The osmotic microbial fuel cell (OsMFC) is formed by combining forward osmosis (FO) and microbial fuel cells (MFC). Its excellent performance undoubtedly provided a new research direction for the application of FO and MFCs. However, the reverse solute flux (RSF) in FO also follows. RSF will cause a series of negative effects such as loss of draw solutions, decrease of water flux, and electricity generation. Therefore, inhibition of RSF is very necessary for the practical application of OsMFCs. Currently, there are much research and related reviews on RSF inhibition in FO but few studies on OsMFCs. To provide some new ideas for the inhibition of RSF in OsMFCs, we focused on three aspects: operating conditions, draw solutions, and FO membranes. Existing studies have demonstrated that some strategies are effective in OsMFC, including inhibitory effect of current generation, conventional inorganic draw solutions, membrane materials and orientation, and modified membranes. But there are many more strategies available. Using FO as a reference, strategies such as pressure-assisted osmosis and temperature control may be feasible and need to be investigated further. In the future, the inhibition or utilization of RSF can further improve the reactor performance and promote the early practical application of OsMFCs.

Membrane bioreactors (MBR) have become prevalent in wastewater treatment because of their high effluent quality and low sludge generation. Sludge retention time (SRT) is an important parameter in the operation of MBR, and it has a direct effect on the microbial community. In this study, microarrays were used to analyze the microbial communities of three different MBRs at short SRTs. The results showed that MBR at SRT 5 days (CS5) has the highest operational taxonomic units (OTUs) richness, but the lowest diversity and uniformity compared to SRT 3 days at continuous CS3 and the sequencing batch (SS3). Proteobacteria were the dominant phylum of three reactors. Bacteroidetes were the second dominant phylum in MBRs at the continuous model, instead of Actinobacteria at the sequencing model. At the class level, the dominant group of Proteobacteria exhibited a remarkable difference between the three MBRs. γ-Proteobacteria was the dominant group in CS5 and CS3, while α-Proteobacteria was the main group in SS3. The samples from the three MBRs had similar compositions of α-, β- and δ-Proteobacteria. However, γ-Proteobacteria showed different community compositions at the order level between the three MBRs. Enterobacteriales were the dominant group in CS5 and CS3, while Pseudomonadales were the dominant group in SS3. The bacterial community concentration of SRT 5 days was generally higher than that of the other two MBRs. The community composition of CS5 was significantly different from that of CS3 and SS3, and the phylogenetic relationships of the three MBRs were relatively different.

Membrane biofilm reactors (MBfRs) have attracted more and more attention in the field of wastewater treatment due to their advantages of high mass transfer efficiency and low-carbon emissions. There are many factors affecting their nitrogen removal abilities, such as operation time, electron donor types, and operation modes. The operation time is directly related to the growth status of microorganisms, so it is very important to understand the effect of different operation times on microbial composition and community succession. In this study, two parallel H2-based MBfRs were operated, and differences in microbial composition, community succession, and NO3−-N removal efficiency were investigated on the 30th day and the 60th day of operation. The nitrogen removal efficiency of MBfRs with an operation time of 60 days was higher than that of MBfRs with an operation time of 30 days. Proteobacteria was the dominant phylum in both MBfRs; however, the composition of the microbial community was quite different. At the class level, the community composition of Proteobacteria was similar between the two MBfRs. Alphaproteobacteria was the dominant class in MBfR, and Betaproteobacteria and Gammaproteobacteria were also in high proportion. Combined with the analysis of microbial relative abundance and concentration, the similarity of microbial distribution in the MBfRs was very low on day 30 and day 60, and the phylogenetic relationships of the top 50 dominant universal bacteria and Proteobacteria were different. Although the microbial concentration decreased with the extension of the operation time, the microbial abundance and diversity of specific functional microorganisms increased further. Therefore, the operation time had a significant effect on microbial composition and community succession.