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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/329202427 Comparison of fouling between aerobic and anaerobic MBR treating municipal wastewater Article in H2Open Journal November 2018 DOI: 10.2166/h2oj.2018.109 CITATIONS 26 5 authors, including: Kanming Wang Zhejiang University of Technology 16 PUBLICATIONS 165 CITATIONS SEE PROFILE Bruce Jefferson Cranfield University 306 PUBLICATIONS 19,219 CITATIONS SEE PROFILE READS 718 Ana Soares Cranfield University 100 PUBLICATIONS 3,536 CITATIONS SEE PROFILE Ewan james Mcadam Cranfield University 119 PUBLICATIONS 2,702 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Fatberg Science (#fatbergscience): understanding FOG deposits and their management (physical and biolgocial) View project Anaerobic membrane bioreactor (AnMBR) View project All content following this page was uploaded by Kanming Wang on 17 December 2018. The user has requested enhancement of the downloaded file.  2018 The Authors 131 H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Comparison of fouling between aerobic and anaerobic MBR treating municipal wastewater K. M. Wanga, N. Martin Garciab, A. Soaresa, B. Jeffersona and E. J. McAdama,* a Cranfield Water Science Institute, Cranfield University, Bedfordshire MK43 0AL, UK b Cetaqua, Centro Tecnolgico del Agua, Crta. de Esplugues, 75, 08940 Cornell de Llobregat, Spain *E-mail: e.mcadam@cranfield.ac.uk Abstract The key driver for anaerobic membrane bioreactors (AnMBR) 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 m), 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. Key words: biogas, biomass characteristics, energy demand, membrane bioreactor INTRODUCTION Anaerobic membrane bioreactors (AnMBRs) hold significant potential to reduce the overall energy demand of municipal (domestic) wastewater treatment through obviating the need for aeration, increasing energy recovery from methane production and producing lower secondary sludge (Martin-Garcia et al. 2011). Several reviews have been published on AnMBR over the past ten years. Liao et al. (2006) focused on AnMBR technology for high strength industrial wastewater treatment. More recent reviews have since updated the position of AnMBR for industrial treatment (Skouteris et al. 2012; Lin et al. 2013), which include a particularly insightful narrative on the thirty years of AnMBR commercial development for industrial application (Lin et al. 2013). Development of AnMBR for municipal wastewater treatment has been comparatively limited due to the past perception that municipal wastewater could not be treated anaerobically since the low organic concentration was insufficient to support microbial growth (Lester et al. 2013). Integrating This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 132 doi: 10.2166/h2oj.2018.109 membrane technology into anaerobic systems helps respond to this challenge by decoupling hydraulic retention time (HRT) from solids retention time (SRT), thereby diminishing washout. The capability of AnMBR to achieve chemical oxygen demand (COD) and suspended solids compliance to international discharge standards has been experimentally demonstrated on municipal wastewater (Martin Garcia et al. 2013). The impact of reactor configuration and operating conditions on organic biodegradation, coupled with a description of potential methods for integrating AnMBR technology into the flowsheet for municipal wastewater, is presented in recent reviews (Smith et al. 2012; Ozgun et al. 2013). While the capability of AnMBRs to treat municipal wastewater has been demonstrated, the principal driver for their implementation is in enabling the transition toward energy neutral wastewater treatment. McAdam et al. (2011) demonstrated this conceptually for anaerobic treatment of temperate settled wastewater, evidencing around 0.28 kWh m3 of additional energy production (including recovery of the dissolved methane fraction) (Cookney et al. 2016). However, through comparison of the typical energy demand of membrane operation in commercial aerobic MBR technology (0.19-0.70 kWh m3; Judd 2011; Krzeminski et al. 2012), it can also be evidenced that energy conservation must be prioritised to achieve the proposed energy neutral ambition. Consequently, membrane fouling can be considered one of the critical barriers in achieving commercially viable AnMBR for municipal wastewater treatment, as this governs the energy demand (Judd 2011). The mechanisms underpinning membrane fouling in aerobic MBR for municipal wastewater have been studied extensively and summarised (Le-Clech et al. 2006; Meng et al. 2009). The factors that influence membrane fouling in both aerobic and anaerobic MBR can be generally defined by the membrane characteristics, biomass properties, reactor and membrane operational conditions. Very few studies have directly compared aerobic and anaerobic MBR experimentally, for either municipal wastewater or blackwater (Baek & Pagilla 2006; van Voorthuizen et al. 2008; Martin-Garcia et al. 2011). These studies acknowledge an important distinction in fouling behaviour between aerobic and anaerobic systems, however, an in-depth characterisation and comparison of foulants has not yet been completed. What is clear from the literature is that size distribution and organics concentration in the AnMBR mixed liquor determines the characteristics and mechanisms of fouling. Providing greater resolution on the characterisation can thus inform the most effective fouling control strategies, as compared with the more extensively studied aerobic MBRs (AeMBRs). This review therefore proposes to complement existing knowledge through characterisation of biomass properties within AnMBRs for municipal wastewater treatment, evaluating their impact on membrane fouling and comparing to those in AeMBR, so as to ascertain the main factors that determine differences in fouling behaviour and characteristics between these two systems. Since membrane fouling strongly influences energy demand, published data on both energy production from AnMBR for municipal wastewater treatment and membrane specific energy utilisation was further analysed to ascertain the position of the existing literature relative to the aspiration of `energy-neutral' wastewater treatment. BIOMASS CHARACTERISTICS Differences in characteristics between aerobic and anaerobic sludge are most readily attributed to the different mechanisms involved in the biological process. Aerobic biological suspensions mainly comprise microorganisms, decay products and influent solids forming microbial aggregates which are held together by high molecular weight polymers secreted by bacteria. This allows them to exist at high population densities (Laspidou & Rittmann 2002) in virtue of their high biomass yield and growth rates. It has been reported that the structure, morphology and surface properties of such suspensions can be altered by changes in physiological state of the biomass induced by changes in bioreactor operational parameters such as SRT and food to microorganism (F:M) ratio (Liao et al. 2001). Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 133 doi: 10.2166/h2oj.2018.109 On the other hand, anaerobic degradation of wastewater with dissolved, colloidal and particulate organic matter involves several sequential steps such as hydrolysis, acidogenesis, acetogenesis and methanogenesis (van Lier et al. 2008). Hydrolysis is thought to be an extracellular reaction where solids are converted to simple monomers by extracellular enzymes secreted by hydrolytic and fermentative bacteria (Sanders et al. 2000; Vavilin et al. 2008). As a result, and due to the low hydrolysis rates and biomass yield of anaerobic bacteria in high rate anaerobic reactors, the reactor solids inventory is considered to be mainly constituted by influent particles (Soto et al. 1993) that are of reduced particle size (Elmitwalli et al. 2001) and density (Lant & Hartley 2007). Therefore, as opposed to aerobic systems, sludge properties are probably more dependent on influent characteristics than on bioreactor operational parameters. Mixed liquor suspended solids Although sludge flocs are not considered the main contributors to fouling under low flux operation, mixed liquor suspended solid (MLSS) concentration has been shown to negatively affect membrane fouling in AnMBRs (Table 1). For instance, Jeison & van Lier (2006) found that changing biomass concentration had a greater impact on the formation of a cake layer than varying