MEASUREMENT OF AEROBIC WASTE DEGRADATION PROCESSES IN DAILY COVERED WASTE

摘要

Greenhouse gas emissions from landfills contribute to anthropogenic global warming. In landfill emission studies, it is assumed that waste is degraded under strictly anaerobic conditions with some methane oxidation in the landfill cover. Recent publications showed that aerobic activity in active landfill cells might be underestimated. The aim of this paper is to measure the extent of aerobic degradation in a laboratory scale bed of MSW covered with soil, representing conditions in an active landfill cell. Degradation studies were performed in four 0.47m diameter respirometers each packed with 30 kg of shredded unsorted MSW to a depth of approximately 0.9 m and a density of approximately 0.2 kg.L-1. The headspace in each reactor was flushed hourly with measurements of CH4, CO2 production and O2 consumption measured online. Each bed was supplemented with 10 L of tap water to allow slow recirculation of leachate. The MSW bed in one reactor was covered with a 100mm clayey-loam soil layer. The other three reactors contained the MSW waste bed only, with one of these reactors fed a steady flow of methane to the base to simulate uprising methane from lower layers in a landfill ans another periodically flushed with 2-bromoethanesulfonate (BES) to suppress methanogenic activity and therefore methanotrophic activity. CH4 production was only evident in the soil covered reactor, with only trace levels of CH4 in the flushed headspace gases from the uncovered MSW beds. However, vertical profiles indicated that O2 was depleted by mid-depth in all waste beds, suggesting that the soil cover only marginally restricted O2 mass transfer into the waste. In order to investigate the reaction processes further, a mass balance was applied to four gaseous components, CH4, CO2, O2 and 13C-CO2 using the reactor as a control volume and incorporating anaerobic digestion, CH4 oxidation and composting within the beds as source and sink terms for each component. The mass balances indicated that anerobic digestion and composting occurred in all waste beds. Methane oxidation activity thrived in the uncovered beds but was suppressed in the soil covered bed, which was attributed to excessive moisture levels in the soil layer sustained by leachate recirculation. The calculated rates were consistent with COD balances based on CH4 production and O2 consumption. These results indicate that the extent of anaerobic and aerobic activity within a shallow waste bed can be calculated from gas profiles.