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Review

Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment, Sewage Sludge Digestion and Biogas Upgrading: A Review

by
Yemei Li
1,2,
Yuanyuan Ren
2,
Jiayuan Ji
3,
Yu-You Li
2 and
Takuro Kobayashi
1,*
1
Material Cycles Division, National Institute for Environmental Studies, Tsukuba 305-8506, Japan
2
Department of Civil and Environmental Engineering, Graduate School of Engineering, Tohoku University, 6-6-06 Aoba, Aramaki-Aza, Sendai 980-8579, Japan
3
Institute of Future Initiatives, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(20), 15129; https://doi.org/10.3390/su152015129
Submission received: 21 August 2023 / Revised: 7 October 2023 / Accepted: 17 October 2023 / Published: 22 October 2023
(This article belongs to the Section Sustainable Water Management)
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Versions Notes

Abstract

:
Anaerobic membrane bioreactors (AnMBRs) are formed through the combination of anaerobic digestion and membrane technology. The upgraded technology separates SRT from HRT in the anaerobic digester, shortening the treatment period, reducing the digester’s volume and improving effluent quality. Furthermore, AnMBRs have a strong tolerance for the existing forms of objects and can handle liquids, high-solid materials and gases. Up to now, AnMBRs have been utilized in the treatment of various types of industrial and municipal wastewater, organic solid waste and also biogas upgrading, and they have achieved excellent performance. However, there are few studies which have discussed their multiple utilization, especially following the flow of wastewater treatment. This study summarizes the application of AnMBRs in their diverse roles in the municipal wastewater treatment process. The discussion revolves around energy generation and the fouling issue of AnMBRs in the treatment of municipal wastewater, the digestion of sewage sludge generated in conventional municipal wastewater treatment and the upgrading of biogas after anaerobic digestion. In addition to controlling operating parameters, strategies used to improve the treatment effectiveness are also introduced. Lastly, online methods for preventing membrane fouling, which is the main operational obstacle to AnMBRs’ wider spread, are also discussed. This review aims to provide a fresh perspective on how AnMBRs are utilized in waste treatment.

1. Introduction

Membrane bioreactor technology is considered a well-established and mature technology that is used in many full-scale plants worldwide that handle municipal and industrial wastewater treatment [1]. The technology features high effluent quality, a small footprint and the separation of solids retention time (SRT) from hydraulic retention time (HRT) [2]. Given the performance of the technology in aerobic conditions, it is possible to expand the working area of the membrane to anaerobic systems where microorganisms are slow to grow but higher economic and environmental sustainability can be achieved [3]. The membrane bioreactors operated under anaerobic conditions are called anaerobic membrane bioreactors (AnMBRs). The AnMBRs are an improvement to the conventional anaerobic digesters, but they are still based on anaerobic digestion where microorganisms convert organic matter into methane through a series of biochemical processes.
AnMBRs have been proven to be effective in treating a wide range of wastewaters with high organic concentrations and full-scale applications have also been reported [4,5]. The high concentration of microorganisms in AnMBRs also ensures reliable performance in treating wastewater containing specific substances, such as emerging contaminants [6]. Furthermore, AnMBRs provide an alternative to anaerobic treatment of municipal sewage. Although municipal wastewater is the most abundant type of wastewater, it is difficult to achieve the desired performance with conventional anaerobic digesters because of their relatively low organic strength [7]. The low organic loading of municipal wastewater is a challenge that restricts the growth of slow-growing anaerobic microorganisms, while increasing the organic loading rate (OLR) by shortening the HRT also results in a rapid loss of microorganisms. With this inadequate reaction, neither the stability of the microbial community nor the good quality of the digested products can be achieved. AnMBRs resolve these problems and simultaneously produce particle-free permeate with a high degree of pathogen removal [8]. Furthermore, the effluent from a semi-industrial-scale AnMBR for municipal wastewater treatment was found to have a significant nutrient recovery potential [9].
In addition to wastewater treatment, AnMBRs have also demonstrated reliable performance in the treatment of organic solid waste with a high solids content, such as sewage sludge produced by the conventional activated sludge process at traditional municipal wastewater treatment plants (WWTPs). Compared to conventional anaerobic digesters, AnMBRs have advantages in organic conversion efficiency, effluent quality, organic tolerance and process stability [10]. However, the gas products of anaerobic digestion contain a significant amount of CO2, especially for organic solid waste up to 30–50%, which significantly reduces the calorific value of biogas [11]. Consequently, several methods have been used to increase the concentration of methane in biogas, most of which require considerable energy and resources [12]. Biological upgrading, which converts CO2 into methane by artificially introducing exogenous hydrogen to improve methanogenesis in the system, is considered an environmentally friendly method. AnMBRs can enhance upgrading by overcoming the problem of insufficient hydrogen transfer to liquid phases and improving the contact between microorganisms and hydrogen [13].
Although there are many studies related to AnMBRs, most of them focus on only wastewater treatment or solid waste treatment [9,14,15,16]. There is little in the literature that discusses the role of AnMBRs in their positioning in the wastewater treatment plant. Therefore, this review summarizes the use of AnMBRs in the municipal wastewater treatment plant from the perspective of sustainability. Their performance in energy generation and the fouling issue in the treatment of municipal wastewater, in the digestion of sewage sludge generated in the conventional activated sludge (CAS) process and in the upgrading of biogas after anaerobic digestion are discussed (Figure 1). In addition to the effects of operational parameters, strategies used to enhance treatment effectiveness are also introduced. Finally, online methods to mitigate membrane fouling, which is the main operational obstacle to AnMBRs’ wider spread, are discussed. This review aims to provide a fresh view on the utilization of AnMBRs in waste treatment and to foster the widespread use of AnMBRs for environmental sustainability.

