Influence of operating pH on biodegradation performance and fouling propensity in membrane bioreactors for landfill leachate treatment

•Effects of pH on DOM transformation and membrane fouling in MBR were systematically analyzed.•Operating pH influenced organic transformation in MBR but total organic removals were insignificantly different.•PCR-DGGE revealed microbial community changes in MBR sludge when pH varied.•Unsettled SS strongly related to membrane fouling rate.•EPS formation caused membrane fouling at lower pH of 5.5 but inorganic precipitates predominated at high pH of 8.5.

This research has investigated the effects of varying mixed liquor pH (i.e. 5.5, 6.5, 7.5, 8.5) on the landfill leachate biodegradability and fouling behavior in two submerged membrane bioreactors (MBR) for a 150-day period. In the experiments, variations in organic removal and dissolved organic matters (DOM) transformation, microbial community and mixed liquor properties as well as the fouling behavior were monitored and systematically analyzed. The findings indicated that the organic removal in both MBRs was insignificantly different for the entire experimental pH range and that the pH levels influenced the DOM transformation. In the analysis of microbial community, PCR-DGGE revealed considerable structural changes, pointing to an effect of pH on the dynamic shift in bacterial species. In addition, clone library analysis showed that Proteobacteria was the dominant phylum in the MBRs and that the dominant bacterial community belonged to the classes Betaproteobacteria and Gammaproteobacteria, both of which were more abundant at pH 8.5 than at pH 5.5. The experiment also showed that the mixed liquor properties were affected by the pH variations. The extracellular polymeric substance (EPS) formation in the mixed liquor at pH 5.5 significantly contributed to membrane fouling while hindering the inorganic and humic substances fouling. Nevertheless, the organic fouling under the low pH condition could be effectively removed by membrane chemical cleaning. It is expected that the experimental results could be applied to enhance the biodegradability and mitigate the fouling in the MBR process.