Effect of thermal hydrolysis pre-treatment on anaerobic digestion of municipal biowaste: A pilot scale study in China

Co-digestion of wasted sewage sludge, restaurant kitchen waste, and fruit–vegetable waste was carried out in a pilot plant with thermal hydrolysis pre-treatment. Steam was used as heat source for thermal hydrolysis. It was found 38.3% of volatile suspended solids were dissolved after thermal hydrolysis, with digestibility increased by 115%. These results were more significant than those from lab studies using electricity as heat source due to more uniform heating. Anaerobic digesters were then operated under organic loading rates of about 1.5 and 3 kg VS/(m3 d). Little difference was found for digesters with and without thermal pre-treatment in biogas production and volatile solids removal. However, when looking into the digestion process, it was found digestion rate was almost doubled after thermal hydrolysis. Digester was also more stable with thermal hydrolysis pre-treatment. Less volatile fatty acids (VFAs) were accumulated and the VFAs/alkalinity ratio was also lower. Batch experiments showed the lag phase can be eliminated by thermal pre-treatment, implying the advantage could be more significant under a shorter hydraulic retention time. Moreover, it was estimated energy cost for thermal hydrolysis can be partly balanced by decreasing viscosity and improving dewaterability of the digestate. ► 38.3% of VSS was hydrolyzed in thermal hydrolysis, with methane production rate increased 115%. ► Thermal hydrolysis process using steam showed advantages over lab studies using electricity. ► Digester performance was improved as a lag phase of 4–6 h was eliminated by thermal hydrolysis. ► Reaction rate was improved by 134% and 57% under organic loading rates of 1.5 and 3 kg VS/(m3 d). ► Energy for thermal hydrolysis can be partly recovered as viscosity and dewaterability changed.