Integrating spatial heterogeneity into sustainability assessments of biofuels from wastewater algae

Abstract

Microalgae are a promising biofuel feedstock whose potential to replace petroleum fuels has improved with the application of hydrothermal liquefaction (HTL) and the use of municipal wastewater. It was hypothesized that the sustainability of biofuels from wastewater algae is sensitive to geographic factors. The availability of land and wastewater near WWTPs in urban and rural areas, the life cycle greenhouse gas emissions of producing algal biofuel through HTL sited at a WWTP and at a refinery, and the life cycle climate change impacts of changing potentially available land near WWTPs to algal ponds were investigated. GIS and life cycle assessment (LCA) methods were developed, and literature data and primary data collected from algae cultivation in wastewater effluent and conversion to biocrude were used in these analyses. The results of the geographic analysis showed trade-offs in water and land availability for algae cultivation at rural and urban WWTPs. The LCA of algal bio-jet fuel using geographic input data showed that siting HTL at a WWTP so that only biocrude is transported from the plant instead of minimally dewatered algae resulted in lower life cycle greenhouse gas emissions. To further incorporate spatial heterogeneity into assessing the life cycle climate change impacts of algal biofuels, methods were developed to integrate direct land use change (LUC) impacts, transportation distances, and an algal growth model into Python models for a baseline LCA, sensitivity analysis, and Monte Carlo analysis of algal renewable gasoline. This methodology was applied to the Level II ecoregions of the continental United States using data collected from GIS datasets. The results show that LUC impacts can have similar magnitudes as foreground process impacts, and that these impacts can differ significantly even between two ecoregions that have nearly identical algal productivities. The inclusion of LUC impacts increases the uncertainty range in Monte Carlo results. Additionally, the higher foreground process impacts arising from the lower annual algal productivity of colder ecoregions were offset by the cooling albedo change impacts of the empty ponds during the months when production is halted. The geographically sensitive analyses that comprise this dissertation show the necessity for incorporating spatial heterogeneity in sustainability assessments of biofuels from wastewater algae.