This combined lab-scale and full-scale study investigated how the anaerobic digestion process of sewage sludge was affected in the lower mesophilic temperature range (32°C). The aim of the study was to achieve a more positive heat balance by reducing the operational temperature in the full-scale mesophilic digesters in a Swedish WWTP with a preserved methane production. The effect on methane yield and degradation rate were examined by a BMP-experiment with three different temperatures (32, 34.5, 37.5°C). The BMPexperiment showed no significant difference between 37.5°C and 34.5°C in methane yield or degradation rate. At 32°C the production of biogas was decreased by 11% compared with 37.5°C. Consequently, the temperature in the full-scale digesters was gradually adjusted from 37.5°C to 35°C during two months. The heat balance for the reactors was calculated and revealed that it is possible to save up to 13% in heat consumption per year with an operational temperature of 35°C compared to 37.5°C. No change in the production of biogas could be observed during and after the temperature adjustment.
Keywords renewable energy, biogas, heat reduction, mesophilic digestion, Wastewater Treatment Plant