(757f) Algae Cultivation in a Space Station-Based Biorefinery

In a biorefinery for isolated environments such as a space station, human activity-derived waste can be treated using biological systems to produce recovered potable water, usable returned cabin air, and edible protein cake. This biorefinery concept in confined spaces is similar to a municipal wastewater treatment where there are processes to treat solids by anaerobic digestion, to reduce chemical oxygen demand in the water through aerobic biotreatment, to reduce nitrogen and phosphorus by algae cultivation and to remove the metals, chlorophyll in algae and other organic substances by adsorption. In this study, algae cultivation systems are designed to produce algae biomass for protein, carbohydrates, and lipids, to reduce nitrogen and phosphorus in the water, and to convert carbon dioxide from the space station cabin air to oxygen. Visual observations and gravimetric measurements of the cell concentration is preferred over spectrophotometry because the algae cells aggregate. Contamination of other microorganisms and maintaining a viable algae culture were challenges in algae cultivation studies. Experiments to measure the biomass yield for cultivation of the algae were conducted. The Chlorella vulgaris UTEX 2714 strain was grown in suspended growth photobioreactors using three media for 18 days of cultivation. Experimental results showed that the media containing sulfur has the higher percent utilized phosphate and nitrogen for algae growth, which is explained by higher algae biomass concentrations than the biomass yield in another media without sulfur. Another experiment was performed to formulate a maintenance media for acclimation of the algae for 18 days to prevent any nutrient deficiency prior to feeding the algae with effluent from the aerobic biotreatment or anaerobic digestion processes. Nutrients are also fed intermittently to prevent any substrate inhibition. A review of studies reported in the literature on the effect of sodium chloride, sodium bicarbonate, ammonium salt or nitrate salts on the growth of Chlorella vulgaris suggested a range of concentrations as nutrient requirements for growth but inhibitory effects are specific to the algae strain, which were experimentally determined under the cultivation conditions for wastewater treatment. A factorial design of experiments approach on two levels of 3 nutrient sources (carbon, nitrogen and phosphorus) were performed without contamination. A regression model from statistical analysis from the experimental data serves as an effects model on the levels of carbon, nitrogen, and phosphate with the interaction parameters, which are valuable for future algae cell viability studies. Hence, a maintenance media is formulated for effluent from aerobic biotreatment and can replace the commercially available maintenance media. The known levels of carbon, nitrogen and phosphorus for maintaining a viable algae culture would be the basis for intermittent feeding rates of the actual wastewater effluent for algae cultivation. Moreover, an analysis of the carbohydrates, lipid, and protein content of the harvested biomass show the relationship of the substrates in the media to the biochemical pathways for production of metabolites in the algae. These metabolites provide nourishment for astronauts in a space station. Furthermore, a viable algae culture maintained in a photobioreactor for wastewater treatment in a space station would serve as a platform for studies on measuring conversion of carbon dioxide to oxygen, thereby producing recovered usable oxygen-enriched cabin air. As far as a literature review on carbon dioxide sequestration and photosynthesis efficiency of algae has revealed that studies on oxygen production are limited. Experiments to show oxygen production are also performed to know the feasibility of enrichment of the cabin air with oxygen. Ultimately, the goal of this project is to have data on material and energy balances in a closed loop system where the human-derived wastes are utilized by biorefinery systems to replenish food, water and oxygen for astronauts in a space station.