2. Application of AnMBRs

2.1. Configurations of AnMBRs

2.1.1. Membrane for Filtration

In most AnMBRs, the membrane module is utilized for filtration to achieve separation of the HRT and the SRT. This type of AnMBRs can be configured in two basic ways: external crossflow or side-stream AnMBRs, and submerged AnMBRs. The external crossflow AnMBRs (Figure 2a) place the tubular membrane outside of the digester in a recirculation loop where the mixed liquor passes from the inside to the outside of the tube at high velocity [17]. Although the membrane surface can be conveniently cleaned by the mixed liquor, a high energy consumption is necessary to achieve the desired velocity [18]. Submerged AnMBRs (Figure 2b) place the membrane directly into the digester, which reduces energy consumption by using a vacuum to permeate. The membranes are generally flat-sheet membranes or hollow-fiber membranes, but also tubular membranes. The membrane surface is scoured with biogas to reduce membrane fouling. External submerged AnMBRs were proposed to connect the configurations of crossflow AnMBRs and submerged AnMBRs (Figure 2c). In this configuration, the membrane is contained in an external small unit that reduces membrane fouling by concentrating high shear. The main digesters in external submerged AnMBRs may be various anaerobic digesters such as the upflow anaerobic sludge blanket (UASB) [19] and the continuous stirred-tank reactor (CSTR) [20].

2.1.2. Membrane for Injection

The membrane is used to inject exogenous hydrogen into the system in AnMBRs for biogas upgrading, often along with internal biogas circulation. The use of membrane modules can achieve bubble gas transfer, which enhances the efficiency of hydrogen use [21]. In this scenario, the membrane modules are usually the hollow-fiber type and submerged in the digestate. The use of tubular membranes has also been reported [22]. The system can be divided into in situ and ex situ upgrading types depending on the location of the substrate feed in relation to the hydrogen injection. The substrate and hydrogen are fed in the same digesters for in situ upgrading (Figure 2d). The injection membrane can also be inserted into a separate upgrading reactor fed with biogas as a post-treatment after the main digesters for ex situ upgrading (Figure 2e). Although the in situ process can decrease building investment, excessive hydrogen can result in the accumulation of volatile fatty acids in the anaerobic system, resulting in system failure. The ex situ system is a risk-free option, but it does require more building construction and greater operation and maintenance costs.

2.2. AnMBRs for Municipal Wastewater Treatment

2.2.1. Effects of Operating Parameters

Operating conditions have a significant impact on the performance of AnMBRs (Table 1). HRT, SRT and temperature are crucial parameters to maximize the performance of AnMBRs for municipal wastewater treatment [9]. AnMBRs have shorter HRTs for superior performance and faster startup compared to other digesters [19]. A decrease in HRT can lead to a decrease in structure volume, which can lower construction costs. However, in addition to the biological performance, the filtration performance that distinguishes AnMBRs from other digesters is also worth noting [23]. Plevri et al. reported a 40 L AnMBR with a submerged flat-sheet membrane for municipal wastewater treatment in which the COD removal efficiency was decreased from 89% to 78% when HRT was decreased from 48 h to 12 h [24]. The transmembrane pressure remained at extremely low levels at HRT 48 h. Its values slowly increased at HRT 24 h but rapidly increased at HRT 12 h. Another pilot-scale AnMBR with hollow-fiber membrane for sewage treatment had similar performance. Kong et al. reported that the 5000 L AnMBR achieved a COD removal efficiency of over 90% and a methane yield of 0.192 L/g-CODrm at a low HRT of 6 h [25]. When the HRT was decreased from 24 to 6 h, the system SRT decreased from 63 to 29 d, which had little influence on digestion performance. The COD removal efficiency was at 89.5–93.2% and the methane content was at 76.7–79.1%, while the transmembrane pressure increased significantly at a fouling rate of 1.39 kPa/d at the HRT of 6 h and the fouling was not recoverable by chemical cleaning.
AnMBRs used for municipal wastewater treatment are commonly operated at ambient temperature (5–30 °C). However, seasonal temperature fluctuations make it difficult to maintain stable operation. On the one hand, temperature control requires a significant amount of energy. On the other hand, temperature influences the microbial community and promotes treatment performance and the membrane filtration performance in AnMBRs. Low operating temperatures have a negative impact on treatment performance by reducing methane yield, increasing sludge yield and increasing the membrane fouling rate, particularly at short HRTs. Previous research found that the COD removal efficiency of a 20 L submerged AnMBR for sewage treatment decreased from 90% to 77% when the temperature decreased from 25 °C to 15 °C [26]. The methane yield decreased from 0.17 to 0.06 L/g-COD and the sludge yield (g-VSS/g-CODrm) increased by 215% as a result of shortening the SRT from 94 to 21 d. Furthermore, the fouling rate jumped sharply as the temperature reached 15 °C, while the acetoclastic methanogens Methanosaeta predominated in all conditions. Similar negative effects were also observed at the pilot scale. A decreasing methane yield from 0.244 to 0.205 L/g-CODrm was reported in a 5000 L submerged AnMBR with a hollow-fiber membrane module when the temperature decreased from 25 °C to 15 °C [27]. Though the methane in the biogas was not significantly influenced by the temperature drops at the pilot scale, the dissolved methane increased from 29% to 43% in the total methane yield. The value was similar to the results reported for another psychrophilic AnMBR with submerged flat-sheet membrane in which dissolved methane in the permeated took up 40–50% of the total methane generation [28]. Dissolved methane presented in the effluent of anaerobic digestion not only releases greenhouse gas to the environment but also loses energy generated from the anaerobic process [29]. Due to the mechanical stirring and biogas sparging, AnMBRs have a lower methane supersaturation degree than other types of digesters [30]. However, considering the large proportion of dissolved methane, its recovery is still the key to achieve an energy-neutral AnMBR system for municipal wastewater treatment and could be another great barrier to AnMBRs’ implementation [31,32]. Regarding the membrane fouling issue, more soluble microbial products (SMPs) and extracellular polymeric substances (EPSs) were produced at lower temperatures even after weekly chemical backwashing [27]. SMPs are released during substrate metabolism and decay in biological processes, while EPSs are formed by active secretion from microorganisms, cell surface material shedding, cell lysis and sorption from the environment [33,34]. SMPs and EPSs play key roles in the formation of membrane fouling, with complicated mechanisms and negative impacts being observed [35]. In addition, smaller sludge particle size and more fine flocs under lower temperatures increased the membrane fouling potential of AnMBRs [36].
Table 1. Performance of AnMBRs for municipal wastewater treatment.
Table 1. Performance of AnMBRs for municipal wastewater treatment.
System Configuration *Scale (L)Temperature
(°C)		CODinfluent (mg/L)HRT (h)SRT
(d)		Methane Yield (L/g-COD)COD Removal Efficiency (%)Reference
S-FS-AnMBR4018/24428–47748–1250-69–89[24]
S-HF-AnMBR20154126–2421–4910.21–0.2390.5[26]
S-HF-AnMBR16502576211–211000.292[37]
ES-HF-AnMBR34,40010–27755–140325–41700.07–0.1793[9]
S-HF-AnMBR500015–25203–490820–710.205–0.24477–93[27]
ES-TM-AnMBR/UASB160 + 150188927–17-0.199–0.23573–90[38]
* S: submerged; ES: external submerged; FS: flat sheet; HF: hollow fiber; TM: tubular membrane.

