of PEG chains alone in comparison [107]. Although PEGylation for managing unwanted adhesions is still the most widely used first-generation antifouling approach, PEG is easily oxidized or eliminated in most biochemically significant situations. Aldehyde-terminated chains are created due to amine-functionalized proteins’ cleavage of EO units. PEG’s short-term stability enables it to suppress protein resistance for extended periods [108]. Practically grafting PEG onto various chemical substrates is getting harder and harder. It frequently requires complex surface chemistry, which can be expensive on industrial sizes or more extensive substrates [102]. Due to their highly hydrated nature, PEG coatings also expand under damp conditions, reducing their mechanical strength and limiting their usefulness [109]. The adsorption of cationic foulants (such as lysozyme), which highlights that PEG is not a universal antifouling agent, is another drawback of PEG coatings [110]. Finally, PEG coatings weaken the protein resistance at high temperatures, which is significant in many biological applications (>35 °C) [111].

3.5.2. Bottle Brushes

Although most recent attempts to create anti-fouling polymer coatings focused on linear polymer brushes (such as PEG), protein resistance is superior to surfaces that have been bottlebrush-lacquered. Comparable to linear polymers, bottlebrush polymers have tightly packed polymeric side chains as their backbone. This design offers a thicker, more impenetrable covering, making it extraordinarily difficult to copy [112]. PVP bottlebrush surfaces have a greater antifouling capacity than linear PVP brush surfaces with equal polymer layer thickness and grafting densities. The PVP bottlebrushes significantly decreased the adsorption of various proteins to pure and brush-coated PVP on a linear surface (up to 44%). Because the grafting density was equivalent, the smoother and thicker impermeable layer [113] can account for the better repair of the PVP bottled brush.

When the coatings’ film thickness was more than 14 nm, they demonstrated complete protein resistance to a range of substrates (golden, glass, and polymer-coated) for fibronectin, bovine serum albumin, and lysozyme [111]. More notably, the serum adsorption levels on these coatings were below the ellipsometry detection limit of 0.1 nm, and the associated cells did not glow [114]. The poly(2-methyl-2-oxazoline) family also contains a significant bottlebrush component (PMOXA). It has several unique properties, including resistance to oxidative degradation, more accessible synthesis, noncytotoxicity, and protein-repellent features comparable to those seen in PEG.

Based on this knowledge, Zheng et al. have spin-coated gold surfaces with bottlebrush polymers with a thiol finish, which have outstanding anti-platelet adhesion and robust protein resistance compared to bare gold, with polymer (PMAA -g-PMOXA) and thiol end-caped polymers. They also showed how changing the backbone and side chain lengths might optimize the PMOXA bottle brush’s protein resistance. Shorter PMAA backbones and longer PMOXA side chains have been shown to increase surface coverage, improve hydrophilicity, and increase protein resistance [115]. Long, compact brushes can be produced more quickly thanks to the cyclical macromolecules’ reduced hydrodynamic range and faster adsorption. The brush’s high density increases the steric barrier more than a typical linear brush’s entropic shield, preventing biomolecular penetration.

Additionally, when two brush-mounted surfaces are sheared, there are no dangling stops at the brush interface, which causes lubrication [116]. Despite the unique qualities of these cyclic polymer brushes, cyclic polymers are still costly and challenging to produce in large quantities with high purity. In addition to the methods necessary to change the surface to make particularly dense and chemically resistant brushes, it has a limited capacity for large-scale applications [117].

3.5.3. Nanoparticle Thin Films Coatings

Another technique for making anti-fouling brushes is to cover surfaces with hairy nanoparticles in addition to the linear, bottle, and cyclic polymer burns. Three methods have been used to prepare the SB-modified surfaces: grafting the SB into a SiO₂ substrate, grafting the SB to a thin film of silica NP in gold, and grafting the SB to silica NPs suspended then applied to gold. Three different strategies have been used. On SB-functionalized silica NP surfaces, protein adsorption and bacterial adhesion were significantly reduced (up to 96%) compared to non-functional silica NP surfaces. Still, they were unable to match the resistance of the SB-fitting SiO₂. The NPs had advantages even if they did not show increased antifouling performance. Without a catalyst, particle functioning and the creation of coatings on various substrates and in a wide range of pH conditions are accomplished. The materials are desirable for frequently used antifouling applications since they are inexpensive and highly manipulable. Chemical procedures are simple to scale up and do not require organic solvents or surface treatments [118].

