Assimilatory and dissimilatory sulfate reduction in the bacterial diversity of biofoulant from a full-scale biofilm-membrane bioreactor for textile wastewater treatment

Assimilatory and dissimilatory sulfate reduction (ASR and DSR) are the core bacterial sulfate-reducing pathways involved in wastewater treatment. It has been reported that sulfate-reducing activities could happen within biofoulants of membrane bioreactors during wastewater treatment. Biofoulants are mainly microbial products contributing membrane fouling and subsequent rising energy consumption in driving membrane filtration. Biofoulants from a full-scale biofilm-membrane bioreactor (biofilm-MBR) treating textile wastewater were investigated in this study. During a 10-month operation, sulfate concentrations in the effluent of the biofilm-MBR gradually decreased alongside with the creeping up sulfite concentrations when biofoulants were also building up on membrane modules. Sulfide had no apparent increases in the effluent during this period. Metagenomic analysis revealed diverse microbial communities residing in the biofoulants. Further analysis on their genetic traits revealed abundant ASR's and DSR's functional genes. A plethora of sulfate-reduction bacteria (SRB), including the well-known Desulfovibrio, Desulfainum, Desulfobacca, Desulfobulbus, Desulfococcus, Desulfonema, Desulfosarcina, Desulfobacter, Desulfobacula, Desulfofaba, Desulfotigum, Desulfatibacillum, Desulfatitalea, Desulfobacterium, were detected in the biofoulants. They were believed to play some important carbon and sulfur-cycling roles in our study. Based on metagenomic analysis, we also deduced that ASR was a functionally more important sulfate-reducing route because of the high abundance of assimilatory sulfate reductases detected. Also, the "AMP (adenosine monophosphate)→sulfite" step was a key reaction shared by both ASR and DSR in the biofoulant. This step might be responsible for the sulfite accumulation in the biofilm-MBR effluent. Overall, ASR functional genes in the biofoulants were more abundant. But the bacteria possessing complete DSR pathways caused the sulfide production in the biofilm-MBR.