Generally, MBRs displayed excellent removal efficiency for organics and nutrients via biodegradation and membrane rejection, but showed limited rejection of smaller-sized, dissolved substances (such as humic-like substances, low molecule weight organics, heavy metals, micropollutants, etc.) that readily pass through the porous membranes (Judd, 2010). Towards achieving wastewater reclamation, further membrane separation processes, such as NF or RO, have been employed to further purify the MBR permeate. Many research studies listed in Table 1 have proven that the MBR + NF/RO hybrid processes could achieve high efficiency and stable performance, which allows them as the most promising technologies for wastewater reclamation.

Compared to RO processes, the NF processes are capable to provide a higher permeate flux or higher recovery ratio with lower operating pressure and cost (Sert et al., 2016). The NF membrane may be suitable to remove low salinity wastewater where high ion selectivity is not required, inducing monovalent-divalent ion selectivity (Nativ et al., 2021). The NF membranes can provide very high removal of macromolecules that include those associated with EPS, SMP and NOM. However, the RO membrane can remove all inorganic chemicals and low-molecular weight organic chemicals that would pass through the NF membrane. Thus, the MF/UF-MBR + NF process is considered as an alternative approach for wastewater reclamation that can compromise between permeability and selectivity. As indicated in Table 1, the MF/UF-MBR + NF process has been successfully applied to treat municipal wastewater (Jacob et al., 2010; Chon et al., 2011; Chon et al., 2013; Kappel et al., 2014; Chon et al., 2015; Woo et al., 2016; Arola et al., 2017; Hacıfazlıoğlu et al., 2019; Arola et al., 2021; Yacouba et al., 2021) and various types of industrial wastewater (Wintgens et al., 2002; Dialynas and Diamadopoulos, 2009; Andrade et al., 2014; Wang et al., 2015a; Hosseini et al., 2016; Li et al., 2016; Reis et al., 2017; Sert et al., 2017; Lan et al., 2018; Moser et al., 2018; Cinperi et al., 2019; Li et al., 2020a; Reis et al., 2020). In most of the studies, the treated water quality could meet the standards of reclaimed water quality for different uses, such as for urban uses, for industrial uses, for groundwater recharge, and for farmland irrigation (WHO, 2006).

Towards further improving the treated water quality (e.g., producing ultrapure water as fresh water source), a combination of MBR and RO was adopted, which could exhibit superior organic removals compared to MBR + NF systems. The total nitrogen (TN) in the permeates of MBR + RO and MBR + NF were different, possibly due to different rejection efficiencies of nitrogen species, for example, 71.7% for NF90% and 86.2% for BW30 membrane by both NF and RO membranes (Cinperi et al., 2019; Hacıfazlıoğlu et al., 2019). It was also reported that the MBR + RO rejected TN about 10% higher than MBR + NF (Cinperi et al., 2019). Also, there was only 3% higher in TN removal efficiency by MBR + RO than MBR + NF process operated at same transmembrane pressure (TMP) (Hacıfazlıoğlu et al., 2019). Thus, the conventional biological processes such as anaerobic and/or anoxic reactors are generally implemented with an aerobic MBR for elimination of both organics and nutrients (nitrogen and phosphorus) before the MBR permeate is fed into the RO system (Li et al., 2020b). Alternatively, a biofilm MBR is adopted for simultaneous removals of organic and nitrogen and its permeate is further purified by a RO membrane, which shows several advantages such as low energy consumption, small footprint, and limited sludge production (Wang et al., 2019). Whether the RO membranes would experience more fouling potential than the NF membranes in the MBR + NF/RO systems are still under debate. Some research work highlighted that the RO membrane displayed excellent rejection of both cations and anions, which could be responsible for more inorganic fouling of RO membranes (Hacıfazlıoğlu et al., 2019). While other studies pointed out the low molecular weight organic molecules in the MBR permeate could potentially contribute to internal pore blocking of NF membranes, which led to more serious NF membrane fouling, especially at lower water recovery ratios and for more tight NF membranes (Jacob et al., 2010).

Effluent quality from the MBR is critical in biofouling on NF/RO membranes (Al-Amoudi and Lovitt, 2007; Jiang et al., 2017; Matin et al., 2021). Nevertheless, studies to make direct comparison of fouling behaviors between NF and RO membrane are still very limited. Effect of surface roughness on biofouling was more pronounced than operational conditions for both NF and RO membrane (Alturki et al., 2010). Given its relatively high rejection efficiency, biofouling would be formed more preferentially by RO than NF membrane from MBR permeate (Kimura et al., 2009). Moreover, when RO and NF membranes were exposed to the same biofouling condition with Pseudomonas aeruginosa, final cell concentration on RO membrane was lower than NF membrane in the presence of pharmaceutically active compounds (Yang et al., 2018).

