The generation of CO₂ in sewage sludge treatment systems: life cycle assessment

Life cycle assessment (LCA) on total sewage sludge treatment systems including thickening, anaerobic digestion, dewatering, and incineration/melting was performed to estimate a global warming impact as carbon dioxide (CO₂) emission. As representative processes for the assessment, the most typical and widely used processes were selected. Generally, the life cycle of actual treatment facilities consists of three stages: construction, operation and dismantlement. In this study, the amount of CO₂ produced from the life cycle steps of existing unit sludge treatment processes, was calculated by inventory analysis of input materials. In addition, for all processes investigated, individual CO₂ emission units (CEUs), i.e. the amount of emitted CO₂ for treating a unit weight of sludge, were also calculated. Byusing the obtained CEU matrix, it was possible to simulate the CO₂ production from various types of complex treatment systems as well as to trace the dominant cause of CO₂ production in individual treatment processes.