Effect of hydraulic retention time and sludge recirculation on greenhouse gas emission and related microbial communities in two-stage membrane bioreactor treating solid waste leachate

•Long term (>500 days) of CH4 and N2O emissions from two-stage MBR was investigated.•More than 90% of CH4 emissions were contributed from first stage anaerobic reactor.•N2O emission were at the same level from anaerobic and aerobic reactors.•Effect of HRT and sludge recirculation was more pronounced for CH4 than N2O.•Microbial diversity and abundance were less when hydraulic loading was increased.

Methane (CH4) and nitrous oxide (N2O) emissions and responsible microorganisms during the treatment of municipal solid waste leachate in two-stage membrane bioreactor (MBR) was investigated. The MBR system, consisting of anaerobic and aerobic stages, were operated at hydraulic retention time (HRT) of 5 and 2.5 days in each reactor under the presence and absence of sludge recirculation. Organic and nitrogen removals were more than 80% under all operating conditions during which CH4 emission were found highest under no sludge recirculation condition at HRT of 5 days. An increase in hydraulic loading resulted in a reduction in CH4 emission from anaerobic reactor but an increase from the aerobic reactor. N2O emission rates were found relatively constant from anaerobic and aerobic reactors under different operating conditions. Diversity of CH4 and N2O producing microorganisms were found decreasing when hydraulic loading rate to the reactors was increased.