Effect of inoculum source and initial concentration on the anaerobic digestion of the liquid fraction from hydrothermal carbonisation of sewage sludge

Hydrothermal carbonisation (HTC) is a relatively new alternative for the management of sewage sludge that allows obtaining a HTC char (hydrochar) with a high heating value (approximate to 22 MJ/kg). The aim of this work has been to study the anaerobic digestion of the liquid fraction generated as by-product during HTC (LFHTC) of dewatered sewage sludge, to get more value to the overall process. For this purpose, three different inocula: granular biomass from industrial reactors treating brewery and sugar beet wastewaters and a flocculent biomass from a full-scale digester of municipal sewage sludge, at two initial inoculum concentrations (IC) (10 and 25 g COD/L) were tested. ANOVA test was applied to evaluate the ultimate methane yield for each IC. The effect was different for each inoculum studied: an increase from 10 to 25 g COD/L increased the methane yield by 23% for brewery waste, achieving the highest value obtained (177 +/- 5 mL STP CH4/g CODadded), while declining to 99 +/- 2 mL STP CH4/g CODadded for sugar beet; it is not affected by the municipal sludge, yielding around 135 mL STP CH4/g CODadded. Therefore, among the inocula tested, brewery waste was the most appropriate for the anaerobic digestion of the LFHTC of dewatered sewage sludge at high IC. (C) 2018 Elsevier Ltd. All rights reserved.