Thermal hydrolysis of primary sludge and waste activated sludge mixture: Biogas production and formation of inhibitors

Abstract

In this study, a mixture of primary sludge and waste activated sludge was pre-treated using thermal hydrolysis at 145, 160, 175, and 190 °C, with processing time varying from 5 to 30 min before anaerobic digestion. The aim was to analyse the biogas production and the formation of compounds inhibiting methanogenesis in the sludge under each pre-treatment condition. For the mixed sludge (55% primary sludge and 45% waste-activated sludge) used in this study, thermal hydrolysis at 145 °C for 30 min is the condition that provided the highest biogas yield (389 ± 6 mL biogas/g VSadded). Thermal hydrolysis at 145 °C did not increase the free ammonia concentration in sludge. Also, the treated condition created the highest amount of acetic acid (1.32 × 103 mg/L). By contrast, the level of propionic acid was lower than 0.59 × 103 mg/L, which is the most suitable of the conditions tested for anaerobic digestion. However, the condition did not provide the highest solubilisation degree for sewage sludge.