Influence on anaerobic digestion by intermediate thermal hydrolysis of waste activated sludge and co-digested wheat straw

•Intermediate thermal hydrolysis mediates further energy recovery.•Straw for co-digestion boosts methane production.•Thermal hydrolysis increases VFA concentration but lowers pH.•Thermal hydrolysis without flash has optimum at 170 °C and 30 min. This paper analyses time (30 and 60 min) and temperature (120–190 °C) effects of intermediate thermal hydrolysis (ITHP) in a two-step anaerobic digestion of waste activated sludge (WAS) with and without wheat straw as a co-substrate. Effects were analyzed by measuring biochemical methane potential for 60 days and assessing associated kinetic and chemical data. Compared to non-treatment, ITHP increased the secondary step methane yield from 52 to 222 L CH4 kg VS−1 and from 147 to 224 L CH4 kg VS−1 for pre-digested WAS and pre-co-digested WAS respectively at an optimum of 170 °C and 30 min. The hydrolysis coefficients (khyd) increased by up to 127% following treatment. Increasing ITHP time from 30 to 60 min showed ambiguous results regarding methane yields, whilst temperature had a clear and proportional effect on the concentrations of acetic acid. The energy balances were found to be poor and dewatering to increase total solids above the values tested here is necessary for this process to be energetically feasible.