In order to maintain the reliable operation in membrane bioreactor (MBR), a concept of moderate solids retention time (SRT) was introduced and particular attention was paid to the identification of moderate SRT in this paper. It was difficult to choose a singly constant SRT as the moderate SRT due to the different type of MBRs. However, the performance in MBR such as sludge concentration, extracellular polymeric substances (EPS) concentration, soluble microbial products (SMP) concentration and removal efficiency of nitrogen, presented different variations and therefore could be chosen as indicators to provide information about and predict arrival of the moderate SRT. Results indicated that the moderate SRT could be defined as a range of SRT, which balanced the advantages and the drawbacks of SRT for reliable operation in MBR. Moreover, this paper suggested that lots of information should be collected and used to build a valid database about the moderate SRT for reliable operation in MBRs. (C) 2011 Elsevier B.V. All rights reserved.