Date of Award:

2013

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biological and Irrigation Engineering

Advisor/Chair:

Ronald C. Sims

Abstract

In the quest for a feedstock for the production of biofuels, microalgae are showing potential. High photosynthetic efficiency, combined with high lipid content and low fresh water requirement, has contributed to the 'biofuels feedstock' status of microalgae. In some communities, microalgae have also been cultivated in wastewater in facultative lagoons to remove phosphorus and nitrogen through the growth of microalgae. With such systems in place, complete biological wastewater treatment can be achieved and the harvested microalgae could provide feedstock for biodiesel and various other bioproducts.

Due to small cell size, low culture concentrations, and the electrostatic repulsive forces that keep the cells in suspension, harvesting microalgae entails high energy inputs and associated high costs. Of the several harvesting methods tested, chemical precipitation has been shown to be the only method to harvest microalgae on a large scale. Although effective in wastewater treatment, the use of inorganic metal coagulants for microalgae harvesting leads to high dosage requirements, excess volume of sludge, and high costs, and due to the presence of associated metal hydroxide, the harvested biomass is unsuitable as feedstock for bioproducts.

The drawbacks of inorganic coagulants for microalgae harvesting can be overcome by using cationic starch. Corn and potato starch were cationized using 3-methacryloyl amino propyl trimethyl ammonium chloride and biogenic amines. Flocculation efficiencies of the cationic starches were tested in a jar test apparatus using single strain microalga, Scenedesmus obliquus, and mixed culture wastewater from the Logan City, Utah lagoons. Cationic starches showed better or comparable removal of total suspended solids compared to aluminum sulfate. Total phosphorus removal efficiencies for cationic starches were lower compared to aluminum sulfate. Effect of cationic starch harvested and alum harvested S. obliquus on biodiesel, acetone, butanol, ethanol production, and Escherichia coli growth was also studied. Results suggested significantly higher yields of bioproducts when cationic starch was used to harvest microalgae and the biomass was used as feedstock.

Cationic starches are an organic, sustainable, and renewable form of coagulant/flocculant. The use of cationic starch for harvesting microalgae eliminates the need for metal salts while enhancing the production of algae-based bioproducts. Cationic starch along with advanced technologies in the processing of microalgae is the way forward in the realization of the “microalgae to biofuels” initiative.