Membrane bioreactors fed with different COD/N ratio wastewater: impacts on microbial community, microbial products, and membrane fouling

It is known that an increase of COD/N ratio can result in an enhanced removal of nutrients in membrane bioreactors (MBRs); however, impacts of doing so on membrane filtration performance remain unclear. In this work, comparison of membrane filtration performance, microbial community, and microbial products under low temperature was carried out in anoxic/oxic (A/O) MBRs with COD/N ratios of 9.9 and 5.5 g COD/g N in influent. There was no doubt that an improvement of nitrogen removal under high COD/N ratio was observed; however, severer membrane fouling was found compared to the MBR fed with low COD/N ratio wastewater. The increase of COD/N ratio resulted in an elevated production of humic acids in soluble microbial product (SMP) and carbohydrates, proteins, and humic acids in loosely bound extracellular polymeric substance (LB-EPS). Quartz crystal microbalance with dissipation monitoring (QCM-D) analysis showed that the adsorption capability of SMP and LB-EPS was higher in the MBR with higher COD/N ratio. Four hundred fifty four high-throughput pyrosequencing revealed that the higher COD/N ratio led to the enrichment of Bacteroidetes at phylum level and Azospira, Thauera, Zoogloea, etc. at genus level. Bacteroidetes are considered to potentially release EPS, and Azospira, Thauera, and Zoogloea, etc. have denitrification activity. The change in microbial communities is consistent with MBR performance.