Energy yield optimization of co-hydrothermal carbonization of sewage sludge and pinewood sawdust coupled with anaerobic digestion of the wastewater byproduct

Abstract

High energy recovery efficiency is expected to be obtained from biomass waste by coupling hydrothermal carbonization (HTC) with anaerobic digestion (AD). This study focused on optimizing the total energy yield (TEY) from sewage sludge (SS) and pinewood sawdust (PS) in a HTC-AD coupling process, which has attracted little attention. Response surface methodology (RSM) was used to gain the optimal operation parameters including PS proportion, liquid–solid mass ratio (L/S) of raw feedstock (SS-PS mixture slurry), HTC temperature, and residence time and discuss how the parameters influenced the TEY. The results showed that the PS proportion, HTC temperature, and residence time had significant effects on the TEY (p < 0.05), while the influence of the L/S of raw feedstock on TEY was limited. The hydrochar mass yield (HCY), higher heating value (HHV) of hydrochar, and the cumulative methane production from the AD of HTC wastewater (HTCWW) increased with PS proportion increasing. By contrast, when HTC temperature increased, HHV increased while both HCY and cumulative methane production decreased. Prolonging the HTC residence time decreased the TEY of hydrochar and methane, because the HCY decreased considerably although the carbonization degree of the fed organics increased, and the AD of HTCWW was inhibited due to the increasingly generated toxic substances in a long-term HTC process. The optimal TEY, with a value of 89.61%, was obtained at the following conditions: PS proportion of 75%, original L/S of feedstock of 7:1, HTC temperature of 190 °C, and residence time of 3 h. Among which, the energy yields of hydrochar and methane were 86.46% and 3.15%, respectively. HTCWW showed good AD performance with a cumulative methane yield of 255.64 mL-CH4/g-COD in a 16-day AD process.