Nevertheless, improving the quality of MBR permeate (i.e., RO feed water) is crucial in alleviating RO membrane fouling. A feasible approach via integrating a bioreactor with an external porous NF membrane (i.e., NF-MBR) has been attempted as pretreatment of wastewater for the RO process (Tay et al., 2018; Tay et al., 2020). Apparently, compared to MF/UF-MBR, the NF-MBR produced the permeate with lower organic/inorganic substances, accordingly, the subsequent RO membrane receiving the NF-MBR permeate displayed better performance. Importantly, under the comparable energy consumption scenario, the NF-MBR-RO system could achieve a water recovery ratio up to 90%, higher than that of the MF/UF-MBR-RO system (recovery ratio of 75%).

In this decade, AnMBRs have received great attention due to their high-quality effluent, limited sludge production, and energy production (i.e., methane) (Wu and Kim, 2020). A combination of anaerobic MBR with RO process have been explored to produce high-grade reclaimed water for both non-potable and potable use (Gu et al., 2019; Li et al., 2020b). As stand-alone AnMBRs are ineffective for removing nutrients (e.g., ammonium and phosphate) and RO membranes have limited ammonium rejection efficiency, integration of an additional polishing process with AnMBR + RO for ammonium removal is suggested. For example, an ion exchange process was supplemented to purify AnMBR + RO permeate (Gu et al., 2019); a zeolite adsorption process was employed to treat an AnMBR permeate water before its feeding to a RO process (Li et al., 2020b).

Generally, porous membranes such as MF or UF, are employed in MBRs to retain active biomass effectively. To obtain high permeate quality for wastewater reclamation, feasibility of NF membrane is also explored by directly combining NF membrane with bioreactor. Since physicochemical separation employed by the NF membrane is dominated by both size exclusion and solution-diffusion mechanisms, various contaminants such as colloidal and organic compounds can be rejected effectively (Choi et al., 2002). However, monovalent ions or nanoscale- organics present in wastewater are difficult to be retained by NF membrane, accordingly, they may not be accumulated easily in bioreactors. This enables the NF-MBR to be operated under relatively lower suction pressures (than RO membranes), while preventing the inhibition against microbial growth and activity (Choi et al., 2002). Additionally, the NF membrane in the MBR process did not provide any adverse impact on biological conversion of NH₄-N (Cao et al., 2022).

However, NF-MBRs suffer low permeability nature, e.g., ∼0.01 to 0.04 LMH of permeate flux was reported in the submerged NF-MBR with commercial NF membrane (Cao et al., 2022). To enhance membrane permeability, novel NF membranes with tailored properties have been adopted in the NF-MBR. For example, the loosely structured hollow-fiber membrane was applied in the NF-MBR, allowing to be operated at ∼2 LMH with more than 95% COD removal efficiency (Cao et al., 2022). In a moving bed biofilm reactor with a side-streamed NF membrane module (using layer-by-layer polyethersulfone NF membrane) under ∼22 h of HRT, about 10 LMH of set-point flux was achieved (Tay et al., 2018). The new NF membrane with high permeability and selectivity should be developed for MBR. With the side-stream membrane configuration, generation of the concentrate stream from the NF membrane that returns into bioreactor is not avoidable, requiring additional treatment unit such as chemical oxidation or electrodialysis to mitigate the concentrate load and protect microbial activity (Kappel et al., 2014). Research works need to be directed into the NF-MBR to maintain desired cell concentration in MBR without wash-out and potential inhibitory effect caused by transportation of monovalent ions or nano-scaled organic compounds on microbial activity.