3.6. Novel Membrane Materials

Recent studies concentrate on developing new formulas to make RO membranes more fouling-resistant. Potential materials include nanoporous metal oxide, carbon nanotubes (CNT), zwitterionic materials, and nanoporous graphic oxide [57]. Standard polymeric membranes are modified to contain nanoparticles dispersed inside polymer nanocomposite membranes. Thin-film nanocomposite membranes (TFNC), which are mixed, make up nanocomposite polymers. By using dip-coating methods and pressure-assisted depositions on the membrane surface, the thin coatings that surround TFNC membranes are created. On the other hand, during the membrane casting process, the polymer and nanoparticles of mixed nano-composite membranes are dispersed in a casting fluid [119]. The detail of novel membrane materials used for antifouling performance are shown in

Table 7. Novel membrane materials for antifouling performance.

Modifier	Test Conditions	Permeate Flux	Salt Rejection %	References
Carboxylated CNf	500 mgL−1 BSA, 200 mg L−1 Nacl solution at pressure of 15 bar with temperature of 25 °C	-	94	[120]
CNf	200 mgL−1 NaCl solution at pressure of 15 bar with temperature of 25 °C	25.9	96	[72]
Zwitterionic diamine monomer N, amino-ethyl piperazine	Nacl solution of 2000 ppm at pressure of 1.5 Mpa with temperature of 25 °C	54.5	98.3	[121]
Zwitterionic colloid nano-particles	Nacl solution of 2000 ppm at pressure of 1.5 Mpa with temperature of 25 °C	37.3	96.5	[122]
Zwitterionic Polymer	0.85 wt% solution at pressure of 1.5 Mpa with temperature of 30 °C	50.48	96.9	[123]
GO zinc oxide	200 mgL−1 Nacl solution at the pressure of 20 bar with temperature of 25 °C	31.42	96.3	[124]
Cu and Fe nano particles	100 mgL−1 Nacl solution at pressure of 300 psi with temperature of 25 °C	3 (Cu NP) and 8.4 Fe NP	74.36 (Cu NP) and 92.6 (Fe NP)	[125]
Graphene oxide	800 mgL−1 CaCl2 and Na2SO4 at 25 °C and pressure of 20 bar	-	98	[126]

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3.7. Zwitterial Materials

Zwitterionic polymers dynamically adjust their charge distribution and intermolecular interactions in response to environmental changes such as pH and salinity variations, thereby maintaining their anti-fouling and oleophobic properties. Thus, modifying material surfaces with zwitterionic polymers is considered a particularly promising strategy for combating material fouling [127].

Zwitterionic materials have a neutral, positive, and negative charge [128]. To diminish protein adhesion as an anti-fouling material, typical polymers of the betaine groups are illustrated in Figure 4. The quaternary ammonium, anion phosphate, sulfonic, or carboxylic cations are polyurethanes. To form dense hydration layers or deter protein adsorption on the polymers, the electrically charged groups interact with the water. However, polymer chains display a neutral load due to the tight coupling between positive and negative charges when exposed to proteins with a nanometer-sized structure; it blocks any beneficial interactions with proteins that have a specific surface charge [3].

Zwitterionic antifouling coatings are robust techniques in research, but it is limited to a lab-scale and difficult to scale up at industries [129].

3.7.1. Super Hydrophilicity in Zwitterionic Materials

A new class of materials called zwitterion has evolved with outstanding antifouling capabilities. The antifouling procedure in zwitterions and PEG-based materials is examined in Figure 5. PEG has one oxygen atom (-CH₂CH₂O-) which can combine with one water molecule to form a hydrogen binder. As opposed to PEG/water systems, stations may generate electrostatic forces in eight stronger water molecules than hydrogen bonds. Therefore, hydrophilic zwitterions are more prevalent than PEG [130]. Higher hydrophilicity than PEGDA has been found in studies of zwitterionic monomer copolymer hydrogels, such as SBMA [131]. Hydrogels containing sulfobetaine are being researched. When the chains are compressed, the zwitterionic strands exhibit elastic pressures against foulants similar to PEG, increasing their anti-fouling characteristics [132]. Figure 5 shows the interaction of zwitterions and PEG with water.

