4.1. Coagulant Addition
The addition of coagulants to water and wastewater treatment systems facilitates the formation of large flocs from fine particulates in solution. In MBRs, the coagulants help the formation of larger size sludge flocs which enhance membrane filtration. Alum [Al2
] and ferric chloride (FeCl3
) were reported to enhance filterability of MBR mixed liquor and ultimately controlled membrane fouling [128
]. Wu et al.
] studied the effects of polymeric coagulants on membrane fouling in MBRs using three coagulants: polymeric ferric sulphate (PFS), polymeric aluminium chloride (PACl), and polymeric aluminium ferric chloride (PAFC). They reported that the addition of these coagulants resulted in membrane fouling control through the reduction of the initial TMP and the rate of TMP increase. This was attributed to the ability of the coagulants to restrain the formation of gel layer, decelerate foulants development, and remove stable foulants from the surface of the membrane. PFS was found to be the most effective in controlling membrane fouling with an optimum dose of 1.05 mM Fe. The reduction of initial TMP and delayed rate of TMP increase when coagulants were added can be explained in terms of charge neutralisation and bridging of the SMPs. Coagulation could also minimise membrane fouling due to the flocculation of the particulates (colloids) in the MBR reactor brought about by coagulant addition. Wu et al.
] further investigated the effect of PFS on membrane fouling characteristics and performance in a long-term (60 days) MBR operation. Findings from their study indicated that the addition of PFS efficiently impeded membrane fouling in long-term MBR operation; and, the optimum PFS dosage and dosing interval were 1.0 mM Fe and 15 to 30 days at an MLSS 7–10 g/L. Additionally, PFS increased the flocs size through the supply of positive charges for organic particles, hence enhanced charge neutralization.
In another study, Zhang et al.
] investigated the ability of FeCl3
to retard membrane fouling in MBR. They reported that an optimum dose of 1.2 mM Fe(III) remarkably improved the filterability of the MBR mixed liquor; which, they attributed to the fact that Fe(III) supplied positive charges for soluble macromolecular substances and sludge flocs, and enhanced the function of charge neutralisation. The added Fe(III) interacted with the negatively charged EPS groups and enhanced the bioflocculation of small particles.
Furthermore, Mishima and Nakajima [133
] investigated the mitigation of membrane fouling in MBR by coagulant addition in laboratory-scale batch and MBR experiments. They reported that ferric chloride performed slightly better than aluminium sulphate. Thus, using ferric chloride in the MBR process, they reported that during the 40-day MBR experiment, the reference MBR tank (without coagulant addition) was cleaned 18 times; the tank with 2.26 g/L coagulant addition was cleaned nine times; and the MBR tank with 4.52 g/L addition was cleaned only five times [133
]. This indicates that, with coagulant addition, the fouling rate reduced appreciably. It has also been found that the addition of a very low-dose of green bioflocculant could achieve significant membrane fouling mitigation after 70 days of operation (TMP development of 2.5 kPa only) with less backwash frequency [134
However, the addition of coagulants to the MBR mixed liquor can decrease the pH. The decrease in pH may affect the bioactivities of the MBR mixed liquor. In addition, coagulant overdosage can result in the deposition of the excess coagulant on the membrane surface. Research is, thus, needed on finding sustainable dosages that mitigate membrane fouling without lowering the pH.
4.2. Adsorbent Addition
Adsorbents provide a large surface area for the adsorption of materials in water and wastewater. In MBR, adsorbents offer the potential to adsorb dissolved organic polymers, notably SMPs, hence reducing membrane fouling propensity. Powdered activated carbon (PAC) is typically applied in MBRs for the purpose of reducing organic fouling and biofouling. PAC also serves as media for bacterial attachment and subsequent growth [5
], hence, reducing their attachment onto the membrane surface and pores.
