In this study, a SCA reactor was employed for municipal wastewater treatment at a mesophilic temperature (30 degrees C) under different hydraulic retention times (HRT) and upflow velocities (V-up) to investigate granule sludge stability and spatial microbial distribution. The stable COD removal efficiency readied at HRT of 15, 12, 9 and 6 h, and V-up ranging from 0.6 to 5.9 m h(-1). EPS fraction analysis of granule sludge shows that municipal wastewater was mainly attributed to the enrichment influence of polysaccharide and tightly bound-EPS. SEM images exhibited that the stability and floating of anaerobic granular sludge may be promoted in the primary three-phase separator area because the channels of the granules was clogged by EPS. The SMA and high-throughput sequencing analysis indicated acetoclastic methanogens and hydrogenotrophic methanogens played an important role in formation and maintenance of the anaerobic granule sludge in low and high organic load rate operation conditions.