Environmental Impacts of Biodiesel Produced from Wastewater Greases

Abstract

This thesis evaluates the technical, economic, and environmental impacts of producing biofuels from greases that accumulate in wastewater systems. The research in this thesis is accomplished through performing four tasks: (1) identification of the statistical variability in wastewater grease composition and its subsequent impact on biodiesel production capacity, (2) exploration of processing methods and their performance in meeting biodiesel fuel specifications, (3) evaluation of the environmental performance of biodiesel produced from wastewater grease feedstock, and (4) analysis of economic and environmental feasibility of producing biodiesel from wastewater greases. The two wastewater greases investigated in this thesis are grease trap waste (GTW), which is collected at restaurants, and sewage scum grease (SSG), which is collected at wastewater resource recovery facilities (WRRFs). Because wastewater greases are heterogeneous, degraded, and contain large amounts of water, solids, and impurities, GTW and SSG require different chemistry and additional processing steps for biodiesel production compared to conventional biodiesel feedstocks. The composition variability and a variety of parameters including wastewater quality are assessed during a year-long longitudinal study of GTW and SSG. GTW is primarily composed of water and has low lipid content (4%); however, ambient settling of GTW produces a floating grease layer that concentrates the lipids (34%). The average lipid content SSG (21%) is comparable to the float grease in GTW; however, SSG lipid content exhibits seasonal variability that is not observed in GTW. SSG has higher lipid content in cooler months (15-40%) and lower lipid content in warmer months (3-21%). Both GTW and SSG lipids have similar free fatty acid content (75%) affects the reaction pathways used for conversion into biodiesel. Technical feasibility of biodiesel production is assessed using a variety of reactors and distillation techniques. A major hurdle to producing biodiesel is reducing sulfur content to meet fuel specifications; approximately 56% of wastewater grease biofuel samples in this project contain between 15-30 ppm sulfur, and only 23% are below the required fuel specification of 15 ppm sulfur. Sulfur contents are shown to decrease throughout biodiesel production with an overall sulfur reduction of 75-96%. This thesis presents life cycle assessment (LCA) and techno-economic analysis to determine the environmental impacts and economics of biodiesel produced from wastewater greases. A process model is used to incorporate experimental biodiesel processing results and to create an inventory of the materials and energy required for biodiesel production. Monte Carlo simulation is used to perform a sensitivity analysis utilizing the longitudinal study data for variability of composition and biodiesel plant capacities. LCA is used to compare the greenhouse gas emissions (GHG) of biodiesel production to current raw grease disposal (business as usual) and a variety of solid waste disposal…