Thermal balance analysis of activated-sludge process for pig slurry treatment

•A detailed heat balance model for activated-sludge reactors (ASRs) was established.•The model had 12 heat transfers via biological or physicochemical processes.•Accuracy of reproducing ASRs temperature by the model was ±0.60 °C.•Simulated temperature change caused by heat gain or loss was 4.5 or 4.3 °C d−1.•The heat budget of ASRs can be accurately simulated by the model.

The potential for operating an activated-sludge process (ASP) at higher temperatures with the aim of improving the hygienic qualities of the treated effluent was studied in terms of thermal balance. A thermal balance model for activated-sludge reactors (ASRs) was developed to simulate thermal changes in treating pig slurry, especially with respect to heat exchanges and temperature variations within the reactor. The model was validated by using data from a treatment plant and weather station. The developed model matched in situ temperature data with a root mean square error of 0.60 °C. It indicated the heat gained from solar radiation (308.435 MJ d−1) and biological reaction (217.773 MJ d−1) was significant, as was the heat dissipated from the slurry surface (273.847 MJ d−1) and slurry flow (214.522 MJ d−1). The model accurately reproduced the thermal balance of ASR and these results showed that the total heat gain resulted in a temperature increase of 3.87–5.15 °C d−1 in the reactor, but this effect was offset by heat loss. These results indicated the need for effective thermal insulation to be installed if an ASP was to be operated at higher temperatures.