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Optimal Harvesting Strategy For Haematococcus Pluvialis Using A Stella-Based Model

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Title: Optimal Harvesting Strategy For Haematococcus Pluvialis Using A Stella-Based Model
Authors: Tsang, Shuki
Issue Date: Dec 2004
Abstract: One method for reducing the atmospheric concentration of carbon dioxide, C02, a greenhouse gas that plays a role in global warming, is to capture it from stationary combustion systems that burn fossil fuels and sequester it underground, in the deep ocean or in biological sinks such as photosynthetic microalgal species. The cost of capture and sequestration of CO2 is not insignificant and this poses a serious hurdle to the implementation of this greenhouse gas emissions mitigation strategy. Toward this end, research and development is being conducted to develop low-cost, high efficiency CO2 gas separation systems. Another alternative that has been explored is utilizing captured C02 in commercial processes to generate an offsetting income stream, while displacing C02 that would otherwise need to be generated (by oxidation of fossil carbon) for these processes. Under adverse environmental conditions, the photosynthetic microalgae Haematococcus pluvialis produces a high-value compound called astaxanthin, a carotenoid pigment that provides health benefits to humans and that is also used in mariculture feed to enhance the color of salmon flesh. Mera Pharmaceuticals, Inc. has an industrial-scale astaxanthin production facility on the island of Hawai'i. In their production process, Haematococcus pluvialis is first grown in a photobioreactor called a Mera Growth Module (MGM). A portion of the culture is then transferred to open ponds where astaxanthin accumulates in the cells under imposed environmental stress. The harvesting strategy for the transfer of cells from the MGM to the pond has not been optimized. The goal of this study was to explore utilization of captured C02 for Haematococcus pluvialis cultivation and to assess the benefits of this in terms ofCO2 displacement (utilization) and biomass production (i.e., revenue generation). The specific objective of the research described in this thesis was to develop a process model that could be applied to optimize the harvesting strategy from the perspective of maximizing carbon uptake. The model employed STELLA, a commercially available simulation software package. Results of the present study suggest that the carbon capture efficiency of the photobioreactor system is modest. About 25% of the carbon sparged into the media is eventually assimilated into the cell biomass. Most of the remaining 75% is lost via degassing and venting. To maximize carbon capture efficiency, a closed system in which vented gas is recirculated back into the photobioreactor should be explored. Simulations were conducted to identify harvesting scenarios that would maximize cell biomass yield (and, hence, carbon capture). It was determined that reducing harvesting quantities while increasing target cell concentrations in the photobioreactor could provide significant increases in cumulative yield (about 10%) compared to the harvesting strategy currently applied by Mera Pharmaceuticals, Inc. Unfortunately, this would require daily harvests and additional ponds. A target cell concentration of 5.5 x 105 cells/mL and a harvesting cell quantity of 3.2 x 1012 cells produced a realistic (i. e., achievable with the current labor force and ponds) maximum cumulative yield of 3.12 x 1013 cells, which is 2.57 x 1011 cells greater than the cumulative yield of the current harvesting strategy. This is an insignificant gain; hence, the current harvesting strategy appears to be very close to the best scenario under the real constraints imposed by labor force and available ponds. Although the number of harvesting scenarios tested was limited, the results suggest that the process model can be a valuable tool in optimizing microalgae production operations from the perspective of profit or carbon capture.
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Appears in Collections:M.S. - Bioengineering

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