Aerobic Membrane Bioreactor Research Articles

Quorum quenching in anaerobic membrane bioreactor for fouling control

Quorum quenching (QQ) is an effective method to control membrane biofouling in aerobic membrane bioreactors (AeMBRs). However, it is not clear if QQ is feasible in an anaerobic membrane bioreactor (AnMBR). In this study, Microbacterium. sp that has QQ capability was embedded in alginate beads, known as QQ beads (QQB), and applied in a lab-scale AnMBR to investigate their potential in fouling control. With the addition of QQB, the operating period of AnMBR-QQB reactor was prolonged by about 8–10 times at constant flux operation before reaching the pre-set maximum transmembrane pressure (TMP). The concentration of Acyl-homoserine lactones (AHLs) in the bulk liquid was significantly higher during the ‘TMP jump’ period compared to QQB and control phases, while AHLs in the membrane foulants were remarkably lower in QQB phase compared to control phase. Furthermore, a much lower level of soluble microbial production (SMP) was observed in QQB phases. Extracellular polymeric substance (EPS), protein in particular, was reduced by 39.73–80.58% in the cake layer of the membrane from QQB phases. Significant changes of organic functional groups were observed in cake layer from QQB membrane as compared with that from control membrane. At the end of operation, bio-polymer (BP), building blocks (BB) and low molecular weight (LMW) organic matters increased in the foulant from control phases but such increase was not observed in QQB phase. After long-term operation, revival of QQB is required due to the declined activity for AHLs degradation.

Comparative evaluation of ultrafiltration and dynamic membranes in an aerobic membrane bioreactor for municipal wastewater treatment.

This study investigated the applicability of self-forming hollow fiber dynamic membrane (DM) as a low-cost alternative to ultrafiltration (UF) membrane. A hollow fiber polyester fabric was used as a support material to form the DM layer. Submerged DM and UF hollow fiber membrane were placed in the same reactor in order to compare the treatment and filtration performance of each membrane. Morphological analyses were also carried out for DM surface. The system was operated continuously at a flux of 5L/m2h for 85days. High COD removal efficiency and total suspended solids (TSS) rejection were achieved by the DM. Transmembrane pressure (TMP) of the DM was higher in comparison to the UF membrane, which was related with the formation of cake layer in DM. DM technology can be used as an alternative to UF membrane for municipal wastewater treatment.

Insight into the roles of packing carriers and ultrasonication in anaerobic side-stream reactor coupled membrane bioreactors: Sludge reduction performance and mechanism

The sludge in situ reduction process by inserting an anaerobic side-stream reactor (ASSR) in a sludge return line provides a cost-effective approach to reduce sludge production in activated sludge systems. In this study, four pilot-scale membrane bioreactors (MBRs), including an AO-MBR for control, ASSR coupled MBR (ASSR-MBR), a MBR with ASSR packed with carriers (AP-MBR) and an AP-MBR with part of sludge ultrasonicated before fed into ASSR (AUP-MBR) were operated in parallel to investigate enhancing effects of ultrasonication and packing carriers on sludge reduction and pollutants removal performance under both normal and low temperature. Low temperature showed negligible impact on COD removal, deteriorated NH4+N and TN removal from 98.3% to 69.7% at 21.6 °C to 92.5% and 48.8% at 2.6 °C, and decreased sludge reduction efficiency (SRE) in ASSR-MBR. Packing carriers and ultrasonication both enhanced sludge reduction, especially under low temperature with SRE values increased from 8.2% of ASSR-MBR to 17.1% of AP-MBR and 32.6% of AUP-MBR at 4.5 ± 2.5 °C. Packing carriers and ultrasonication increased cell rupture by 11.1% and 14.5% in aerobic MBR, enhanced protease activity in ASSR by 60.0% and 116.3%, and reduced ATP content for heterotrophic metabolism by 31.4% and 7.3%, respectively. MiSeq sequencing results showed that packing carriers enriched hydrolytic bacteria (Terrimonas, Dechloromonas and Woodsholea), slow growers (Sulfuritalea, Thauera and Azospira) and predatory bacteria (Bdellovibrio and norank_Saprospiraceae), while ultrasonication further enriched hydrolytic bacteria (norank_Saccharibacteria and Ferruginibacter). Packing carriers is more cost-effective than ultrasonication to enhance sludge reduction by partial damage to bacterial cells and promoting better interaction between bacteria, enzymes and substrates to favor particles hydrolysis.