gas sparging intensity. Le Clech et al. (2003) also reported that the effect of MLSS was higher than gas sparging in an aerobic MBR. However, in their study, critical flux increased at higher biomass concentrations of 12 g MLSS L1 as compared to the lowest MLSS concentration of 4 g L1. Robles et al. (2013a) also indicated that higher MLSS concentration led to lower membrane permeability, which supports the observations of Jeison & van Lier (2006). Martin Garcia et al. (2013) compared a flocculated AnMBR configured as a continuous stirred tank reactor (CSTR) and a granulated AnMBR configured as an upflow anaerobic sludge blanket (UASB) reactor for settled municipal wastewater treatment, which presented MLSS concentrations for membrane filtration of 7.7 and 0.1-0.6 g L1, respectively. Higher fouling rates and lower critical fluxes have been determined in CSTR configured AnMBR with higher MLSS concentrations. The contradicting influence that MLSS concentration has on membrane hydraulic performance between aerobic and anaerobic MBRs can be attributed to differences in the relationship between biomass and dissolved/colloidal compounds in the mixed liquor. In aerobic MBRs, it has been widely reported that higher levels of soluble microbial products are found at lower MLSS when short sludge retention times are applied (Mass et al. 2006; Liang et al. 2007) while in anaerobic systems, soluble microbial products (SMP) tend to accumulate together with biomass (Harada et al. 1994) or at high sludge ages. Particle size distribution According to Lant & Hartley (2007), anaerobic sludge particle sizes are one order of magnitude lower than aerobic floc (activated sludge floc), even though the range of particle sizes covers three orders of magnitude as opposed to only one for aerobic biomass. Data collated from the literature identified similar median particle size ranges of 5.2-220 and 0.8-138 m in aerobic and anaerobic MBRs respectively (Table 2), however, a significant difference is the presence of a population of fine colloidal matter in AnMBR which has been shown to negatively affect membrane performance. This was confirmed by Martin-Garcia et al. (2011) who found a higher colloidal content in a flocculated AnMBR (CSTR) and granulated AnMBR (UASB), when compared directly with AeMBR. The authors attributed the higher degree of dispersive growth to higher fouling propensity. Particle size analysis in AnMBRs (Table 2) indicates that externally configured (side-stream) membrane systems yield considerably lower particle sizes when compared to immersed systems using gas sparging (Imasaka et al. 1989; Bailey et al. 1994; Hu & Stuckey 2006; Jeison & van Lier 2006) Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 134 Table 1 | Overview of operating conditions and membrane performance in immersed anaerobic MBRs treating municipal wastewater (real and synthetic) Source Reactor type/ Sludge Crua Crub Crub Crub Crua Crua Crua Crua Cruc Cruc UASB/F CSTR/F CSTR/F CSTR/F CSTR/F CSTR/F CSTR/F CSTR/F IAFMBR IAFMBR Cru (GA) CSTR/F Black Black Set Set Set UASB/F CSTR/F UASB/F UASB/G UASB/G Set CSTR/F Set CSTR/G Set CSTR/F Reactor size RE. FI. l Material/ Geo./ Pore size m 17.7d 1,300 800 - 50 - 50 550 80 550 80 550 80 60d 7.6d 7.6d 350 - PE/HF/0.03 PVDF/HF/0.05 PVDF/HF/0.04 PVDF/HF/0.04 PVDF/HF/0.04 PVDF/HF/0.04 PVDF/HF/0.04 PVDF/FS/140f -/HF/0.4 -/HF/0.4 PES/FS/0.038 MLSS g L1 16-21.5 10-30 21.3 21.3 12.8f 5.6f 11.3-23 6.4-9.3 - - 15 5 - PVDF/Tub/250e - 4d PVDF/Tub/250e - 160 150 PVDF/HF/0.045 0.5,5-7f 326 175 PVDF/HF/0.045 2, 12-14f 94 31 PVDF/HF/0.04 0.1-0.6 900 300 PVDF/HF/0.04 7.7 40d 51 -/HF/0.1 PES/PF/0.45 6.5-6.8 8.0-13.6 SGD m3 m2 h1 Jc L m2 h1 Jc,20 Flux TMP L m2 h1 mbar 0 0.23 1.3 0 0.146 0.146 0.146 0.12 0 0 - - - 10-12 12-14 - - 21-27 - - - 7 - - 10.3-12.3 12.3-14.4 - - 20-25 - - - 7 5,10 9-13.3 10 10 17 17 17 12 11.3 11.3 7 ,700 - ,140 ,140 ,215 ,88 ,250 - ,300 ,300 ,280 - - 0.2, 0.4-1.0 0.07-0.13 1.2 - - - - 4.3-13.4 - - - - 4.6-14.2 10g - 8g - 2.5,10-15 50, ,550 12, 15.7 - 6g ,100 0.4 1.9-9.7 2.0-10.3 6g ,700 - - 40 - - - - - - ,300 Fouling rate mbar min1 0.033,0.058 - 0.002-0.075 0.001-0.027 0.002 0.001j 0-0.004 - 0.014 0.009k ,0.002 kPa h1 0.200,0.350 - 0.012-0.45 0.007-0.16 0.010 0.004j 0.001-0.025 - 0.086 0.052k 0.001-0.011 - - - 0.001-0.013 0.001 - - - 0.004-0.079 0.004-0.013 0.033 0.198 0.004-0.007 0.005-0.014 0.025-0.042 0.031-0.083 Top h Ref. 336,144 14,400 60 .200 2,160 2,160 4,272 - 348 576 2,352 - - 4,320 - .300 ,200 - 360-960 Wen et al. (1999) Robles et al. (2013b) Ruigmez et al. (2016b) Ruigmez et al. (2016b) Dong et al. (2015) Dong et al. (2015) Dong et al. (2016b) Lin et al. (2011) Gao et al. (2014b) Gao et al. (2014b) Martinez-Sosa et al. (2011) van Voorthuizen (2008) van Voorthuizen (2008) Gouveia et al. (2015a) Gouveia et al. (2015b) Martin Garcia et al. (2013) Martin Garcia et al. (2013) Fawehinmi (2006) Huang et al. (2013) (Continued.) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 135 Table 1 | Continued Source Reactor type/ Sludge Reactor size RE. FI. l Material/ Geo./ Pore size m MLSS g L1 SGD m3 m2 h1 Jc L m2 h1 Jc,20 Flux TMP L m2 h1 mbar Fouling rate Top mbar min1 kPa h1 h Ref. Set SAF-MBR/F 0.245 0.245 PVDF/HF/0.1 - 0 - - 9 ,400 ,0.001-0.035 0.002-0.213 9,500 Yoo et al. (2014) Set SAF-MBR/F 990 770 PVDF/HF/0.03 0.6-1.2f 0 - - 4.1-7.5g ,500 - - 11,640 Shin et al. (2014) Syn UASB - PVDF/FS/0.22 - 1.8 - - 25 ,300 0.055-0.530l 0.330-3.180l - Wu et al. (2009) Syn UASB/G 4.7d PE/0.1 - 0 - - - - - - 480 Chu et al. (2005) Syn UASB/F 30d PVDF/FS/0.22 0.14 0-1.2 - - 25 ,300 0.417-1.250 2.5-7.5 4-12 An et al. (2010) Syn CSTR/F 5 1 PES/PF/0.45 5.6-10.5 - - - 5.3-7.9 ,300 ,0.006 0.002-0.036 840-2,400 Huang et al. (2011) Syn CSTR/F 3d PE/FS/0.4 4.3-4.8 3 - - 20 ,270 0.003-0.005k 0.018-0.031k 384 Hu & Stuckey (2007) Syn CSTR/F 3d PE/FS/0.4 6-19,12-16 3 - - 10 120-230 - - 360 Akram & Stuckey (2008) Syn CSTR/F 7d PES/FS/0.2 - 7.24 - - 7 - ,0.001m ,0.001m 720 Smith et al. (2013) Syn AFBR AFMBR 0.245 0.245 PVDF/HF/0.1 0.09-0.13f 0 - - 9 ,100 - - 2,400 Bae et al. (2014) Syn ABR AFMBR/F 3.93 2.0 PVDF/HF/0.1 - 0 - - 7 ,30h, , 320i 0.444i 2.667i - Kim et al. (2011) Syn ARMBR 4d PE/FS/0.22 5.5 0 - - 11 ,50 - - 2,400 Kim et al. (2014) Acronyms: ABR AFMBR: anaerobic baffle reactor anaerobic fluidised bed membrane bioreactor; AFBR: anaerobic fluidised bed reactor; ARMBR: anaerobic rotate disk MBR; BW: backwash; Cru: Crude; CSTR:continuous stirred tank reactor; F: flocculent sludge; FI: filtration; FS: flat sheet; G: granular sludge; GA: glucose addition; HF: hollow fibre; IAFMBR: integrated anaerobic fluidised-bed; MWCO: molecular weight cut off; PE: Polyethylene; PES: polyethersulfone; PF: plate and frame; PVDF: polyvinylidene fluoride; RE: anaerobic reactor; SAF-MBR: staged anaerobic fluidised MBR; Set: Settled; SGDm: specific gas demand; Syn: Synthetic; Top: time of performance; Tub: tubular; UASB: upflow anaerobic sludge blanket. aCrude: after screening. bCrude: pre-treatment including screening, degritter and grease removal. cCrude: from septic tank. dMembrane submerged in the anaerobic reactor. eMWCO (kDa). freported MLVSS. gNet flux. hWith GAC. iWithout GAC. jWith FeCl3 addition. kWith GAC (granular activated carbon) or PAC (powered activated carbon) addition. lWith addition of PAC (powered activated carbon)/zeolite/polyamide/polyaluminum chloride. mBackwash 4 min every 4 h. H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 136 doi: 10.2166/h2oj.2018.109 Table 2 | Particle size distribution of sludge/biomass in aerobic and anaerobic MBRs in contact with the membrane Source Reactor Type Geo./Mode/Config. Temp SRT C d Average dp50 m Ref. Anaerobic MBRs Industriala Industrialb Settled Settled Settled Synthetic Syntheticc Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Aerobic MBRs Industrial Crude Crude Crude Settled CSTR CSTR UASB CSTR CSTR UASB UASB UASB UASB UASB CSTR CSTR CSTR CSTR CSTR CSTR CSTR CSTR CSTR CSTR CSTR PF/pumped/Side Tubular/pumped/Side HF/gas sparged/Sub HF/gas sparged/Sub PF/gas sparged/Sub Tubular/pumped/Side FS/pumped/Side Tubular/pumped/Side Tubular/pumped/Side Tubular/pumped/Side -/pumped/Side Tubular/pumped/Side Tubular/gas sparged/Sub FS/gas sparged/Sub FS/gas sparged/Sub FS,HF/gas sparged/Sub PF/gas sparged/Sub HF/gas sparged/Sub MT/pumped/Side HF/gas sparged/Sub HF/gas sparged/Sub 53-55 52 - - 25-30 30 - 25 25 15 35 25 30,55 35 35 35 25-30 .10 20 9-21 20 Settled Synthetic Synthetic CSTR HF/gas sparged/Sub - CSTR HF/gas sparged/Sub - CSTR HF/gas sparged/Sub - - - 