Ying and Ping [135
] studied the effect of PAC dosage on membrane fouling using PAC dosages of 0, 0.75, and 1.5 g/L of wastewater, respectively. They reported that PAC application was effective in reducing the amount of EPS inside the microbial floc at PAC dosage of 0.75 g/L; and, the addition of PAC decreased the EPS deposited on the membrane. The cake resistance decreased as the PAC dosage increased. The optimum PAC dosage for organics removal and filtration flux was found to be 0.75 g/L. Similarly, Satyawali and Balakrishnan [136
] studied the effect of PAC addition on sludge properties in MBR treating high-strength wastewater from an alcohol distillery in long-term operation (over 180 days). They reported that PAC addition decreased the SVI (sludge volume index) which improved sludge dewaterability. They further found that PAC addition did not substantially affect total EPS concentration; however, the composition of the SMPs in terms of proteins/polysaccharides ratio was altered resulting in a high proteins/polysaccharides ratio.
In another study, Remy et al.
] investigated the effect of PAC on membrane fouling in two pilot-scale MBRs treating municipal wastewater. Findings from their research revealed that low PAC dosage (500 mg/L of sludge) combined with a relatively long SRT (50 days) resulted in about 10% improvement of the critical flux and a strong filtration period increment without significant fouling at high fluxes (50–70 L/m2
h). Other positives reported from their research include: Easier removal of gels deposited on the membrane at high fluxes; a reduction in the deposition of gel on the membrane surface after a long period of filtration; and a slight increase in the permeate quality. Torretta et al.
] investigated the optimum dose of PAC in an MBR pilot plant by using low PAC doses: 0, 2, 5, 10 and 20 mg/L. Findings from this study showed that PAC addition was effective at the low doses (2 and 5 mg/L) by reducing the permeate flux loss (from 16% up to 27%, respectively). Rezaei and Mehrnia [116
] found that the addition of the zeolite (clinoptilolite) resulted in significant improvement of sludge properties including 22.5% increase in MLSS, more accumulation of large particles (7%), 50% reduction in SMPs, and 66% reduction in TMP. The increase in flocs size, reduction in SMPs and TMP translate to remarkable membrane fouling abatement. Similarly, another study found that MBR with PAC in the mixed liquor exhibited low fouling tendency and prolonged filtration as compared to conventional MBR [138
]. The authors further indicated that PAC stabilised the biomass in form of biological activated carbon with porous cake structure that prolonged filtration.
It has been indicated that higher concentration of fresh PAC in submerged MBR would offer enhanced simultaneous adsorption and biodegradation effects for the reduction of EPS, fine colloids and planktonic cells in MBR mixed liquor supernatant [139
]. Membrane fouling abatement in MBRs through the addition of PAC has been attributed to the combined action of flocculation and adsorption [138
]. The adsorption opportunity provided by PAC in MBRs is expected to enhance organics removal. In general, PAC addition to MBR acts as mobile carriers for active biomass, reduces membrane cake layer formation, and retains microorganisms by making the MBR both attached growth and suspended growth [140
]. An additional advantage offered by enhancing MBRs with adsorbents (particularly PAC) is the removal of recalcitrant pollutants from wastewater [141
Furthermore, Deng et al.
] reported that the addition of sponge to the MBR resulted in lower biomass growth, less filamentous bacteria, reduced sludge viscosity, larger sludge flocs, and lower concentration of EPS and SMPs, leading to lower pore blocking resistance and lower cake formation when compared to conventional MBR [142
]. This clearly shows that sponge addition can significantly alleviate membrane fouling in MBRs.
While the addition of adsorbents mitigates membrane fouling in MBR, further research is needed to establish optimum dosages of the various adsorbents. Operating above the optimum dosage can be counterproductive as it may rather increase sludge apparent viscosity, aggravate fouling through deflocculation, reduce mass transfer and sludge dewaterability [141
]. It has also been indicated that overdose of adsorbents (especially PAC) may increase membrane fouling due to their potential to become foulants themselves through cake layer formation over the membrane and/or by blocking the membrane pores [135
]. It is necessary to determine the optimum dosage in batch experiments for any wastewater. Low PAC dosages (in the neighbourhood of 0.5 g/L) coupled with short SRTs have been recommended for membrane fouling mitigation in MBRs [144
]. The optimum dosage of the adsorbents will also allow the striking of a balance between the cost savings arising from membrane fouling abatement and the cost of the additives and handling of the resulting sludge.
