Date of Award

2012

Document Type

Honors Thesis (Open Access)

Department

Colby College. Environmental Studies Program

Advisor(s)

F. Russel Cole

Second Advisor

Peter Countway

Third Advisor

Loren McClenachan

Abstract

The global reliance on fossil fuels is an unsustainable practice that has led to the depletion of finite resources and the accumulation of greenhouse gases in the atmosphere, leading to global climate change (Demirbas 2010). Transportation accounts for about a third of carbon dioxide (CO2) emissions in the United States. Demand for oil continues to increase and resources are becoming more uncertain. Increased production of renewable fuels such as biofuels can help ease the dependence on fossil fuels. Although interest and production in biofuels has been increasing in the past decade, current feedstocks do not meet efficiency needs and some may even lead to environmental damage and increased greenhouse gas emissions (National Research Council 2010). Microalgae are arguably the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels due to their high productivity and ability to store large amounts of lipids (Demirbas 2010). Algal biofuels have strong advantages over first-generation feedstock such as corn, palm oil, and soybeans. Algae culturing systems can be located on land that is unsuitable for agriculture, can be coupled with flue gas CO2 mitigation, wastewater treatment, and the high-value byproducts from algae biofuel production ease production costs (Li et al. 2008). The potential for algae as biodiesel is great, as algae are highly productive, and the oil content can exceed 80 percent by weight compared to 5 percent for the best agricultural oil crops; however, microalga-based systems still have poor volumetric efficiencies, which make them costly compared with petroleum fuels (Amaro et al. 2011). Research and innovation into algae biofuels, specifically biodiesel, could help the industry grow and become a major provider of liquid fuel. Most of the algae known to produce large amounts of lipids (more than 20 percent of their biomass) are of the Divisions Cryptophyta, Chorophyta, and Chromophyta (Darzins et al. 2010). Dunaliella tertiolecta is a green alga species that has been studied for biofuel production. Dunaliella tertiolecta is relatively easy to cultivate and has a high growth rate and lipid content (Tang et al. 2011). In this study, a photobioreactor was designed and constructed for the cultivation of microalgae in a laboratory, with attention to feasible scale-up possibilities for industrial productions. Dunaliella tertiolecta was cultivated in "! the bioreactor and experiments were performed to study the algal growth rate, lipid production, and potential harvesting techniques. The first objective of this experiment was to determine the effects of carbon dioxide on the growth rate of Dunaliella tertiolecta. The CO2-enhanced (~1-2% CO2) cultures had a growth rate of 2 doublings per day compared to the ambient air (~0.04% CO2) cultures with a growth rate of 1.4 doublings per day, indicating that algae farm integration with CO2-emitting industrial plants could both increase biofuel production as well as mitigate CO2 and harmful pollutant emissions. Lipids are the cellular components that are extracted from microalgal cells for the conversion to biodiesel and new, inexpensive methods to increase lipid content are needed to make microalgae a viable feedstock. Both a salt shock (5%) and decane treatment (1%) showed an increase in lipid production during the stationary phase of growth compared to untreated cultures. Finally, experiments were performed to compare the effectiveness of autoflocculation and Chitosan flocculation with samples of Dunaliella tertiolecta. Chitosan is a non-toxic flocculant made from grinding and processing the exoskeletons of crustaceans to acquire the polysaccharide chitin (Lavoie and de la Noue 1983). Chitosan flocculation at a concentration of 2 g/L was the most effective means of separating the algal cells from the medium, compared with autoflocculation and chitosan at a concentration of 1 g/L. Mixing helped dissolve the powder and effectively bind to the algal cells and over 99% of the cells coagulated on the bottom of the container after three days, making them more available to be harvested. The growth, lipid induction, and flocculation experiments have implications for algae biofuel productions and scalability to commercial-size operations. Optimizing biomass production simultaneously with increasing lipid content will result in the highest net biodiesel yields (Packer et al. 2011). Finding effective, affordable, and environmentally- friendly harvesting methods is also an area of important algae biofuel research (Chen et al. 2011). Using innovation and integrating production systems with other industries can help the algal biofuel industry become a viable option for renewable liquid fuel production.

Keywords

microalgae, biofuel, climate change

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