2.2.2. Co-Digestion

From the perspectives of environment, economy and engineering, it has been found that the organic fraction of municipal solid waste is a suitable co-substrate for municipal wastewater [39,40]. A full-scale case study at a WWTP in Germany showed that co-digesting with food waste resulted in an increase in energy production by 5.6 kWh/PE/a [41]. The strategy not only increased the energy self-sufficiency of WWTPs but also became a profitable investment with a 10-month payback period. The application of AnMBRs further enhanced the process. Previous research reported an AnMBR pilot plant fed with municipal wastewater only or co-digestion with food waste under different HRTs and SRTs [42]. The AnMBR system consisted of a 0.9 m3 anaerobic reactor and two 0.6 m3 membrane tanks with a hollow-fiber ultrafiltration membrane module whose pore size was 0.05 µm. When the substrate changed from pure municipal wastewater to co-digestion with food waste, the amount of which was calculated as 40% of population using a food waste disposer, the obtained methane yield increased from 51.2 to 80.4 L/kg-CODrm while the methane content was similar at around 45%. During this period, the HRT decreased from 30 to 18 h while the SRT remained at 40 d. When the SRT was extended to 70 d, the total methane production further increased 67% and the methane content increased to 72.9%. Further increasing the percentage of population using food waste disposers to 80%, no obvious increase in the methane content (from 72.9 to 74.7) was observed but methane production was enhanced by 66%. The stable permeate quality also needs to be noted since the COD concentration of the effluent was kept at 49–54 mg/L and COD removal efficiency was 85–94%. Further energy consumption showed a similar power requirement of AnMBRs under similar filtration conditions while energy recovery from dissolved methane and biogas increased with a higher food waste ratio [3]. However, the higher fouling intensity that resulted from the addition of food waste increased the energy demand by 0.34 kWh/m3 and costs by 0.067 EUR/m3, while the total costs were reduced by 0.012 EUR/m3 due to a higher biogas production [43]. Life cycle assessment (LCA) results also presented the enhanced environmental sustainability of AnMBRs by conducting co-digestion that reduced environmental impacts related to abiotic depletion, global warming, acidification and marine aquatic ecotoxicity [3].

