A simulation model of ammonia volatilization in composting

A model to simulate substrate decomposition and ammonia volatilization during the high-rate stage of composting was developed. The model linked the dynamics of microbial populations to carbon and nitrogen transformation during the composting process. The model described microbial growth, metabolism, and heat production on substrates using Monod kinetics. Substrates are partitioned into three groups based on chemical components and different decomposition rates. Ammonia volatilization was related with interstitial carbon dioxide concentration, and calculated based on the analysis of a CO2-NH3-H2O multi-solute aqueous system. The pH value was the consequence of chemical species released during decomposition and affected the vapor-liquid composition of ammonia, which further influenced ammonia loss from the vapor phase. The model simulated evolution of carbon dioxide and ammonia, water loss and temperature according to user-specified feedstock characteristics. Simulation of ammonia volatilization from a small-scale composter was in general agreement with experimental data. The model demonstrated that ammonia volatilization was influenced inversely by C/N ratio and positively by aeration rate. Simulated effects of different factors corresponded satisfactorily with measured effects reported in the literature. Sensitivity analysis revealed that ammonia emission was sensitive to maximum microbial growth rate on non-fiber substrate, microbial death rate, the heat transfer coefficient, and the extent of ammonia gas-liquid phase equilibrium.