Zwitterion has more stable anti-fouling properties than PEG-based polymers under high salinity environments. Due to PEG’s amphiphilic nature, high salinities may cause the surface of the PEG chains to break and become hydrophilic. Zwitterionic chains, however, prefer to maintain an open and robust hydration layer in salts [108].

3.7.2. Surface Coating Using Dense Zwitterions

On the membrane’s surface, hydrophilicity may instantly cover zwitterionic molecules. For instance, UF membranes have been used to make and cover PTFE-co-SBMA copolymers. The copolymers can automatically mix to form nanochannels with a diameter of about 1 nm. There was more water streaming than there was on the unaltered UF membranes. When tested with one gL⁻¹ of BSA and 1500 mg/L of oil, the modified membranes exhibit just 4% reductions in water flow, demonstrating robust antifouling capabilities from the zwitterionic coating [133]. On the membrane surface, in situ polymerization techniques like initiated chemical vapor deposition (iCVD) can also produce thin films of zwitterionic polymers [134]. As a result, in the vapor step, the initiator and monomer enter a chamber at a high temperature. Breaking and sticking to the low-temperature membrane surface, the initiator starts the polymerization process that produces thin films. This method makes it possible to use a range of non-liquid phase-covered substrates, including those made of non-planar materials [135]. Better antifouling capabilities from zwitterions were evident when BSA, humic acid, and sodium alginate were evaluated on the changed surfaces as opposed to naked ones.

3.7.3. Surface Grafting of Zwitterions

It is also possible to transplant zwitterionic monomers onto membrane surfaces [136]. Depending on the type of initiator, many polymerizations can be used, including photoinitiated, ozone-inducted, plasma-induced, and physisorption of radical graft polymerization [136]. For instance, the polymerization poly(2-methacryloyloxyethyl-phosphorylcholine) (PMPC) was used to graft the photo-instructed polymerization into the PES membrane, and the modified surface contained significantly fewer bacteria than the total accumulations [137,138]. The modified membranes acquired a high flux recovery ratio of 90% when handling BSA solutions [137]. Through glue-like PDA, the membrane surface can likewise be grafted [131]. Figure 6 shows the PDA-g-PMPC coatings on a range of substrates.

The decisive factors in achieving stronger antifouling qualities from zwitterions, such as grafted surfaces with PSBMA, were clarified by computational modeling. The concentration, bonding coverage, polymer chain form, and chemical composition may impact the antifouling capabilities [139]. Ionic groups are a vital building block for enhancing the anti-fouling properties of the membranes in zwitterionic materials because of their flexibility. The three most commonly used materials are PSBMA, PMPC, and PCBA. Given that actual wastewaters may contain a variety of foulants, including proteins, bacteria, and algae, more research is required to find the best zwitterion for these applications [140].

3.7.4. Hydrophilic Surface Modification

The surface coating is altered by adding hydrophilic materials to the hydrophobic film surface to improve the films’ anti-fouling performance. Infrared spectroscopy and X-ray photoelectroscopy were used to analyze the changed membrane after a PSF ultrafiltration membrane was applied to a comb polymer (XPS). The characterizations [141] supported the coating layer’s existence. Tests have also shown that the modified membrane’s flux recovery rate has significantly increased. Additionally, the antifouling effectiveness of the modified membrane was increased by more than 80% when hydrophilic elements were coated on its surface. Zuthi et al. [142] developed the antifouling membrane by immersing plasma pretreated PVDF polymer with 0.4 solution. According to Yoon et al. [143], this method is suitable for developing antifouling membranes.

