Modeling of aerobic biodegradation of feces using sawdust as a matrix

Composting in the bio-toilet system is a continuous thermophilic-aerobic biodegradation process. Unlike to the traditional composting systems, biodegradation rates of organic matter are very important because feces are daily added into the composting reactor of the bio-toilet and an accelerated decomposition is aimed. The models developed for conventional composting processes include simple formulations of biodegradation kinetics and deal mainly with energy and water balances. Therefore, formulation of kinetics that can reasonably describe the biodegradation process in the bio-toilet system is required for better modeling predictions. In this work, a bio-kinetic model was introduced to describe the aerobic biodegradation of feces in the bio-toilet system. This model includes three processes for carbonaceous material degradation and is prepared by using the activated sludge modeling techniques and formulations. Stoichiometric parameters were adopted from literature on activated sludge processes. Kinetic parameters were estimated by conducting batch tests for several organic loadings and by using respirometry, curve-fitting techniques, and sensitivity analysis. Feasibility and applicability of these parameters were assessed by conducting feces intermittent-feeding tests and by simulating the experimental respiration rates. Model, stoichiometric and kinetic parameters proved to be affordable for describing the biodegradation of feces in the bio-toilet system.