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 L1 d1. 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.