Anaerobic digestion of sludge not only decreases the mass/volume of sludge, but also increases the production of energy-rich biogas. The effects of a thermal hydrolysis process (THP) on the solubilization of main organics of sludge, as well as the performance of the followed biochemical methane potential (BMP) tests under mesophilic condition (35 °C), were systematically evaluated. Organic compounds such as proteins and carbohydrates which have relatively high yield coefficients are solubilized efficiently during the THP. Good linear relations were obtained between the amounts of soluble chemical oxygen demand and soluble proteins/carbohydrates. The viscosity of sludge dramatically reduced after the THP, particularly at higher temperatures. In contrast to the treatments by low-temperature THP (60–90 °C), the treatments by high-temperature THP (120–160 °C) accelerated the digestion rate and increased the biogas yield in the following BMP tests. Meanwhile, the solid retention time could be reduced from 18–20 days to 12–14 days when high solid samples were pre-treated under THP at 140–160 °C based on the corresponding results of acceleration of methane production after the treatment. As a result, this study elucidated the appropriate operational conditions for high solid sludge (TS > 10%) under THP, and their mechanisms of the acceleration of biogas production.
