Visualization of Chlorella Algal Cells at Bubble Surfaces

Abstract

This thesis examines flotation as a method for harvesting the green algae chlorella for its use as a feedstock for biofuel. Lipids extracted from algae can be converted to biodiesel for use as an alternative renewable energy source. Visualization and image processing techniques were utilized to study algae flocculation and flotation both qualitatively and quantitatively. It is hypothesized that imaging techniques may be used to gain insight into the process of algal flotation. Flocculation of algae with iron nitrate, alum, chitosan, gelatin, and cellulose was explored. Samples of flocculated algae were imaged with light microscopy and processed to determine the average equivalent floc diameter. Iron nitrate and chitosan chemistries produce larger flocs than alum and gelatin chemistries. Algae flocculated with 50 ppm iron nitrate and a 10:1 cellulose to algae mass ratio was found to produce the largest flocs of 190 µm diameter. The smallest average equivalent diameter of 24 µm was observed for alum and gelatin concentrations of 100 ppm and 6.25 ppm respectively. Increasing the concentration of the secondary flocculant decreased the average equivalent diameter. For iron nitrate and chitosan chemistries, the addition of cellulose can replace the need for chitosan. Algal cell and bubble interactions were imaged in three bubble facilities: The stationary bubble facility which suspends a bubble on the tip of a needle in a quiescent fluid, the suspended bubble facility which suspends a bubble in a down flow of fluid, and the electrochemical flotation cell which generates very small diameter bubbles. No algal adsorption to bubbles was observed in the stationary bubble facility. This was attributed to the large diameter of the bubbles produced in this facility (approximately 1 mm). Images were useful to visualize the approach of individual algae cells and larger algae flocs to bubble surfaces. Algae adsorption to bubbles was observed in the suspended bubble facility during filling of the facility but adsorption was not observed during normal operation. Algae adsorption to bubble surfaces was successfully imaged in the electrochemical flotation cell. Flotation runs were conducted in an electrochemical flotation cell and a Denver D-12 flotation cell. Algae foam collected in these facilities was processed into algae pads via vacuum filtration and the mass of algae floated was determined. The flotation efficiencies for various flocculation chemistries was evaluated. Iron nitrate and chitosan concentrations of 50 ppm and 1.25 ppm respectively produced the highest flotation efficiency of 83%. Flotation efficiency increased with increasing average equivalent diameter.