Sustainable Cultivation of Microalgae Using Diluted Anaerobic Digestate for BiofuelsProduction
ABU HAJAR, HUSAM A. Ph.D. August 2016, Civil Engineering Sustainable Cultivation of Microalgae Using Diluted Anaerobic Digestate for Biofuels Production Director of Dissertation: R. Guy Riefler Microalgae cultivation has gained considerable interest recently as a potential source for the production of biofuels. Nevertheless, many obstacles still face the industrial application of microalgal biofuels such as the high production costs due to nutrient requirements and the high energy input for cultivating and harvesting the microalgal biomass. In this study, the utilization of the anaerobic digestate as a nutrient medium for the cultivation of two microalgal species was investigated. The anaerobic digestate was initially characterized and several pretreatment methods such as hydrogen peroxide treatment, filtration using polyester filter bags, and supernatant extraction were applied to the digestate. It was found that the supernatant extraction was the simplest and most effective method in decreasing the turbidity and COD of the diluted anaerobic digestate while maintaining sufficient nutrients (particularly nitrogen) for microalgae cultivation. The microalga Neochloris oleoabundans was cultivated using the diluted anaerobic digestate supernatant on a bench scale. It was found that 100 mg N/L dilution was sufficient to maximize the biomass concentration of this microalga. It was attempted to scale up the cultivation of the microalga N. oleoabundans to 100 L raceway ponds; however, the culture was contaminated with other algal species. 4 The microalga Scenedesmus dimorphus was then cultivated using the anaerobic digestate supernatant on a bench scale. The highest biomass concentration recorded was 654 mg/L. Furthermore, it was found that 50 100 mg N/L dilutions were sufficient to maximize the specific growth rate of this microalga while still producing relatively high biomass concentrations. As a result, the microalgae cultivation was scaled up to 100 L raceway ponds using 100 mg N/L dilution at 454 and 317 mmol/m/s light intensities and 50 mg N/L dilution at 384 and 234 mmol/m/s light intensities. The highest biomass concentration achieved was 432 mg/L in the 100 mg N/L 454 mmol/m/s culture. Nitrogen removal efficiencies were in the 65 72% range with complete ammonia removal. Phosphorus removal efficiencies were in the 63 100% range while COD removal efficiencies as a result of the bacteria presence in the unsterilized nutrient media were in the 78 82% range. The effect of mixing on the growth of the microalga S. dimorphus was evaluated by cultivating this microalga in a raceway pond at 0.1, 0.2, and 0.3 m/s water surface velocities. It was concluded that the biomass concentration and growth rate increased with an increase in the mixing velocity. However, by balancing the power required to operate the pond at different velocities with the potential energy yield from biodiesel synthesis, it was found that operating the pond at 0.1 m/s surface velocity was the only case with a positive net energy. Finally, two biomass growth models were developed and tested on the growth of the microalga S. dimorphus. It was found that the logistic model, which assumes a maximum bearing capacity of the culture, represented the biomass growth better than the 5 exponential growth model, which assumes that the microalgae grow exponentially at specific growth rates equal to or less than the maximum specific growth rate depending on the culture conditions.