Bench-Scale Study for Optimization of Co-Digestion of Creamery and Poultry Waste for City of Petaluma

The City of Petaluma is planning to implement a co-digestion program using creamery waste and poultry waste to increase the current TWAS-only biogas production of 30 to 40 scfm to approximately 100 scfm to cost effectively implement a biogas to biofuel (vehicle fuel) program. Approximately 1,500 gpd of creamery waste and 6,600 gpd of poultry waste are available for digester addition. Prior to implementation of the full scale program, the City performed a bench scale study using four 10 L digesters to determine i) if the available creamery and poultry wastes can increase biogas production to target levels, ii) the impact of co-digestion on digester stability and plant operations and iii) the optimum organic waste loading between its two 550,000 gallon digesters to maximize the renewable energy credit (Renewable Identification Number or RIN credit) as well as the overall economics of the program. The bench scale study data indicated that the addition of the organic wastes increased volatile solids reduction from approximately 30 in the TWAS only digester to about 65% in the digester with the creamery and poultry waste addition. The unit biogas production increased by 30 % due to organic waste addition. The data indicated that addition of available organic wastes can result in the production of 125 to 150 scfm biogas. However, such an addition will increase the digester ammonia levels to about 4,500 mg/L and the composite centrate ammonia levels to about 2,300 mg/L. Although the digesters operated without any upsets at such high levels of ammonia, this may potentially cause challenges for the plant to meet its nutrient discharge limits. A loading scenario involving operating one of the digesters with only thickened waste activated sludge (TWAS) and operating the second digester with about 5,000 gpd TWAS and all of the creamery waste and only the DAF waste (~2000 gpd) was recommended. This loading is estimated to produce about 93 scfm biogas, keep the ammonia level at approximately 1,170 mg/L and maximize the Renewable Identification Number (RIN) credit. A preliminary economic evaluation indicated a simple payback period of 10 years for this project.