Microbial community structure of anoxic–oxic-settling-anaerobic sludge reduction process revealed by 454-pyrosequencing

•An A+OSA sludge reduction system was investigated by 454-pyrosequencing.•A+OSA system achieved good performances of pollutants removal and sludge reduction.•The A+OSA system showed higher relative abundance of microbial communities.•Microbial community in A+OSA system was provided with higher evenness and stability.•The A+OSA process had a significant effect on microbial community structure.