(175aw) Algae Species on Turf Filter Reactor for Space Explorations
The Space-Based Biochemical Conversion System (BIOSYS) is an integrated system designed for an almost complete material recycling, providing life-support products (water, oxygen and food) from waste streams. At the center of BIOSYS are symbiotic biochemical processes that include anaerobic and aerobic digestion, and algal system. Anaerobic digestion converts waste streams into organic acids (acetic, butyric, lactic acids), and biogas (methane, hydrogen, carbon dioxide). The effluent (with organic acids) is then fed to an aerobic fermentation process to produce microbial lipids. The produced biogas (from anaerobic digestion) and the liquid effluent (from the aerobic fermentation) are then passed through an algal reactor to convert the carbon dioxide to oxygen and for further water clean-up. In addition, the algal reactor is also used for removal of the carbon dioxide in the air in the space station. Literature search revealed that among the many species of algae (~51 total) that have been flown in space, the genus Chlorella, as well as some cyanobacteria, were studied for their survival and growth. Most of these algal species can grow on both suspended and attached growth systems. A screening metrics was established to select a list of algae species to be considered as well as to determine the type of algae reactor for growing algae for further studies. A published literature review by Tsinghua University researchers on studies on attached growth reactors have shown to have higher biomass productivity, easier harvesting, and better water treatment capability than suspended growth reactors. In our project, three algae species have been chosen for studies on attached growth: Spirulina platensis, Botryococcus braunii and Chlorella vulgaris. Spirulina platensis is a filamentous algae capable of growing on surfaces in the presence of organic acids in wastewater. Botryococcus braunii is a high lipid producer that has grown in wastewater containing fatty acids. Chlorella vulgaris and Spirulina platensis can be used as food during space exploration. Based on the various reactor designs reported in the literature, an ideal design for attached algal growth was established for further studies. The design involves a tray or plate modular scrubber, as a biochemical reactor for attached algae growth. The reactor trays would include a medium where the algae grows (e.g. cloth or metal mesh) that can be easily removed for harvesting biomass. Tray reactors could tremendously improve carbon dioxide uptake rate by algae by decreased mass transfer resistance, compared to suspended growth reactors. In addition, a tray reactor will provide large area for attached growth with relatively small footprint. The modular design will allow continuous reactor operation since each module can easily be removed and replaced in case maintenance or harvesting is needed.