Date of Award

6-2014

Degree Name

MS in Civil and Environmental Engineering

Department

Civil and Environmental Engineering

Advisor

Tryg Lundquist

Abstract

Microalgal biomass is a candidate feedstock for biofuel production. To improve the sustainability of algae biofuel production, following biofuel recovery, the biomass nutrients should be recycled for additional algae growth. Anaerobic digestion of algae or oil-extracted algae is a means of recovering carbon and other nutrients, while offsetting algae production electricity demand. The major limiting factor in microalgae digestion is the low biodegradability of the cell walls. In the present study, various pretreatment technologies were tested at bench scale for their ability to improve raw, non-lipid-extracted algae biodegradability, which was assessed in terms of methane yield, volatile solids destruction, and solubilization of N, P, and K. The microalgae were harvested by sedimentation from outdoor wastewater-fed raceways ponds operated in coastal southern California. Four pretreatment methods (sonication, high-pressure homogenization, autoclaving, and boiling) were used on the algae slurries, each followed by batch anaerobic digestion (40 days at 35oC). Biomass sonication for 10 minutes showed the highest methane yield of 0.315 L CH4/ g VSIN, which is a 28% increase over the untreated control. Conversely, autoclaved algae slurry inhibited methane production (0.200 vs. 0.228 L CH4/ g VSIN for the treatment and control). A preliminary energy balance indicated that none of the pretreatments led to a net increase in energy conversion to biomethane. However, pretreatment did increase the initial N and P solubilization rates, but, after digestion, the ultimate N and P solubilization was nearly the same among the treatments and controls. After 40 days of digestion, solubilization of N, P, and K reached, respectively, 50-60% of average total Kjeldahl N, 40-50% of average total P, and 80-90% of average total K. Descriptive first-order models of solubilization were developed. Overall, certain pretreatments marginally improved methane yield and nutrient solubilization rate, which cast doubt on the efficacy of, or even the need for, algae biomass pretreatment prior to anaerobic digestion.

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