An anoxic/oxic membrane bioreactor (AO-MBR) and three anaerobic side-stream reactor (ASSR) coupled MBRs (ASSR-MBR) were operated to investigate effects of side-stream ratio (SR) on sludge reduction and microbial community structure of ASSR-MBRs. The ASSR-MBR achieved efficient COD and ammonium nitrogen removal. SR increased from 0.2 to 1.0 favored nitrogen removal, and increased sludge reduction from 6.0% to 49.7%. The total released COD in the ASSR increased with the rising SR and was inversely proportional to sludge yield of ASSR-MBR. Pyrosequencing analysis showed that phyla Chloroflexi and Armatimonadetes surviving in anaerobic conditions were enriched in the ASSR, while Nitrospirae was dominant in the MBR. Comparison at the genus level revealed that higher SR favored the growth of slow growers, while lower SR enriched hydrolytic and predatory bacteria. The results suggested that SR has a profound effect on nitrogen removal, sludge reduction and microbial community structure in the ASSR-MBR. (C) 2017 Elsevier Ltd. All rights reserved.