Characterization of foaming and non-foaming sludge relating to aeration and the implications for membrane fouling control in submerged membrane bioreactors

In this study, two parallel aerobic MBRs were operated with different aeration diffusers (coarse-bubble aeration vs. fine-bubble aeration). It was found that fine-bubble aeration would cause serious foaming in the MBR system. The specific filtration resistance (SFR) of non-foaming sludge was similar to that of the sludge suspension in the foaming MBR, but almost 2–3 times larger than that of the foaming sludge liquor from the top layer of the foaming MBR. The intermediate pore blocking was the dominant fouling mechanism in batch filtration tests. The filtration performance of the sludge mixtures was governed by the combined effect of several major factors. There were more smaller flocs, less soluble microbial products (SMP) and less bound extracellular polymeric substances (EPSs) in the foaming sludge samples. Interfacial interaction calculation showed that the foaming sludge had a weaker adhesion ability. Comparison of foaming sludge and overflow foaming sludge revealed that the degree of foaming could affect the filtration behavior and physicochemical properties of the foaming sludge. The findings regarding the combined effect of aeration bubble size on foaming and fouling gave some implications for foaming and fouling control. Since fine-bubble aeration was detrimental for foaming control but beneficial for fouling mitigation, the selection of the optimal aeration intensity and diffuser would be a potentially effective strategy for the membrane fouling and foaming control in the MBR system.

Brief review of operation of anaerobic wastewater treatment with membrane bioreactors

In recent years, anaerobic membrane bioreactor (AnMBR) technology has been considered as a very appealing alternative for wastewater treatment due to its significant advantages over conventional anaerobic treatment and aerobic membrane bioreactor (MBR) technology. The paper provides an overview of the current status of the anaerobic membrane bioreactor technology with a special emphasis on its performance and drawbacks when applied for domestic and municipal wastewater treatment. According to the reported data, the renewable energy produced at the plants (i.e. from methane) covered the energy demand for membrane filtration while the excess energy can be further utilized. Anaerobic membrane bioreactors are an attractive technology that needs further research efforts and applications at an industrial scale.

Perspectives on Membrane Bioreactor Potential for Treatment of Pulp and Paper Industry Wastewater: A Critical Review

The pulp and paper wastewater contains various organic and inorganic compounds in considerable amounts. Membrane bioreactor (MBR) technology has been used extensively for various industrial wastewater treatments due to its ability to produce appropriate effluent according to water quality regulations. Pulp and paper wastewater can be successfully treated by MBR in different conditions according to the wastewater and MBR characteristics. However, a major drawback in the use of MBR is membrane fouling, which leads to the reduction of effluent permeate flux. Fouling factors need to be considered because they are the major problems affecting the MBR performance. Besides, the performance of MBR can be increased by wastewater pretreatment methods and addition of a fouling reducer. In this paper, the application of aerobic and anaerobic MBR for pulp and paper wastewater treatment and membrane fouling has been reviewed. It has been found that the advantages of MBR over conventional biological treatment processes have made this technology popular for pulp and paper wastewater treatment.

Comparison of fouling between aerobic and anaerobic MBR treating municipal wastewater

AbstractThe key driver for anaerobic membrane bioreactors (AnMBRs) for municipal wastewater treatment is enabling the transition to energy neutral wastewater treatment. However, municipal wastewater delivers a comparatively constrained methane yield, which means energy conservation must be prioritised to achieve the proposed energy neutral ambition. A critical focus on membrane fouling is therefore warranted, as membrane operation represents the primary energy demand in MBRs. This review seeks to quantify the characteristics of the prevailing AnMBR biological suspension and to ascertain whether knowledge transfer exists between fouling characteristics in aerobic and anaerobic MBRs for municipal applications. Analysis of literature data revealed that the level of extractable extracellular polymeric substrate is slightly higher in aerobic MBRs than in anaerobic MBRs. However, AnMBR comprises considerably higher soluble microbial product concentrations, which have been widely reported to increase fouling propensity in aerobic systems. More distinct is the difference in the colloidal and fine solids fraction (between 1 and 10–15 μm), which is likely to dominate fouling in anaerobic systems and limit knowledge transfer from aerobic MBRs. Literature data on energy production was compared to that employed for membrane operation, and evidences that despite the challenging character of the particle matrix, energy neutral operation is achievable for AnMBR applied to municipal wastewater treatment.