Despite the advances in NF or RO that promise to increase MBR permeate quality, it consumes high operational energy. In recent years, combining a FO membrane process with a bioreactor, termed as FO-MBR or osmotic MBR has been emerging as a result of recognition of FO to provide excellent permeate (effluent) qualities and reduction in operational cost over traditional high-pressure driven membrane processes (Holloway et al., 2015; Wang et al., 2016; Pathak et al., 2021; Moktadir et al., 2023). The FO filtration uses a semipermeable membrane and is driven by natural osmotic pressure generated by draw solution to produce permeate from feed solutions (Ndiaye et al., 2021). Since the FO filtration relies on the osmotic pressure caused by concentration gradient through membrane materials, an energy demand required for TMP to operate the system is very low. As filtration progresses with time, however, the draw solution becomes diluted, so that both feed and bulk solution in the MBR should be concentrated subsequently. The waste product concentrated by FO membrane is easy to be disposed or reused downstream as useful energy and resource source. Since the wastewater is transported from a feed side toward draw solution, it can be recovered for the production of high quality of permeate given that draw solution regeneration step is integrated, for example, a double-barrier membrane (Holloway et al., 2014; Lutchmiah et al., 2014; Im et al., 2020). Nevertheless, the purity of the permeate depends on the rejection features of FO membranes and the separation effectiveness of permeate water from draw solution strongly. Depending upon the characteristics of the effluent stream and the effectiveness of the entire treatment train, the extracted permeate can also be reused for various purposes such as cooling, rinsing or even irrigation and drinking water. In particular, this unique characteristics in FO membrane process is also beneficial for Zero Liquid Discharge (ZLD) waste disposal system because it leaves much less water to be removed by evaporators, so that energy demand can be further reduced significantly. Here, the product water extracted by a FO membrane can be reused for other purposes such as process water for industrial manufactures (Im et al., 2018). The FO-MBR shows acceptable permeate flux and superior removal efficiency of organic contaminants as well as other emerging contaminants without applying external pressure (Zhu and Li, 2013; Jang et al., 2018; Sivodia and Sinha, 2023). Therefore, the FO-MBR should have a great potential to replace the conventional MBR processes for wastewater reclamation conditioning that sources suitable for draw solution, i.e., seawater can be obtained easily nearby the MBR plants (Arnaldos et al., 2023).

For the FO-MBR process, there are two functions designated by FO membranes such as wastewater concentration and water recovery or extraction (Wu et al., 2021). The wastewater concentrated by the FO process should be particularly helpful in anaerobic MBR systems because it can retain anaerobes completely in anaerobic bioreactor (Chen et al., 2014; Anjum et al., 2021). Additionally, the FO membrane process has been employed for the treatment of AnMBR effluents containing high nutrient loadings such as ammonia nitrogen as they are applied as post-treatment (Kwon et al., 2020). Although biodegradation and/or biotransformation is the main mechanism for trace organic contaminants removal by both aerobic MBR (AeMBR) and AnMBR, the removal of some specific materials can be different since the microbial community differs between the two (Liu et al., 2020). While there is no significant difference in permeate qualities produced by desalination technologies with AnMBR, some portion of methane produced can be dissolved in permeate particularly AnMBR treats domestic sewage. In terms of water extraction, the FO membranes can be applied to separate the water from MBR effluents or directly from bioreactor. Nevertheless, the regeneration step for the recovery of draw solution should be required, and this will be the bottleneck to be overcome for FO-MBR processes (Qiu et al., 2016; Song et al., 2020).

The selection of draw solution acts as an important role in FO-MBR processes because it can determine not only permeate flux but also water quality as well as microbial activities. Especially, the reverse salt transport may increase the amount of salt in bulk suspension of MBRs, which would provide adverse effects on microbial community and bacterial growth (Nawaz et al., 2013; Nawaz et al., 2016). Subsequently, the salt concentration in the MBR increased by the FO membrane varies depending upon the growth rate of biomass, particle size distribution and sludge filterability (Nguyen et al., 2015; Adnan et al., 2019). Therefore, more studies are needed to understand effect of draw solution concentrated by FO membrane in bioreactor performance, such as microbial activities. Also, proper selection of draw solution should be suggested to confirm operational energy.

Like other membrane processes, the membrane fouling observed with FO membrane is a necessary phenomenon because biofouling can be formed particularly on the surface of FO membrane (Qin et al., 2010; Bao et al., 2019). In fact, the FO membrane can be fouled more easily than other types of high pressure-driven membrane due to relatively lower crossflow velocity on membrane surface. Furthermore, the fouling rate on FO membrane can be severe as it is combined with bioreactor because both organic/inorganic loadings increase gradually by FO filtration (Parida and Ng, 2013; Aftab et al., 2017). High organic loading can increase microbial by-products such as EPS while reducing sludge dewaterability (Aftab et al., 2015). A recent study (Olives et al., 2023) on granular sludge based AnMBR proved that fouling occurred less frequently than MF. It was also confirmed that the change in salinity within the MBR caused the draw solution’s reverse salt flux. The use of FO resulted in complete and very high purity in effluent, but the presence of MF was necessary for the removal of salts. A method combining thermal osmotic backwashing and air scouring and osmotic backwashing were evaluated as non-chemical methods to remove biological contamination of FO-MBR (Satterfield et al., 2021). Considering energy efficiency, optimization of backwashing period and frequency was found to be the most economically efficient way to remove FO-MBR fouling.

