Sewage-derived substances and dead microbial cells cause gel biofilm formation in membrane bioreactors treating real sewage

[Display omitted] •Cake biofilm gradually developed on membrane surface under stable SLR condition.•Gel biofilm is cloud-like structure with dead cell layer and few live cells.•Gel biofilm is formed by accumulation of sewage derived substances.•Stable SLR condition is important for mitigating gel biofilm formation. Gel fouling is frequently reported as one of the biggest obstacles in membrane bioreactors (MBRs) and is observed under unstable conditions. However, the mechanism and the cause of gel fouling are unknown. In this study, gel fouling was induced and compared with normal fouling (cake fouling) in MBR to identify the source of the substances and microorganisms which form the basis of the gel biofilm. Two anoxic/oxic MBRs were operated under changing and stable sludge loading rate (SLR) to induce gel and cake fouling, respectively. As SLR increased from 0.07 to 0.11 kg-COD/kg-MLSS, transmembrane pressure increased rapidly, and a gel biofilm formed on the membrane surface. By contrast, a cake biofilm was developed under stable SLR of 0.07 kg-COD/kg-MLSS with slow transmembrane pressure increase. Excitation-emission matrix-parallel factor analysis indicated that simple aromatic proteins and soluble microbial byproduct-like substances in the filtered supernatant were efficiently removed in both reactors; however, proteins and polysaccharides accumulated in the supernatant with flocs. Scanning electron microscopy analysis revealed that the gel biofilm had a cloud-like structure, with aggregates of filamentous bacteria and microcolonies. The gel biofilm had a thick dead-cell layer and a small number of dispersed live cells, whereas the cake biofilm had a three-dimensional structure of live and dead cells. In the context of the microbial community in gel biofilms, sewage-derived bacteria, such as Campylobacteria and Clostridia, were highly dominant, and the microbial community resembled more sewage than activated sludge. These results indicate that the accumulation of sewage-derived substances and dead microorganisms might have formed gel biofilm in the MBR.