Improved membrane sequencing batch reactor: effect of carbon and nitrogen volumetric loading rate on dephosphatation

ABSTRACTA lab-scale anaerobic–anoxic–aerobic membrane bioreactor (A2NO-MBR) fed with synthetic wastewater was operated to investigate the impact of influent carbon and nitrogen volumetric loading rate (VLR) on dephosphatation, and the corresponding influent concentration was 100–300 mg L−1 (COD), 24–50 mg L−1 (NH4+–N) and 4.8–6.0 mg L−1 (TP), respectively. The results demonstrated that carbon VLR had a negligible effect on the COD removal with effluent below 50 mg L−1, and high and stable removal capacity for phosphorus were also obtained, regardless of carbon VLR change. Whereas TN removal efficiency was positively correlated with carbon VLR reduction, and lower carbon VLR produced a negative effect on TN removal. In addition, since nitrate served as an electron acceptor for denitrifying phosphorus removal (DPR), a significant effect on nitrogen and phosphorus removal was observed with different nitrogen VLR. The TN and TP removal efficiency was 68.30 ± 1.36%, 70.70 ± 1.23%, 45.19 ± 1.72% and 41.63 ± 3.09%, 98.14 ± 0.53%, 53.34 ± 2.68% with influent nitrogen VLR of 0.024 ± 0.001, 0.034 ± 0.001 and 0.045 ± 0.001 kg-N/(m3 d), respectively. Moreover, bacterial community structure of sludge samples in Run I and V from anaerobic–anoxic–aerobic-SBR (named A2OSBR_1 and A2OSBR_2) and membrane bioreactor (named N-MBR_1 and N-MBR_2) revealed that Candidatus_Accumulibacter was the most dominant genus in A2OSBR_1 (21.50%) and A2OSBR_2 (18.98%). The relative lower carbon VLR favoured the enrichment of Saprospiraceae, which was related with DPR, with the proportion of 9.31% and 14.61% in A2OSBR_1 and A2OSBR_2. Besides, Nitrospira and Nitrosomonas with proportions of 11.14%, 5.38% in N-MBR_1 and 10.72%, 6.77% in N-MBR_2 were observed, which were likely responsible for the nearly complete nitrification.

Magnetically induced membrane vibration (MMV) system for wastewater treatment

Membrane bioreactors (MBRs) are gaining more attention as they provide a viable alternative for conventional wastewater treatment. To operate efficiently, however, it is still key to select appropriate antifouling strategies. Fouling mitigation through vibration, as in the magnetically induced vibration (MMV) system, has been conceptually proven recently, but was never further optimized. This work presents a 2nd generation vibration system which was validated in both an aerobic MBR (AeMBR) and anaerobic MBR (AnMBR) improving the filtration time respectively by a factor of 133, at 60 L/m2 h, and a factor 13, at 20 L/m2 h. To prolong this filtration time even further, a combination with backwashing proved to be more effective compared to relaxation, allowing a 10-days long continuous operation for an AnMBR at a flux of 20 L/m2 h.

Development of a combined anaerobic and aerobic membrane bioreactor for wastewater treatment and reclamation in terrestrial-based controlled ecological life support system

Abstract A novel wastewater treatment and reuse system (WTRS) combining an anaerobic membrane bioreactor (AnMBR) and an aerobic membrane bioreactor (MBR) with the design capacity of 115 L/d was developed for a terrestrial-based controlled ecological life support system (CELSS). Results clearly showed that the WTRS realized mineralization of organic compounds and reservation of nitrogenous nutrient, therefore converting the effluent into replenishment for the hydroponic system. Trace gas emission from the WTRS could meet requirements for the whole CELSS. Compared with physico-chemical processes, the specific consumables consumption of the WTRS was advantageous but its specific energy consumption is still in need of improvement. Results of microbial community analysis were consistent with the running state of the AnMBR and the MBR.

