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Long-term monitoring and treatment of landfill leachate (LFL) and landfill gas (LFG) is required until landfilled municipal solid waste (MSW) is sufficiently stabilized and post-closure care can be terminated. Monitoring data of methane (CH4) emissions in LFG from a marine landfill over 30 years were compared with the IPCC first order decay (FOD) model estimates. The observed changes in CH4 showed a similar attenuation trend to the estimates, but the observed CH4 emissions were only about 30% of the estimate over 30 years; LFL is considered to be another pathway for organic carbon to be released to the environment, but the total organic carbon in discharged LFL was only about 0.2% of CH4 carbon in LFG emission over the same period. The increase in the CO2/CH4 ratio in LFG over time suggests that the discrepancy between estimated and observed emissions is due to methane oxidation in the overlying soil, in addition to the high coefficient values used in the FOD model. Total organic carbon (TOC) in LFL discharged as effluent reached a maximum value in the early stages of the landfill and gradually decreased, but only to about one-third of the maximum value after more than 30 years and a decrease in the amount of effluent. As incineration of MSW is expected to reduce organic carbon and nitrogen, the CH4 reduction effect of incineration of business and household waste and sewage sludge was investigated using FOD model estimates. [ABSTRACT FROM AUTHOR]