Codigestion Decreases Oneida County WPCP Energy Consumption by 50 Percent

The Oneida County, New York Department of Water Quality and Water Pollution Control (County) is responsible for the operation and maintenance of 45 miles of interceptor sewers, the Sauquoit Creek Pump Station (SCPS), and the Water Pollution Control Plant (WPCP). In 2007, the New York State Department of Environmental Conservation (NYSDEC) entered into a consent order due to sanitary sewer overflow at the SCPS. As a result of the consent order, the County is required to double the capacity of the WPCP from 55 million gallons per day (mgd) to 111 mgd. Over the past ten years, the County has embarked on a $325 million program and upgrade the WPCP to double the peak flow capacity. As a result of the increased flow to the plant the entire solids handling system was rehabilitated. In 2012, an evaluation recommended modifications to the existing incinerators. On a 20-year net present worth basis (Table 1), this was the most economically viable alternative. In 2011, the USEPA enacted 40 CFR 60, Subpart MMMM — Emission Guidelines and Compliance Times for Existing Sewage Sludge Incineration Units. At the same time, The Oneida Herkimer County Solid Waste Authority, located next door to the WPCP, was in the midst of installing equipment to receive and pre-process source separated organic food waste from local hospitals, educational facilities, etc. With a renewed focus on regionalization and sustainability, the County became willing to invest more upfront capital in an approach that would provide more favorable long term benefits. The 40-year net present worth analysis (Table 2) suggested new anaerobic digesters to replace the existing incinerators would have a lower net present worth cost, and would provide the additional benefits of supporting regional growth and sustainable infrastructure. Following the re-evaluation, the County made the decision to pursue the anaerobic digester approach. Design was completed in 2016, and construction was completed in 2020. This project represented a complete change in operational philosophy for the WPCP. The plant had been utilizing fluidized bed incinerators for 50 years and had no experience with anaerobic digesters. The original solids handling system at the WPCP, shown schematically on Figure 1, featured gravity thickeners and belt filter presses. The upgraded system, as shown in Figure 2, separated the primary and WAS sludge thickening systems. Egg shaped digesters were chosen over traditional tanks for their mixing efficiency, ability to prevent grit accumulating in the cone, and increased volatile solids destruction/gas production. The primary digesters are mixed with draft-tube style to maintain proper operating temperature (95 degrees F) for mesophilic anaerobic digestion. A secondary digester was constructed for short-term storage of digested sludge and biogas. The ability to receive food waste directly from the Solid Waste Authority was included in the design. A photograph of the constructed facilities is shown on Figure 3. The combined heat and power system features hydrogen sulfide and siloxane removal equipment. Treated biogas is compressed and conveyed to three microturbines, capable of producing 200 kW of electricity each. Each microturbine has an integral heat exchanger, sized to capture 500,000 BTU/hour when the corresponding microturbine is online. The heat exchangers are connected to a hot water loop, which provides building heat for three buildings at the WPCP in addition to the digester sludge heat exchangers. Energy production has been higher than anticipated due to the success of the Solid Waste Authority's food waste collection program. In 2022, New York State began prohibiting any large generator of food waste from disposing of that waste as garbage. Large generators are able to haul their source separated organic food waste to the Solid Waste Authority, where it is preprocessed with screening equipment and macerators. Since commissioning in 2020, digester gas production routinely exceeded the capacity of the 600 kW microturbine system. In 2021, grant funding was secured to install an additional two microturbines — increasing the total capacity of the CHP system to 1,000 kW electrical production and 2,500,000 BTU/hour heat recovery. The two additional microturibines were operational in 2023. As shown on Table 3, the annual WPCP utility costs have decreased significantly as a result of the solids handling system and CHP. Once the savings from the two additional microturbines are realized, the plant's net energy consumption will decrease by over 50%. The original 40-year net present worth analysis suggested the annual O&M cost of the solids handling system at $860,000. The actual O&M cost has been less than $500,000 as shown on Table 4. Over the 40-year planning period, this has resulted in a $10 million savings on the net present worth of the project. As a result of the project, greenhouse gas reduction is estimated at 2,900 metric tons CO2e/year.