Recent developments in biofouling control in membrane bioreactors for domestic wastewater treatment

• Biofouling is a long-standing problem in MBR for wastewater treatment. • Recent advances in anti-biofouling technologies in MBR are critically reviewed. • Integration of MBR with hybrid approach can mitigate biofouling effectively. • Biofouling properties are compared between aerobic and anaerobic MBR systems. • Biofouling is controlled by functionalized media in particle-sparged MBR. Biofouling is a long-standing problem in membrane bioreactors (MBRs) for domestic wastewater treatment because it deteriorates membrane permeability, thus demanding frequent chemical cleaning which can shorten membrane life-time. Recently, several anti-biofouling strategies have been suggested under scientific and engineered attentions from aerobic or anaerobic MBR systems. Nevertheless, researches are still needed to better understand biofouling and develop novel approaches to control this inevitable phenomenon. In this review, recent advances and emerging issues associated with biofouling control in aerobic and anaerobic MBR technologies are critically discussed. Existing challenges and future research perspectives are also addressed to achieve MBR sustainability with biofouling control. This also suggests that integration of MBR with hybrid approach could effectively enhance MBR performance in terms of biofouling mitigation. Future works should elucidate biofouling behavior in integrated MBRs more clearly for sustainable wastewater treatment applications.

Brewery wastewater treatment using MBR coupled with nanofiltration or electrodialysis: biomass acclimation and treatment efficiency.

Breweries release significant amounts of wastewater loaded with various organic and mineral materials. Prior studies of membrane bioreactor (MBR) wastewater treatment have been conducted with very little interest granted to the conditions of biomass acclimation. This study displays biomass behavior during brewery wastewater treatment by an aerobic MBR. In addition, nanofiltration and electrodialysis have been studied as potential post-treatment to decrease mineral concentrations and permit further water reuse for agriculture. An anoxic/aerobic laboratory MBR, associated with a flat sulfonated polyether membrane was used for synthetic brewery wastewater treatment. Biomass acclimation was performed using a feeding substrate. Organic concentrations in the MBR influent varied from 700 mg COD/L to 10,600 mg COD/L (COD: chemical oxygen demand) for 110 days. The results indicate a good acclimation to effluent with high salts and organic matter loads. Steady evolution of biomass concentration and activities was achieved after 90 days of operation. A reduction of COD of around 95% was obtained with MBR and up to 99% with nanofiltration post-treatment for the reconstructed brewery effluent with an organic loading rate of 7 g COD/L·d and a solid and hydraulic retention time of 30 days and 36 hours. A good reduction of the salt content was also recorded primarily with the nanofiltration and electrodialysis processes.

An integrated membrane bioreactor system with iron-dosing and side-stream co-fermentation for enhanced nutrient removal and recovery: System performance and microbial community analysis

An integrated membrane bioreactor (MBR) system was developed for enhanced nutrient (N and P) removal and effective P recovery in wastewater treatment. The system consisted of an iron-dosing MBR and side-stream fermentation for P removal and recovery and side-stream denitrification for N removal. Around 98.1% of the total phosphorus (TP) in wastewater was removed by ferric iron-induced precipitation and membrane filtration in the aerobic MBR, and nearly 53.4% of the TP could be recovered via anaerobic fermentation from the MBR sludge. In addition, the fermenter that allowed acidogenic co-fermentation with food waste provided sufficient soluble organics for biological denitrification, and an overall 91.8% total N removal was achieved through the side-stream denitrification. High-throughput sequencing was applied to analyse the microbial communities in the integrated system, and important functional bacteria were identified for nitrification, denitrification, acidogenic fermentation and dissimilatory iron reduction through the different components of the system.

Membrane integrated process for advanced treatment of high strength Opium Alkaloid wastewaters.

In this study, an integrated aerobic membrane bioreactor (MBR)-nanofiltration (NF) system has been applied for advanced treatment of Opium processing wastewaters to comply with strict discharge limits. Aerobic MBR treatment was successfully applied to high strength industrial wastewater. In aerobic MBR treatment, a non-fouling unique slot aeration system was designed using computational fluid dynamics techniques. The MBR was used to separate treated effluent from dispersed and non-settleable biomass. Respirometric modeling using MBR sludge indicated that the biomass exhibited similar kinetic parameters to that of municipal activated sludge systems. Aerobic MBR/NF treatment reduced chemical oxygen demand (COD) from 32,000 down to 2,500 and 130 mg/L, respectively. The MBR system provided complete removal of total inorganic nitrogen; however, nearly 50 mgN/L organic nitrogen remained in the permeate. Post NF treatment after MBR permeate reduced nitrogen below 20 mgN/L, providing nearly total color removal. In addition, a 90% removal in the conductivity parameter was reached with an integrated MBR/NF system. Finally, post NF application to MBR permeate was found not to be practical at higher pH due to low flux (3-4 L/m2/hour) with low recovery rates (30-40%). As the permeate pH lowered to 5.5, 75% of NF recovery was achieved at a flux of 15 L/m2/hour.