2.3. AnMBRs for Sewage Sludge Treatment

2.3.1. Effects of Operating Parameters

In AnMBRs for sewage sludge treatment, the OLR, SRT and operating temperature are important operating parameters (Table 2). An AnMBR with a higher OLR and lower HRT that leads to a reduction in reactor volume and thus reduces capital costs is expected in the application. The increase in OLR is found to increase methane production and COD levels in the permeate. Previous research reported that when OLR was enhanced from 0.1 to 3 kg-COD/m3/d, the methane yield of an AnMBR increased from 0.19 to 0.45 L/g-COD [44]. A further improvement in the OLR did not lead to a noticeable improvement in methane yield, while the concentration of COD in the permeate increased from 1.1 to 3.8 g/L when the OLR went from 0.1 to 10 kg-COD/m3/d. In an AnMBR fed with high OLR and high-solid substrate, a rapid increase in the solid concentration of mixed liquor in the system necessitates a shortening of the SRT. However, SRT has a direct impact on the reduction of organic matter. AnMBRs can achieve better COD and VS removal efficiency than conventional digesters under the same HRT by maintaining active biomass and particulates under independent control of SRT and HRT [45]. In addition to SRT, operating temperature has a significant effect on the degradation efficiency of organic matter due to the catalytic effect on sludge hydrolysis of raising the temperature. Pileggi et al. reported an increasing methane yield from 0.19 to 0.28 and to 0.34 Nm3/kg-VS when the temperature increased from 25 to 35 and to 55 °C, respectively, under the same HRT of 7 d [46]. However, the effect was not apparent if the AnMBR was operated under sufficient SRTs. Meabe et al. achieved a methane content of 67.5% and similar methane yields of 0.242 and 0.245 L/g-COD under mesophilic and thermophilic conditions, respectively [47]. During the same 7 d HRT, the SRTs reached the maximum acceptable values of 30 and 50 d for mesophilic and thermophilic AnMBRs, respectively. It is worth noting that high temperatures increase levels of irreversible fouling and deteriorate permeate quality with higher levels of soluble matter [47,48].
Unlike operating conditions, the system configuration has more of an impact on membrane filtration performance than on digestion performance. AnMBRs equipped with crossflow membrane modules can handle higher levels of mixed liquor total solids, making them more frequently utilized in sludge digestion. In recent years, submerged membrane modules, both flat-sheet membranes and hollow-fiber membranes, have become more popular. A previous study found that the external submerged hollow-fiber membrane and the external tubular membrane had comparable COD removal efficiency, but the latter had a higher critical flux [17]. The composition of membrane foulant is also related to the mode of membrane operation and the solid concentration of the mixed liquor in the system, as well as the configuration of the system. Hafuka et al. reported an external crossflow hollow-fiber membrane with a mixed liquor solid concentration of 2% under a flux of 0.01–0.07 m/d in which physically irreversible fouling was the dominant form of membrane fouling [49], while under the same temperature, cake layer and organic substances were found to be the main foulants in another submerged AnMBR with a permeate flow of 0.08–0.1 m/d under a mixed liquor solid concentration of 25–40 g/L [50].
Table 2. Performance of AnMBRs for sewage sludge digestion.
Table 2. Performance of AnMBRs for sewage sludge digestion.
System Configuration *Scale (L)Temperature
(°C)		OLR
(g-COD/L/d)		HRT (d)SRT
(d)		Methane YieldCOD Removal (%)Reference
ES-HF-AnMBR/CSTR15351.72–3.7215–3025–600.239 L/g-VS99.0[50]
ES-HF-AnMBR/CSTR15554.3115-0.31 L/g-VS96.7[51]
ET-AnMBR50024, 35, 553.4–3.76.9–7.322–390.19–0.34 L/g-VS-[46]
ET-HF-AnMBR/CSTR2.37 + 0.1370.21–0.2730–33102–1070.19–0.24 L/g-VS-[52]
ET-CM-AnMBR/CSTR2535 & 554.8–10.43–730–500.213–0.245 L/g-COD-[47]
* ES: external submerged; HF: hollow fiber; ET: external tubular; CM: ceramic membrane.

2.3.2. Co-Digestion

In order to increase biogas production, co-digestion is also performed in sludge digestion. Food waste, which is easily accessible and has a high methane potential, is the most commonly used co-substrate in full-scale applications. The use of AnMBRs in the co-digestion of food waste and sewage sludge is usually with high-solid AnMBRs which are fed with a high-strength substrate and operated under high-solid conditions. Li et al. reported a high-solid AnMBR for co-digestion of food waste and sewage sludge in which the methane yields were enhanced 53% and 21% with substitution percentages of food waste of 75% and 50%, respectively, compared with pure sludge digestion [20]. The economic evaluation of a large-sized AnMBR-WWTP showed that the application of co-digestion with food waste increased both the capital and operating costs and the revenue [53], while it reduced the net cost compared with AnMBRs for only sludge treatment, and the net cost decreased with the enhancement of the OLR. Furthermore, the gate/delivery fee was a key factor in the net cost of AnMBRs for sludge digestion with food waste addition.
In addition, sewage sludge also serves to stabilize the anaerobic digestion of other substances. In the co-digestion of microalgae, primary sludge is used to increase the C/N ratio. Serna-García et al. reported a co-digestion of primary sludge and microalgae grown in the effluent from an AnMBR treating urban wastewater with TS ratios of 62% and 38% in which methane yields of 391 mL/g-VS and 370 mL/g-VS were obtained at the lab scale and pilot scale, respectively [54]. Furthermore, the addition of primary sludge prevents the inhibition of free ammonia nitrogen in microalgae digestion, which was more evident in AnMBRs than in CSTR under thermophilic conditions [55]. The superiority of AnMBRs to CSTR was also observed in the digestion of a lipid-rich substrate with the co-substrate as sludge. The addition of sludge prevented the inhibition caused by the accumulation of long-chain fatty acids in the digestion of the lipid-rich substrate [10]. The effect was strengthened by extending the SRT through the application of AnMBRs. It also should be mentioned that the accumulation of long-chain fatty acids led to an increase in sludge hydrophobicity and a decrease in membrane fouling [56]. Sludge can also serve as a trace metals provider in co-digestion. Qiao et al. reported that a thermophilic AnMBR for the digestion of coffee grounds failed under an OLR of 5.39 kg-COD/m3/d due to the accumulation of volatile fatty acids caused by the shortage of trace metals [57], while it achieved a 67.4% COD removal under an OLR of 11.8 kg-COD/m3/d with a 15% sludge addition.

