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.