Microbial Community Structure of an Anaerobic Side-stream Coupled Anoxic-aerobic Membrane Bioreactor (AOMBR-ASSR) for an In-situ Sludge Reduction Process

The in-situ sludge reduction process via the insertion of an anaerobic side-stream reactor into the sludge return circuit is an efficient approach to reduce the sludge yield in the activated sludge process. In this study, with the anoxic-aerobic membrane bioreactor (AO-MBR,CP) as a reference, high-throughput sequencing technology was used to reveal the characteristics of the microbial community structure in the anaerobic side-stream anoxic-aerobic membrane bioreactor sludge reduction process (AOMBR-ASSR,SRP). After the stable operation of two processes for 120 days, the average removal efficiencies of TN and TP in the effluent of SRP were increased by 5.63% and 29.85%, respectively. However, there was no significant difference between the two processes in the removal effect of COD and ammonia nitrogen. It is worth noting that the observed sludge yields (Yobs) of the two processes were 0.14 and 0.17 gMLSS/(gCOD), respectively, and the sludge reduction rate of the AOMBR-ASSR reduction process was 19.5%. Compared to the CP, the microbial richness and diversity index of SRP increased significantly. Among 14 major phyla, Proteobacteria and Bacteroidetes were the dominant microorganisms. Chloroflexi , which is responsible for the degradation of organic substances under an anaerobic condition, seemed to be reduced in the SRP. Meanwhile, other phyla that involved in the nitrogen cycle, such as Nitrospirae and Planctomycetes , were found to be more abundant in the SRP than in the CP. A total of 21 identified classes were observed, and primarily hydrolyzed fermented bacteria ( Sphingobacteriia, Betaproteobacteria, Actinobacteria , and Deltaproteobacteria ) and slow-growing microorganisms ( Bacilli ) were accumulated in the SRP. At the genus level, the inserted anaerobic side-stream reactor favored the hydrolyzed bacteria ( Saprospiraceae, Rhodobacter and Candidatus_Competibacter ), fermented bacteria ( Lactococcus and Trichococcus ), and slow-growing microorganisms ( Dechloromonas and Haliangium ), which play a crucial role in the sludge reduction. Furthermore, the enrichment of bacterial species related to nitrogen ( Nitrospir and Azospira ) provided the potential for nitrogen removal, while the anaerobic environment of the side-stream reactor promoted the enrichment of phosphorus-accumulating organisms.