Aerobic processes in landfill: an Australian field trial

Abstract

Modern landfills are often thought of as exclusively anaerobic systems, with landfill gas generation and regulatory emission models typically describing waste degradation as a first order decay process. However, in most instances, observed biogas production from landfills fall short of predictions by these landfill gas models. To improve biogas estimates, a closer examination of the configuration and operation of landfills is required, to identify if a significant portion of waste is degraded aerobically. An improved understanding of aerobic processes will contribute towards more accurate landfill gas estimation and emissions reporting.It was hypothesised that the rate and extent of anaerobic digestion (rAD), CH4 oxidation (rOX), and composting (rCOM) within the soil cover and the fresh waste immediately below the cover could be determined by the combination of stable isotope and molecular mass balances for carbon species (CH4 and CO2). The primary objective of this thesis was to determine the extent of simultaneous in-situ aerobic (CH4 oxidation and composting) and anaerobic processes occurring in an operational landfill cell. An 18-month field trial was established on a fresh layer of waste placed on a sloped face at the edge of a landfill cell and covered with an interim soil cover (30-50cm).The evolution of CH4, CO2 and O2 through the waste profile and the surface emissions were monitored by gas samples collected from gas probes and static flux chambers. Stable isotopes (2δH for CH4, 13δC for CH4 and CO2) were monitored. The developed model consisted of four mass balances for CH4, CO2, δ13C-CH4 and δ13C-CO2 over a control volume that extended approximately 1.5m below the surface, incorporating the soil cover and the uppermost portion of the waste layer. The model was applied to data collected from two locations over four separate sampling campaigns, each representing a climatic season in Brisbane. It was necessary to conduct companion laboratory work to characterise key isotopic parameters in the model, these being (1) CH4 oxidation fractionation factor for the landfill cover soil, (2) anaerobic digestion fractionation factor for the waste, (3) the composting signature of the landfill cover and waste. The sensitivity of predictions of rAD, rCOM and rOX to the values of these parameters as well as the stoichiometry of the three reaction processes was assessed by randomly varying each parameter by ±5% over 500 simulations for each data set. The mass balance model revealed that aerobic activity forms a large proportion of early degradation activity. On average over the 18 month monitoring period, approximately 30% of the organic carbon that was bio-gasified was degraded by the composting and 70% by anaerobic digestion with a variable fraction of the resulting CH4 subsequently oxidised. A range of 1.3 to 44.5 g CO2 m-2 d-1 resulted from composting (rCOM) with higher rates typically observed at the crest of the slope. The average rate of CH4 oxidation ranged from rOX =1.6 to 8.6 g CO2 m-2 d-1. Thereby, CH4 oxidation was therefore the least important of the three reactive processes generating CO2 for this landfill cell. rAD spanned averages from 10.6 to 45.3 g CO2 m-2 d-1, with lower activity tending to occur at the crest of the slope.These predictions by the mass balance model were further supported by long-term monitoring data for O2, where ongoing O2 ingress was evidenced by sampling snapshots and by 2 months of continuous monitoring of O2 concentrations immediately below the soil cover. Daily rises and falls in O2 concentration at the top of the waste layer were evident in the continuous data. Overall, this thesis has highlighted that CH4 oxidation, composting and anaerobic digestion are significant in newly deposited waste lifts. This has a significant impact on anticipating the available CH4 resource and estimating CH4 capture efficiencies and greenhouse emissions.