Integration of microalgae cultivation with coal-based flue gas is a widely proposed approach to supply carbon dioxide (CO 2). The utilization of coal derived CO 2 in microalgae growth systems will introduce heavy metals (toxic metals and metalloids), originally present in coal, into the cultivation system. This study evaluates the effect of 10 heavy metals (As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Se and Zn) at four different concentrations on the growth and lipid yield of microalgae, Scenedesmus obliquus. Results show at the baseline heavy metal concentration of 1X, corresponding to the concentrations expected from a coal fired power plant, a 12% higher biomass and 61% higher lipid yield when compared to a control containing no heavy metals. Higher concentrations of heavy metals, twofold (2X), fivefold (5X) and tenfold (10X), are shown to inhibit both growth and lipid production. Analysis shows most heavy metals added to the culture were mainly sorbed to biomass with Hg mostly lost to the environment. Discussion focuses on how flue gas-derived heavy metals could result in both positive and negative outcomes for biomass and biodiesel production and the environmental implications of a commercial microalgae growth system that utilizes coal based flue gas as a CO 2 source.

Processing of microalgal biomass to biofuels and other products requires the removal of the culture from a well-controlled growth system to a containment or preprocessing step at non-ideal growth conditions, such as darkness, minimal gas exchange, and fluctuating temperatures. The conditions and the length of time between harvest and processing will impact microalgal metabolism, resulting in biomass and lipid degradation. This study experimentally investigates the impact of time and temperature on Nannochloropsis salina harvested from outdoor plate photobioreactors.The impact of three temperatures, 4, 40, or 70°C, on biomass and lipid content (as fatty acid methyl esters) of the harvested microalgae was evaluated over a 156 h time period. Results show that for N. salina, time and temperature are key factors that negatively impact biomass and lipid yields. The temperature of 70°C resulted in the highest degradation with the overall biofuel potential reduced by 30% over 156 h. Short time periods, 24 h, and low temperatures are shown to have little effect on the harvested biomass.

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Sustainability assessments have shown that integrating CO2 from power plants with microalgae production systems can reduce costs and greenhouse gas emissions. However, coal fired flue gases contain contaminants (heavy metals) that could have a deleterious effect on products derived from such systems. To address this concern, photobioreactors (PBRs) were designed to test the hypothesis that metals in a microalgae cultivation system at concentrations derived from flue gas will limit end uses of the biomass and spent medium. Scenedesmus obliquus were grown in PBRs spiked with 10 metals found in such flue gases at concentrations representing typical (1X) and high (5X and 10X) metal loading scenarios. Results show that contamination levels can be modulated by managing the harvesting time and by leaching, with EDTA being effective at early growth stage (approximately ≤ day 6) and acidified methanol effective afterwards. Although metals limit biomass and medium uses, some uses fall within regulatory limits (e.g., 1X spent medium is suitable for irrigation, and 1X biomass is suitable for bio-fertilizer and select animal feed uses). Results showed