Biofuel production by two green microalgae utilizing wastewater and recycled nutrients for growth
Halverson, Luke Daniel
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Microalgae have received considerable attention in recent years as a viable feedstock for biofuel production. In order for algal biofuel to displace a significant amount of traditional fossil-based fuel, non-potable water must be used to avoid additional stress on dwindling freshwater supplies. Additionally, adequate nutrient sources must be available, and productive, robust strains need to be utilized. The work performed towards this thesis aims to determine the ability of two green microalgae and one diatom to grow and produce biofuel and biofuel precursors in untreated wastewater. Scenedesmus sp. strain WC-1 (WC-1), Chlorella sp. strain SLA-04 (SLA-04) and Navicula sp. strain RGd-1 (RGd-1) were initially screened for growth in untreated primary clarifier effluent and diluted anaerobic digestate. WC-1 and SLA-04 were able to grow in each condition, but RGd-1 was unable to sustain growth. After WC-1 and SLA-04 displayed successful growth, experiments were performed attempting to increase the lipid content in WC-1 and SLA-04 by varying the form and concentration of dissolved inorganic carbon present in the cultures. The addition of supplementary inorganic carbon did not increase cellular triacylglyceride (TAG) content as expected, but WC-1 and SLA-04 were able to achieve considerable fatty acid methyl ester (FAME) content. The final experiments conducted toward this thesis involved the use of recycled harvest water amended with anaerobic digestate for nutrients. WC-1 grew without inhibition during the first two generations of growth in recycled harvest water, but high ammonium concentrations due to an error during the addition of anaerobic digestate in the third generation caused reduced growth rates. SLA-04 was able to grow without inhibition during each of the three generations of growth in recycled harvest water. The results of this work may strengthen the outlook of microalgae's potential as a biofuel feedstock. Identifying robust algae that can utilize low-cost nutrients while requiring minimal supplies of freshwater is a large step towards the commercialization of algal biofuels.