The Tale of Two Digesters: Hydraulic Versus Compressed Biogas Mixing

3Fairfield Suisun Sanitary District (FSSD) uses two identical 500 M3 (1.764 million gallons) anaerobic digesters to treat primary and secondary sludges as well as a small quantity of high-strength industrial waste. Digesters are well-instrumented, continuously monitored, and frequently tested for process control parameters. In September 2010, FSSD retrofitted a new mixing system in Digester #1: a 75 kilowatt (KW) (100-horsepower [hp]) Vaughan chopper pump and 12 Rotamix® nozzles. Digester #2 uses a 45 KW (60 hp) biogas compressor and bottom nozzles for mixing.Digesters were operated near a loading of 0.8 kilograms per day per cubic meter (kg/day/M3) (50 pounds per day of volatile solids per 1,000 cubic feet) (ppd/1,000cf) of digester volume. Volatile solids loading rates and other environmental and chemical conditions were the same for both digesters. We compared the two mixing methods for biogas and methane production, volatile solids reduction, and energy efficiency.