3.8. Role of Economic Membrane Technology in Sustainable Water Generation

Drinking water is treated by desalination and wastewater treatment through membrane-based technology. Large portions of water on earth are mainly saline and not fit for drinking purpose and only 2.5% is available as fresh water [144]. The increased demand for fresh water lead to the development of various techniques for treating wastewater during the past decades, such as multi-stage flash distillation (MSF), vacuum distillation (VD), multiple effect distillation (MED), and membrane-based technologies such as membrane distillation, reverse osmosis, etc., for seawater distillation. Membrane-based technologies such as RO, MD, and forward osmosis, because of low energy consumption, lower operating, and maintenance expenses and reduced capital needs, are the appealing substitutes among the other technologies. Fresh water is produced through desalination from either brackish or seawater by removing the salt content through the thermal distillation process or membrane-based technology. Among many nations worldwide, desalination, particularly in Gulf countries, played a sustainable role in their water supplies. For instance, in many commercial areas of Qatar and Kuwait, water is produced through the desalination process [145]. The RO process is gaining commercial interest on a global scale due to recent improvements in the process features that cause the reduction in operating costs and improvements in feed pre-treatment membrane materials and energy recovery [146]. In the desalination sector, the membrane distillation process is primarily used. It requires low temperature during operation and gained interest in industrial sectors, including medical sectors, for the sterilization of biological fluids, and in the food industry for the concentration of fruit juices and the removal of heavy metals [147].

3.9. Interaction Between Membranes and Emerging Pollutants

When these PEPs come into contact with water, they can contaminate it and harm aquatic life. Toxic chemicals and metals have been separated from effluents using a variety of techniques. Aquatic life is at risk when PEPs are in contact with water and separation of metals are necessary. Membrane-based technology such as membrane bioreaction and membrane separation can effectively remove the emerging contaminants [148].

With the rapid development of materials science, membrane-based-coupled technologies such as catalytic membranes and adsorption membranes have also achieved significant success, offering effective solutions for the treatment of refractory organic contaminants. There are several techniques for the treatment of effluents, but membrane technology such as microfiltration (MF), nanofiltration (NF), ultrafiltration (UF), and reverse osmosis (RO), is one of the most robust technologies for providing clean water at an affordable cost with minimum energy requirement.

3.10. Potential of New Methods for Wastewater Treatment and Their Advantges and Disadvantages

The treatment methods adopted for the elimination of both organic and inorganic contaminants in wastewater are important factors that include: biological methods adopting plants and microorganisms such as bacteria, fungi, and algae to convert complex toxic compounds to less toxic products as these organisms use the organic pollutants as a carbon source for the growth of cells.

During biological treatment methods, the effluent generated is used in agricultural purpose for irrigating the crops [149]. During water crisises, the reusability of treated water is essential and beneficial. The treatment of reused water requires low-cost technologies in industries in aerobic and anaerobic mode where biological processes are performed in a reactor depending on the bacterial population. Aerobic methods for treating wastewater includes trickling filter, activated sludge, membrane bioreactor, oxidation and lagoons; in contrast, anaerobic treatment includes anaerobic baffled reactor, fluidized bed reactor, anaerobic filter, and anaerobic contact process [150,151]. The aerobic and anaerobic biological process combine with each other to treat industrial wastewater where high quality water generated can be used in industries. MBRs are one of the most prominent technologies where the activated sludge and membrane are combined and excess sludge production is reduced. The combination of the upflow anaerobic sludge blanket (UASB) and membrane bioreactor in a pilot scale study for wastewater treatment reduces the COD from 2350 mg/L to 155 mg/L [152]. Some advantages and disadvantages of wastewater treatment technology are given in

Table 8. Some advantages and disadvantages of wastewater treatment technology.

Treatment Technology	Advantages	Disadvantages	References
Chemical coagulation	Ferric chloride percoagulation is one of the possible pretreatment step for chemical oxidation causes the lower toxicity and higher samples biodegradability.	Organic compounds are are not degraded through this technology.	[153]
Electrocoagulation process	In a small treatment facility, the high treatment particulate removal effifiency is achived. It is the rapid process in eliminating colloidal, suspended and charged particles.	It is not good efficient for removal of persistent organic compounds.	[154]
Chemical precipitation	Due to high metal selectivity, potential removal efficiency and ease of its use, chemical precipitation is considered as a superior technique.	It is the more expensive due to the use of chelating agents and unable to reduce concentrations at acceptable limit.	[155]
Advanced oxidation processes	Wastewater biodegrability and toxicity is improved, organic pollutants are completely miniralized into H2O, CO2 and inorganic ions.	Large concentrations of FeSO4 and H2O2 are needed during Fenton treatment.	[153]
Membrane process	This method is much more robust in treating wastewater. It separates high concentrated metals and other valuable chemicals without changing their state. This is also promising technology in separating heavy metals.	Damage of membrane affects on the cost and serious membrane fouling issue which requires frequent membrane cleaning.	[156]