2.3.3. Pretreatment

Due to the hard cell wall and complex structure of sewage sludge, hydrolysis is the rate-limiting step in its anaerobic digestion. The pretreatment process is applied to increase the accessible surface area and speed up hydrolysis, thereby improving methane production [58]. Thermal hydrolysis is a well-developed pretreatment for sludge digestion. Even low-temperature thermal hydrolysis increased the soluble COD content of sludge by 2.56 times and resulted in a 2.96-fold increase in the methane production rate compared to raw sewage sludge [59]. In a long-term experiment with a 10 L AnMBR equipped with a 0.116 m2 PVDF flat-sheet membrane, the methane content of biogas also increased from 47.9% to 68.3% when the substrate changed from raw sludge to sludge after 125 °C thermal pretreatment. The organic biodegradation and methane production were also upgraded by the introduction of pretreated sludge due to its positive effects on the growth of organic-degrading bacteria and methanogens which also alleviate transmembrane pressure. Another AnMBR used a flat-sheet membrane provided by Kubota Membrane Cartridge with a normal pore size of 0.2 µm and operated under an initial flux of 0.12 m3/m2/d, and it was fed with hydrolysis-pretreated sludge [60]. The AnMBR showed better TS and VS removal efficiency, methane yield and specific methanogenic activity than CSTR. An integrated continuous system comprising a pretreatment reactor and an AnMBR was also studied. A temperature-phased hyperthermophilic-mesophilic AnMBR achieved the highest methane yield of 246 L/kg-VS at a 15 d HRT (5 d for hyperthermophilic pretreatment and 10 d for mesophilic AnMBR) [61]. The hyperthermophilic pretreatment reactor and AnMBR had working volumes of 12 L and 7 L, respectively. The AnMBR was fitted with a flat-sheet microfiltration membrane module and the flux was set as 4–5 LMH initially. Severe membrane fouling, as evidenced by a rapid and unrecoverable rise in transmembrane pressure, occurred due to the high solid concentration of 55% in the membrane unit under a short AnMBR-HRT of 5 d.

2.4. AnMBRs for Biogas Upgrading

In WWTPs, achieving energy recovery from anaerobic digestion is a key step in minimizing the overall operation cost. Efficient biogas utilization can reduce the environmental risk of greenhouse gas emission and the carbon footprint for WWTPs. Combustion of biogas in boilers and/or flare biogas is the most direct way to realize biogas energy recovery in most WWTPs conducting anaerobic digestion but it has a low energy recovery rate [62]. Combined heat and power (CHP) technology is what is most often adopted in self-sufficient WWTPs generating biogas [63]. The CHP electric efficiency is about 30% and the thermal efficiency is about 40%. However, its utilization is restricted because it needs a large investment to build and operate a relatively mature system and the pretreatment process. The biogas upgrading process, the bioconversion of CO2 with exogenous hydrogen to methane by anaerobic microorganisms, is thought to be a better strategy with higher benefits than anaerobic digestion followed by a CHP process, especially when biomethane is generated [64,65]. Compared to the direct use of hydrogen, such a conversion avoids the problem of the low volumetric energy content of hydrogen and the difficulty for transportation, and also increases the calorific value of the biogas, allowing for a wider range of its utilization, and it even can be transferred through the natural gas pipeline network [13,66]. Depending on the purpose of use, the involvement of AnMBRs in biogas upgrading can be divided into three forms (Figure 3). One is where the membrane module maintains the role of separating SRT and HRT in a conventional AnMBR while exogenous hydrogen is passed into the AnMBR with another pipeline (Figure 3a). In such a system, both solids-free permeate and upgraded biogas can be obtained. Hafuka et al. reported that the methane content reached 92% in upgraded biogas in a mesophilic AnMBR for digesting waste-activated sludge with the addition of 11 equivalents of hydrogen relative to CO2 [52]. In the second type, the membrane module is used as a medium for hydrogen to enter the digestion process and for gas sparging by biogas recirculation (Figure 3b,c). The utilization of the membrane improves the mass transfer within the gas and liquid phases and the contact between microorganisms and hydrogen, thus promoting hydrogen utilization and biogas upgrading [13]. Alfaro et al. reported a mesophilic digester with a submerged hollow-fiber membrane module for in situ upgrading of sludge digestion [67]. The concentration of CO2 in the upgraded biogas was 11%, which was a 66% decrease compared to conventional digesters, at an OLR of 1.5 g-VS/L/d and hydrogen flow rate of 0.87 L/L/d. At the same time, the VS removal efficiency was not affected, with both being 48.5%. An ex situ process has a higher gas production rate and better biogas quality compared to an in situ one. Díaz et al. reported a successfully operated ex situ AnMBR that upgraded biogas at a rate of 25 L/L/d and increased methane concentration from 60% to 95% [68]. The AnMBR had a working volume of 31 L and operated at thermophilic conditions. The injection membrane was a hollow-fiber type with a total membrane surface of 0.93 m2 and pore size of 0.4 µm. In addition, it can also combine these two functions of the membrane to form a dual-membrane structure (Figure 3d). Deschamps et al. reported a pilot AnMBR with two membrane modules for in situ biogas upgrading and the system reached a biogas productivity of 1.7 Nm3/m3/d with a high methane content of 98% [22]. Both of the membranes were external tubular ceramic modules with a surface area of 0.25 m2 and pore size of 0.1 µm. The hydrogen injection membrane had a fluorinated silanes coat while the filtration membrane had none.

