Available at: https://digitalcommons.calpoly.edu/theses/1389
Date of Award
MS in Civil and Environmental Engineering
Civil and Environmental Engineering
Shallow, mixed raceway ponds can be used to grow microalgae for the dual purposes of wastewater treatment and biofuel feedstock production. To improve the environmental sustainability of microalgae biofuels and to alleviate resource limitations, nutrients remaining after biofuel production should be recycled for additional cultivation. This thesis considers three topics: wastewater treatment by algae, nitrogen and phosphorus assimilation by algae, and algae cell disruption to facilitate nutrient recovery.
The main experimental work was done in pilot raceway ponds growing polycultures of microalgae on clarified municipal wastewater. In addition, two lab-scale pretreatment technologies were tested for their ability to disrupt cells, as indicated by subsequent biomass organic nitrogen and particulate phosphorus degradation during sequential anaerobic and aerobic digestion. The two pretreatment technologies were sonication and high-pressure homogenization.
The raceway pond research was conducted at the City of San Luis Obispo Water Resource Recovery Facility (WRRF). Nine 30-m2, 0.3-m deep raceway ponds were operated continuously from March 1, 2013 to August 31, 2014. The ponds were arranged in three sets of triplicates. One set was operated at a 2-day hydraulic residence time (HRT) on clarified wastewater throughout the study. A second set (“Round 1” of ponds-in-series) was operated at a 3-day HRT, also on clarified wastewater. Its effluent was clarified and then discharged into the third set (“Round 2” of ponds-in-series), which initially operated at a 4-day HRT but then later a 3-day HRT.
The nutrient removal and assimilation data were compared seasonally—summer (March–October) and winter (November–February). The triplicate raceways operating at a 2-day HRT achieved average total ammonia nitrogen (TAN) removal efficiencies of 11% in the winter and 71% in the summer, while dissolved reactive phosphorus (DRP) removal remained similar throughout seasonality. In the first ponds-in-series experiment (3-day HRT followed by 4-day), average summer TAN removal efficiencies for Round 1 and 2 were 88% and nearly 100%, respectively. Round 1 and 2 average summer DRP removal efficiencies were 29% and 67%, respectively. The first ponds-in-series experiment was not conducted in the winter. In the second experiment, the Round 2 HRT was changed to 3 days. Average TAN removal efficiencies for Round 2 in the winter and summer were 88% and 100%, respectively. DRP removal for Round 2 increased from 38% in the winter to 66% in the summer.
Total nitrogen (TN) mass balances on the raceway pond experiments were useful to illustrate the fate of influent nitrogen, including losses. In the first ponds-in-series experiment, 76% of the influent soluble nitrogen was converted to organic nitrogen by assimilation, while 6% of the influent ammonia was lost by volatilization. In the second ponds-in-series experiment, 81% of the influent soluble nitrogen was converted to organic nitrogen by assimilation and only 1% of the influent ammonia was lost by volatilization. The 2-day HRT raceway experiment achieved 41% conversion of influent soluble nitrogen to organic nitrogen by assimilation, with influent ammonia losses of 3% by volatilization.
In addition to these pilot-scale raceway pond experiments, laboratory experiments were conducted on re-solubilization of algae biomass nutrients to support additional algae growth. Algae harvested from the pilot ponds was pre-treated with either sonication or high-pressure homogenization. The pretreated biomass was then subjected to anaerobic digestion and then aerobic digestion to increased nitrogen and phosphorus solubilization. The laboratory anaerobic digestion simulated pilot digestion, also conducted at the pilot facility, and the aerobic digestion was meant to simulate further re-solubilization that would occur when algae digestate was returned to the aerobic raceway ponds to promote further algae growth. Neither pre-treatment technologies had a significant impact on degradation of biomass organic nitrogen and particulate phosphorus compared to controls. It was found that simple anaerobic digestion followed by aerobic digestion resolubilized 90% of organic nitrogen and 50% of particulate phosphorus.