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Updated Clean Water Discharge Permit for Merrimack Station in Bow, N.H. Issued for Review

Release Date: 09/29/2011
Contact Information: David Deegan, , (617) 918-1017

(Boston, Mass. - Sept. 29, 2011) An updated permit governing discharge of waste water by the Merrimack Station power plant in Bow, N.H. has been released for public review and comment.  The draft permit proposes updated requirements for controlling discharges of waste water to, and withdrawals of river water from, the Merrimack River. If finalized, the new permit will replace the facility’s current permit, issued in 1992, and dramatically improve protections for the Merrimack River.

Merrimack Station discharges pollutants to, and takes water for cooling from, the Hooksett Pool segment of the Merrimack River. At present, these operations seriously impair water quality which has resulted in adverse impacts to the aquatic habitat and fish community in this segment of the river.

The new Draft National Pollutant Discharge Elimination System (NPDES) Permit was developed by EPA and the New Hampshire Department of Environmental Services (NHDES) to address these problems and meet the requirements of the federal Clean Water Act and the State of New Hampshire’s water pollution control laws. The new Draft Permit’s conditions call for reduced pollutant discharges and water withdrawals that will improve habitat quality in the Hooksett Pool and are expected to allow for the recovery of the resident fish community. 

Merrimack Station is a 470 megawatt, predominantly coal-fired power plant. In the process of generating electricity, the power plant discharges a variety of pollutants to, and takes large quantities of water from, the Merrimack River. Each day of operations, Merrimack Station burns coal to heat up water in its boilers and produce the steam needed to drive its electrical generating turbines. It also takes up to 287 million gallons of water from the Hooksett Pool each day to use in its “open-cycle” cooling system. This “cooling water” is used to condense the steam back to water so that additional steam can be generated and used to produce more electricity. 

In the process of condensing the steam, the cooling water absorbs the power plant’s waste heat and it is then discharged back to the river at much higher temperatures. This heated water, in turn, raises water temperatures in the river, which degrades the river’s aquatic habitat and harms its aquatic life. Data show that fish abundance in Hooksett Pool has substantially declined since the plant’s second power generating unit came on line in 1968, and that the existing community of resident fish is now dominated by species that favor warmer water. 

At the same time, the up to 287 million gallons of water per day that Merrimack Station takes from the Hooksett Pool for its cooling needs contains millions of aquatic organisms, including fish eggs and larvae. These tiny organisms are killed as a result of being pulled with the water through the plant’s cooling system and exposed to severe physical impacts, extreme water temperatures and chlorine. Cooling water withdrawals also create a water velocity at the intake pipes which traps many juvenile and mature fish against the intake screens, causing injury or death to these fish. 

In developing the new draft permit, EPA determined that the best technology available for minimizing the mortality to fish eggs and larvae resulting from water withdrawals from the river would be to convert the existing, decades-old open-cycle cooling system to a closed-cycle system. A closed-cycle system allows for cooling water to be recycled so that water withdrawals from the river, and the mortality to fish eggs and larvae, can be reduced by 95 percent or more. The new draft permit limits water withdrawals based on the use of this improved technology.

The significant reduction in water withdrawal associated with the operation of closed-cycle cooling will result in an equally significant reduction in intake flow velocity at the cooling water intake structures.  This reduction in flow velocity will allow many fish that would have been impinged on the intake screens to successfully avoid impingement.  In addition, to reduce harm to juvenile and mature fish caught against intake screens, the permit requires that the cooling water intake structures be modified to (a) include operational controls to reduce chlorine exposure, (b) use low pressure spray washes, and (c) construct a new fish return system so that fish which are caught on the screens can be safely returned to the river.

EPA also decided that converting the power plant to a closed-cycle system is the best available technology for reducing the facility’s discharges of waste heat to the river. Consistent with the use of this technology, the draft permit includes limits to reduce the amount of heat that Merrimack Station discharges to the river by 99 percent.  By largely eliminating the waste heat by Merrimack Station, the aquatic habitat will be allowed to return to a more natural state, thus providing the opportunity for a more robust and balanced fish community to exist in Hooksett Pool.  

To convert to closed-cycle cooling, Merrimack Station would need to install cooling towers at the facility. EPA understands that converting to closed-cycle cooling will entail a significant construction project and that a reasonable amount of time will be needed to complete the project.

EPA estimates that the cost to install cooling towers and operate them year-round over a 20-year period at Merrimack Station to control thermal discharges would be approximately $112 million (present value) (or about $9 million per year). If PSNH passes the cost on to its customers, EPA has estimated that the average residential customer would see an average monthly rate increase of approximately $1.15 - $1.35.

The permit also contains provisions to limit pollution from other specific waste water discharges by Merrimack Station, including its metal cleaning waste discharges and future discharges anticipated from the facility’s new flue gas desulfurization unit that is being installed to reduce air pollution.

EPA and the NHDES last reissued Merrimack Station's federal and state waste water permit in June 1992.

EPA has placed Merrimack Station's Draft Permit on Public Notice for 60 days, from September 30 to November 30, 2011. During the Public Notice period any individual, agency, organization, etc., can submit written public comments on the Draft Permit.  EPA will also host a combined public meeting and public hearing on Nov. 3 at NH Dept. of Environmental Services headquarters auditorium in Concord, N.H. starting at 6:30 p.m. 

More information: Merrimack Station Draft NPDES permit (

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