This experimental study investigates the use of process liquor from hydrothermal carbonization (HTC) of sewage sludge as sole substrate for anaerobic digestion (AD). The process performance at 37 degrees C (mesophilic) and 55 degrees C (thermophilic) was determined based on two identical continuously-fed anaerobic filters (26 L each) operated for 20 weeks. During operation, the organic loading rate was stepwise increased from 1 to 5 g(COD) L-1 d(-1). This decreased the hydraulic retention time from 34 to 5 days. Significant differences in methane production were not observed as both reactors yielded up to 0.18 L-CH4 g(COD)(1). Increased temperature had no effect on the steady-state COD removal efficiency with both reactors stabilized at 68-75%. Macro-and micronutrients were provided by HTC liquor in sufficient concentrations throughout the experiment. Methanogenesis was identified as the speed-limiting step in anaerobic digestion of HTC liquor. Hydrolysis was of an order of magnitude faster than methanogenesis. (C) 2015 Elsevier Ltd. All rights reserved.