Hydrothermal carbonization of sewage sludge : potential for utilization and environmental impact reduction in Sri Lanka

Abstract

Hydrothermal carbonization (HTC) is a technique on thermochemical conversion of biomass in wet state including sewage sludge. It results a carbonaceous solid product called hydrochar, large quantity of process water and a slight amount of gas. It operates in a closed HTC reactor at comparatively low temperatures and under autogenously saturated pressures with presence of water in subcritical state. In finding technical feasibility of HTC process as a novel technology to treat sewage sludge in Sri Lanka, total dewatered, sewage sludge generated from centralized wastewater treatment plants (WWTPs) was estimated. It was approximately 51 000 tonnes/year ensuring its technical feasibility. Moisture, ash, organic carbon and total phosphorous percentages in sewage sludge samples in the study were in line with scientific literature, in which sewage sludge was tested under HTC process. In characterization of hydrochar by using data from literature, majority of elemental carbon and phosphorous in feedstock have been retained in hydrochar in compared with process water. The main limitation of HTC process at present is that scientific data is available mostly from experiments under laboratory conditions and not in mass scale applications. HTC was identified as an energy saving dehydration technique combined with mechanical dewatering to increase TS% of sewage sludge while reducing its volume. Potential applications of HTC end products were mainly focused for hydrochar within Sri Lanka to be used as a soil amendment, recovery of nutrients and energy recovery as a solid fuel. In assessing potential environmental impacts, global warming potential (GWP) is reduced approximately by 33 and 1430 t CO2 eq./year by HTC, in compared with composting and landfilling respectively which are current sewage sludge management methods in Sri Lanka.