LIFE CYCLE ASSESSMENT OF A NOVEL BIOREFINERY FOR SOLID ORGANIC WASTE IN HALIFAX, NOVA SCOTIA

Abstract

In this study, a novel biorefinery process is proposed for municipal solid organic waste. Source Separated Organics (SSO), Mechanically Separated Organic Fractions (MSOF) of garbage, and dewatered sewage sludge are considered the main waste stream in this project. Anaerobic co-digestion and a novel digestate valorization system make up the suggested biorefinery aiming to maximize energy and nutrient recovery from input waste streams. After digestion of feed streams, generated biogas is sent to combined heat and power (CHP) unit to generate electricity as the main product and heat as a by-product while digestate is sent to a phase separation unit. The solid digestate is sent to composting facilities to further stabilize and sold as the compost product. Liquid digestate is sent to the proposed valorization unit consisting of struvite recovery, ammonia stripping, and algae cultivation. The products of valorization systems are struvite, ammonium sulfate, and algal biomass. Chlorella sorokiniana was the algal species selected for nutrient recovery. The protein content of harvested biomass is a suitable substitute for soybean cultivation as an animal feed. A life cycle assessment was conducted to compare the environmental impacts of the proposed biorefinery with those of the conventional Halifax regional municipality (HRM) waste management system. HRM waste management implements composting and alkaline stabilization to treat SSO and sludge streams, respectively. Site-specific information was used to compile the inventory data. Additional information from the literature and Ecoinvent database was retrieved when necessary. Seven different impact categories were used to compare novel biorefinery and HRM scenarios, including fossil resource scarcity, freshwater eutrophication, global warming potential, land use, marine eutrophication, terrestrial acidification, and water use. OpenLCA software was employed to carry out the impact assessment stage of the LCA and perform further analysis. The results of the study showed a dominant contribution in environmental saving associated with electricity generation from the CHP plant in all evaluated impacts except land use. After electricity generation, ammonium sulfate, and struvite products showed higher contributions in reducing the environmental burdens of the waste management system. Produced algae (which substituted soybean cultivation) did not show a significant impact except for the marine eutrophication category.