Accelerated high-solids anaerobic digestion of sewage sludge using low-temperature thermal pretreatment

Abstract

High-solids anaerobic digestion (HSAD) of sewage sludge is a promising alternative to conventional anaerobic digestion due to its use of smaller digesters and its reduced energy requirement; however, long digestion time is a key shortcoming. This study investigated whether low-temperature thermal pretreatment of sludge could accelerate high-solids digestion. The results showed that after 30 min treatment at 60, 70 and 80 °C, 9.1, 13.0 and 16.6% of sludge solids, respectively, were disintegrated and sludge flow indices increased from 0 to 0.098, 0.195 and 0.249, respectively. As a consequence, thermal pretreatment at 60, 70 and 80 °C increased the quantity of accessible substrates and decreased sludge viscosity such that, compared to the control, biogas production in the batch anaerobic digestion experiments increased by 7.3, 15.6 and 24.4%, respectively, and the apparent kinetic constants increased from 0.19 d−1 to 0.29, 0.39 and 0.39 d−1, respectively. A high concentration of volatile fatty acids and a low pH resulted from thermal pretreatment, but neither of these exerted a significant negative effect on the subsequent HSAD. The optimal pretreatment condition was a temperature of 70 °C for 30 min. In the semi-continuous pilot scale HSAD experiment with optimal pretreatment, biogas yield increased by 11% compared to that with no pretreatment, and the digestion time decreased from 22 to 15 days. The improved digestion following low-temperature thermal pretreatment confirmed the efficacy of this technique to accelerate high-solids anaerobic digestion, as well as increase biogas yield.