Diverse and distinct bacterial communities induced biofilm fouling in membrane bioreactors operated under different conditions.

We conducted a laboratory-scale experiment using real municipal wastewater with identical submerged membrane bioreactors (MBR) operated under different conditions (sludge retention time (SRT) and membrane flux) for nearly 6 months. Membrane biofilm samples were periodically retrieved, and cultivation-independent molecular approaches were used to systematically elucidate the community composition and diversity of microorganisms responsible for biofilm formation in the MBRs. Membrane fouling occurred earlier and faster in the low-SRT reactors which had more active mixed liquor biomasses and higher concentrations of dissolved organic matters. Denaturing gradient gel electrophoresis (DGGE) analysis and comparative rRNA sequencing revealed that diverse and distinct bacterial communities significantly differing from those in the planktonic biomass developed on the microfiltration membrane surfaces, with phylotypes from the Proteobacteria (particularly the alpha and beta subdivisions) and Bacteroidetes dominating the 16S rRNA gene libraries. This indicated that specific groups of bacteria were preferentially growing in the membrane habitats. Membrane flux had great impact on the predominant populations selected in the fouling biofilms. At lower fluxes, biofilm community composition was quite similar independent of sludge ages and biofilm formation seemed to be the result of a more natural process of colonization and biofilm development. In contrast, distinct biofilm communities developed on membrane surfaces at high fluxes. Despite the high convective forces, the biofilm composition was significantly different from the planktonic biomass, and selective enrichments of certain species were observed. Our study suggests that the microbial communities responsible for membrane biofouling in MBRs are far more complex and variable than expected and thus could be challenging to control.