The City operates an advanced wastewater treatment facility with a capacity to treat 34 mgd (average flow) and 66 mgd (peak flow), serving a population base of 120,000 from more than 14 entities in and around the city.  The principal water pollution control facility upgrade project began with major plant improvements, including temperature-phased anaerobic sludge digestion of biosolids that yields combustible biogas. The combustible gas is then treated for hydrogen sulfide and siloxane removal prior to cogeneration of electricity and hot water. The electrical power produced from the digester gas provides significant operational cost savings by satisfying approximately 60 percent of the electrical demand of the treatment plant.

The digester gas contains a high concentration of hydrogen sulfide (2,000 to 3,000 ppm); therefore, it requires a customized gas cleaning system.  The unique cleaning system designed employs biological treatment (supported by thiobacillus bacteria) to the digester gas in a packed tower to remove hydrogen sulfide. Siloxanes are then removed using activated carbon following compression and drying of the digester gas.

Methane content of the digester gas is continually monitored by instrumentation mounted adjacent to the engine generator. The panel-enclosed methane monitoring systems are automatically and regularly calibrated using a gas calibration system.  The 450 kW generator delivers power at a voltage and frequency to parallel the nominal 4.16 kV, 60-cycle, three-phase utility power that is delivered to this part of the treatment facility.  Paralleling of plant-generated power with utility-generated power to match voltage, frequency, and phase is critical to avoid damage to other plant electrical equipment.

Heat from the generator cooling fluid is automatically delivered to a centralized hot water system. This system serves heat exchangers to maintain digester temperature, high strength waste tank jackets, and heating coils in the building heating system. The cogeneration system is supplemented and backed up by a system of existing and new boilers.  The boiler and cogeneration systems  are monitored and controlled through an integrated plant-wide SCADA system that was modified to serve these new improvements.

This project achieves the City of Fond du Lac’s goals of maximizing the efficiency of the anaerobic digestion process at the treatment plant, reducing the cost of treating high strength waste from local industry, improving the overall energy efficiency of the plant, and reducing the plant’s carbon footprint in a timely and cost-effective manner.

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