Carbon Footprint Analysis of Sewage Sludge Thermochemical Conversion Technologies

Thermochemical conversion technology for sewage sludge (SS) management has obvious advantages compared to traditional technologies, such as considerable volume reduction, effective pathogen elimination, and potential fuel production. However, few researchers conducted comparative research on the greenhouse gas (GHG) emission performances of these technologies. This paper evaluates the lifecycle carbon footprints of three SS thermochemical conversion technologies, including hydrothermal liquefaction (HTL) (Case 1), pyrolysis (Case 2), and incineration (Case 3) with software OpenLCA and Ecoinvent database. The results show that Case 1 has the smallest carbon footprint (172.50 kg CO2eq/t SS), which indicates the HTL process has the best GHG emission reduction potential compared to other SS disposal routes. The biggest contributor to the carbon footprint of SS thermochemical conversion technologies is indirect emissions related to energy consumption. So the energy consumption ratio (ECR) of the three cases is calculated to assess the energy consumption performances. From the perspective of energy conversion, Case 1 shows the best performance with an ECR of 0.34. In addition, element balance analysis is carried out to deeply evaluate the carbon reduction performance of the three cases. This study fills the knowledge gap regarding the carbon footprints for SS thermochemical conversion technologies and provides a reference for future technology selection and policymaking against climate change in the SS management sector.