AbstractBACKGROUNDThe mesophilic anaerobic digestion of the liquid fraction from hydrothermal carbonisation (208°C, 1 h) of dehydrated sewage sludge has been studied. Two initial inoculum concentrations (IC) (10 and 25 g COD L‐1) and four inoculum to substrate ratios (ISR) (2, 1, 0.5 and 0.4 on a COD basis) have been selected to analyse their influence on the evolution of the anaerobic digestion process.RESULTSThe substrate is characterised by a high COD (95.5 g L‐1) and TKN (8.7 g N L‐1) values. High inoculum concentration (25 g COD L‐1) and/or low ISR (≤ 0.5) inhibited methanogenesis due to the high ammonia nitrogen (1.4 g TAN L‐1) and VFA (>4 g COD L‐1) released. For the inhibited samples final COD removals lower than 15% and IA/TA ratios higher than 0.3 were found. The greatest methane yield (177±5 mL CH4 STP g‐1 CODadded) was achieved at 25 g COD L‐1 of IC and at an ISR of 2.CONCLUSIONDuring anaerobic digestion of the liquid fraction from the hydrothermal carbonisation of sewage sludge, the IC and ISR must be adequately selected for proper operation of the process and successful valorisation. According to the results, working at an ISR ≥ 1 is recommended. © 2017 Society of Chemical Industry