Right-Sizing a Cogeneration System for a Middle Sized Wastewater Treatment Plant

Johnson County Wastewater, Overland Park, Kansas, is expanding its solids processing facilities at the 14.5 million gallon per day (mgd) Douglas L. Smith Middle Basin Wastewater Treatment Plant (Middle Basin WWTP) to match a recent expansion of its liquid stream capacity, accept additional trucked in sludges from other wastewater treatment plants, and provide a readily accessible disposal site for fat, oil, and grease (FOG) wastes in support of the County's aggressive FOG waste management program in the collection system. The comprehensive cogeneration evaluation conducted during design included a payback analysis that determined the economic viability of cogeneration for the Middle Basin WWTP and the optimum size and operating scenarios for the cogeneration units. The study evaluated three engine sizes in five engine configurations (with and without standby units), and two operating scenarios (with and without supplemental natural gas), for a total of nine different engine/operational alternatives. Engine sizes ranged from 633 kW to 1060 kW and capital construction costs for the nine alternatives ranged from 3.6 to 6.7 million. Electrical power cost savings were determined from projected reductions in facilities charges, summer and winter demand charges, and summer and winter energy charges. The 20-year life cycle analysis included projected engine down times for scheduled and unplanned maintenance and anticipated variability in daily digester gas production.