Human health impact of non-potable reuse of distributed wastewater and greywater treated by membrane bioreactors

We assessed the annual probability of infection resulting from non-potable exposures to distributed greywater and domestic wastewater treated by an aerobic membrane bioreactor (MBR) followed by chlorination. A probabilistic quantitative microbial risk assessment was conducted for both residential and office buildings and a residential district using Norovirus, Rotavirus, Campylobacter jejuni, and Cryptosporidium spp. as reference pathogens. A Monte Carlo approach captured variation in pathogen concentration in the collected water and pathogen (or microbial surrogate) treatment performance, when available, for various source water and collection scale combinations. Uncertain inputs such as dose-response relationships and the volume ingested were treated deterministically and explored through sensitivity analysis. The predicted 95th percentile annual risks for non-potable indoor reuse of distributed greywater and domestic wastewater at district and building scales were less than the selected health benchmark of 10−4 infections per person per year (ppy) for all pathogens except Cryptosporidium spp., given the selected exposure (which included occasional, accidental ingestion), dose-response, and treatment performance assumptions. For Cryptosporidium spp., the 95th percentile annual risks for reuse of domestic wastewater (for all selected collection scenarios) and district-collected greywater were greater than the selected health benchmark when using the limited, available MBR treatment performance data; this finding is counterintuitive given the large size of Cryptosporidium spp. relative to the MBR pores. Therefore, additional data on MBR removal of protozoa is required to evaluate the proposed MBR treatment process for non-potable reuse. Although the predicted Norovirus annual risks were small across scenarios (less than 10−7 infections ppy), the risks for Norovirus remain uncertain, in part because the treatment performance is difficult to interpret given that the ratio of total to infectious viruses in the raw and treated effluents remains unknown. Overall, the differences in pathogen characterization between collection type (i.e., office vs. residential) and scale (i.e., district vs. building) drove the differences in predicted risk; and, the accidental ingestion event (although modeled as rare) determined the annual probability of infection. The predicted risks resulting from treatment malfunction scenarios indicated that online, real-time monitoring of both the MBR and disinfection processes remains important for non-potable reuse at distributed scales. The resulting predicted health risks provide insight on the suitability of MBR treatment for distributed, non-potable reuse at different collection scales and the potential to reduce health risks for non-potable reuse.

Energy and greenhouse gas life cycle assessment and cost analysis of aerobic and anaerobic membrane bioreactor systems: Influence of scale, population density, climate, and methane recovery

This study calculated the energy and greenhouse gas life cycle and cost profiles of transitional aerobic membrane bioreactors (AeMBR) and anaerobic membrane bioreactors (AnMBR). Membrane bioreactors (MBR) represent a promising technology for decentralized wastewater treatment and can produce recycled water to displace potable water. Energy recovery is possible with methane generated from AnMBRs. Scenarios for these technologies were investigated for different scale systems serving various population densities under a number of climate conditions with multiple methane recovery options. When incorporating the displacement of drinking water, AeMBRs started to realize net energy benefits at the 1 million gallons per day (MGD) scale and mesophilic AnMBRs at the 5 MGD scale. For all scales, the psychrophilic AnMBR resulted in net energy benefits. This study provides insights into key performance characteristics needed before an informed decision can be made for a community to transition towards the adoption of MBR technologies.

