Resource recovery from wastewater by anaerobic membrane bioreactors: Opportunities and challenges

This review critically discusses the potential of anaerobic membrane bioreactor (AnMBR) to serve as the core technology for simultaneous recovery of clean water, energy, and nutrient from wastewater. The potential is significant as AnMBR treatment can remove a board range of trace organic contaminants relevant to water reuse, convert organics in wastewater to biogas for subsequent energy production, and liberate nutrients to soluble forms (e.g. ammonia and phosphorus) for subsequent recovery for fertilizer production. Yet, there remain several significant challenges to the further development of AnMBR. These challenges evolve around the dilute nature of municipal wastewater, which entails the need for pre-concentrating wastewater prior to AnMBR, and hence, issues related to salinity build-up, accumulation of substances, membrane fouling, and membrane stability. Strategies to address these challenges are proposed and discussed. A road map for further research is also provided to guide future AnMBR development toward resource recovery.


Introduction
In a paradigm shift towards the circular economy, wastewater can no longer be viewed as the culprit of environmental pollution but rather a source of valuable resources, including clean water, renewable energy and nutrients. The economic value of key resources in wastewater can help to offset the cost of wastewater treatment (Burn et al., 2014). Indeed, reclaimed water has been considered as an alternative source to augment clean water supply and address issues caused by water shortage (Shannon et al., 2008). Energy can be extracted from the organic content in wastewater by anaerobic treatment to produce biogas, which is a renewable fuel.
Nutrients in wastewater can also be recovered to produce fertilizers for sustainable agriculture production, particularly given the finite availability of phosphorus from mining (Koppelaar & Weikard, 2013). Recent interest in these resources has spurred new research aiming to convert wastewater treatment plants into resource recovery facilities.
Nutrient recovery from wastewater can also reduce the maintenance cost of wastewater treatment facilities and avoid environmental impacts. During wastewater treatment, phosphate and ammonium (which are abundant in wastewater) can react with magnesium to form crystalline precipitate, known as struvite (MgNH4PO4 . 6H2O), causing blockage and scaling of plant equipment (Doyle et al., 2002). Moreover, both nitrogen and phosphorus are important contaminants that can result in eutrophication of natural waterways if they are discharged to the environment.
Membrane bioreactor (MBR) has been deployed at an increasing speed to advance wastewater treatment and reuse on a global scale . MBR is an integration of membrane filtration with conventional activated sludge (CAS) treatment. Compared to CAS treatment, MBR exhibits several advantages, including higher effluent quality, smaller footprint, as well as easier operation and management (Judd, 2016). Indeed, MBR is more effective for the removal of trace organic contaminants (TrOCs) than CAS treatment for advanced water reuse (Luo et al., 2014). TrOCs occur ubiquitously in municipal wastewater and are of particular concern to water reuse. It is noteworthy MBR is energy-intensive since aeration is necessary for the growth and activity of activated sludge. Furthermore, energy and nutrients in wastewater are dissipated as released gases (e.g. carbon dioxide and nitrogen gas) in MBR treatment.
An alternative MBR configuration, namely anaerobic MBR or AnMBR, has also been explored for energy neutral wastewater treatment (Gao et al., 2008;Verstraete et al., 2009). AnMBR integrates anaerobic digestion treatment with membrane filtration.
During AnMBR treatment, organic substances in wastewater are biologically converted to methane-rich biogas. The produced biogas can offset the energy demand for wastewater treatment . Since anaerobic treatment converts nutrients to chemically available forms (e.g. ammonia and phosphate), AnMBR can also facilitate nutrient recovery via subsequent precipitation. Nevertheless, there remain several significant challenges in the development of AnMBR for resource recovery from wastewater, particularly municipal wastewater. These include low organic and nutrient contents in municipal wastewater as well as issues associated with salinity build-up, membrane stability, membrane fouling, and the occurrence of inhibitory substances.
In this paper, the performance of AnMBR for wastewater treatment and resource recovery is critically reviewed. Several key challenges to the further development of AnMBR are delineated. Potential strategies to address these challenges are proposed.
This review paper provides important insight to the development of AnMBR for the management of water, energy, and nutrients.

