Analysis of the mixing performance of a full-scale membrane bioreactor for municipal wastewater treatment

•For the entire plant, 8.02% of the vessel volume were wasted as dead zones.•The entire plant could be considered as a cascade of 2.15 CSTRs.•The membrane tanks were away from completed mixed condition.•The mixing energy utilization distribution was evaluated in different aspects.•The membrane separation unit accounted for the energy utilization difference. High energy consumption remains to be a key problem for application of membrane bioreactor (MBR). Optimization of MBR to save energy requires a compressive understanding of the performance of the reactor, among which the mixing performance is a significant parameter, however received little attention. In this study, a tracer experiment was carried out in a full-scale MBR for municipal wastewater treatment in China. The mixing performance of both the entire plant and the membrane tanks were evaluated. The entire plant was found to be a cascade of 2.15 continuous stirred tank reactors (CSTRs) with 8.02% of dead zones. The membrane tanks were also found to deviate from CSTR. The mixing energy was analyzed and compared with literature data from three aspects: the specific power used per unit of tank volume (Ps,v), per unit of permeate volume (Ps,p), and per unit of membrane area (Ps,m).