Public Service Co. Selects Siemens' FGD Scrubber

Public Service Company of New Hampshire has selected Siemens Water Technologies to provide a system to treat wastewater from a Flue Gas Desulfurization (FGD) scrubber being constructed at the Merrimack Generating Station in Bow, N.H.

The installation of the wet scrubber is required by state law and will significantly reduce the station's emissions of mercury and sulfur dioxide. The wastewater treatment system will de-saturate the wastewater and remove suspended solids and metals, including mercury, from the scrubber waste stream so the water can be safely discharged. Designed for a maximum flow rate of 50 gallons per minute (11.36 m3/h), the system is scheduled to be completed in 2011.

Scrubbers are used in numerous U.S. power plants to meet emissions standards set by the U.S. Environmental Protection Agency and state agencies. Flue gas systems frequently use limestone-forced-oxidation (LSFO) scrubbers to convert sulfur dioxide in the flue gas to gypsum, which can be recycled and sold for wallboard manufacturing, cement additive, or agricultural applications.

The scrubber at Merrimack Station is being specifically designed to remove mercury from plant emissions, also. The proper design of scrubber wastewater treatment systems and the selection of materials of construction are critical. Both of these key items can have a major impact on the treatment plant’s operation and reliability.

Siemens’ scope of supply on the new physical/chemical wastewater treatment system includes system design, equipment supply and installation, system start-up, staff training, and system commissioning. The system will include equalization tanks, reaction tanks, chemical feed systems, clarifiers, gravity sand filters, storage tanks, sludge holding tanks, and recessed chamber filter presses. Equalization tanks receive the waste stream and equalize the flow to eliminate spikes in flow rates and concentration. Next, two trains, each with two reaction tanks in series, continue the treatment process to de-saturate the wastewater, reduce heavy metals, and prepare the wastewater for clarification. Reaction chemicals are added to the reaction tanks. Following the reaction tank trains, polymer is added and the wastewater enters the clarifier process, where suspended solids are coagulated and settled. Solids from the clarifiers are directed to sludge holding tanks prior to dewatering in the filter presses. The treated water from the clarifiers is sent to gravity sand filters for final treatment and then to treated water storage tanks prior to discharge.

Siemens is partnering with Northern Peabody LLC of Manchester, N.H., which will provide installation services, including manpower and materials to construct the wastewater treatment system.

Merrimack Station is Public Service Company' prime base-load plant, operating continuously to meet its customers’ electrical demand. It consists of two coal-fired, steam-electric generation units with a generating capacity of 443 megawatts, providing low-cost energy for customers.

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