Membrane biofouling in pilot-scale membrane bioreactors (MBRs) treating municipal wastewater: impact of biofilm formation.

For more efficient control and prediction of membrane biofouling in membrane bioreactors (MBRs), a fundamental understanding of mechanisms of membrane biofouling is essential. In this study, we operated full-scale submerged MBRs using real municipal wastewater delivered from the primary sedimentation basin of a municipal wastewater treatment facility over 3 months, and the adhesion and formation of biofilms on 0.4-microm pore size polyethylene hollow-fiber microfiltration (MF) membrane surfaces, separated from simple deposition of sludge cake, were monitored using scanning electron microscopy (SEM). In addition, the compositions of planktonic and biofilm microbial communities in the MBR were analyzed using culture independent molecular-based methods (i.e., fluorescent in situ hybridization (FISH) and 16S rRNA gene sequence analysis). The SEM and LIVE/DEAD staining analyses clearly showed that the biofilms gradually developed on the membrane surfaces with time, which had a strong positive correlation with the increase in trans-membrane pressure (TMP). This indicated that the biofilm formation induced the membrane fouling. The FISH results revealed that the microbial communities on membrane surfaces were quite different from those in the planktonic biomass in the mixed liquor. Moreover, FISH and 16S rRNA gene sequence analyses revealed that a specific phylogenetic group of bacteria, the Betaproteobacteria, probably played a major role in development of the mature biofilms, which led to the severe irreversible membrane biofouling.