30,200 30,200 30,60 - - - - - 90-360 - 250 250 - 30- 3-16 0.8,4.0 60.3,1.4 42.8,7.6 28,21 36,16d 75,13d 465,138e 80-137f 25-88f 13 50.6,25.3d 70-90g 23.5 9.6-16.3h 60-65 24-31 - 60 5,20,40 10,30 30,200 20,40,60 30 31.8-35.6 50-300 14,48,31 120-220, 70-100 32.4,14.0 5.2-6.6 57.6 Choo & Lee (1998) Imasaka et al. (1989) Martin-Garcia et al. (2011) Martin-Garcia et al. (2011) Huang et al. (2013) Bailey et al. (1994) Cho & Fane (2002) Ozgun et al. (2015a) Ozgun et al. (2015b) Ozgun et al. (2015b) Elmaleh & Abdelmoumni (1997) Ho & Sung (2009) Jeison & van Lier (2006) Akram & Stuckey (2008) Akram & Stuckey (2008) Hu & Stuckey (2006) Huang et al. (2011) Sun et al. (2011) Defrance et al. (2000) Huang et al. (2001) Mass et al. (2006) Martin-Garcia et al. (2011) Lee et al. (2003) Zhou et al. (2014) Acronyms: CSTR: continuous stirred tank reactor; FS: flat sheet; HF: hollow fibre; MT: multiple tube; PF: plate & frame; UASB: upflow anaerobic blanket reactor. aAlcohol-distillery wastewater. bConcentrated thermophilic fermentation broth of evaporator condensate discharged from a kraft pulp mill. cNot mention synthetic, but assume according to the influent characteristics. dParticle size before and after shear induction. eUASB sludge particle size before and after membrane addition. fParticle size in the UASB effluent. gEstimate from graph. hWith addition of PAC (powered activated carbon). which can be attributed to the shear imposed by the pumping demanded to maintain cross-flow velocity (Imasaka et al. 1989; Jeison et al. 2009). Analogous effects have been demonstrated following conversion of conventional anaerobic reactors to AnMBR (Ho & Sung 2009; Ozgun et al. 2015a) and it is suggested that such particle reduction can also decrease biomass activity (Brockmann & Seyfried 1997). Apart from hydrodynamic conditions, operating temperature has been shown to influence biomass characteristics. Ozgun et al. (2015b) observed a reduction in median particle size from 80-137 to 25-88 m when the temperature was reduced from 25 to 15 C during the treatment of synthetic municipal wastewater. This was corroborated by Robles et al. (2013a), following the transition from mesophilic to psychrophilic conditions. The authors proposed that the smaller particle sizes led to lower cake layer porosities, and higher cake layer tortuosity, which resulted in considerable cake layer resistance. Transients in particle size distributions have been noted in both anaerobic Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 137 doi: 10.2166/h2oj.2018.109 and aerobic MBR systems following changes to SRT. Huang et al. (2011, 2013) reported a decrease in median particle size for AnMBR when SRT increased from 30 to 60 d, which is similar to the observations of Martin-Garcia et al. (2011). Similar effects of particle size reduction in aerobic MBR in response to an increase in SRT have been observed (Huang et al. 2001; Martin-Garcia et al. 2011). Organic fouling by EPS Extracellular polymeric substances (EPS), composed mainly of polysaccharides, proteins and lipids, have been widely reported as being responsible for organic fouling in both aerobic (Pollice et al. 2005; Fan et al. 2006; Rosenberger et al. 2006) and anaerobic MBRs (Harada et al. 1994; Cho & Fane 2002; Hu & Stuckey 2006; van Voorthuizen et al. 2008). For example, Rosenberger et al. (2006) revealed high polysaccharide concentrations in the sludge supernatant corresponded to high membrane fouling rate in aerobic MBR. In AnMBR, Huang et al. (2011) indicated higher protein and polysaccharides accumulated at long SRT, which resulted in increased fouling. These biopolymers have been fractionated according to whether they are found in the sludge supernatant as SMP or bound to the sludge flocs and are thus extracted from the cell walls (eEPS). Although the term SMP implies that these substances are of bacterial origin, they may also be the result of recalcitrant or partially transformed influent organics. This is particularly the case for anaerobic systems at lower temperatures where lower biodegradation rates apply, which increase SMP concentration above those of aerobic biomass (Lettinga et al. 2001). However, independent of their origin, analysis of eEPS and SMP has contributed to further characterisation both of the solid and the colloidal/soluble fractions of biological suspensions respectively. Bound/extractable EPS The surface properties of the sludge are primarily determined by eEPS. The hydrophobicity and surface charge have been correlated with the total EPS concentration and the ratio of proteins to carbohydrates in both conventional activated sludge and MBR systems (Liao et al. 2001). For instance, a higher ratio of proteins to carbohydrates in activated sludge has been reported to enhance bioflocculation through the reduction of surface charge and increase in hydrophobicity (Liao et al. 2001). On the other hand, high proportions of carbohydrates in the eEPS are associated with a more dispersed sludge structure due to the greater repulsion between sludge particles and interaction with the aqueous phase resulting from the higher negative surface charge and reduced hydrophobicity (Liao et al. 2001). Literature data regarding surface properties of anaerobic sludge do not correlate as consistently as those from aerobic systems, and no conclusions can be drawn regarding comparative concentrations of eEPS and fractions thereof, to allow comparison between aerobic and anaerobic sludges with respect to surface charge and hydrophobicity. Morgan et al. (1990) reported that aerobic sludge was more negatively charged, contained higher levels of total eEPS and lower ratios of proteins to carbohydrates than anaerobic sludge. Similarly, analysis of eEPS literature data from MBR operated with municipal wastewater (Crude, Settled and Synthetic) demonstrates a considerably higher protein to carbohydrate ratio in AnMBR when compared to AeMBR (Table 3). In terms of specific eEPS concentration, the distinction between aerobic and anaerobic systems is less clear. Comparison between anaerobic and aerobic MBRs operated with complete retention of solids and fed with settled sewage (Baek & Pagilla 2006) showed levels of EPS to continuously decrease. In a further study on AnMBR, Fawehinmi et al. (2004) observed an increase in specific resistance to filtration as the eEPS content of crushed granular sludge increased from 20 to 130 mg g VSS1. Cern-Vivas et al. (2012) demonstrated the highest eEPS Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 138 Table 3 | Concentration and composition of EPS in aerobic and anaerobic MBR sludge/biomass Source Reactor type/sludge Anaerobic MBRs Crudea CSTR/F Crudea CSTR/F Crudea CSTR/F Settled CSTR/G Settled CSTR/F Synthetic UASB/G Synthetic UASB/G Synthetic UASB/G Synthetic UASB/F Synthetic CSTR/F Synthetic CSTR/F Synthetic CSTR/F Synthetic Synthetic Synthetic Synthetic CSTR/F IAFMBRd/F IAFMBRd/F IAFMBRd/F Aerobic MBRs Crudeb CSTR Crudeb CSTR Crudeb CSTR Config. Sub Sub Sub Sub Sub Sub Sub Sub Side Sub Sub Sub Sub Sub Sub Sub Sub Sub Sub Temp. C 20 33 17-29 12 25-30 11-25 11-25 21-24 25 15 25-30 25-30 25-30 35 35 35 16-22 - - SRT HRT MLSS d h g L1 70 70 28.6-41.1 120f 30-90 - - - - 300 30 60 - - - 24.5 5.5-16.5 12.1-28.4 6 10 3.5-5.7 3.5-5.7 8 6 16 8-12 8-12 8-12 6 6 6 18-28 14-32 10-25 5.9 8.0-13.6 - - - - - 5.6-7.1 5.7-8.9 6.5-10.5 - - - 13.3 40-45 16 30-75 42 5.2-10.5 10-31.2 2.2-5.6 EPStot mg g VSS1 98h 155h 161i 11-32i,n 42-50i 5-7.7i,j 17.4-20.4i,k 219i,l 4i 57-81h,k 55.5i 55.5i 49.5i 60i,m 37.5-42.5i,m 70i,k,m 25.5-79.8i 34-157i 38-114i EPSp mg g VSS1 74 121 126 10.1-31.5n 32-40 0.6-1.6 5.8-7.0 108-244 3.7 32-46 38-48 37-45 35-42 45m 30-35m 52m 20.4-64.5 - - EPSc mg g VSS1 24 34 35 0.9n 10 4.4-6.1 11.6-13.4 27-60 0.3 25-35 10-15 11-18 11 15m 5-10m 18m 3.4-34.0 - - Ratio EPSp/EPSc 7.0 (3.1)o 16.4 (3.6)o 3.6 10.7-33.3 4-5 0.18-0.25 0.48-0.55 4.1 12.3 1.3 2.5-2.85 2.0-2.85 2.5 3 4.3 2.8 2.3 - - Ref. Robles et al. (2013a) Robles et al. (2013a) Gimnez et al. (2014) Fawehinmi (2006) Huang et al. (2013) Chu et al. (2005) Chu et al. (2005) Cern-Vivas et al. (2012) Ozgun et al. (2015a) Smith et al. (2013) Huang et al. (2011) Huang et al. (2011) Huang et al. (2011) Gao et al. (2014b) Gao et al. (2014b) Gao et al. (2014b) Bella et al. (2011) Holba et al. (2012) Holba et al. (2012) (Continued.) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 139 Table 3 | Continued Source Temp. SRT Reactor type/sludge Config. C d HRT h MLSS g L1 EPStot mg g VSS1 EPSp mg g VSS1 EPSc mg g VSS1 Ratio EPSp/EPSc Ref. Crudeb Crudeb Crudeb Crudec Settled Settled Settled Synthetic Synthetic CSTR CSTR CSTRe CSTR CSTR CSTR CSTR CSTR CSTR Sub 11-23.8 5-12 6 Sub 13-26 10 10 Sub 13-26 10 10 Sub - - - Sub - 10-30 - Sub 20 10 16 Sub 20 53 16 Sub - 20,40,60 7.8 Sub - 10-80 - 10-11.2 3.8-4.2 3.8-4.3 6.0 12-18 1.9 6.0 2.8-5.5g 1.7-3.7 90-140h 162.7i 165.2i 6.9h 81-115i 45-70i 20-40i 63-70i - - 58.7 58.6 3.5 57-88 - - 30-36 ,2 - 42.9 41.8 3.4 24-29 - - 30-35 3-6 - 1.4 1.4 1.0 2.4-3.2 2-4 2-4 1.0 0.33-0.67 Fan et al. (2006) Liu et al. (2012a) Liu et al. (2012a) Gabarrn et al. (2013) Trussell et al. (2007) Mass et al. (2006) Mass et al. (2006) Lee et al. (2003) Duan et al. (2015) Acronyms: CSTR: continuous stirred tank reactor; F: flocculent sludge; G: granular sludge; GAC: granular activated carbon; IAFMBR: bioreactor; UASB: upflow anaerobic sludge blanket. aCrude: pre-treatment including screening, degritter and grease removal. bCrude - after screening. cData from full scale plant, Crude - after coarse screen, grit chamber, buffering and fine screen. dWith 40 g GAC addition. eWith biofilm carrier. fNot report SRT, but no sludge is wasted during the tests except sampling. SRT can be assumed the same as experiment duration time. gReported MLVSS. hReported as extracted EPS. iReported EPS. jEPS of sludge on granules. kEPS of sludge on membrane. lEPS in the UASB effluent. mNo MLSS or MLVSS, data is reported with initial unit mg L1. nUse MLVSS/MLSS ratio 0.63 to convert the unit from mg g SS1 to mg g VSS1. oValues in the bracket were calculated directly from the average value. integrated anaerobic fluidised-bed membrane bioreactor; SAF-MBR: staged anaerobic fluidised membrane H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 140 doi: 10.2166/h2oj.2018.109 values above 200 mg g VSS1 in a synthetic wastewater. Analysis of the arising cake layer revealed that the specific eEPS deposited on the membrane surface was twice that found in the granular sludge (Chu et al. 2005) with higher eEPS protein to carbohydrate ratio on the membrane, whilst Lin et al. (2009) reported a lower eEPS protein to carbohydrate ratio in the fouling layer when compared to the bulk material. Regardless of the major fouling component, the discrepancies between biomass and cake layer eEPS composition found in these studies suggest that soluble or colloidal compounds are also responsible for the increase in membrane resistance in AnMBRs. Soluble-colloidal EPS: SMP It has been reported that soluble organic matter in the effluent from the biological treatment processes is predominantly SMP, which comprises the soluble cellular components released during cell lysis, lost during synthesis, or otherwise secreted for some purpose (Laspidou & Rittmann 2002). Soluble microbial products are classified according to their origin as products associated with biomass growth and are produced at a rate proportional to substrate utilisation and non-growth associated products related to cell lysis. In conventional (i.e. non-membrane based) systems, the concentration of SMP normalised to influent COD is higher in aerobic than anaerobic processes (Barker & Stuckey 1999). This can be explained by the lower biomass uptake and decay rates of anaerobic microorganisms compared to aerobic biomass. The SMP concentration within aerobic and anaerobic MBR is higher than for conventional processes (Aquino & Stuckey 2006; Mass et al. 2006). Ozgun et al. (2015a) directly compared the SMP of conventional UASB with UASB coupled with membrane and reported that SMP increased over three times after membrane addition, from about 37 to 120 mg L1. This is due to an increased retention of the high molecular weight organic fraction by the membrane (Mass et al. 2006) as well as an increase in net SMP productivity through endogenous decay and cell lysis which are enhanced by long SRT and high organic loading rates (Harada et al. 1994). Anaerobic MBRs have considerably higher SMP concentrations than are present in aerobic MBR (Table 4). Normalising literature SMPCOD data to influent COD (w/w), suggests arising SMPCOD ranges from 10 to 48% and from 9 to 59% for aerobic and anaerobic MBRs, respectively. This has been confirmed through comparative experimental study of anaerobic and aerobic MBR systems for municipal wastewater treatment, which has evidenced SMP concentrations up to five times higher in anaerobic MBR (Martin-Garcia et al. 2011). The higher colloidal content within AnMBR, compared with that of AeMBR, may reflect the higher levels of free bacteria in mixed liquor, together with the lower biodegradation rates or SMP biodegradability expected under anaerobic conditions (Ince et al. 2000). There is also evidence of high molecular weight polymeric material of up to 1,000 kDa being retained by the cake layer in AnMBRs, which presumably decreases permeability and potentially increases rejection capability (Harada et al. 1994). Feed temperature is also a factor, where decreased temperature from 25 to 15 C in an AnMBR increased SMPCOD from 50 to 150 mg L1 (Ho & Sung 2010). FOULING CONTROL STRATEGIES While much of the early research on AnMBR for industrial applications sought to pursue externally configured MBR, the development of AnMBR for municipal applications has been generally more focused on immersed MBR technology, which is presumably due to the lower energy penalty that this configuration can achieve (Judd 2011). Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 141 Table 4 | Concentration and composition of soluble microbial products in aerobic and anaerobic MBRs Source Reactor typea/ Sludge Anaerobic MBRs Crude Crudec UASB/F IAFMBRj Black UASB/F Black Settled CSTR/F UASB/Gk Settled CSTR/G Settled CSTR/F Settled CSTR/F Settled CSTR/F Synthetic UASB/G Synthetic UASB/F Synthetic UASB/F Synthetic UASB/F Synthetic CSTR/F Synthetic CSTR/F Synthetic CSTR/F Synthetic CSTR/F Synthetic CSTR/F Aerobic MBRs Cruded CSTR Config. Side Sub Sub Sub Sub Sub Sub Side Sub Sub Sub Side Side Sub Sub Sub Sub Side Sub Temp C 22 35 37 37 10-25 12-35 - 25 25-30 21-24 35 25 - 35 35 35 25-30 35 13-26 SRT d 180 - - - 250l 120l 100 233 30-90 - - - - 150 250 250 30,60, 190l 10 HRT h 6 6 12 12 16 6 16 24 10 8 12 6 4-12 6 15 6 8-12 48-120 10 MLSS g L1 0.4 - - - 0.1-0.6 5.9-6.8 7.7 7.1 8.0-13.6 - 0.14 0.5 - 3.7m 12-16 6-19 5.6-10.5 15m 3.8-4.2 SMPCbOD mg L1 - - 327 (0.29) 269 (0.24) 198 (0.59) 180 (0.40) 598 (1.77) 51 (0.61)n - - - 122 (0.23) 40 (0.11) 180o (0.40) 1,787 (0.45) 228-360o (0.09) - 1,200 (0.24) - SMPc mg L1 SMPp mg L1 Ratio SMPp/SMPc Ref. 99.2 3-5 81 45 18 8 47 - 18-19 0.5-1.2 1.2-16.6 25-40 - 31 - - 4.5-14 80 133.2 14-15 70 69 50 59 108 - 40-50 1.1-3.3 7.4-15.1 60-80 - 58 - - 7-18 400 1.3 3.5-3.8 0.9 1.5 2.8 7.4 2.3 - 2.2-2.5 2.6 0.7-6.5 2.1 - 1.9 - - 0.9-2.0 5 Herrera-Robledo et al. (2011) Gao et al. (2014b) van Voorthuizen et al. (2008) van Voorthuizen et al. (2008) Martin Garcia et al. (2013) Fawehinmi (2006) Martin Garcia et al. (2013) Baek & Pagilla (2006) Huang et al. (2013) Cern-Vivas et al. (2012) An et al. (2010) Ozgun et al. (2015a) Salazar-Pelez et al. (2011) Aquino et al. (2006) Akram & Stuckey (2008) Akram & Stuckey (2008) Huang et al. (2011) Harada et al. (1994) 3.5 (1.8)p 3.6 (1.9)p 1.0 (1.1)p Liu et al. (2012a) (Continued.) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 142 Table 4 | Continued Source Temp SRT Reactor typea/ Sludge Config. C d HRT h MLSS g L1 SMPCbOD mg L1 SMPc mg L1 SMPp mg L1 Ratio SMPp/SMPc Crudee,f Crudeg Crudee,h Crudee,i Settled Settled Settled Settled Synthetic CSTR CSTR CSTR CSTR CSTR CSTR CSTR CSTR CSTR Sub 10-25 - - - - 3.6 Sub 11-12 5-12 6 9.7-11.9 - 17-38 Sub - 14-38 12-41 2.2-13.5 - 3-18 Sub - - - 6.0 - 3.7 Sub - 10-30 - 12-18 37-82 (0.11-0.24) 12-26 Sub - 10-30 - 12-18 33-166 (0.10-0.48) 16-27 Sub 20 10-110 16 1.9-7.2 45-110 (0.12-0.30) 37.2 Sub - 100 16 8.7 99 (0.29) 18 Sub 28 10-40 10 3.1-7.8 - 8-12 2.4 10-58 ,5 2.7 10-79 12-140 8.9 18 5-9 0.7 1.2 0.5-1.7 0.7 0.5-3.0 0.6-5.4 0.2-0.6 1.0 1.4 Acronyms: CSTR: continuous stirred tank reactor; F: flocculent sludge; G: granular sludge; IAFMBR: integrated anaerobic fluidised-bed membrane bioreactor; UASB: upflow anaerobic sludge blanket. aReport SMP from UASB effluent, CSTR and IAFMBR from mixed liquor. bValues in brackets correspond to normalised SMPCOD with respect to influent COD. cCrude: from septic tank. dCrude: after sand settler and screening. eData from full scale plant. fCrude: after coarse screen, grit chamber, grease trap and fine screen. gCrude: pre-treatment with screening. hCrude: after screening. iCrude: flow after coarse screen, grit chamber, buffering and fine screen. jWith 40 g GAC addition. kReported SMP in the membrane tank. lNo reported SRT, but no sludge is wasted during the tests except sampling. SRT can be assumed as same as experiment duration time. mReported MLVSS. nNormalised against influent soluble COD. oWith addition of PAC (powered activated carbon). pValue in bracket is with bio-carrier. Ref. Lyko et al. (2007) Fan et al. (2006) Shen et al. (2012) Gabarrn et al. (2013) Trussell et al. (2007) Trussell et al. (2007) Mass et al. (2006) Martin-Garcia et al. (2011) Liang et al. (2007) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 143 doi: 10.2166/h2oj.2018.109 Specific gas demand and operation flux The operational costs related to membrane operation in immersed MBRs are mainly determined by the relationship between the specific gas demand (SGDm) and operating flux, with the SGDm being the gas flow rate per unit membrane area (the specific aeration demand, SADm, for aerobic systems). This reflects the relationship between the convective flow towards the membrane produced by permeate suction and the back transport induced by the gas sparging and tangential shear at the boundary layer (Liu et al. 2003). In AeMBRs, sustainable or critical fluxes have been reported to increase by increasing gas intensity up to a certain threshold value beyond which no further increase in flux is observed for flat sheet (Guglielmi et al. 2008), hollow fibre (Guglielmi et al. 2007) and multi-tubular membranes (Le-Clech et al. 2006). For instance, Chen et al. (2016) reported an increase in critical flux from 23 to 47 LMH for an AeMBR when SADm increased from 4.5 to 9.0 m3 m2 h1. Analysis of full scale immersed AeMBRs indicated an operational SADm range of 0.21-0.88 m3 m2 h1, corresponding to fluxes between 24 and 31 LMH (Verrecht et al. 2008). Increasing membrane flux has been shown to increase fouling rates and decrease the duration of the slow fouling phase in aerobic MBRs under conditions of sub-critical flux operation, prior to the widely reported `TMP jump' (Pollice et al. 2005; Le Clech et al. 2006). For example, under sub-critical conditions, a decrease in flux from 10 to 2 LMH caused an exponential decrease in the fouling rate, from 19.8 to 0.46 kPa h1, and prolonged the time before the TMP jump to up to 8 days (Brookes et al. 2006). Results from a pilot scale study suggest that the duration before the TMP jump decreases linearly at fluxes close to the critical flux and that an asymptote exists at a certain flux below which operation can be extended to long filtration cycles (Guglielmi et al. 2007). Research into immersed AnMBR for municipal wastewater treatment (Table 1) has employed a wide range of SGDm up to 7.2 m3 m2 h1 (Smith et al. 2013). Similar to AeMBR, optimum hydrodynamic conditions have been identified by increasing gas sparging until a threshold is reached, or the sustainable or critical flux is identified (Martin-Garcia et al. 2011, 2013; Robles et al. 2012). Robles et al. (2012) observed a linear increase in critical flux from 12 to 19 LMH when SGDm was increased from 0.17 to 0.50 m3 m2 h1. Operational fluxes for AnMBR (Table 1) between 2.5 and 25 LMH have been reported with the higher fluxes achieved using synthetic wastewater (Wu et al. 2009; Gouveia et al. 2015a). Similar to AeMBRs, an increase in membrane flux, albeit below the critical flux, also leads to an increased fouling rate in AnMBRs (Vallero et al. 2005). An increase in gas sparging intensity appears to be effective in extending membrane operation in both AeMBRs and AnMBRs, but does not obviously enhance permeability. For instance, Weinrich & Grlot (2008) reported sustained permeability for an AeMBR of 600 LMH bar1 for around two months at a flux of 25 LMH and SADm of 0.20-0.35 m3 m2 h1. Robles et al. (2013b) reported sustained permeability above 100 LMH bar1 in an AnMBR for over 30 days while operating at a sub-critical flux of 13.3 LMH and SGDm of 0.33 m3 m2 h1. Three years of AnMBR operation has been reported, by adopting an operating flux of 12-14 LMH, which sustained TMP between 0.35 and 0.6 bar, without extra physical and chemical cleaning (Gouveia et al. 2015b). Overall, the permeability is still lower than for AeMBRs which for full scale municipal wastewater treatment plants is between 150 and 250 LMH bar1 ( Judd 2011), even when the quantity of gas provided to the membrane is up four times higher and fluxes between two and three times lower. Physical and chemical cleaning: reversible, irreversible and irrecoverable fouling Membrane fouling can be generally classified as external fouling (cake and gel formation) and internal fouling (pore clogging) (Judd 2011; Wang et al. 2014). In AeMBRs, cake and gel layer fouling are Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 144 Table 5 | Hydrodynamic conditions and specific energy demand of immersed anaerobic MBRs treating municipal wastewater (real and synthetic) Source Reactor type/ Sludge Crua Crua Crua Crub Crub Crub Crub Cruc Cru (GA) Black Black Set Set Set Set Set Set Syn Syn Syn Syn Syn CSTR/F CSTR/F CSTR/F CSTR/F CSTR/F CSTR/F CSTR/F IAFMBR CSTR/F UASB/G CSTR UASB/G UASB/G UASB/F UASB/G CSTR/F SAF-MBR/F UASB/G UASB/G CSTR/F CSTR/F CSTR/F Material/ Geo./ Pore size m PVDF/HF/0.05 PVDF/HF/0.05 PVDF/HF/0.05 PVDF/HF/0.04 PVDF/HF/0.04 PVDF/HF/0.04 PVDF/FS/140d -/HF/0.4 PES/FS/0.038 PVDF/Tub/250d PVDF/Tub/250d PVDF/HF/0.04 PVDF/HF/0.04 PVDF/HF/0.045 PVDF/HF/0.045 PVDF/HF/0.08 PVDF/HF/0.03 PVDF/Tub/100d PVDF/Tub/100d PS/Tub/0.2 PS/Tub/0.2 PE/FS/0.4 Temp C 33 20-33 17-29 18.9 18.9 23 30 15-35 20 37 37 10-25 10-25 18 18 10-25 8-30 21-24 21-24 55 30 35 MLSS g. L1 Flux LMH Filtration cycle FI min R min 23 12-19 250s 50s 23 10-13.3 250s 50s 10-25 7-11 250s 50s 21.3 10 Con. - 21.3 10 Con. - - 17 8 2 6.4-9.3 11 9 1 - 11.3 Con. - 9.5-14.7 7 10 0.5 - 10f 8 1 - 8f 8 1 0.1-0.6 6f 10 - 0.1-0.6 6f 10 - 5.95e 10-14 7.5 5s 2e 8 15 10s 7.7 6f 10 - - 5.1-6.2f 30 5 - 4.2 10 1 - 5.2 4 1 25-50 16-23 10 - 25-50 5-21 10 - - 5-8 Con. - BW min Others SGD m3.m2.h1 Ug m h1 30sh 40 s V 30 s Dj 0.17-0.5 - 30sh 40 s V 30 s Dj 0.23 - 30sh 40 s V 40 s Dj 0.23 - - - 1.3 - - - 0 - 4i - 0.146 - - - 0.11 - - - 0 - 1 - - 62 1 - - 8-16 1 - - 40 1 - 1.17 148 1 - 1.17 148 15s 5sR 0.32-0.48k 40-60 1 10 s R 0.07k 9 1 - 0.39 277 - - 0 - - - 5.29 - - - 5.29 - 0.5 - 0.29-1.02l 12-42 0.5 - 0.85-1.70l 35-70 - - 3 - Gas sparging cycle Gas On min Off min Energy consump. kWh m3 Ref. B Con. B Con. B Con. B Con. -- B Con. B Con. -- B Con. - Con. - Con. N 10s 10s N1 10 B Con. B Con. N 10s 10s -- N1 10 N1 4 B Con. B Con. B 10 5 0.10-0.46 0.19-0.26 0.23-0.37 2.10m 2.30m 0.10 0.10 - - - - 0.88 0.16 0.26-0.39 0.09 0.29 0.23n 1.13 2.27 0.12-0.63 0.40-3.35 2.2-3.6 Robles et al. (2012) Robles et al. (2013a) Gimnez et al. (2014) Ruigmez et al. (2016b) Ruigmez et al. (2016b) Dong et al. (2016a) Lin et al. (2011) Gao et al. (2014)a, 2014b) Martinez-Sosa et al. (2012) van Voorthuizen (2008) van Voorthuizen (2008) Martin Garcia et al. (2013) Martin Garcia et al. (2013) Gouveia et al. (2015a) Gouveia et al. (2015b) Martin Garcia et al. (2013) Shin et al. (2014) Cern-Vivas et al. (2012) Ceron-Vivas et al. (2012) Jeison & van Lier (2006) Jeison & van Lier (2006) Vyrides & Stuckey (2009) (Continued.) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 145 Table 5 | Continued Source Reactor type/ Sludge Material/ Geo./ Pore size m Temp C MLSS g. L1 Flux LMH Filtration cycle FI min R min BW min Others SGD m3.m2.h1 Ug m h1 Gas sparging cycle Gas On min Off min Energy consump. kWh m3 Ref. Syn CSTR/F PE/FS/0.4 35 - 5-8 Con. - - - Syn CSTR/F PE/FS/0.4 - - 7.2 Con. - - - Syn CSTR/F PES/FS/0.2 15 - 7 30 (240)g - 0.5 (4)g - Syn CSTR/F PES/FS/0.2 3-15 - 1.2-12 5 - 1 - Syn ARMBR PE/FS/0.2 30 5.5 11 8 2 - - Syn ABR AFMR PVDF/HF/0.1 35 - 7-10 Con. - - - 3 - B Con. 2.4 - B Con. 7.24 14 B Con. 5.8 - B Con. 0 - -- 0 - -- 3.4-5.4 3.0 9.6 6.5-64.8 0.10n 0.058n Vyrides & Stuckey (2009) Fox & Stuckey (2015) Smith et al. (2013) Smith et al. (2015) Kim et al. (2014) Kim et al. (2011) Acronyms: ABR AFMBR: anaerobic baffle reactor anaerobic fluidised bed membrane bioreactor; ARMBR: anaerobic rotate disk MBR; AT: ambient temperature; B: biogas; BW: backwash; Cru: Crude; CSTR: continuous stirred tank reactor; F: flocculent sludge; FI: filtration; FS: flat sheet; G: granular sludge; GA: glucose addition; HF: hollow fibre; IAFMBR: integrated anaerobic fluidised-bed; MWCO: molecular weight cut off; N: Nitrogen; PE: Polyethylene; PVDF: Polyvinylidene fluoride; R: relaxation; SAF-MBR: staged anaerobic fluidised membrane bioreactor; Set: Settled; Syn: Synthetic; Tub: tubular; UASB: upflow anaerobic sludge blanket. aCrude: pre-treatment including screening, degritter and grease removal. bCrude: after screening. cCrude - from septic tank. dMWCO (kDa). eReported MLVSS. fNet flux. gvalues in the bracket are the second filtration conditions. hbackwash 30 s every 50 min. ibackwash weekly with chemicals. j40 s ventilation every 50 min, 30 s degasification every 250 min. kZW-10 module, use membrane cross sectional area of 74 cm2 (Martin Garcia et al. 2013). lAssume 50% of the membrane package ratio and membrane mount area is double size of the membrane fibre cross sectional area. mEnergy demand provided in the paper. H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 146 Table 6 | Overview of operating conditions and membrane performance in side-stream anaerobic MBRs Source Inde Cru Cru Cru Cru Cru Set Set Set Set Set Syn Synf Syn Syn Syn Syng Synh Reactor type/ Sludge CSTR UASB/F UASB/F UASB/F UASB CSTR UASB/G UASB/G CSTR/F CSTR/F CSTR/G UASB/F UASB UASB/F UASB/F UASB/F CSTR CSTR Temp C 53-55 4 4 4 22 37 - - - - 12-35 - - 25 25 15 55 35 Material/Geo./ Pore size m Mode MLSS g L1 CFVa m s1 Ugb m s1 Jc Flux TMP L m2 h1 L m2 h1 kPa Fouling rate mbar min1 c kPa h1 c L m2 h1 d1 d TOP h -/-/20i PAN/Tub/- PAN/Tub/- PAN/Tub/- PVDF/Tub/100i -/-/100i PVDF/Tub/0.03 PVDF/Tub/0.03 PVDF/Tub/0.03 PVDF/Tub/0.03 PE/Tub/0.1 PVDF/Tub/100i PVDF/-/0.22 PES/Tub/0.03 PES/Tub/0.03 PES/Tub/0.03 Ceramic/Tub/0.2 Ceramic/Tub/0.2 Pumped 0.3-3.0j Pumped - Pumped - Pumped - Pumped - Pumped 0.5-10j Pumped - Gas-lift - Pumped - Gas-lift - Pumped 5 Pumped - Pumped 0.3-0.55 Pumped 0.5 Pumped 0.3-0.5 Pumped 0.3-0.5 Pumped - Pumped - 0.2-1.0 0.4 0.4 0.4 2.25 3 0.4-2.0 - 0.4-2.0 - 0.7 2 0.93 1 1 1 1-1.5 2 - - - - - - - 0.02-0.14 - 0.02-0.14 - - - - - - 0.1 - - - - - - - 4-41 4 4-19 4 - - 50 - 41-70 34-41 - - 10-70 10.5 10.5 10.5 45-50 9-13 - 11-12 - 11-12 9-20 - 30 12.3 12.3 12.3 20 14-19 100-200 - ,50 0.067 ,50 0.040 ,15 ,0.001 - - 100-200 - - - - 1-2 - - - 8-25 - 0.001-0.002 103.4 - ,40 0.008-0.042 8.7 - ,150 0.001-0.003 ,450 0.003-0.009 - - 40-50 - - 0.400 0.238 0.007 - - - 6-12 - 48-150 0.004-0.011 - 0.05-0.25 - 0.008-0.021 0.017-0.055 - - - - - - - 0.056 - - - - - - - - - - 4,800 ,125 ,210 2,150 - 4,080 - - - - - ,6.7 .400 - 720 720 - 1,680 Filtration cycle FI (min) R (min) BW (min) Ref. Con. 10 10 Con. Con. Con. Con. Con. Con. Con. Con. Con. Con. 3 3 5 Con. Dep relax k [1] - - [2] 20s - [2] - 20s [2] - - [3] - - [4] - - [5] - - [5] - - [5] - - [5] - - [6] - - [7] - - [8] - - [9] - 20s [10] - 20s [10] - 20s [11] - - [12] (Continued.) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 147 Table 6 | Continued Source Reactor type/ Sludge Temp C Material/Geo./ Pore size m Mode MLSS g L1 CFVa m s1 Ugb m s1 Jc Flux TMP L m2 h1 L m2 h1 kPa Fouling rate mbar min1 c kPa h1 c L m2 h1 d1 d TOP h Filtration cycle FI (min) R (min) BW (min) Ref. Syn CSTR/F 35 Syn CSTR 35 Syn CSTR/F 25 Syn CSTR/F 35 PS/PF/3000i Ceramic/0.2 PTFE/Tub/1 PVDF/HF/0.03 Pumped 15j Pumped 1.6-22 Pumped 6-12 Pumped 6 0.8 - 2 - 0.1-0.2 - 0.1-0.3 - - 21-75 49 - - - 25-65 35 - - - 5 7-55 - - 10-12.5 6 ,5 0.200 1.2 1.9-9.2 168-240 Con. - - [13] - 120 Con. - - [14] - - Con. - - [15] - 2,160 9 1 - [16] Acronyms: BW: backwash; Cru: Crude; CSTR: continuous stirred tank reactor; F: flocculent sludge; FI: filtration; G: granular sludge; Ind: Industrial; MWCO: molecular weight cut off; Tub: Tubular; PAN: polyacrlonitrile; PE: polyethylene; PF: flat plate; PES: polyethersulfone; PTFE: poly-tetrafluoroethylene; PVDF: Polyvinylidene fluoride; R: relaxation; Set: Settled; Syn: Synthetic; UASB: upflow anaerobic sludge blanket. Reference: [1] Choo & Lee (1996); [2] An et al. (2009); [3] Herrera-Robledo et al. (2011); [4] Saddoud et al. (2007) [5] Martin-Garcia et al. (2011); [6] Fawehinmi (2006); [7] Salazar-Pelez et al. (2011); [8] Cho & Fane (2002); [9] Ozgun et al. (2015a); [10] Ozgun et al. (2015b); [11] Jeison et al. (2009); [12] Cadi et al. (1994); [13] Harada et al. (1994); [14] Beaubien et al. (1996); [15] Ho & Sung (2009); [16] Wei et al. (2014). aLiquid cross flow velocity. bGas scouring rate. cFouling rate under constant flux. dPermeate drop down (fouling rate under constant transmembrane pressure). eAlcohol-distillery wastewater. fNot mention synthetic, but assume according to the influent characteristics. gVFA mixed. hSynthetic wastewater containing starch. iMWCO (kDa). jReported MLVSS. kDepressurisation (down to about 0.3bar) relaxation (flow stopping). H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 148 doi: 10.2166/h2oj.2018.109 generally considered to govern membrane fouling (Liu et al. 2012a). Dominant cake layer resistance has similarly been reported by a number of authors in AnMBR for municipal wastewater (Liu et al. 2012b; Martinez-Sosa et al. 2012; Ozgun et al. 2015b), which is therefore comparable to the general trend for AeMBR. Chu et al. (2005) reported that 90% of total resistance for an AnMBR corresponded to cake resistance while only 9% was internal fouling. Ozgun et al. (2015b) reported that the relative contribution of cake layer resistance in their AnMBR study decreased with a decrease in operating temperature, which suggests transients in the bulk particulate and colloidal characteristics will influence cake deposition. Resistance to filtration due to membrane fouling can also be classified as reversible, irreversible or irrecoverable, depending on whether it can be removed physically during operation (by relaxation or backflushing), chemically, or if it remains after chemical cleaning ( Judd 2011). In immersed AeMBRs, physical cleaning procedures like relaxation and backwashing have shown to be effective in extending membrane operation compared to continuous filtration reducing the chemical cleaning frequency (Judd 2011). By applying physical cleaning procedures, it is possible to apply fluxes that result in an increase of resistance to filtration as long as the cake layer deposited onto the membrane surface can be removed by relaxation or backflushing. For instance, operational cycles of 10 minutes of filtration followed by 1 minute relaxation at fluxes between 22.3 and 28.5 LMH resulted in fouling rates during filtration cycles of 1.39 and 1.8 mbar min1 respectively, while the irreversible fouling rates were two orders of magnitude lower (Guglielmi et al. 2007). Similarly, in AnMBRs, relaxation and backwashing have also been applied and proved to be effective for membrane fouling mitigation (Tables 5 and 6). Smith et al. (2013) also showed that a 4 min backwash every 4 h reduced membrane fouling from 0.045 to below 0.001 mbar min1, with similar observations made for side-stream AnMBR (An et al. 2009). A four times decrease in fouling rate was observed following implementation of backwash at double the permeate flux, which indicates backwash flux specification is also important. Once a tenacious cake layer is formed, it is hard to remove just with physical cleaning which is evidenced by cake layer growth on membranes exposed to high shear flow (Imasaka et al. 1993) or high shear in combination with relaxation and depressurization (Choo & Lee 1996). Permeability recovery can be variable following chemical cleaning in both aerobic and anaerobic MBRs, which is illustrative of irrecoverable fouling. Gouveia et al. (2015a) reported a chemical cleaning protocol for an AnMBR comprising 1,000 mg L1 sodium hypochlorite (NaClO), which achieved recovery of only 61% of initial permeability. However, more typically only around 7-20% of the total resistance is regarded irrecoverable following incorporation of chemical cleaning policies in AnMBR (Chu et al. 2005; Ozgun et al. 2015b; Dong et al. 2016a). ANAEROBIC MBR FOR INDUSTRIAL VERSUS MUNICIPAL WASTEWATER TREATMENT Industrial wastewaters are generally of higher organic strength and comparatively smaller flows than for sewage (van Lier et al. 2008). Organics in industrial wastewater are predominantly soluble and readily assimilable (Liao et al. 2006); whereas sewage comprises a considerable particle fraction, with a proportion of the organics known to be recalcitrant (Lettinga et al. 2001). Due to the smaller flows and higher organic strength, higher methane yield (m3CH4 m3 permeate) in industrial applications is sufficient for heating the influent to mesophilic temperatures, which improves biodegradation kinetics, and