Fundamentals of anaerobic membrane bioreactor
AnMBR differs intrinsically from aerobic MBR in terms of the biological component.
The anaerobic biological process involves four integrated stages, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Degradation of organic matter and their conversion to biogas depend on the symbiotic relationship among the different groups of microorganisms (e.g. fermentative bacteria, syntrophic acetogens, homoacetogens, hydrogenetrophic methanogens and aceticlastic methanogens) (Chen et al., 2016). Of these microorganism groups, methanogens play arguably the most important role for biogas production by converting intermediate products from previous stages to methane gas. However, methanogens are slow-growing microorganisms and can be easily washed out from conventional anaerobic bioreactors. By integrating membrane separation processes, commonly including microfiltration (MF) and ultrafiltration (UF), the hydraulic retention time (HRT) can be decoupled from sludge retention time (SRT). Thus, AnMBR can produce more biogas than conventional anaerobic treatment (Liao et al., 2006). In many aspects (e.g. energy consumption, contaminant removal efficiency, and volume throughput), AnMBR differs considerably from aerobic MBR (Table 1). Since aeration is not required, AnMBR has a significantly lower energy input to the bioreactor compared to aerobic MBR. In addition, the energy footprint of AnMBR can be offset by produced biogas (Smith et al., 2012). Nevertheless, Martin et al. (2011) reported that the energy demand in submerged AnMBR varies considerably from 0.03 to 5.7 kWh/m 3 due to different energy requirements for gas sparging to control membrane fouling. Indeed, AnMBR is usually operated at high biomass concentration as well as long SRT and HRT to treat complex wastewater (Skouteris et al., 2012), resulting in more severe membrane fouling in comparison with aerobic MBR. As such, the reported flux of AnMBR is commonly in the range between 5 and 12 L/m 2 h, which is considerably lower than the flux of 20 -30 L/m 2 h typically for full-scale aerobic MBR . Without oxygen as an electron acceptor, anaerobic digesters release electrons onto methane (CH4) rather than using them for microbial growth. Thus, AnMBR produces less sludge than aerobic MBR (Liao et al., 2006).
Since anaerobic degradation is a slow process, AnMBR has a lower contaminant removal efficiency and volume throughput (i.e. treatment capability) than aerobic MBR.

Configurations of anaerobic membrane bioreactor
There are several AnMBR configurations depending on the anaerobic treatment process ( Figure 1). Excellent reviews of anaerobic bioreactors for AnMBR are In the side-stream AnMBR, membrane module is integrated outside of the bioreactor.
Mixed liquor in the bioreactor is transferred to the membrane unit for clean water extraction. In the submerged AnMBR, membrane unit can be directly immersed into the bioreactor (Figure 1 E) to extract treated water through the membrane.
The submerged AnMBR can be deployed as a two-stage system by submersing the membrane module in a chamber separated from the working bioreactor ( Figure 1F).
The two-stage AnMBR configuration facilitates membrane maintenance and cleaning by intensive shear force and chemicals. Retentate from the membrane tank can also be recirculated to the anaerobic reactor for further contaminant biodegradation. As such, the two-stage configuration can be potentially used for full-scale AnMBR applications.
Indeed, Shin and Bae (2018) reported that ten out of eleven recent pilot-scale AnMBR studies have adopted the two-stage configuration. As a notable exception, Gouveia et al. (2015b) developed a single-stage AnMBR system, in which a submerged membraned housed at the supper part of the USAB reactor. In their study, two baffles were placed between the three-phase (i.e. gas/liquid/solid) separator and the UF membrane to improve solid settleability. AnMDBR is an integration of membrane distillation (MD) and anaerobic treatment.
MD is a thermally driven separation processes, in which the thermal gradient between a feed solution and distillate drives the transportation of water vapour through a hydrophobic, microporous membrane. The competitive advantages of anaerobic processes can be readily utilized when they are combined with the MD process, because the thermophilic operation for anaerobic treatment can reduce extra heat requirement for MD operation (Kim et al., 2015).
AnOMBR, which combines forward osmosis (FO) with anaerobic treatment, is also attractive for advanced wastewater treatment and reuse. In FO, water transports from a feed solution, across the semi-permeable membrane, to a draw solution with the osmotic pressure difference between these two solutions as the driving force. During AnOMBR operation, a desalination process, such as nanofiltration (NF) and reverse osmosis (RO), can be used to regenerate the draw solution and produce clean water.
Compared to conventional MF and UF membranes, FO has higher selectivity, lower membrane fouling propensity and better membrane fouling reversibility (Xie et al., 2015).

Organic removal
The performance of AnMBR for water reuse has been extensively studied in recent years. AnMBR is best suited for the treatment of wastewater with a high organic content. Indeed, there have been a number of pilot demonstration and full-scale AnMBR systems for treating effluents from field crop processing (e.g. sauerkraut, wheat, maize, soybean, and palm oil), dairy processing, and the beverage industry (e.g. winery, brewery, and distillery) ( Table 2).  up-flow anaerobic sludge reactor; CSTR: continuous stirred-tank reactor; N.A: information is not available.

Biogas production
Chemical energy in wastewater in the form of organic carbon can be recovered by AnMBR to produce biogas (Table 2). It has been well established that biogas produced by AnMBR consists of more than 80% of CH4 (Skouteris et al., 2012).
During AnMBR treatment, the CH4 yield increases linearly with the organic loading rate (Yeo et al., 2015). Under an optimized condition, AnMBR can convert up to 98% of the influent COD into biogas, which is equivalent to seven times of the energy

Nutrient removal and recovery
During AnMBR treatment, nutrient removal depends largely on microbial assimilation and is limited due to low biomass yields of anaerobic microbes. Dai et al. (2015) reported that AnMBR could only remove 10% of the total nitrogen. On the other hand,

Factors underlying key challenges to further develop anaerobic membrane bioreactors
Despite the high potential of AnMBR for resource recovery from wastewater, there remain some challenges, particularly for treating municipal sewage. They include the dilute nature and temperature difference of municipal wastewater, salinity build-up when diluted wastewater is preconcentrated, membrane fouling and stability, and inhibitory substances (e.g. free ammonia and sulphide) ( Figure 3). Thus, future studies are required for the development of effective strategies to address these challenges for further development of AnMBR.

