Treatment of concentrated leachate with low greenhouse gas emission in two-stage membrane bioreactor bio-augmented with Alcaligenes faecalis no. 4.

Methane (CH4 ) and nitrous oxide (N2 O) emissions from two-stage membrane bioreactor (MBR) bio-augmented by Alcaligenes faecalis no. 4 during municipal solid waste leachate treatment were investigated. The system was operated at hydraulic retention time (HRT) of 2.5 and 1 days in each reactor under the presence and absence of sludge recirculation. Alcaligenes faecalis no. 4 bio-augmentation helped improving organic carbon and nitrogen removals while reducing CH4 and N2 O emissions. CH4 and N2 O emissions were decreased by 46% and 85% when A. faecalis no. 4 was introduced at HRT of 2.5 days. Under the presence of A. faecalis no. 4, the operation of two-stage MBR with sludge recirculation could reduce CH4 and N2 O emissions by 51% and 54% as compared to its operation without sludge recirculation. An operation under short HRT of 1 day also yielded high organic carbon and nitrogen removals of more than 85% while emitting lower CH4 and N2 O emission of 6.7% C and 0.04% N when operated with sludge recirculation.

IMPLICATIONS: A two-stage membrane bioreactor was effectively applied to the treatment of concentrated leachate (BOD~20,000 mg/L) at a short hydraulic retention time of 2.5 days and 1 day. About 80% of CH4 and N2 O was emitted from the anaerobic and aerobic reactors, respectively. Introduction of Alcaligenes faecalis no. 4 reduced CH4 and N2 O emissions in both reactors as it became the predominant microorganism under an elevated pH condition. Lower CH4 and N2 O emissions were achieved under a sludge recirculation operation, as Alcaligenes faecalis no. 4 could suppress methanogenic activities in the anaerobic reactor and converted a majority of nitrogen into its cell mass, thus reducing N2 O production through a biological nitrification-denitrification pathway.