Life cycle assessment of biosolids processing options

Abstract

Biosolids are the reusable organic materials removed from sewage during treatment. Different options for biosolids handling in Sydney, Australia, are compared using life-cycle assessment. Two key comparisons are made: of system scenarios (scenario one - local dewatering and lime-amendment; scenario two - a centralized drying system) and of technologies (thermal drying versus lime-amendment). The environmental issues addressed are energy consumption, global warming potential (GWP) and human toxicity potential (HTP). Scenario two would consume 24% more energy than scenario one. This is due to the additional electricity for pumping and particularly the petrochemical methane that supplements biogas in the dryer. A centralized system using the same technologies as scenario one has approximately the same impacts. The GWP and HTP of the different scenarios do not differ significantly. The assessment of technology choices showed significant differences. The ample supply of endogenous biogas at North Head Sewage Treatment Plant (STP) for the drying option for allows reductions, relative to the lime amendment option, of 68% in energy consumption, 45% in GWP and 23% in HTP. Technology choices have more significant influence on the environmental profile of biosolids processing than the choice of system configurations. Controlling variables for environmental improvement are the selection of biogas fuel, avoidance of coal-sourced electrical energy, minimization of trucking distances and raising the solids content of biosolids products.