Influence of organic loading on the performance and microbial community structure of an anaerobic stage reactor treating pharmaceutical wastewater

► UASR treating pharmaceutical wastewater showed efficient substrate removal at low OLRs by promoting efficient COD reduction. ► Methanosaeta dominated UASR stages at low OLR, while Methanosarcina dominated at high OLR. ► Desulfovibrio cells contributed to the overall degradation of organic substances in the pharmaceutical wastewater.