3. Membrane Fouling Mitigation

Membrane fouling is the most important issue hampering the widespread application of AnMBRs, and it is also the key factor for virus removal capacity [69,70]. Fouling decreases the lifespan and the performance of the membrane, resulting in higher energy requirements and operating costs [69]. As mentioned previously, submerged membrane modules are not convenient to clean compared to crossflow types. Moreover, the composition and structure of foulants are complex. The formation of fouling layers and mitigation strategies have received extensive attention. In hydrogen-assisted AnMBRs, especially for sludge digestion with long HRTs, though the biofilm formed on the membrane contributes to hydrogen consumption, it also increases the resistance to hydrogen diffusion into liquid phase [21]. However, the mitigation of these types of fouling has not been much discussed compared to that for membranes used for filtration. Instead, more attention has been paid to characterizing the communities of microorganisms enriched on membrane surfaces [71,72,73]. For membrane modules used as permeate extractors, the decline in flux due to membrane fouling reduces the productivity of the AnMBRs, directly limiting the popularity of the technology [1]. The properties of the feedstock and mixed liquor, the operating conditions and the characterization of membranes affect the formation of membrane fouling [74]. In addition to the offline cleaning that removes reversible foulants, there are also some strategies conducted online to mitigate the development of fouling (Table 3).

3.1. Filtration Mode

Sub-critical flux operation, membrane relaxation and backwashing are the common fouling control strategies [75]. Sub-critical flux operation results in the AnMBR flux being below the highest flux, causing no or little membrane fouling under operating conditions [76]. Robles et al. reported that a submerged AnMBR for municipal wastewater was operated for almost two years at sub-critical levels without any irreversible fouling problems and no chemical cleaning was conducted [77]. However, for industrial applications, the balance between the costs of membrane cleaning and the costs of operating an extremely low flux should also be carefully considered. According to a previous study, doubling the flux would reduce the capital costs of an AnMBR system by 46% [78]. To achieve lower fouling rates under higher flux operation, the relaxation phase, which is a temporary cessation that allows the dislodgement of foulants from the membrane surface and the recovery of flux, is added to the continuous filtration mode [79]. At this point, excessive instantaneous flux and insufficient relaxation will lead to substantially irreversible fouling [80]. To maintain sustainable AnMBR operation, it is suggested to maintain a moderate filtration to relaxation time at higher operational flux, with periodic online chemical backflushing [80]. Online backflushing is applied to reduce channel blockage on the membrane surface and sodium hypochlorite is the common cleaning chemical [81]. However, the toxicity of chemicals to microorganisms also needs to be noted in addition to permeability recovery [27]. Yue et al. tested four NaClO concentrations for backwash in an AnMBR for domestic wastewater treatment and found that increased NaClO concentrations enhanced the removal of proteins from the membrane surface [82]. For the condition of mixed liquor in the membrane unit, a low concentration benefited the biodegradability of organics and microbial activities while a high concentration of NaClO deteriorated cell metabolism and promoted the production of cell lytic products. A balance of foulant removal and treatment performance was achieved at 1 mg-NaClO/L.

3.2. Biogas Sparging

Biogas sparging is a widely accepted fouling mitigation strategy by promoting turbulence around the membrane by means of bubbles [83]. Biogas sparing scours the membrane surface, reducing the deposition of particles and microorganisms, leading to constant transmembrane pressure, higher critical flux and a thinner cake layer [84,85]. From the perspective of engineering, suitable sparging rates fall within two critical sparging rates [83,86,87]. A dramatic rise in transmembrane pressure would be caused by a further decrease beyond the lower one, while increasing further beyond the upper limit would not improve the membrane filterability but have negative effects on microbial growth and fouling control. Over-sparging has also resulted in worse permeate quality with higher COD levels [88]. On the other hand, biogas sparging also contributes to the mixing of bulk suspensions and requires a significant amount of energy. LCA results showed that AnMBRs recovered 49% more energy as biogas than a high-rate activated sludge system followed by anaerobic digestion of produced sludge, but they had significantly higher energy demands of which 86% was attributed to biogas sparging for fouling control [78]. Therefore, it is necessary to optimize the biogas sparging strategy in combination with the specific conditions of the digesters. Alternating sparging was found to be a more effective option than continuous sparging and intermittent sparging. Zhang reported that an alternating strategy of 30 s each for 3 L/min and 5 L/min reduced the membrane fouling rate by 60% compared with a constant sparging at 4 L/min [87], while in an intermittent sparging mode, the fouling increased rapidly during the non-sparging period and a high sparging rate could not remove the foulants within a short period. Furthermore, biogas sparging intensity was one of the determining factors for the filtration process costs of AnMBRs. Economic analysis showed that the minimum filtration process costs ranged from 0.03 to 0.12 EUR/m3 and happened when the biogas sparging intensity was at 0.05 to 0.3 m/h and the mixed liquor suspended solids concentration was at 5–25 g/L [89].

3.3. Anaerobic Fluidized-Bed Membrane Bioreactors

Anaerobic fluidized-bed membrane bioreactors (AFMBRs) are developed by combining anaerobic fluidized-bed bioreactors with submerged membrane filtration [90]. The media used to implement the fluidized bed in AnMBRs are mainly carbon-based materials, iron-based materials and other materials [91]. Although different types of materials have different functions due to their properties, the main objective of their use is to reduce membrane fouling. Carbon-based materials, such as activated carbon and biochar, are the most widely used and studied. Carbon-based materials have been found to promote interspecies electron transfers, buffering capacity and enzymatic activity in anaerobic digestion, thereby increasing methane yield and enhancing biogas and digestate quality [92]. In AFMBRs, their function of effectively preventing membrane fouling is also attractive. Yang et al. reported that granular activated carbon significantly delayed the sudden change in transmembrane pressure and extended membrane service time [93]. The carbon-based materials are still capable of functioning under low-temperature conditions. Lei et al. reported a 200% prolonged operation duration of a submerged biochar-assisted AnMBR operated at 18 °C due to a decreased filtration resistance [94]. Membrane scouring, foulant adsorption and sludge status regulation are the mitigation mechanisms of AFMBRs with carbon-based materials. However, it is also important to be aware of the possible negative effects of inappropriate dosage and excessive scouring during the operation [95,96].

