Impact of sludge retention time on the fine composition of the microbial community and extracellular polymeric substances in a membrane bioreactor. Effect of sludge retension time on membrane bioreactor

Membrane bioreactors (MBRs) are an advanced technology for wastewater treatment whose wide application has been hindered by the fouling of the membranes. MBRs can be operated with long sludge retention time (SRT), a crucial parameter impacting microbial selection in the reactor. This also affects filtration performance, since the major fouling agent are the extracellular polymeric substances (EPS), specific of a microbial community. In this study the impact of the SRT on the ecophysiology of the MBRs and, consequently, on membrane fouling was evaluated. A MBR was operated under a SRT of 60d followed by a SRT of 20d. A comprehensive description of the microbial population structure and EPS proteins and polysaccharides profiles, of the mixed liquor and cake layer were monitored throughout time. The imposition of a shorter SRT led to a shift in the dominant bacterial populations. The mixed liquor and cake layer communities were very different, with Actinomycetales order standing out in the cake layer. Higher protein and polysaccharide concentrations, particularly the former, were found at 20d SRT. Furthermore, EPS profiles were clearly affected by the SRT: it was possible to correlate a group of soluble EPS proteins with the SRT of 60d; and a lower sludge age led to a lower diversity of polysaccharide monomers, with an increase of glucose and galactose in the cake layer, as well as the succinyl group. This study improves our knowledge regarding the molecular reasons for fouling, which may contribute to improve MBR design and operation.