A mass balance model to estimate the rate of composting, methane oxidation and anaerobic digestion in soil covers and shallow waste layers

Although CH4 oxidation in landfill soil covers is widely studied, the extent of composting and CH4 oxidation in underlying waste layers has been speculated but not measured. The objective of this study was to develop and validate a mass balance model to estimate the simultaneous rates of anaerobic digestion (rAD), CH4 oxidation (rOX) and composting (rCOM) in environments where O2 penetration is variable and zones of aerobic and anaerobic activity are intermingled. The modelled domain could include, as an example, a soil cover and the underlying shallow waste to a nominated depth. The proposed model was demonstrated on a blend of biogas from three separate known sources of gas representing the three reaction processes: (i) a bottle of laboratory grade 50:50% CH4:CO2 gas representing anaerobic digestion biogas; (ii) an aerated 250 mL bottle containing food waste that represented composting activity; and (iii) an aerated 250 mL bottle containing non-degradable graphite granules inoculated with methanotrophs and incubated with CH4 and O2 to represent methanotrophic activity. CO2, CH4, O2 and the stable isotope 13C-CO2 were chosen as the components for the mass balance model. The three reaction rates, r (=rAD, rOX, rCOM) were calculated as fitting parameters to the overdetermined set of 4 mass balance equations with the net flux of these components from the bottles q (=qCH4, qCO2, qO2 and qCO2×δ13C-CO2) as inputs to the model. The coefficient of determination (r2) for observed versus modelled values of r were 1.00, 0.97, 0.98 when the stoichiometry of each reaction was based on gas yields measured in the individual bottles and q was calculated by summing yields from the three bottles. r2 deteriorated to 0.95, 0.96, 0.87 when using an average stoichiometry from 11 incubations of each of the composting and methane oxidation processes. The significant deterioration in the estimation of rCOM showed that this output is highly sensitive to the evaluated stoichiometry coefficients for the reactions. r2 deteriorated further to 0.86, 0.77, 0.74 when using the average stoichiometry and experimental measurement of the composition and volume of the blended biogas to determine q. This was primarily attributed to average errors of 8%, 7%, 11% and 14% in the measurement of qCH4, qCO2, qO2 and qCO2×δ13C-CO2 relative to the measurement of the same quantities from the individual bottles.