Hydrothermal carbonization of sewage digestate at wastewater treatment works: Influence of solid loading on characteristics of hydrochar, process water and plant energetics

Nowadays the sludge treatment is recognized as a priority challenge to the wastewater industry due to the increasing volumes produced and tighter environmental controls for its safe disposal. The most cost-effective process for sewage sludge is the anaerobic digestion but raw digestate still contains high levels of organic matter that can be transformed into an energy carrier by using processes like Hydrothermal Carbonization (HTC). In this work, the influence of solid loading (2.5, 5.0, 10.0, 15.0, 17.5, 20.0, 25.0 and 30.0% solids w/w) on the composition of hydrochar and process water was studied, together with an evaluation of product yields, solubilisation of organic carbon and biomethane potential of process waters from HTC processing (250 °C, 30- minute reaction time). Hydrochar yields ranged from 64 to 88%wt, whereas the concentration of soluble organic carbon increased from 2.6 g/L in the raw digestate to a maximum of 72.3 g/L in the process water following HTC at the highest solid loading. Furthermore, process modelling with Aspen Plus shows that the integration of AD with HTC to wastewater treatment works provides a significant positive energy balance when process water and hydrochar are considered as fuel sources for cogeneration.