Aerobic degradation processes in active landfill cells

Abstract

A landfill is built up in cells with each cell filled in layers. Layers are covered with soil, typically 200 to 300mm thick as a temporary measure to maintain sanitary conditions. A thicker cover, typical 500mm, would be placed over a completed cell. Previous researchers have measured O2 concentrations at shallow depth beneath soil covers as well as high concentrations of CO2 relative to CH4 from the surface of soil layers. Despite evidence of the ingress of O2 through soil covers, the extent that the uppermost waste beneath soil covers is degraded aerobically has not been quantified. A major challenge in the quantification of aerobic processes in beds of soil covered waste is that CH4 oxidation, composting and anaerobic digestion could occur simultaneously in different zones close to the surface of the bed. The objectives of this study were: • To develop and verify a mass balance model for a waste bed exposed to atmosphere at the upper surface to determine the rate of CH4 oxidation (rox), composting (rcom) and anaerobic digestion (rAD) based on surface fluxes of gas components and concentrations of the same components at the base of the bed; and • To apply and validate the model on packed beds of fresh waste with a surface exposed to atmosphere in order to determine the contribution of aerobic processes to waste degradation. The mass balance model that was developed is a steady state mass balance of the gaseous components CH4, CO2, O2 and the stable isotope 13C-CO2 over a control volume with the net flux of these components in or out of the control volume as inputs and the three reaction rates, r (= rAD, rOX, rCOM), as fitting parameters to the overdetermined set of 4 mass balance equations. The model was validated by analysing a blend of gases arising from a composting reactor, a CH4 oxidation reactor and a stream of bottled CH4:CO2 (50:50 v/v) gas representing anaerobic digestion. The model estimations showed a coefficient of determination (r2) of 1.00, 0.97 and 0.98 for rAD, rox and rcom when independently determined stoichiometries and an arithmetic sum of the production or consumption of each gas component from each source was used as input to the model. The fit of the model deteriorated to 0.86, 0.77 and 0.74 when using experimentally measured values of the composition and volume of the blended gas as inputs for the net production or consumption of each gas component, illustrating the sensitivity of the model to input fluxes. The mass balance model was then applied to interpret the reaction processes occurring in four packed beds of 35kg of MSW. The nature of degradation in each bed was altered as follows: • Reactor 1 was fed with CH4 at the base of bed. • Reactor 2 was flushed with 2-Bromoethanesulfonate to partially suppress CH4 generation and therefore CH4 oxidation. • Reactor 3 contained a bed covered by a 10-15cm layer of soil to limit O2 intrusion. • Reactor 4 was the control reactor, containing the same amount of waste and operated in the same manner as the other reactors. The consumption rate of O2 and the production rates of CH4 and CO2 were measured on-line while the production of 13C-CO2 was measured by manual sampling and analysing at 10-15 day intervals. The mass balance model was used to estimate r at each 13C-CO2 sampling event over the course of experiment (26 weeks). The result of this experiment showed that anaerobic digestion, CH4 oxidation and composting occurred simultaneously in both the covered and uncovered beds. The emitted biogas in the uncovered beds implied that they were fully aerobic. The results of mass balance model, however, revealed that both rAD and rOX were significant and based on an integration of rAD, 49% of the beds COD was degraded anaerobically. The generated CH4 was subsequently oxidized leading to a minimal CH4 emissions from the uncovered beds. Similarly, approximately 35% of the total emitted C from the soil covered bed was CH4, superficially implying this was the only bed supporting anaerobic activity. However, the mass balance model showed rAD was only marginally enhanced by the presence of soil layer with 68% of the solids (COD basis) degraded anaerobically. The mass balance demonstrated that the high level of CH4 emission from the soil covered bed relative to the uncovered beds was due to the suppression of rOX, which was an order of magnitude less in Reactor 3 compared to the uncovered reactors. The accuracy of the mass balance model was validated by comparing the extent of COD removal according to the model with COD assays of the fresh waste and digestate of each reactor. Furthermore, the occurrence of rAD, rOX and rCOM where consistent with vertical gas concentration and microbial community profiles.