Removing refractory organic substances from landfill leachate using a nanofiltration membrane bioreactor: Performance and mechanisms

Membrane bioreactors (MBRs) are typically used to dispose landfill leachate, but they are challenged by the low removal of refractory organic substances. To address this challenge, in this study, we replaced the commonly used microfiltration (MF) membranes in conventional MBRs with nanofiltration (NF) membranes, i.e., MBRNF. We operated a one-step MBRNF for 127 days and benchmarked against a conventional MBRMF in terms of membrane fouling, pollutants removal, and bulk solution properties. The results showed that the NF membrane could effectively reject refractory organic substances, leading to a longer organic retention time (ORT) and a higher concentration of refractory organic substances in the bioreactor. Consequently, the MBRNF outperformed MBRMF by 20 %-30 % in removing of refractory organic substances, mainly by the enhanced biodegradation of refractory organic substances with low molecular weight. Further investigation suggested that the enhanced biodegradation in the MBRNF could be ascribed to 1) more biomass in the bioreactor; 2) higher concentration of refractory organic substances in the bioreactor; and 3) the enrichment of dominant bacteria such as Chloroflexi. These findings highlight the potential of MBRNF in removing refractory organic substances from landfill leachate and other wastewaters with poor biodegradability.