3.4. Electrochemical AnMBRs

Electrochemical anaerobic digestion has been found to be a promising improvement of traditional digestion. The addition of current enhances organic reduction and maintains system stability by oxidizing organic matter and even non-degradable organic matter. It also improves microbial activity and the electron transfer, which changes the microbial communities and the consumption of volatile fatty acids [97]. The electrochemical AnMBRs combine electrochemical regulation, anaerobic digestion and membrane separation by placing electrodes on both sides of the membrane in a conventional AnMBR. In addition to the improved digestion performance, a delayed membrane fouling rate was also observed [23]. The main cause may be the effect of the applied voltage on the characteristics of the sludge in the AnMBR. The electrochemical AnMBRs had a lower amount of EPSs, which improved their resistance to membrane fouling [98]. The attached sludge was found to have a lower amount of EPSs than that of the suspended sludge [23]. Furthermore, the electrical stimulation significantly decreased the EPS-proteins/EPS-carbohydrates ratio in the attached sludge, reducing the possibility of pollutant adsorption on the cake layer [23]. Another electrochemical AnMBR for co-digestion of sewage sludge and food waste was reported to have a significantly higher content of tightly bound EPSs than of loosely bound EPSs and soluble EPSs when the voltage was applied [99]. With the utilization of Fe as a sacrificial anode, the coagulation increased the size and porosity of the sludge floc, significantly reducing the adhesion capacity of EPSs and alleviating the membrane fouling potential [98]. While electrochemical AnMBRs have been reported to improve methane production and prevent membrane fouling, most of these studies were conducted in the laboratory. Further research is needed on the principles involved and their utilization at a larger scale, for complex substrate treatment and under higher loading rates.
In summary, the main mechanisms of these online membrane fouling control strategies are flux reduction, scouring of the membrane surface and modification of sludge properties. Each of these strategies has its own advantages and limitations and can be used individually or in combination. Optimizing the filtration mode is the easiest strategy, which basically requires no additional equipment. However, exploring the boundary conditions requires extensive experimental validation, and the results can vary with sludge properties and the hydraulic conditions of the bioreactor. Backwashing may also use chemicals that may negatively affect the anaerobic sludge and also increase operating costs. Biogas sparging is the most commonly used adjunct. It is simple to operate and effective, but energy consumption is an issue that has to be considered in practical application. In AFMBRs, the addition of carriers needs to be considered in terms of its impact on the treatment and disposal of digestate. Furthermore, the physical impact of particulate scouring on the membrane surface and membrane service life also need to be considered. In electrochemical AnMBRs, the construction and maintenance costs of adding power generation devices need to be considered. Therefore, in large-scale application, it is necessary to optimize the reactor design and operating conditions according to local conditions, so as to maximize the benefits of production.

4. Future Perspectives

From the perspective of engineering feasibility, economic viability and environmental sustainability, the AnMBR, a combination of anaerobic digestion and membrane separation, is a promising technology for both waste treatment and resource recycling. Further research and improvements are still needed for its promotion and practicality. Firstly, the effectiveness of the treatment and the operation of the membranes in AnMBRs are greatly impacted by the temperature since the technology is based on microbial activity. Despite the increase in the concentration of microorganisms in the AnMBRs and the expansion of the reaction time by membranes, the effects of temperature cannot be ignored. Due to economic considerations, the use of excessive energy input to maintain the facility temperature is not cost-effective for large-scale treatment facilities. Further research is needed to reduce the negative effects of temperature and to use a rational temperature, such as optimizing the treatment temperature, optimizing the operating strategies or using temperature-phased processes. Secondly, although AnMBRs have advantages in microbial retention, strategies for responding to novel pollutants, sudden shocks in terms of both species and quantity, and extreme operating conditions also need to be explored. Most commonly, the question is how to balance the relationship between maintaining treatment efficiency and keeping the membrane in stable operation under high-solid concentrations brought by increasing the HRT and OLR. Finally, there are still some resources contained in the permeate and discharged digestate, such as dissolved methane in the permeate from wastewater treatment, which reduce energy recovery and increase the global warming impact based on LCA results [78], as well as inorganic resources in the permeate and discharged digestate from sewage sludge treatment. Methods for recovering and reusing these resources, such as combining this process with other advanced technologies to achieve zero resource release, also deserve further research. When this is accomplished, AnMBRs can be expected to be utilized in a wider range of applications.

5. Conclusions

Anaerobic digestion is a technology that is environmentally friendly and can both treat pollutants and recover energy simultaneously. The combination of membrane mod-ules and conventional anaerobic digesters in AnMBRs results in improved treatment effectiveness and efficiency. The AnMBRs have a high tolerance for the physical state of the treated objects, including wastewater, sewage sludge and gases. The operating parameters, such as temperature and SRT, have a significant impact on the performance of AnMBRs in wastewater and sewage sludge treatment. AnMBRs for biogas upgrading are mainly used for the treatment of organic solid waste such as sewage sludge with a high quantity of CO2 in the biogas. Although AnMBRs have the advantages of high efficiency, high product quality, low sludge yield, and the recovery of resources and energy, membrane fouling has limited their promotion. Optimization of membrane operation modes, controlled biogas sparging, the addition of carriers and the application of currents can mitigate the occurrence and development of membrane fouling. Further research is still needed to reduce energy input, to operate and maintain them under extreme conditions, to integrate links with other advanced technologies and for large-scale applications.

