Microalgae have the potential of producing biomass with a high content of lipids at high productivities using seawater or saline ground water resources. Microalgal lipids are similar to vegetable oils and suitable for processing to liquid fuels. Engineering Icost analysis studies have concluded that, at a favorable site, microalgae cultivation for fuel production could be economically viable. The major uncertainties involve the microalgae themselves: biomass and lipid productivity and culture stability.The mass culture of microalgae for low cost lipids production requires a detailed understanding of both the physiology of lipid biosynthesis by the selected algal strains and of the factors making these strains competitive in outdoor ponds. Mass culture systems will, because of climatic variability, the day-night cycle, and inherent design factors, exhibit fluctuations in key parameters affecting algal growth, productivity and competitiveness: temperature, pH, po?, pC02' light, and nutrient availability. This project has for ~ts overall objective to quantitate the effects of such fluctuations on culture productivity and species competitiveness. A second objective has been the development of basic information on algal physiology as it relates to lipid productivity.Species competition experiments with several strains of algae isolated by the SERI/DOE Aquatic Species Program, some of which were previously used in outdoor mass cultures, were carried out. In these experiments certain parameters proved to be more important then others in determining species succession: fluctuating p02 was more important than fluctuating temperature, which was more important than fluctuating pH. These results must be extended to other species and conditions to determine how far these findings can be generalized. Future work will concentrate on comparing unialgal to mixed culture experiments and on elucidating the specific environmental factors that impact on species productivity and competitiveness.To help guide this work, a computer model was developed capable of predicting the amplitude and frequency of the environmental parameters (p02' pH, PC02, temperature, and light intensity) over the diurnal cycle throughout the year at any location for which climatic data is available. This model must still be validated with data collected from operating ponds.Using one algal strain, "Nanno Q" (Nannochloropsis sp.), as a test organism we also have investigated strategies for maximizing lipid productivity. Our data suggest a two stage process: the first stage is operated at the highest density permitting maximal productivity and the second stage at a higher light input (lower standing biomass or cell density) and under conditions of nitrogen starvation. Such a two stage system is more productive than a continuous nitrogen limited culture process. Future work in this area should utilize algal strains that have previously been selected for their competitiveness under outdoor conditions.
A process providing a beneficial use for waste heat and excess nutrients in the cooling waters of nuclear reactors and fossil-fueled power generating plants has been developed. The process involves the cultivation of selected strains of thermotolerant microalgae in heated discharge waters and the subsequent harvesting of the algal biomass for nutrient removal, recovery of energy and fertilizer, and extraction of high value products. The design of such a process is presented for a large cooling reservoir receiving a discharge of 109 1 -1 d -1 of secondary cooling water containing 100 ig 1 -1 of available P and 400 Itg 1-1 of available N. Based on this nutrient load, with a 1% P content in the algal biomass and a productivity of 10 g m -2 d -1, a 100 ha region would be needed for the process. Hydraulic barriers (submerged plastic curtains) would isolate the 100 ha algal production area "cultivation zone" in the influent end of the reservoir to create a hydraulic and thermal environment conducive to the selective growth of filamentous, thermotolerant, nitrogen-fixing, blue-green algae. The algal culture would be inoculated into the thermal plume and harvested near the distal barrier of the cultivation zone with rotating, backwashed, fine mesh screens ("microstrainers"). A portion of the harvested biomass would be recycled to the inoculation site to maintain a dense culture. This process could mitigate both thermal and nutrient loadings on receiving bodies of water.
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