Engineered Microalgae Cultivations Systems: Conversion of Wastewater Nutrients Into Biofuels and Bioplastics

Dissolved Nitrogen and Phosphorus in wastewater can contribute to harmful algae blooms if released into the environment. One technology that can be used to recover dissolved nutrients from wastewater is the Rotating Algae Biofilm Reactor (RABR), which supports microalgae growth in an easily-harvested biofilm and produces nitrogenand phosphorus-rich biomass that can be used to produce slow-release fertilizers, biofuels, and compostable bioplastics. This thesis (1) examines the effects of several environmental factors on the biomass production rate and nutrient removal efficiency of RABRs treating municipal wastewater, (2) quantifies the composition and biofuel yields of microalgae biomass cultivated using a RABR at the Central Valley Water Reclamation Facility (CVWRF) in Salt Lake City, Utah, and (3) evaluates the cost of producing biofuels and bioplastics from the same biofilm. The effects of four environmental factors (temperature, light intensity, harvesting period, and hydraulic retention time) on biofilm growth and phosphorus removal were evaluated using a statistical design that allows the estimation of changes in the effect of each factor in response to the other factors. This study found that the effect of harvesting period on biofilm growth was influenced by temperature and by light intensity, the effect of light intensity on biofilm growth was influenced by HRT, and the effect of light intensity on phosphorus removal was influenced by temperature. Phosphorus removal could be accounted for primarily by chemical precipitation, with relatively small contributions from direct uptake by the microalgae biofilm. Results from this study were applied to a 11,400-liter pilot RABR operating at CVWRF and used to help determine operating conditions for use in a full-scale model (600,000 gallons of wastewater per day) of the RABR system. Products like compostable bioplastics, biofuels, and slow-release fertilizers are produced from wastewater-grown microalgae biomass and can be sold to offset wastewater treatment costs. To determine which product should be produced from the algae grown at CVWRF, the biochemical composition and biocrude yields of the CVWRF algae were characterized. Based on this characterization, the technical and economic feasibility of three biomass upgrading processes were selected for evaluation. These processes are (1) the production of bioplastics, (2) the production of bioplastics with a lipid-extraction pretreatment, and (3) the production of biocrude via hydrothermal liquefaction. The production of slow-release fertilizer (dried algae with no further treatment) is also described. Of these processes, the bioplastic production process had the most cost-competitive pricing and the highest carbon and energy efficiency.