Optimizing sludge management: system dynamics modelling for predicting sludge accumulation and solid concentration in a commercial septic tank

Graphical abstract showing system dynamics modelling of a 1,200 L commercial septic tank to predict sludge accumulation and effluent quality. Blackwater inputs are simulated using STELLA and validated with field data. Results indicate 47.4 L/capita/year sludge accumulation, high pollutant removal efficiencies, and optimized desludging intervals based on household size. In Thailand, commercial septic tanks are increasingly used as onsite wastewater treatment systems in new residential buildings due to their ease of installation and effectiveness in collecting and treating blackwater. However, improper emptying of the tanks could reduce treatment efficiency. This study presents a system dynamics model using STELLA software to simulate the performance of a 1,200-L commercial septic tank under baseline influent solids composition derived from field measurement data. Simulations were conducted for households of one, three, five, and eight people. Under the baseline condition, the model predicted a first-year sludge accumulation rate of 47.44 L/capita/year, followed by a gradual decline over time. The removal efficiencies of total solids, total dissolved solids, total suspended solids, and total volatile solids were 59, 36, 86, and 80%, respectively. Sensitivity analysis, by varying the fixed suspended solids (FSS) fraction by ±10%, showed that changes in nonbiodegradable solid content influenced sludge accumulation rates and desludging intervals, with higher FSS fractions leading to increased accumulation and shorter desludging frequencies. This study supports decision-making for desludging intervals. The useful information can be contributed to tank producers, regulators, and service providers for the sustainable management of onsite sanitation systems.