enables application of higher organic loading rates than for sewage (Lettinga et al. 2001; van Lier et al. 2008). The combination of a more complex organic feed composition (coarser particle fraction), comprising a large recalcitrant contribution, coupled with insufficient heat to improve biodegradation kinetics, implies that sewage presents the higher fouling potential. The higher methane yield indicates more electrical power is available to industrial AnMBR making Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 149 Table 7 | Biogas production from anaerobic MBRs treating municipal wastewater (real and synthetic) Volume Source l Inf. COD (BOD/OD) mg L1 Cruf Cru Crug Crug Crug Crug Cruf Cruf Cruh Crui Setj Setk Setj Set Set Setl,m Setl Set Syn Syn Syn Syn 17.7n 98-2,600 34 58-348 1,300 800 445 1,300 800 445 1,300 800 468-598 1,300 800 650 550 80 304-388 60 342-527 5.8 247-449 350 - 630 (0.63) 160 150 892 (0.64) 160 150 892 (0.64) 326 175 978 (0.48) 42.5 30 265 5 1 427 0.25 0.25 154 (0.57) 0.25 0.25 154 (0.57) 990 770 233 (0.50) 4.7n 383-849 30n 390 5n 440 3n 460 SO42 mg L1 CODa % OLRb kg COD m3d1 - - 97 0.5-12.5 - - (77-81) 0.3-0.9 297 5 87 0.9-3.0 297 5 87 0.9-3.0 300o - 90-94 0.6-1.9 315 - - - 50-55 - 88-92 1.1 - - 90 1.0 - - 51-74 1.2-1.4 - - 82,90 0.6-1.1 47 - 74-90 0.8-2.6 47 - 73-90 2.0-4.7 47 - 75-90 0.6-3.2 - - 93 0.2-0.6 - - 84-86 1.0 - - 84 4-6 (1.3)q 63 0 84 3.5 (1.2)q 41 7 91-93 2.5-3.0 - - 85-96 1.6-4.5 - - 89p 0.78 - - 92 0.7 - - 90-95 0.2-3.7 Temp C 14-25 4 33 33 17-29 17-33 23 30 15-35 20,35 18 18 18 14 25-30 25 25 9-25 11-25 35 15 35 HRTc h 4-6 5.5-10 - 6-21 6-21 12-29 - - 8.5 10 6 14.3 13-17 - 7-13 - 10-15 - 12 10 1 1.3 1 1.3 2 2.6 3.5-5.7 12 16 3-48 CH4 % 53-66 59-64 55 55 - - 35 75-85 - 88,80 80-83 83-86 81-83 - - - 40 (54)r - 63-72 81.2 - 60-70 CH4 production (Normalised to STP) l CH4 gCODre1md l CH4 m3 kWh m3 0.02-0.06 0.04-0.07s 0.06s 0.26s,t 0.001-0.05 - 0.04-0.07 0.22s 0.13-0.17s 0.23-0.27s 0.14-0.26 0.09-0.23 0.11-0.19 0.004 0.04-0.10 - 0.05 0.09-0.13 0.06-0.12 - 0.05-0.13 0.20-0.29s 25-82 5.5-10.8 22.8 - 0.7-22.1 1.3-23.6s 14-19 83 23.6-43.4 127-149 57-285 86-231 25-191 1.0 14-38 - 9.1 17-31 32-66 26.3 20-53 83-125 0.10-0.33 0.02-0.04 0.10 - 0.003-0.088 0.005-0.095 0.06-0.08 0.331 0.09-0.17 0.506-0.594 0.23-1.14 0.35-0.92 0.16-0.76 0.004 0.06-0.15 0.03v 0.04 0.07-0.12 0.13-0.27 0.105 0.08-0.21 0.33-0.50 Dissolved CH4 (Normalised to STP) l CH4 m3 kWh m3 20.0x 27.9x 17.2w 17.2w 7.1-10.2w 6.7-13.0w 13.3w 22.3x - 29,21x 28.6x 28.6x 28.6w 13.0w - - 15.8w 12-27w 23.5x 20.8x 29.1w 16.7x 0.080 0.111 0.069 0.069 0.03-0.04 0.03-0.05 0.053 0.089 - 0.12,0.08 0.114 0.114 0.114 0.052 - 0.05-0.06v 0.063 0.05-0.11 0.09-0.10 0.083 0.09-0.14 0.067 Total CH4e kWh m3 0.18-0.41 0.13-0.15 0.160 .0.160 0.03-0.12 0.03-0.15 0.11-0.13 0.420 .0.09-0.17 0.62-0.68 0.31-1.25 0.46-1.04 0.21-0.88 0.056 .0.06-0.15 0.08-0.10v 0.099 0.17-0.18 0.22-0.35 0.189 0.20-0.33 0.40-0.57 Ref. Wen et al. (1999) An et al. (2009) Gimnez et al. (2011) Gimnez et al. (2011) Gimnez et al. (2014) Pretel et al. (2014) Dong et al. (2016b) Lin et al. (2011) Gao et al. (2014a) Martinez-Sosa et al. (2011) Gouveia et al. (2015a) Gouveia et al. (2015a) Gouveia et al. (2015b) Cookney et al. (2016) Huang et al. (2013) Bae et al. (2013) Yoo et al. (2012) Shin et al. (2014) Chu et al. (2005) An et al. (2010) Smith et al. (2013) Hu & Stuckey (2006) (Continued.) H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest 150 Table 7 | Continued Volume Source l Inf. COD (BOD/OD) mg L1 SO42 mg L1 CODa % OLRb kg COD m3d1 Temp C HRTc h CH4 % CH4 production (Normalised to STP) l CH4 gCODre1md l CH4 m3 kWh m3 Dissolved CH4 (Normalised to STP) l CH4 m3 kWh m3 Total CH4e kWh m3 Ref. Syn 2 - 400 Syn 4 - 500 Syn 3.93 2 513 Syn 4n 342 - - 60-90 - - - - - 98 .90 99 (88) 96 0.8-1.6 1.0-2.0 4-6 - 35 25 - 35 30 6-12 80-90 0.08-0.12s 6-12 70-75 0.19-0.20s 2-3 - 86 0.18 - 68 0.14s 30-47 89-93 92u - 0.12-0.19 0.35-0.37 0.368 0.144v - 21w 40w 19.0x 0.055 0.084 0.159 0.076 0.18-0.24 .0.44-0.46 0.527 0.220 Wei et al. (2014) Ho & Sung (2009) Kim et al. (2011) Kim et al. (2014) Acronyms: Cru: Crude; Eff: Effluent; FI: filtration section; Inf: Influent; RE: Anaerobic reactor; Set: settled; STP: standard temperature and pressure, 0 C and 1 bar; Syn: Synthetic. aThe values in bracket means the biological reactor removal. bOrganic loading rate of anaerobic reactor. cSplit tables showed the HRT of biological reactor and membrane tank, combined tables showed the HRT of whole AnMBR. dLCH4 gCOD1 removed based on CODt influent and CODt permeate. eAssume 1 m3 CH4 can generate 10 kWh of energy and CHP efficiency is 40% CH4. fCrude: after screening. gCrude: pre-treatment including screening, degritter and grease removal. hCrude: from septic tank. iCrude wastewater with glucose addition. jWith recirculation. kWithout recirculation. lSettled sewage go through 10 m cartridge filter. mSettled sewage go through 1 mm screen. nMembrane submerged in the anaerobic reactor. oReported as SO42-S. pTOC removal. qOLR for AFBF (OLR of AFMBR). rCH4 composition in SAF (membrane tank). sDirectly reported from literature. tCalculate the methane yield on CODt used for methanogenesis bacterium by subtracting the CODt removed for sulphate reduction bacterium (Lens et al. 1998; Gimnez et al. 2012). uOnly consider the methane from AFBR. vDirectly reported the energy production. wLiterature reported the dissolved CH4 (directly test or estimate from Henry's law). xUse Henry's law to for dissolved CH4 calculation (assume saturation index is 1.00) (Gimnez et al. 2012). H2Open Journal Vol 1 No 2 doi: 10.2166/h2oj.2018.109 Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 151 doi: 10.2166/h2oj.2018.109 sustainable membrane operation easier to achieve with adoption of high cross-flow velocities in sidestream (Lin et al. 2013), and the application of conventional CSTR configured AnMBR with high biomass concentrations (Skouteris et al. 2012) commonly reported that can still deliver energy neutral membrane operation. This practice is not readily translatable into sewage applications, subsequently demanding the development of engineered solutions to facilitate energy neutral operation (Ozgun et al. 2015a; Wang et al. 2018). ESTABLISHING THE ENERGY PROFILE OF ANMBR FOR MUNICIPAL APPLICATION Towards energy neutral wastewater treatment The key facet that drives commercial interest for AnMBR is in the potential to achieve energy neutral wastewater treatment through a reduction in net energy demand for treatment, coupled with an Figure 1 | Combined headspace and dissolved methane production rates from anaerobic MBR treating: (a) settled sewage and (b) crude sewage. Where dissolved methane data was not provided, the dissolved fraction was estimated using Henry's law. Energy data was normalised assuming 40% CHP conversion efficiency. Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 152 doi: 10.2166/h2oj.2018.109 increase in total biogas production, without having to rely on sludge imports (McAdam et al. 2011). Several flowsheets have been proposed for how to incorporate AnMBR technology. Upstream, the decision must be made as to whether primary sedimentation is introduced to divert particulate COD toward anaerobic digestion, leaving only settled wastewater to be processed by AnMBR (McAdam et al. 2011) or whether the full organic load (crude wastewater) is to be treated within the AnMBR (Gimnez et al. 2011; Robles et al. 2013a, 2013b). Downstream of the AnMBR, the decision to have biological or physical separation processes for polishing and nutrient removal is a further consideration (McAdam et al. 2011; Sutton et al. 2011). The merit of these various approaches is beyond the scope of this review. Nevertheless, it is implicit in each of these cases that the AnMBR itself needs to be energy neutral, or better, energy positive (to provide residual energy for the ancillary processes), if overall energy-neutrality is to be achieved. Typical headspace methane yield from the literature is between 0.02 and 0.27 L CH4 . g COD1 for both settled and crude wastewater (Table 7). This is below the expected stoichiometric conversion of 0.35 L CH4 . g COD1. The difference between measured and expected values can be explained by: (i) the fact that yield does not take into consideration the solids which are retained (and accumulate) within the bioreactor (Uemura & Harada 2000; Lester et al. 2013); (ii) differences in HRT or OLR between literature studies which can influence specific yield, and (iii) the dissolved