Author Contributions

Resources, conceptualization, investigation, data curation, formal analysis, and writing—original draft, Y.L.; resources, investigation, and writing—review and editing, Y.R.; investigation, data curation, and writing—review and editing, J.J.; conceptualization and writing—review and editing, Y.-Y.L.; supervision, conceptualization, and writing—review and editing, T.K. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by JSPS KAKENHI grant number 19J11913.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Application of AnMBRs in the municipal wastewater treatment plant, including the treatment of municipal wastewater, the digestion of sewage sludge generated in the conventional activated sludge (CAS) process and the upgrading of biogas after anaerobic digestion.
Figure 1. Application of AnMBRs in the municipal wastewater treatment plant, including the treatment of municipal wastewater, the digestion of sewage sludge generated in the conventional activated sludge (CAS) process and the upgrading of biogas after anaerobic digestion.
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Figure 2. System configurations of anaerobic membrane bioreactors: (a) external crossflow AnMBR; (b) submerged AnMBR; (c) external submerged AnMBR; (d) in situ upgrading; (e) ex situ upgrading. B: blower; P: pump.
Figure 2. System configurations of anaerobic membrane bioreactors: (a) external crossflow AnMBR; (b) submerged AnMBR; (c) external submerged AnMBR; (d) in situ upgrading; (e) ex situ upgrading. B: blower; P: pump.
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Figure 3. System configurations of anaerobic membrane bioreactors for biogas upgrading: (a) AnMBR with membrane for filtration; (b) AnMBR with membrane for in situ hydrogen addition; (c) AnMBR with membrane for ex situ hydrogen addition; (d) AnMBR with dual membrane structure. B: blower; P: pump.
Figure 3. System configurations of anaerobic membrane bioreactors for biogas upgrading: (a) AnMBR with membrane for filtration; (b) AnMBR with membrane for in situ hydrogen addition; (c) AnMBR with membrane for ex situ hydrogen addition; (d) AnMBR with dual membrane structure. B: blower; P: pump.
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Table 3. Characterization of membranes and operational strategies of AnMBRs for municipal wastewater treatment and sewage sludge digestion.
Table 3. Characterization of membranes and operational strategies of AnMBRs for municipal wastewater treatment and sewage sludge digestion.
Feedstock 1System Configuration 2Membrane Material 3Filtration Area (m2)/Pore Size (μm)Membrane Flux (L/m2/h)Filtration Mode (Filtration: Relaxation; min:min)Mixed Liquor Solid Concentration
(g/L)		Biogas SpargingOnline Chemical CleaningReference
S-relatedM-ES-HF-AnMBR/CSTRPTFE0.1/0.163:325–30 (MLTS)Yes, 85 m/hNo[20]
W-relatedP-S-HF-AnMBRPVDF72/0.410.84:110–12 (MLSS)Yes, 0.75 m/hYes, weekly[27]
S-relatedT-ES-HF-AnMBR/CSTR-0.1/0.153:127 (MLTS)Yes, 74 m/hNo[51]
S-relatedT-S-AnMBRCPE0.116/0.22.0–7.64:110–80 (MLTS)Yes, 5 L/minNo[57]
W-relatedP-ES-HF-AnMBR/UASB-0.93/0.0450–157.5/30:0.080–5.95 (MLVS)Yes, 25–60 m/hYes, 15/60 s per cycle[38]
S-relatedM-ET-HF-AnMBRPVDF0.0016/0.11.29–1.46Continuously6–13 (MLTS)0/0.52 L/minNo[52]
S-relatedM&T-ET-CM-AnMBRTiO2/ZrO20.0226/300 kDa7-20 (MLTS)NoNo[47]
S-relatedM-S-FS-AnMBRPolymeric membrane0.232/0.223.6–10.52:138–61 (MLTS)YesNo[61]
W-relatedP-S-HF-AnMBR-30/0.49.5-13–16 (MLSS)Yes, 26 m3/hNo[19]
S-relatedM-ES-HF-AnMBRPVDF0.44/0.033.5–10.51.5:012.3 (MLTS)Yes, 0.15 m/hYes, 40 s per cycle[54]
S-relatedM-ES-HF-AnMBRPVDF0.44/0.034.18–5.63:0.511.5 (MLTS)Yes, 0.15–0.62 m/hYes, 45 s per cycle[54]
1 S-related: sewage-sludge-related feedstock; W-related: municipal-wastewater-related feedstock. 2 M: mesophilic; P: psychrophilic; T: thermophilic; ES: external submerged; S: submerged; HF: hollowfiber; FS: flat sheet; ET: external tubular. 3 PTFE: polytetrafluoroethylene; PVDF: polyvinylidene difluoride; CPE: chlorinated polyethylene.
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Li, Y.; Ren, Y.; Ji, J.; Li, Y.-Y.; Kobayashi, T. Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment, Sewage Sludge Digestion and Biogas Upgrading: A Review. Sustainability 2023, 15, 15129. https://doi.org/10.3390/su152015129

AMA Style

Li Y, Ren Y, Ji J, Li Y-Y, Kobayashi T. Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment, Sewage Sludge Digestion and Biogas Upgrading: A Review. Sustainability. 2023; 15(20):15129. https://doi.org/10.3390/su152015129

Chicago/Turabian Style

Li, Yemei, Yuanyuan Ren, Jiayuan Ji, Yu-You Li, and Takuro Kobayashi. 2023. "Anaerobic Membrane Bioreactors for Municipal Wastewater Treatment, Sewage Sludge Digestion and Biogas Upgrading: A Review" Sustainability 15, no. 20: 15129. https://doi.org/10.3390/su152015129

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