Simulation of dissolved-oxygen distribution in matrix particles during the aerobic composting process of sewage sludge with two-region model

Abstract

The anaerobic cores of compost particles generate and emit harmful off-gases during composting, and the air-immobile region is a significant factor contributing to an anaerobic environment. However, existing research methods cannot predict the aerobic/anaerobic region distribution of compost particles in air-mobile and air-immobile regions under different flow conditions on the compost pile. Therefore, this study obtained the characteristic parameters of the air-immobile region during composting with different bulking agents using a two-region model (TRM). Furthermore, a fitted equation coupled model was developed to numerically simulate the relationship between the surface oxygen concentration (CO2,S) and aerobic/anaerobic layer thickness (LA/LANA). The results showed that adding peanut shells and wheat straw significantly decreased the proportions of air-immobile regions by 0.02 and 0.07, respectively in the curing phase compared with rice bran treatment (increased by 0.06), although there was no significant change in the early stages of composting. And, the addition of peanut shells and wheat straw enhanced the mass transfer of porous gases between the air-mobile and air-immobile regions of the compost piles during the thermophilic phase (rice bran treatment increased by 0, both peanut shell and wheat straw treatments increased by 0.001); however, there were no observable differences in initial-material, temperature-increasing, and curing phases. The O2 concentrations in the air-mobile and air-immobile regions decreased, and the LANA gradually increased as the composting process progressed and the height of the compost pile increased. The fitted equation coupled model simulates the actual situation well and can support the accurate regulation of harmful gas production in composting systems.