Oxygen uptake rate OUR has been
Oxygen uptake rate (OUR) has been widely studied and modeled as a fundamental indicator reflecting the actual O2 demand and the degree of degradation ,  and . There are two main types of OUR simulations (i.e., models based on first-order reaction kinetics and Monod kinetics) . Originally, OUR was assumed to be proportional to the organic 3X FLAG degradation rate by a stoichiometric coefficient. The organic matter degradation rate was described by the first-order approximation  and . The composting models based on this method were relatively simple and could give a good prediction using different correction factors. However, the microbial population and physicochemical properties of composting materials, which are directly related to O2 consumption by microorganisms, required further study. Subsequently, the conventional Monod kinetics approach was introduced to quantify the microbial growth rate (RX). In these models, OUR was determined by RX, which considers the biomass concentration and specific microbial growth rate  and . This method served as a starting point to build a mathematical OUR model. Nevertheless, the application of the Monod equation to simulate RX was only effective in cases in which sufficient substrate was available for microbial metabolism. Inevitably, there were many microsites with insufficient substrate in the composting mixture ,  and .