Biofuel potential, nitrogen utilization, and growth rates of two green algae isolated from a wastewater treatment facility
Eustance, Everett O'Brien
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Nitrogen removal from wastewater by algae provides the additional benefit of producing lipids for biofuel and biomass for anaerobic digestion. As ammonium is the renewable form of nitrogen produced during anaerobic digestion and one of the main nitrogen sources associated with wastewater, experiments focused on the optimization of growth and lipid production when grown on ammonium were evaluated. Scenedesmus sp. 131 and Kirchneriella sp. 92 were grown in a 14:10 light/dark cycle on ammonium, nitrate or urea in the presence of 5% CO 2 and ammonium and nitrate in the presence of air. Growth on nitrate and urea showed similar growth rates, and provided knowledge on the target growth rate for optimizing growth on ammonium. Results showed the pH decreased during exponential growth on ammonium in both 5% CO 2 and air, causing chlorophyll degradation. Growth on nitrate and air increased the pH of the medium and produced an increase in Nile Red fluorescence and biofuel potential for strain 131, but not for strain 92. Biological buffers were implemented to counteract the change in pH to prevent growth inhibition. Cultures were grown on 5% CO 2 or air, which showed that increased levels of CO 2 are required for increased growth, biofuel potential, and ammonium utilization. This increased the growth rates from 0.26 d -¹ to 1.04 d -¹ for strain 131 and 0.45 d -¹ to 1.31 d -¹ for strain 92. pH-controllers using 0.1 M KOH were used in experiments with 5% CO 2 with the understanding that buffers are limited to lab scale experiments and pH control would bridge the gap to industrial processes. The growth rate while utilizing pH-controllers showed similar growth rates to buffered experiments. Growth on nitrate, urea, and buffered ammonium with 5% CO 2 showed an increase in the biofuel potential for strain 92 in comparison to growth with air. Strain 131 had a decrease in biofuel potential when grown on ammonium compared to growth on nitrate or urea. Both strains showed increased levels of CO 2 is required to increase biofuel productivity.