methane fraction which is released with the AnMBR effluent. Several studies have now demonstrated this dissolved fraction can comprise over 50% of the dissolved methane balance (Cookney et al. 2012), with losses exacerbated at lower temperature (Cookney et al. 2016). Downstream membrane technology has been demonstrated that can recover this fraction (Bandara et al. 2011; Cookney et al. 2016) in sufficient concentration for reuse in power generation. Collectively the average dissolved and headspace methane yield from the literature provides around 0.34 kWh m3 of permeate produced (0.80 kWh m3 maximum), which is the typical membrane operation energy consumption of fullscale aerobic MBR (0.19-0.70 kWh m3). Consequently, AnMBR should be delivered at equivalent or better energetic performance to aerobic MBR to achieve `energy-neutral' conditions. Comparison of specific energy production with membrane energy demand estimated from published AnMBR literature (Figure 1) demonstrates that several AnMBR studies can achieve membrane operation within `energy-neutral' conditions (Figure 2). Since the shear promoted at laboratory scale is generally a conservative estimate of that attained at scale (Delgado et al. 2004), such evidence is encouraging. While Figure 2 | Comparison of membrane energy demand to the average (0.34 kWh m3, dashed line) and maximum (0.80 kWh m3, continuous line) energy production reported in the literature to date for anaerobic MBR treating settled wastewater. Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 153 doi: 10.2166/h2oj.2018.109 many studies have sought to optimise hydrodynamics to lower energy (Martin Garcia et al. 2013; Seib et al. 2016; Wang et al. 2018), an explicit focus on achieving energy neutral membrane operation would facilitate further improvements toward this goal. Emerging engineered solutions to constrain energy demand within AnMBR Several authors have now directly compared different AnMBR reactor configurations and have identified that when configured as a UASB, membrane fouling and energy demand is constrained (van Voorthuizen et al. 2008; Martin-Garcia et al. 2011). The authors ascribed the enhanced performance to the lower particle concentration within the downstream membrane tank (Martin-Garcia et al. 2011). Martin-Garcia et al. (2011) also screened a host of membrane geometries and were able to demonstrate a specific gas demand of 0.3 kWh m3 for immersed hollow fibre membranes compared to 3.7 kWh m3 for externally configured membranes. It has been shown that further reduction in specific energy demand can be attained for immersed membranes through inclusion of novel gas sparging strategies (Cern-Vivas et al. 2012; Martin Garcia et al. 2013; Wang et al. 2018). Shin et al. (2016) introduced the anaerobic fluidised bed MBR which incorporates granulated activated carbon (GAC) into the reactor to scour the membrane. Significantly, the authors demonstrated sustainable fluxes could be achieved with an immersed membrane without the need for gas sparging, provided a critical upflow velocity could be achieved to sufficiently fluidise the GAC (Wang et al. 2017). Kim et al. (2011) reported a specific energy demand of 0.028 kWh m3 for this configuration, which evidences a substantive energy reduction when compared to classically commercially configured aerobic MBR technology. One of the future challenges with GAC is to identify a compatible membrane that can withstand the abrasion introduced by the GAC (Shin et al. 2016; Wang et al. 2017). Ruigmez et al. (2016a, 2016b) integrated rotating membrane technology into a CSTR configuration as an alternative method of shear (Zsirai et al. 2016). The authors demonstrated an improvement in critical flux with membrane rotation when compared to conventional gas sparging; specific energy demand for disk rotation is around 0.104 kWh m3 (Kim et al. 2014). Within the context of scaling-up AnMBR, membrane cost must also be considered as the sustainable fluxes identified at present of around 10 L m2 h1 (Table 1) are markedly below those nominally achieved in commercial aerobic MBR. For large scale installations, investment decisions are strongly influenced by initial capital investment. As such, there is a need to establish new paradigms that can access cost efficient water productivities whilst still delivering to the energy neutral objective. One innovative example is anaerobic dynamic membrane bioreactors (Alibardi et al. 2014; Ersahin et al. 2016) which employ woven and non-woven filter substrates as a low cost alternative (less than 13 m2 (Ersahin et al. 2016)) to conventional polymeric membranes. Mesh sizes of 10-200 m have been tested which provide substantial clean water permeability. The developed cake provides the rejection (Alibardi et al. 2014) with .99% COD rejection noted for several prospective applications, coupled with sustainable fluxes, which evidences their considerable potential to making AnMBR both capitally and energetically efficient (Alibardi et al. 2014; Ersahin et al. 2016). CONCLUSIONS The present literature review reveals significant differences with respect to biomass characteristics and fouling behavior between aerobic and anaerobic MBRs which can be summarized as follows: 1. The ratio of proteins to carbohydrates in eEPS is higher in AnMBRs than in AeMBRs. However, the total eEPS appears slightly higher in aerobic MBR. There is a general lack of knowledge regarding the relationship between eEPS concentration and composition and surface properties such as charge and hydrophobicity for AnMBR. Downloaded from https://iwaponline.com/h2open/article-pdf/1/2/131/512611/0010131.pdf by guest H2Open Journal Vol 1 No 2 154 doi: 10.2166/h2oj.2018.109 2. Although similar median particle sizes have been reported in aerobic and anaerobic MBRs, the presence of a population of fine solids with particle sizes ranging from 1 to 10-15 microns have been widely reported in the latter and associated to biomass of high fouling propensity. 3. The SMPCOD concentration contained within bulk sludge is an order of magnitude higher in AnMBR than in AeMBR. The main operational parameters that have shown to enhance SMP production in AnMBRs are low temperature and extended SRT. 4. The effect of turbulent gas sparging on membrane performance in immersed AnMBRs seems to be limited as compared to aerobic systems, indicating that fouling is more determined by sludge properties than by membrane operational conditions 5. AnMBR fluxes are between one-third and one-half of those reported in AeMBRs. Reported permeabilities in AnMBR are around 50% below those of AeMBR and employ specific gas demands between 50 and 300% higher. 6. Although as with aerobic systems the predominant fouling mechanism in AnMBRs has been reported to be cake filtration, contradictory results with respect to the effectiveness of membrane backwashing at reducing membrane fouling and permeability recovery after chemical cleaning have been reported and would require further research. 7. Whilst membrane fouling remains a critical challenge, several research groups have already demonstrated the potential to achieve energy self-sustained conditions at pilot scale. Whilst AeMBR is comparatively mature, continued investment in commercial module and aeration (gas scouring) engineering is still enabling radical reductions in specific energy demand to be realised at scale. Consequently, the potential to achieve energy neutral wastewater treatment at a commercial scale appears inherently viable. ACKNOWLEDGEMENTS The authors would like to thank our industrial sponsors Severn Trent Water, Yorkshire Water, Thames Water, Anglian Water and Scottish Water for their financial and technical support. Dr. Martin-Garcia was supported by a Marie Curie Early Stage Research Training Fellowship of the European Community's Sixth Framework Programme under contract number MEST-CT-2005-021050. REFERENCES Akram, A. & Stuckey, D. C. 2008 Flux and performance improvement in a submerged anaerobic membrane bioreactor (SAMBR) using powdered activated carbon (PAC). Process Biochem. 43(1), 93-102. Alibardi, L., Cossu, R., Saleem, M. & Spagni, A. 2014 Development and permeability of a dynamic membrane for anaerobic wastewater treatment. Bioresour. Technol. 161, 236-244. An, Y., Yang, F., Bucciali, B. & Wong, F. 2009 Municipal wastewater treatment using a UASB coupled with cross-flow membrane filtration. J. Environ. Eng. 135(2), 86-91. An, Y., Wu, B., Wong, F. 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