4.3. Use of Granular Biomass (Aerobic Granulation)
As indicated in Section 3.3.3
, EPS are the construction materials for microbial aggregates. As such, recent research has focused on incorporating biotechnological processes that can use up these foulants. A key innovation in this regard is the integration of aerobic granulation biotechnology with MBR to develop the aerobic granulation membrane bioreactor (AGMBR) as a novel approach to control membrane fouling. Aerobic granulation refers to the process of microbe-to-microbe self-immobilisation without any biocarriers [145
]. The resulting granular biomass are dense microbial consortia packed with different microbial species that can collectively degrade wastewater pollutants [77
]. Compared to the ASP, aerobic granulation technology offers the following advantages: excellent settling properties, smaller footprints, strong microbial structure, higher biomass retention, less sludge production, high resilience to toxic chemicals, and good ability to handle high organic and shock loading rates [77
]. AGMBR offers a distinctive advantage of utilising EPS for granule formation and offering the large size and rigid structure of the granules for bacteria to attach to rather than the membrane surface. The large size and rigid structure of the granules is expected to reduce cake-layer formation, pore blocking and surface deposition on the membrane surface.
The integration of aerobic granulation and MBR was first reported by Li et al.
]. Findings from their study showed that membrane permeability in the AGMBR system was over 50% higher when compared to conventional MBR. Following this was a four-months bench-scale study by Tay et al.
] who compared the processes and performances of AGMBR and submerged MBR. They found that while AGMBR and submerged MBR showed similar treatment efficiencies, the AGMBR showed much better filtration characteristics with the membrane permeability loss (34.5%) in AGMBR being twice as low as that of submerged MBR at constant pressure testing. Constant flux testing also showed that, increasing the flux threefold resulted in membrane permeability loss of 2.4% with the AGMBR mixed liquor (i.e.
, 21 times lower than that of the MBR). In continuous operational mode, the TMP increment in the AGMBR was negligible (3–6 kPa) and the membrane required no physical cleaning; whereas, the TMP increment in the submerged MBR was significant (50–60 kPa) and regular physical cleaning of the membrane module was required. Similarly, in a long-term study (10 months), Tu et al.
] reported a higher removal efficiency of pollutants, as well as improved membrane performance (fouling rate was maintained below 0.1 kPa/day at MLSS > 18,000 mg/L) when aerobic granules were formed in MBR. They indicated that the granule size change and improved settling ability were responsible for maintaining membrane permeability.
Another study by Juang et al.
] investigated membrane fouling in AGMBR and reported that most bacteria cells were retained by the granules thus preventing their penetration through the membrane pores and the chance to cause internal fouling layer. The combination of aerobic granulation and MBR has also been reported to enhance good filtration performance and lower propensity for membrane fouling [115
]. Other reported findings on AGMBR include: stable operation at 20 L/m2
h for 61 days with significant filtration improvement [153
]; extension of filtration period by 78 days without physical cleaning [111
]; and excellent fouling control [154
Regarding treatment performance, aerobic granulation offers diverse microbial aggregates that exhibit superior treatment efficiency when compared to floccular sludge. This is attributable to the strong microbial structure of aerobic granule as well as their high biomass retention and high resilience to toxic chemicals. Tu et al.
] reported a higher removal efficiency of pollutants when aerobic granules were formed in MBR. Aerobic granules exhibit a layered structure with an oxic zone near the granule surface, an anoxic zone in the middle layer, and an anaerobic core at the granule centre [155
]. This layered structure is suitable for simultaneous organics, nitrogen, and phosphorus removal. A study conducted to determine the removal of nitrogen in AGMBR reported about 60% total nitrogen removal in the AGMBR [111
]. Another study to determine the performance of AGMBR reported the removal of COD, total phosphorus, nitrate-nitrogen and total nitrogen as 93.17%, 90.42%, 95% and 95%, respectively [157
However, the major technical problem of AGMBR is the long-term system operation instability of aerobic granulation and granule disintegration problems [151
]. Aerobic granules have been observed to disintegrate after prolonged operation [149
]. The deterioration in granule stability over time impacts the efficiency of wastewater treatment and is a major issue affecting the effectiveness of aerobic granulation in full-scale operations. As applied to AGMBR, the granule disintegration increases the concentration of soluble EPS, consequently increasing the membrane fouling propensity [154
]. Hence, the production of granules with sustainable long-term structural integrity is a key area requiring further research.
