Hydrothermal carbonization of sewage sludge coupled with anaerobic digestion: Integrated approach for sludge management and energy recycling

Abstract

Sewage sludge management is a major operational task that challenges sustainability in municipal wastewater treatment plants. On the other hand, the high level of organic carbon in sewage sludge suggests its utilization potential for renewable energy applications. This study investigates hydrothermal carbonization (HTC) of anaerobic sludge at increasing temperatures (200, 250 and 300 °C) and retention times (30, 60 and 120 min), providing a wide spectrum of reaction severities (logR0 = 4.4–8.0). The goal of the study was to quantify the energy recovery from hydrothermal co-products (i.e., hydrochar and process water) for their use as a solid fuel and a feedstock for anaerobic digestion (AD), respectively. The hydrochar yield was quantified and its quality was evaluated for elemental composition, higher heating value and combustion behavior using thermogravimetric analysis. The hydrothermal process water was analyzed for its chemical oxygen demand and biochemical methane potential. The hydrochar yield was negatively affected by reaction severity with the highest value of 73% at logR0 of 4.4 and the lowest value of 49% at logR0 of 8.0. On the other hand, the hydrochar quality was found to be positively influenced by reaction severity, which enhanced the reduction of oxygen by decarboxylation processes. The biomethane yields were also affected by the reaction severity, with the highest potential value of 227 mL CH4 g COD−1 at the lowest severity and the lowest potential value of 28 mL CH4 g COD−1 at the highest severity. An overall energy balance suggests that mild reaction severities can serve as a suitable platform returning more than 70% of the energy from anaerobic sludge using the HTC-AD process integration.