Utilization of aerobic granulation to mitigate membrane fouling in MBRs

Membrane bioreactor (MBR) is a compact and efficient wastewater treatment and reclamation technology; but, it is limited by membrane fouling. The control of membrane fouling significantly increases operational and maintenance costs. Bacteria and their byproducts - extracellular polymeric substances (EPS) - are major contributors to membrane fouling in MBRs. A recent attempt at fouling mitigation is the development of aerobic granular sludge membrane bioreactor (AGMBR) through the integration of a novel biotechnology - aerobic granulation - and MBR. This paper provides an overview on the development of AGMBR to mitigate membrane fouling caused by bacteria and EPS. In AGMBR, EPS are used up in granule formation; and, the rigid structure of granules provides a surface for bacteria to attach to rather than the membrane surface. Preliminary research on AGMBR using synthetic wastewater show remarkable membrane fouling reduction compared to conventional MBR, thus improved membrane filtration. Enhanced performance in AGMBR using actual municipal wastewater at pilot-scale has also been reported. Therefore, further research is needed to determine AGMBR optimal operational conditions to enhance granule stability in long-term operations and in full-scale applications.

Synergy between anaerobic digestion and a post‐treatment based on Thermophilic Aerobic Membrane Reactor (TAMR)

This work concerns the treatment of the liquid fraction of digestate, derived from two‐stage anaerobic digestion process that treats bovine manure and corn silage, with a thermophilic aerobic membrane bioreactor (called TAMR) at pilot scale. The pilot plant was daily monitored about for 4 months; the temperature was maintained equal to 50°C and the average Organic Loading Rate (OLR) is 5.3 kg COD m−3 d−1. Results highlight high removal efficiencies of COD (> 94%) and total phosphorous (> 87%). The liquid fraction of TAMR (permeate), in view of a treatment in a municipal wastewater treatment plant that works in mesophilic conditions, shows good biodegradability; moreover, high ammonia content could be recovered by stripping the permeate and submit the ammonia rich gas to acidic absorption to produce ammonia salt (a valuable fertilizer). The solid fraction of TAMR (concentrate) shows an important neutralization effect due to the presence of calcium carbonate that derives from the reaction between the lime added and the carbon dioxide produced from organic matter degradation. Moreover, good hydrolytic effect on protein/carbohydrate solid substrate and sulfate reduction suggest the use of TAMR concentrate as supplement in anaerobic digestion process. © 2017 American Institute of Chemical Engineers Environ Prog, 36: 1802–1809, 2017

Eco-efficiency analysis of non-potable water systems in domestic buildings

Energy efficiency in water systems contributes significantly towards achieving sustainable water management. Decentralized anaerobic fluidized bed membrane bioreactor (AFMBR) systems with energy recovery have been proposed for greywater recycling in domestic buildings for non-potable uses, such as toilet flushing. This study developed an eco-efficiency analysis (EEA) framework with the integration of life-cycle assessment (LCA) and economic analysis for the evaluation of different water systems. Four water management scenarios including (1) freshwater flushing system, (2) seawater flushing system, (3) greywater flushing system adopting aerobic membrane bioreactor (MBR), and (4) greywater flushing system adopting AFMBR, were analyzed in a case study in Hong Kong. The EEA results reveal the AFMBR greywater reuse scenario to be the most eco-efficient option as the system is capable of energy recovery, recycling of water resource and reduction of sewage treatment loadings. This study has demonstrated that the EEA framework is an effective tool to guide water management towards sustainability and provides a basis for further research on the application of greywater recycling systems on a larger scale.

Fate of dissolved organic matter and byproducts generated from on-line chemical cleaning with sodium hypochlorite in MBR

On-line chemical cleaning with sodium hypochlorite (NaClO) has been extensively practiced for maintaining the stable permeability of membrane bioreactor (MBR), during which activated sludge is inevitably exposed to NaClO. The authors previously reported the NaClO-induced generation of dissolved organic matter (DOM) and halogenated byproducts, however their fate in terms of biodegradability and removability in MBR has been unknown. Therefore, this study investigated the removal mechanisms of generated DOM and byproducts in an aerobic MBR. It was found that about 39% of DOM produced was removed through biodegradation and membrane rejection. The fluorescence excitation-emission matrix (EEM) coupled with parallel factor analysis (PARAFAC) also revealed that protein-like substances were more readily biodegradable than humic-like substances. Moreover, 25 kinds of chlorinated and brominated byproducts were detected after the contact of biomass with NaClO by ultra performance liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry, while nine of which were confirmed with standard compounds. 62.4–84.5% halogenated byproducts were ended up in permeate of MBR chemically cleaned with 5–20mg/L of NaClO. This study raises a serious concern on reusing and recycling of such MBR permeate.