Engineering design of passively aerated urine-diverting dry toilet chambers based on thermophilic composting of human feces

A diagram illustrating the composting process in a dry toilet chamber. Inputs of human faeces and sawdust undergo mixing and passive aeration, triggering a feedback loop of microbial activity, heat generation, temperature, and moisture content, resulting in heat loss and water vapor. The final compost output is characterized by a graph showing E. coli decay over time, driven by temperatures above 50 °C. The entire process integrates mathematical modelling through mass balance and kinetic equations to optimize chamber volume design. Thermophilic composting of source-separated human feces offers a strategy to reduce storage time and enhance hygienic safety in urine-diverting dry toilet (UDDT) systems. Bench-scale insulated batch reactors were operated for 30 days with three feces-to-sawdust (F/S) ratios (60, 40, and 30%, total solids [TS] basis) and monitored for temperature, moisture, volatile solids (VS), and thermotolerant coliforms (Escherichia coli). Sustained thermophilic conditions (>50 °C) were achieved at F/S = 30%, resulting in >60% VS reduction within approximately 7 days. E. coli concentrations declined below detection by day 14, and a modeled 5-log reduction was achieved within approximately 26 days under sustained thermophilic exposure. A coupled mathematical model describing organic matter degradation, moisture evolution, temperature-dependent kinetics, and pathogen inactivation was calibrated against experimental data from the optimized configuration. Model performance was strong for moisture prediction (R2 = 0.76) and moderate for VS reduction (R2 = 0.57), reflecting substrate heterogeneity. Sensitivity analysis demonstrated that hygienization time is highly dependent on the effective inactivation constant, supporting the incorporation of a safety factor in field applications. The framework enables performance-based engineering design of mixed, passively aerated UDDT composting chambers, providing a process-based alternative to empirical storage guidelines when sustained thermophilic conditions can be verified.