In this study, waste activated sludge (WAS) was used as feedstock to generate hydrochars at different temperatures (220 degrees C and 260 degrees C) and their effect on sludge anaerobic digestion was evaluated. Experimental results showed that the maximum yield of short-chain fatty acid (SCFA) enhanced by hydrochar (220 degrees C) and hydrochar (260 degrees C) were 507.33 and 270.80 mg chemical oxygen demand (COD)/L respectively, which were much higher than that in blank (141.49 mg COD/L). Mechanism investigation confirmed that hydrochar remarkably accelerated the solubilization and hydrolysis of organic matters, enhanced the acidification of hydrolyzed products, and inhibited the activity of methanogenic bacteria as well as promoted the activities of key enzymes. Meanwhile, the organic matters especially humic substances existed in the hydrochar played an important role during anaerobic digestion. (C) 2017 Elsevier Ltd. All rights reserved.