NPDES Permits in New England
Merrimack Station Draft NPDES Permit
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Background: EPA and the New Hampshire Department of Environmental Services (NHDES) have issued a new Draft National Pollutant Discharge Elimination System (NPDES) permit for the Merrimack Station power plant in Bow, New Hampshire. The agencies have designed the permit to meet the requirements of federal and state water pollution control laws. Important permit conditions include those addressing the facility's discharges of waste heat to the Merrimack River, its withdrawals of river water for plant cooling needs, and its discharges to the river of mercury, arsenic, selenium and other pollutants.
Merrimack Station is a 470 megawatt (MW), predominantly coal-fired, electrical generation facility owned and operated by Public Service of New Hampshire (PSNH), a corporate subsidiary of The Northeast Utilities System. The facility takes up to 287 million gallons of water per day (MGD) from the Hooksett Pool section of the Merrimack River to use for cooling in its process for generating electricity. The facility also discharges waste heat and other pollutants to the river. More specifically, Merrimack Station burns coal in its boilers to generate steam to drive its electrical generating turbines. It also takes water from the Merrimack River through two intake structures and uses it to condense the steam back to water (i.e., for cooling) so that more steam and electricity can be generated. In the process of condensing steam, the river water absorbs the facility's steam turbine waste heat. This water, carrying the facility's waste heat, is then discharged back to the Hooksett Pool.
This type of cooling system is referred to as an "open-cycle" (or "once-through") system because the water is used for only one cooling cycle before it is discharged to the river. This type of system tends to maximize both the amount of water that must be taken from a water body for cooling and the amount of waste heat discharged to the receiving water. As an alternative to open-cycle systems, closed-cycle cooling systems include technology to chill heated condenser cooling water so that it can be recycled for additional cooling cycles. As a result, closed-cycle systems tend to minimize water withdrawals for condensing steam and discharges of waste heat.
By raising water temperatures, discharges of heat to a water body (i.e., "thermal discharges") can harm aquatic organisms in many ways. EPA's analysis finds that Merrimack Station's thermal discharges have, indeed, harmed fish in the Hooksett Pool. Data indicates that thermal discharges have contributed to the alteration and depletion of fish populations in the Pool over the last 20 to 30 years. Making matters worse, fish are also killed and injured by the facility's withdrawals of river water for its cooling needs. The water taken from the river contains fish eggs and larvae and these tiny creatures are pulled through the facility's cooling system and killed by exposure to harsh physical impacts, extreme water temperatures and toxic chemicals. This "entrainment" of fish eggs and larvae is a problem from April through August of each year at Merrimack Station because this is the primary period when fish eggs and larvae are found in the Hookset Pool. In addition, withdrawals of river water create a water velocity at the intake pipes that can trap (or "impinge") juvenile and adult fish against the facility's intake screens. The fish caught on the screens may be killed or injured and "impingement" is a year-round concern at Merrimack Station.
Another important issue addressed by the new draft permit is the wastewater discharge expected from Merrimack Station's recently constructed wet flue gas desulfurization (FGD) scrubber system. The wet FGD scrubber system will reduce the facility's emission of air pollutants, such as mercury and sulfur dioxide, but it will also generate a wastewater containing many of these same pollutants. Thus, the discharge of FGD wastewater must be properly controlled.
EPA and the NHDES last reissued Merrimack Station's NPDES permit (Permit No. NH0001465) on June 25, 1992. Originally scheduled to expire on July 27, 1997, the permit has been administratively continued in effect pending reissuance of a new permit.
Major Permit Conditions: Consistent with the requirements of the Clean Water Act, EPA and NHDES have designed the following key permit conditions:
- Reduced Thermal Discharges: EPA has found that Merrimack Station's thermal discharges have contributed to the deterioration of fish populations in the Hooksett Pool. In addition, EPA has determined that upgrading Merrimack Station's decades-old open-cycle cooling system to a closed-cycle system is the best available technology for reducing the facility's discharges of waste heat. Therefore, the Draft Permit includes monthly and yearly limits on the amount of heat that Merrimack Station can discharge to the Hooksett Pool based on the levels achievable by a closed-cycle cooling system. These limits apply year-round and would reduce the facility's thermal discharges by 99.6%.
