Modeling temperature effects in anaerobic digestion of domestic wastewater

•UASB and the combined ST-UASB reactor were simulated at low temperatures.•UASB model applied is quite reasonable in predicting the behavior of AD process.•ST-UASB model underestimates COD accumulation and COD removal.•ST-UASB model overestimates biogas production by up to 15%.•ADM1 based ST-UASB simulation could adequately be used for preliminary design. A combination of septic tank (ST) and up-flow anaerobic sludge blanket (UASB) as treatment for domestic wastewater was modeled with the anaerobic digestion model 1 (ADM1). The model was used to visualize the influence of temperature and organic load. The UASB process alone and the combined ST-UASB were simulated with temperature compensation kinetics for low temperature conditions 10, 15 and 20 °C. The combination of ST and UASB reactor allowed high and predictable overall COD removal even at low temperatures and high organic loads. This model underestimates COD accumulation and COD removal, while overestimating biogas production by up to 15%. However, the UASB model applied is quite reasonable in predicting the behavior of such a process in estimating biogas production and COD removal of domestic wastewater pretreated by a ST. The modeling approach presented can become a useful tool to evaluate and design low cost ST-UASB systems for fluctuating climatic environment such as Nepal.