4.4. Use of Granular Materials with Aeration
To enhance the detachment of foulants from the membrane in MBR, research has focused on using granular materials with air scouring to provide continuous mechanical cleaning. In this regard, Siembida et al.
] reported that the abrasion produced by granular materials introduced into the MBR tank significantly reduced cake layer formation on the membranes. The study further found that this method resulted in a successful long-term operation (more than 600 days) at permeate flux of 40 L/m2
h without membrane chemical cleaning. The introduction of the granular materials also allowed MBR operation at a higher permeate flux (more than 20% higher) compared to the conventional MBR. Similarly, Kurita et al.
] found that the introduction of granular materials (made of polyethylene glycol) into a submerged MBR increased the critical flux by more than 40%; allowed for the stable operation of MBR despite the reduction of aeration by 50%. The reduction of aeration would remarkably reduce MBR operational and maintenance cost. Johir et al.
] studied the effect of different particle sizes of granular activated carbon (GAC) on submerged MBR operated at a filtration flux of 20 L/m2
h. Three size ranges of GAC were used in the study: 150–300, 300–600 and 600–1200 μm. The authors reported that GAC size played a significant role in membrane fouling abatement as the total membrane resistance reduced by 60% with GAC particle sizes of 300–600 μm. In addition, organics removal was up to 95% with the addition GAC.
Furthermore, Pradhan et al.
] found that the addition of granular media in the MBR reactor resulted in the same TMP reduction as doubling the aeration intensity (from 600 to 1200 L/h/m2
). A recent study also found that the rate of aeration in MBR can be reduced by over 50% with the introduction of granular materials [166
]. In the same regard, Krause et al.
] investigated the removal of the membrane fouling layer by continuous physical abrasion by adding granular materials to the activated sludge. They reported no membrane permeability decline throughout the over eight months of operation at fluxes of up to 40 L/m2
The effectiveness of using granular material as a fouling control mechanism in anaerobic fluidised membrane bioreactor (AFMBR) has also been reported in the literature. Kim at al.
] examined the effect of placing GAC directly in contact with membranes in an AFMBR in a 120-day continuous-feed experiment using two-stage anaerobic treatment system: first stage consisting of a fluidised-bed bioreactor without membranes followed by an AFMBR. The fluidised GAC produced scouring action on the membrane surface which reduced membrane fouling evidenced by the cleaning of the membrane only twice during the 120 days of operation. The authors reported an energy requirement of 0.028 kWh/m3
, which is remarkably low compared to the values reported for anaerobic membrane bioreactors using gas sparging for membrane fouling control. Similarly, Aslam et al.
] studied the effectiveness of using GAC and non-adsorbing silica and polyethylene terephthalate (PET) beads as fluidised media in an AFMBR in reducing membrane fouling and for energy requirement for fluidisation. Findings of their study indicated that GAC can reduce membrane fouling both by adsorption of foulants and scouring action along membrane surfaces. Smaller particles demonstrated higher sorption capacity and less energy requirement for fluidisation until sorption capacity was exhausted. Afterwards, membrane scouring became the dominant mechanism; and, fouling reduction was a function of energy expenditure, with larger GAC particles that required more energy for fluidisation providing the best fouling reduction. In addition, increasing the packing ratio of GAC particles from 10% to 50% increased the energy required for fluidisation and also lowered the membrane fouling rate. Non-adsorbing silica particles and PET beads demonstrated similar results to pre-adsorbed GAC, lower fouling was accomplished by the larger media that had a higher energy requirement for fluidisation. Fouling reduction was also somewhat better at a given energy expenditure with lower specific gravity PET beads than with denser and smaller pre-adsorbed GAC particles. Additionally, recently, Kim et al.
