Anaerobic Digestion of Wastewater-Grown Algae Biomass with Optimized Biogas Yields and Nutrient Solubilzation

Author

Tiffany A. Racz, California Polytechnic State University, San Luis ObispoFollow

DOI: https://doi.org/10.15368/theses.2018.172
Available at: https://digitalcommons.calpoly.edu/theses/2245

Date of Award

6-2018

Degree Name

MS in Civil and Environmental Engineering

Department/Program

Civil and Environmental Engineering

College

College of Agriculture, Food, and Environmental Sciences

Advisor

Tryg Lundquist

Advisor Department

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Advisor College

College of Agriculture, Food, and Environmental Sciences

Abstract

Whole-cell algae biomass grown on nutrient-rich wastewater can be anaerobically digested to produce renewable heat and power, and to solubilize nutrients to grow additional algae biomass in a biorefinery system. In this study, algae biomass was grown on clarified primary wastewater in 33-m² ponds at a 4-day residence time with sedimentation harvesting of the biomass. Lab digesters were used to test biogas yields and nutrient solubilization from the anaerobic digestion of algal biomass with and without mixing and with sonication as a pretreatment. Additionally, algae were fed to unheated and unmixed 1135-L pilot digesters to determine the effects of seasonal temperatures and organic loading rates on biogas yields and nutrient solubilization. Finally a scalability experiment was conducted to determine how well lab digesters replicated the nutrient solubilization and biogas yields of pilot digesters when they were operated at the same average daily temperature, and organic load. Overall, the tested conditions included mixing, temperature, feed pretreatment by sonication, scale, and organic loading. It was determined that unmixed, 20˚C digesters fed an average variable organic loading of 0.12 g VS/L-day had the highest yield of 0.3 L CH₄/g VS fed. Compared to similarly operated digesters (30˚C, constant organic load 0.25 g VS/L-day) sonicated feed increased the mass yield of methane by 18% (0.28 L CH₄/g VS_introduced), and mixing increased the mass yield of methane by 4% (0.25 L CH₄/g VS). For the same digesters, sonicated feed increased the average nitrogen and phosphorus solubilization 10% and 11% with 36% N and 28% particulate P remaining, respectively. Eliminating mixing increased the average nitrogen and phosphorus solubilization by 13% and 27%, with 40% and 31% remaining as particulates, respectively. The pilot digesters produced an overall average mass yield of 0.19 L CH₄/g VS, with a summer average of 0.46 L CH₄/g VS and a winter average of 0.15 L CH₄/g VS. For the pilot digesters, the average amount of remaining particulate nitrogen and phosphorus was 36% and 39%, respectively, with an average of 57% volatile solids destruction. Finally for the scale experiment, the pilot digesters exhibited mass and volumetric yields of 47% and 28% (0.19 L CH₄/g VS; 0.011 L CH₄/L-d) greater than the lab digesters. Additionally, the pilot digesters had 2% greater nitrogen solubilization and 29% less phosphorus solubilization with 23% N and 15% P, than the lab digesters. Based on these results, for a low organic load (0.01 – 0.65 g VS/L-day), it is recommended that digesters be unmixed and heated which, and have a longer winter residence time. In addition to benefiting methane yield and nutrient solubilization, these digester operating conditions would allow increased supply of nutrients to ponds during the most productive months when nutrients are being consumed at faster rates in the algae ponds.