- Reduced withdrawals of river water: EPA has determined that converting Merrimack Station's cooling system from an open-cycle system to a closed-cycle system is also the best technology available for minimizing mortality to fish eggs and larvae from entrainment by Merrimack Station's cooling system. Due to seasonal spawning patterns, however, eggs and larvae are present in the Hooksett Pool only from April to August. Therefore, EPA is proposing limits on the facility's water withdrawals based on the use of closed-cycle cooling, but these limits only apply only during the April to August period. The intake flow limits for the rest of the year would allow water withdrawals compatible with open-cycle cooling. (By contrast, the Draft Permit's thermal discharge limits are based on the year-round use of closed-cycle cooling.)
- Improved Fish Return System: To reduce fish mortality from impingement, the Draft Permit specifies that Merrimack Station's cooling water intake structures must be modified to include low pressure spray washes for gently removing impinged fish from the intake screens, a new "fish return system" to safely return impinged fish to the river, and operational controls to reduce exposure of impinged fish to chlorine.
- Controls on Other Internal Wastewater Streams: The new Draft Permit also includes limits to control the discharge of pollutants in wastewater from the wet FGD scrubber system and various other sources at the facility, such as from metal cleaning operations.
Public Comment: EPA has placed Merrimack Station's Draft Permit on Public Notice from September 30 to November 30, 2011. During the Public Notice period any individual, agency, organization, etc., can submit written comments on the Draft Permit to EPA Region 1. EPA will consider and respond in writing to all timely, significant comments. Summaries of all comments received and EPA's responses to them will be posted at www.epa.gov/region1/npdes/merrimackstation/. EPA will also host a Public Hearing scheduled for November 3, 2011, at NH Dept. of Environmental Services headquarters auditorium starting at 6:30 p.m.
- Draft Permit (PDF) (29 pp, 328 K)
(Note: Corrections on pages 6-7)
- Attachment A - Freshwater Chronic and Modified Acute Toxicity Test Protocol (PDF) (7 pp, 49 K)
- Attachment B - Monitoring Location Map (PDF) (1 pg, 244 K)
- Part II Standard Condition (PDF) (25 pp, 142 K)
- Fact Sheet (PDF) (60 pp, 385 K)
(Note: Corrections on pages 12, 33, and 34)
- Attachment A: Map Location of Merrimack Station (PDF) (1 pg, 351 K)
- Attachment B: Map Location of Outfalls (PDF) (1 pg, 436 K)
- Attachment C: Merrimack Station Schematic of Water Flow (PDF) (1 pg, 813 K)
- Attachment D: Clean Water Act NPDES Permitting Determinations for the Thermal Discharge and Cooling Water Intake Structures at Merrimack Station in Bow, New Hampshire (PDF) (391 pp, 2.2 MB)
- Attachment E: Determination of Technology-Based Effluent Limits for the Flue Gas Desulfurization Wastewater at Merrimack Station in Bow, New Hampshire (PDF) (52 pp, 555 K)
- Attachment F: Discharge Monitoring Report Summary Janaury 2005 - December 2010 (PDF) (11 pp, 242 K)
- Administrative Record
- Comments Received During the Public Notice Period
- Public Hearing
- News Release: Updated Clean Water Discharge Permit for Merrimack Station in Bow, N.H. Issued for Review
John Paul King (King.John@epa.gov)
U.S. Environmental Protection Agency
Office of Ecosystem Protection
5 Post Office Square, Suite 100 (OEP06-1)
Boston, MA 02109-3912
Phone: (617) 918-1295
Fax: (617) 918-0295