Influence of digestion temperature and organic loading rate on the continuous anaerobic treatment of process liquor from hydrothermal carbonization of sewage sludge

•Process liquor from HTC of sewage sludge was used in anaerobic mono-digestion.•Different digestion temperatures (37 and 55 °C) had no distinct effect.•Step-wise decreasing HRTs from 34 to 5 days were achieved without performance loss.•COD removal efficiency stabilized at 68–75% even at only 5 days HRT.•Sewage sludge HTC liquor provides sufficient amounts of macro- and micronutrients.

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 °C (mesophilic) and 55 °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 gCOD 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 LCH4 gCOD−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.