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3.11. Future Trends of Membrane

Membrane and membrane-based technologies have been widely used in various water purification and wastewater treatment applications, either individually or in combination with other processes [157]. Currently, membrane processes such as reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), membrane distillation (MD) are being used in seawater desalination, and wastewater treatment applications. The effective utilization of membrane technologies as a sustainable solution for different applications requires novel membrane materials along with customized separation characteristics.

The piezoelectric membrane has gained promising attention influencing the physical and chemical properties of membrane material. Vibrations are generated on a membrane surface when alternating currents are applied on the surface, which reduces membrane fouling [158].

Nanotechnology has played a vital role in treating the high quality of water. This includes polymeric nanomaterials, 2D nanosheets and electrospun nanofibrous membranes for water treatment applications. Due to the outstanding growing technique of nanomaterials, the researchers are more focused on the manufacturing of nanomaterials for various applications. The development of the electrospun nanofibers membrane completely allows synthetic and naturals polymers to be used in the development of innovative membranes. The pressure-driven membrane is one where pressure is applied to membrane for separation of permeate and filtrate. The pressure-driven membrane used in practical applications are: microfiltration (MF), nanofiltration (NF), and ultrafiltration (UF).

4. Conclusions

Microfiltration, ultrafiltration, and reverse osmosis membranes are promising technologies that have been widely used for wastewater treatment. A significant challenge during wastewater treatment is membrane fouling. Membrane fouling appears to be an intrinsic and inevitable issue with membrane technology. A variety of fouling, such as biofouling, organic fouling, inorganic scaling, and colloidal fouling, can appear on membrane surfaces based on conditions of feed water and operating parameters. Membrane material and surface modification of the membrane may affect membrane fouling. This paper presented an overview of membrane fouling, surface modifications of the membrane for antifouling performance, and fouling mitigation measures during wastewater treatment. Even though many foulants may arise in various ways, there may occasionally be no clear distinction between them because of their connections or synergistic effects. Consequently, more research is needed to understand the fouling behavior that causes their formation. This understanding will aid researchers in developing advanced antifouling techniques.

5. Future Recommendations

Despite the improvements for controlling the antifouling performance of membranes through different strategies, there is still a huge gap for further research and development. Most of the research indicates fouling control techniques on a lab-scale operation of membranes and still there is need to determine a wide range of antifouling performances of membranes on an industrial scale. Many studies have been conducted for the fabrication and modification of nanofibers membrane for the antifouling performance during wastewater treatment, however, there is still a gap for the improvement of existing methods. The deposition of nanomaterials over the nanofibers membrane generally causes poor adhesion which could reduce the life span of membrane. The research should still focus on the development of novel membrane materials that could work at low aeration energy and at low-cost fouling control strategies. Although the membrane technology for wastewater treatment is more productive and environmentally friendly, this membrane requires some cost to make it resistive against fouling, so this needs further research to produce cost-effective strategies.

The membrane bioreactor (MBR) process has received increased interest in recent years in academics at the lab-scale. However, there might be obstacles to this MBR scale-up. In order to solve the scientific and technical issues surrounding these novel MBRs and enhance their performance in large-scale applications, further research is needed.

Although the modification of a membrane with blending is a simple technique to change the membrane surface, it is limited by miscibility issues of inorganic nanoparticles of agglomeration and some polymer additives. Due to the poor adhesive dispersion in matrix, this will be the cause of leaching and decrease the antifouling performance of the membrane. In addition, the pores generated on the polymer matrix due to the leached additives cause the higher permeability and create the pores on the surface which result in the lower rejection of pollutants and decreased mechanical strength due to the formation of larger voids on the surface. Moreover, due to the inorganic additive agglomeration, the surface roughness in increased, causing the decrease in steric repulsion against foulants and the significant effects on the antifouling performance [159,160].