Temperature
AnMBR can be operated under either thermophilic (50 -60 ˚C) or mesophilic (30 - could also be attributed to its increased solubility in the effluent when the temperature decreased to 20 ˚C. In addition, the mixed liquor viscosity also increased as the temperature decreased, thus requiring more energy for mixing and pumping.

Salinity build-up
Saline wastewater is a challenge to biological treatment. Indeed, AnMBR performance in terms of biogas production and organic removal decreases when treating highly saline feed, such as wastewater from seafood processing and cheese reported the robustness of AnMBR to short-term, step-wise increase of salinity up to 20 g/L NaCl with significant variation in the microbial community. It is noteworthy that salinity increase exacerbated membrane fouling by reducing sludge particle size in their study.

Inhibitory substances
AnMBR is susceptible to the accumulation of inhibitory substances, such free ammonia and sulphate, in wastewater. Ammonia is generated by the biodegradation of

Membrane fouling
Membrane fouling is a persistent challenge to advance AnMBR given membrane Despite effective strategies to control fouling, membrane cleaning is still necessary.
Membrane cleaning includes physical, chemical, and biological schemes. Physical membrane cleaning can be achieved by backwashing, surface flushing, and ultrasonication (Lin et al., 2013). Chemical cleaning is necessary to further remove fouling layers using suitable agents, such as sodium hypochlorite, hydrochloric acid, nitric acid, citric acid, sodium hydroxide, and EDTA for target foulants.
Chemically-assisted backwashing has also been developed to enhance membrane cleaning for AnMBR. Nevertheless, chemicals that can diffuse back to the bioreactor may inhibit the microbial activity and then biological performance of AnMBR. Mei et al. (2017) reported that utilising 12 mmol/L NaOH to assist in-situ membrane backflush did not adversely affect AnMBR treatment performance given the alkali consumption by anaerobic biomass and buffering capacity of the mixed liquid.

Membrane stability
Chemically and biologically stable polymeric materials are commonly used to fabricate robust membranes for MBR applications. These polymeric materials mainly include polytetrafluoroethylene, polyvinylidenefluoride, and polypropylene (Alkhudhiri et al., 2012). Thus, membrane degradation is not a concern for conventional MBR using the existing low retention UF or MF membranes. By contrast, membrane integrity is a major issue to FO when integrating with biological processes.
Currently commercial FO membranes are made of either cellulose or polyamide.

Future perspectives
AnMBR has a proven capability and can offer a unique opportunity to achieve simultaneous wastewater treatment and resource recovery. However, the adoption and commercialisation of AnMBR at industrial scale is still pending due to the challenges discussed above. Thus, future research should be dedicated to address these issues for the further development of AnMBR ( Figure 3).
FO is a promising approach to produce clean water and pre-concentrate wastewater to Compared to membrane fouling, little is known about the stability of membranes during AnMBR operation. In AnMBR, membranes are exposed to the biologically active conditions with biomass concentration typically higher than 10 g/L. Moreover, given the severity of membrane fouling in AnMBR operation, frequent membrane cleaning with harsh chemicals may be necessary to maintain water production. Thus, it is important to understand membrane degradation in AnMBR operation and develop mitigation strategies to prolong membrane lifespan.
Several techniques have been proposed to further purify AnMBR effluent for clean water production and/or nutrient recovery. They include membrane filtration, ion exchange, electrodialysis, biological processes (e.g. photosynthetic bioreactor), advanced oxidation processes, and electrocoagulation. Nevertheless, further work is needed to evaluate the techno-economic feasibility of these processes in integration with AnMBR to determine an appropriate framework that can facilitate practical application of AnMBR for wastewater treatment and resource recovery. Moreover, the agronomic availability of recovered nutrients should be assessed to emphasize AnMBR potential for resource recovery from wastewater.
Recovering dissolved CH4 from effluent is also strategically important to broaden

Conclusion
AnMBR has the potential to revolutionise current wastewater treatment facilities for simultaneous recovery of clean water, energy, and nutrients. Such revolution can be accelerated by continued efforts to concentrate municipal wastewater to the level suitable for AnMBR treatment and subsequent resource recovery. Issues associated with salinity build-up, membrane stability and fouling, and the occurrence of inhibitory substances (e.g. free ammonia and sulphate) need to be addressed to advance AnMBR for water reuse and resource recovery. Successful recovery of clean water, energy and nutrient also requires the integration between AnMBR and other complementary processes.

Acknowledgement
Xiaoye Song would like to thank the Chinese Scholarship Council and the University of Wollongong for PhD scholarship support. Factors governing the pre-concentration of wastewater using forward osmosis for subsequent resource recovery. Sci. Total Environ., 566-567, 559-566.