Abstract
Membrane bioreactors (MBRs) represent the best available technology for municipal and wastewater treatment, as confirmed by the number of wastewater treatment plants based on MBRs in operation worldwide. However, MBRs are not able to effectively reject some substances, like low molecular weight organic compounds found in wastewaters. Therefore, further improvements are needed. In this respect, coupling MBRs with high-retention membrane processes, like reverse osmosis (RO) and membrane distillation (MD), was investigated in the past years. In this paper, the main MBR features and their commercial development are presented. Their performance when coupled with RO and with MD units is discussed, as well as limitations toward a large-scale implementation, with particular emphasis on the further research needed in the field. Finally, MBR-RO and MBR-MD integrated systems are compared in terms of the most significant parameters. Actions to take in order to overcome linked issues are also proposed.