Modeling of substrate degradation and oxygen consumption in waste composting processes

A multi-component modeling system was developed to simulate substrate degradation and oxygen consumption in waste composting processes. Levels of soluble substrate (Ss), insoluble substrate (Si), active biomass (X), inert material, moisture, temperature, and oxygen concentration were considered as state variables. The relationships among these variables were also incorporated within the modeling framework. Three conversion reactions, including growth of aerobic biomass, decay of aerobic biomass, and solubilisation of insoluble substrate, were considered in the simulation system. The modeling inputs included temperature, moisture, oxygen concentration, and initial conditions of the state variables, while the outputs included oxygen uptake accumulation (OUA), oxygen uptake rate (OUR), Ss, Si, and X for representing the substrate degradation and oxygen consumption status. The effectiveness of the developed model was demonstrated through its application to a case study in a 30 L vessel over 200 h. Through verification-based composting experiments, it was shown that the modeling solutions were consistent with the experimental results with an acceptable accuracy level. Sensitivity analyses of the model showed that an increased maximum microbial growth rate would result in raised OUA, OUR, Ss, and X levels; a decreased biomass decay rate constant would help enhance the composting process. Moreover, variations in the maximum growth rate would affect the composting process more significantly than those of the biomass decay rate constant.