Using CFD to Select Anaerobic Digestion Mixing Systems

As part of a bioenergy master plan (BMP), the city of Des Moines, Iowa, Wastewater Reclamation Authority (WRA) identified facility improvements that included upgrades to the digesters' mixing system. This paper describes the use of computational fluid dynamics (CFD) modeling as a design tool to first evaluate different mixing systems, including mechanical draft tubes, gas draft tubes, and pumped recirculation, and then optimize the size of the selected system. The traditional approach used to determine the appropriate mixing system capacity identified a larger capacity mixing system than the CFD-based recommendation. By using CFD to identify the optimal mixing system, the utility saved approximately 700,000 in capital costs and expects to save up to 800,000 kilowatt hours per year in energy consumption.