Methane and nitrous oxide emissions from a two-stage membrane bioreactor applied in municipal landfill leachate treatment

The emissions of two major greenhouse gases, i.e., CH4 and N2O, during the treatment of partially stabilized municipal solid waste leachate by a practical-scale two-stage membrane bioreactor (MBR) were investigated. The system consisted of anoxic and aerobic reactors that had a total hydraulic retention time (HRT) of 4 days and were operated with internal sludge recirculation at 100% of the influent flow rate. At steady operation, high organic and nitrogen removals of 99% for BOD, 97% for COD, 99% for TOC and 94% for TKN were achieved. During operation, high direct CH4 emissions from the anoxic reactor, accounting for 90% of the total emissions from the system and 16.2% of the total organic carbon mass removed, were observed. N2O was emitted to a much lower extent, and N2O emissions varied much less between the reactors. The presence of methane- and nitrous oxide-producing bacteria was confirmed by PCR-DGGE and real-time PCR analyses. The operation of the two-stage MBR with sludge recirculation enabled the co-existence of CH4- and N2O-producing microorganisms in the sludge from the anoxic and aerobic reactors, with an approximately 30% difference in their microbial communities.