Valorization of the liquid fraction of co-hydrothermal carbonization of mixed biomass by anaerobic digestion: Effect of the substrate to inoculum ratio and hydrochar addition

The effect of the substrate to inoculum ratio (SIR) on the anaerobic digestion (AD) of the liquid fractions (LFs) of co-hydrothermally treated mixed biomass (sewage sludge (SS) and the organic fraction of municipal solid waste (MSW)) was evaluated. The impact of SS + MSW-hydrochar was also studied at different hydrochar concentrations (0, 1, 5, 10, 15, 20 and 25 g_hydrochar/L), in two of the LFs studied. The SIR had a significant impact on methane yield (YCH₄) and organic matter degradation, with low methane production (4–44 NmLCH₄/g tCOD_added) observed for all LFs at SIR = 1:3. Hydrochar significantly improved YCH₄ and specific methane production rate. Compared with the AD without hydrochar, the YCH₄ improved up to 1.95 times at 15 g/L. Hydrochar doses from 1 to 10 g/L shortened the lag phase, while higher concentrations (15 to 25 g/L) showed an increment with respect to the control reactor without hydrochar.