Energy Efficient Urban Wastewater Treatment Using Galdieria Sulphuraria

This dissertation research was undertaken to develop and validate the fundamentals of a photosynthetically oxygenated waste to energy recovery (POWER) system that can potentially render urban wastewater treatment energy-positive and sustainable. Experiments conducted in the first phase of the studies demonstrated that, Galdieria sulphuraria can be cultivated in primary-settled urban wastewater, achieving high nutrient removal efficiencies at removal rates comparable to other strains. In the lab scale reactors, the strain achieved ammoniacal-nitrogen removals greater than 95%; and phosphate removals greater than 96% in 7 days. Biomass yield in these experiments averaged 27.42 g biomass per g nitrogen removed while similar data reported in the literature averaged 25.75 g biomass per g nitrogen. The high biomass yield recorded under laboratory conditions as well as the high areal productivity achieved under outdoor conditions in closed photobioreactors, hold promise for Galdieria sulphuraria as a preferred strain for use in the POWER system. Growth studies conducted in the second phase of the research with the aqueous product of hydrothermal liquefaction of algal biomass confirmed that Galdiria sulphurauia could be grown at rates comparable to that in the baseline artificial medium. This study confirmed another premise of the POWER system that recycling of the aqueous product of hydrothermal liquefaction could increase biomass productivity and net energy yield: biomass productivity recorded with initial N-NH3 level of 80 mg L−1and 20 mg L−1 of phosphate was 0.241 g L−1 d−1 whereas, that with initial N-NH3 level of 40 mg L−1 and 10 mg L−1 of phosphate typical of primary settled wastewater was 0.201 g L−1 d−1. Heterotrophic growth of Galdieria sulphuraria cultivated in the aqueous product of hydrothermal liquefaction conducted over a range of temperatures (180 to 300°C) and dilutions showed that biomass productivity recorded with recycled AP was greater than that with the standard growth medium, by a factor as much as 1.86 and confirmed the potential of carbon/nutrient recycling using the aqueous product (AP) of hydrothermal liquefaction. Estimates of net energy yields over this range of temperatures indicate the optimal HTL temperature for Galdieria sulphuraria to be 300°C. Results of this study confirmed the premises upon which the POWER was developed. Based on the experimental results of the study and the energetic analysis, the proposed POWER system holds promise for energy-efficient algal-based urban wastewater treatment and nutrient recovery.