MD is the thermally driven hybrid membrane process. The MD process could concentrate the non-volatile impurities in the feed solution and transport volatile matters to the permeate solution though the hydrophobic pores of membranes. The schematic diagram of the MD process for wastewater treatment is presented in Figure 1. Due to the relatively lower fouling potential than that of pressurized-driven membrane process, the MD process is combined with a bioreactor has been applied as main or side streams for various wastewater treatment and water reuse (Julian et al., 2022; Kharraz et al., 2022), treatment of industrial wastewaters (petrochemical (Santos et al., 2020), flue gas (Li et al., 2021), desulfurization (Zheng et al., 2022), coking (Guo et al., 2022), textile (Elcik et al., 2021), seawater brine (Kim et al., 2019) and radioactive (Jia et al., 2021)) as well as resource recovery (Julian et al., 2022; Kharraz et al., 2022). In the MD-MBR, the MD process could be used as final barrier in the water treatment process because it allows the transports for the volatile matters such as water. However, feed solution containing volatile impurities such as nitrogen (ammonia) sometimes required a pH control to regulate ammonia transport through membrane (Tun et al., 2016; Lee et al., 2021b).

As mentioned, the main purpose of MBR process is to separate biomass and improve permeate quality by controlling HRT and SRT independently. When the MD process is combined with MBR in municipal wastewater treatment, the deionized water could be produced by municipal wastewater (Tun et al., 2016; Song et al., 2018; Lee et al., 2021a; Simoni et al., 2021; Dow et al., 2022). Like pressure-driven membrane process, the MD membrane experiences severe fouling caused by particulates (Choudhury et al., 2019), organic compounds (Guo et al., 2019; Elcik et al., 2021; Jeong et al., 2021), inorganic compounds (Kim et al., 2019; Kim et al., 2022), and microorganisms (Zheng et al., 2022). Additionally, the fouling rate can be accelerated by the membrane wetting (Choudhury et al., 2019; Kim et al., 2019; Madalosso et al., 2022) during long-term operation of MD membrane. To improve the membrane permeability of MD membrane or antifouling capability, the modified MD membrane materials have been adopted (Günay et al., 2023). The performance of MD membrane could be enhanced by fabricating the membranes with proper membrane pore size and thickness to prevent membrane wetting or heat loss across the membrane (Tibi et al., 2020; Julian et al., 2022; Kharraz et al., 2022). It is reported that higher hydrophobicity and porosity of the MD membrane results in higher rejection with organic and inorganic species (Tibi et al., 2020; Julian et al., 2022; Kharraz et al., 2022). The PVDF, polypropylene (PP), and polytetrafluoroethylene (PTFE) polymers have been used to fabricate a flat-sheet or hollow fiber type MD membranes, which have been used in various MD module configurations such as direct contact membrane distillation (DCMD), air-gap membrane distillation (AGMD), solar driven membrane distillation (SGMD), and vacuum membrane distillation (VMD) (Tibi et al., 2020; Julian et al., 2022; Kharraz et al., 2022). Specifically, the aramid layer was suggested to be coated on the PVDF membrane to enhance chemical resistances (e.g., harsh pH conditions) (Ji et al., 2021). Moreover, the MD membrane can be modified for anti-wetting (Madalosso et al., 2022), self-cleaning efficiency (Yan et al., 2022), and photothermal property (Lee et al., 2022) to achieve its high sustainability for hybrid MD-MBR processes. Nevertheless, the techno-economic analysis should be conducted for hybrid MD-MBRs to accelerate commercialization. Operational energy required for MD processes may be still high due to maintaining required temperature driving force, but if the renewable energy or waste heat is available from anaerobic bioreactor, for example, the energy demand to operate the MD-MBRs can decrease significantly. As reported in a recent study of pilot-scale MD-MBR (treating 10 m³/day wastewater) using single-solar power panel without and with the heat pump, the costs were expected to be 56.5 and 87.3 $/m³ produced water respectively, considering the gain output ratio (GOR) of 0.5–0.8 (Choi et al., 2022). It has been well documented, for the multi-staged MD system, the water production cost was from 6.1 to 7.4 $/m³ with reduction of GOR from 4.6 to 4.4 as the number of stages increased from 8 to 16 (Dudchenko et al., 2021). Both evaporation efficiencies (48%-69%) and the GOR (2.0-3.4) of the multi-stage MD system could be increased with increasing wind speed (8-11 m/s). Moreover, waste heat can be produced by the wind turbine, which is available for energy source to operate MD system (Memon et al., 2022). As a result, it is worthwhile to develop MD-MBR processes to simultaneously capture renewable energy and perform wastewater reclamation.