A decade ago, membrane bioreactors (MBR) were considered as a potential technology for replacing conventional activated sludge (CAS) processes. Indeed, MBR experienced a period of rapid development around 2009, especially in China. In recent years, however, there has been a sharp drop in the number of MBR installations all around the world - with the exception of in China. It is important to understand the reasons for this difference between the reactions of China and other countries. High-quality effluent and a smaller MBR footprint are also associated with high energy consumption and operational costs. Therefore, it is necessary to carefully evaluate the advantages and disadvantages of MBR. In this study, a model (including capital and operational costs, operational efficiency and stability, resources saving and recovery and an evaluation of the sustainability index (S_I) of MBR over CAS) was established to comprehensively evaluate the effects of MBR on economy, technology and management. The model calculations of S_I demonstrate S_I > 1.0. This means, briefly put, that MBR is not a sustainable process when compared to CAS, which is extensively applied but generally thought to be an unsustainable process. The sensitivity analysis of both related factors and weighted coefficients in the model reveals that there is almost no possibility of attaining S_I < 1.0 (more sustainable than CAS). For this reason, it is not recommended that MBRs extensively replace CAS when upgrading and constructing wastewater treatment plants (WWTPs). Alternative technologies may be considered, such as aerobic granular sludge (GAS).