Anaerobic co-digestion of the process water from waste activated sludge hydrothermally treated with primary sewage sludge. A new approach for sewage sludge management

Hydrothermal carbonization (HTC) is a suitable technology for managing wastes with a high moisture content, providing a carbon-rich and high energy density material called hydrochar and a process water (PW) with significant organic matter content. The aim of this work was to develop a new approach to sewage sludge management involving anaerobic digestion (AD) of the PW of dewatered waste activated sludge (DWAS) with primary sewage sludge (PSS). The process was optimized by performing semi-continuous experiments with different feed mixture compositions (10% PW/90% PSS and 5% PW/95% PSS, on a COD basis), organic loading rates (OLR; 1.5 and 2.5 g COD L−1 d−1), and temperature regimes (mesophilic and thermophilic). The combination of mesophilic conditions, a 10% PW/90% PSS feed mixture and OLR of 1.5 g COD L−1 d−1 provided concentrations of volatile fatty acids <400 mg COD L−1 in addition to a methane yield (172 ± 11 mL CH4 g−1 CODadded), 1.15 times the value for the control test (100% PSS). Therefore, the energy content of hydrochar from HTC of DWAS followed by AD of the process water with primary sewage sludge enhances the valorization of this renewable residue.