] used GAC as the fluidised particles for scouring the membrane and as a support for bacterial growth. They found that membrane fouling was successfully controlled by GAC fluidisation as the TMP was low, and could be maintained below 0.12 bar by a daily removal of excess solids. With the scouring action of the GAC and daily withdrawal of solids, membrane cleaning was not required. These results open up a new possibility of using this method to make chemical cleaning of the membrane a seldom activity in MBR operation. However, granular materials have a tendency to damage the membrane material. Further research is needed to find the optimum aeration intensity and the right granular material that would not cause damage to the membranes.
4.5. Quorum Quenching
Another innovative approach to membrane fouling control in MBRs is the use of quorum quenching. The regulation of microbial group behaviours by cell-to-cell communication (quorum sensing) is involved in biofilm formation [171
]. Bacteria produce autoinducers, which are used in intercellular communication [172
]. When the concentration of autoinducer attains a threshold level, it combines with the receptor protein and activates the transcription of specific genes to induce group behaviours such as biofilm formation and EPS production [171
]. On this basis, the concept of quorum quenching was introduced in MBR for biofouling control through quorum sensing (QS) control by the blocking intercellular communications.
Yeon et al.
] utilised acylase attached to magnetic carrier to inhibit QS in MBR; and, they reported reduced biofouling and enhanced the membrane permeability. Another study found that disrupting the energy metabolism and subsequently producing QS signaling molecules effectively controlled biofouling [175
]. Similarly, Jahangir et al.
] and Oh et al.
] investigated membrane biofouling control by encapsulating quorum quenching bacteria into a hollow fiber membrane. Findings from these two separate studies showed that membrane biofouling was successfully inhibited. The study of Jahangir et al.
] further found that as the recirculation rate (of the mixed liquor between the bioreactor and the membrane tank) increased, the biofouling inhibition (quorum quenching effect) increased. This was attributed to the facilitated transport of signal molecules from the biofilm into the bulk mixed liquor, and then to the microbial-vessel.
Kim et al.
] immobilised acylase onto the membrane surface and reported that the MBR system showed mitigation of membrane biofouling. Similarly, Jiang et al.
] immobilised acylase into sodium alginate capsules for enzymatic quorum quenching in MBRs. They reported that quorum quenching influenced sludge characteristics and membrane biofouling through better sludge settleability, smaller sludge particle size, less production of EPS and SMPs, lower apparent viscosity and higher zeta potential of mixed liquor. Quorum quenching further altered the characteristics, behaviour and function of EPS and SMPs, ultimately weakened biofilm formation ability but enhanced membrane filterability.
Another quorum quenching approach reported in the literature is the use of free-moving beads entrapped with quorum quenching bacteria [179
]. The cell entrapping beads (CEBs) were prepared by entrapping quorum quenching bacteria (Rhodococcus
sp. BH4) into alginate beads. The introduction of CEBs in the MBR significantly delayed the TMP rise (the time to reach a TMP of 70 kPa was 10 times longer than the control MBR). This membrane biofouling mitigation was attributed to both physical (abrasion) and biological (quorum quenching) effects of CEBs. The explanation is that microbial cells in the biofilm produced fewer EPS and thus formed a loosely bound biofilm due to the quorum quenching effect of CEBs. This enabled the biofilm to slough off easily from the membrane surface from the abrasion of the CEBs.
Additionally, recently, Wood et al.
] engineered a beneficial biofilm that mitigates biofouling (through limiting its own thickness) by sensing the number of its cells that are present via a QS circuit. This was based on the secretion and uptake of a communication signal; hence, biofilm formation was limited by self-monitoring and selective dispersal. The beneficial biofilm prevented the formation of biofilm by deleterious bacteria through the secretion of nitric oxide, a general biofilm dispersal agent. The engineered beneficial biofilm further removed the environmental pollutant, epichlorohydrin, through the production of epoxide hydrolase. Thus, the engineered beneficial biofilm plays two significant roles: mitigation of membrane biofouling and provision of a platform for biodegradation of recalcitrant organic pollutants.
However, practical issues on the cost and stability of enzymes that are used in QS are yet to be determined. The use of free-moving beads entrapped with quorum quenching bacteria can open the door to practical application but this quorum quenching method needs pilot-scale testing. Engineered living biofouling-resistant membrane system needs further testing at pilot-scale.