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Clean Water Discharge Permit Issued for Mirant Canal Station in Sandwich, Mass.


An updated permit governing discharge of waste water by the Mirant Canal Station power plant in Sandwich will provide better protections for the environment. The new requirements are contained in a Final National Pollutant Discharge Elimination System (NPDES) permit, developed jointly by EPA and the Massachusetts Dept. of Environmental Protection (MassDEP).

In order to meet the requirements of the federal Clean Water Act, the new permit contains several new provisions to improve protection of water quality, aquatic life and designated uses of the Cape Cod Canal that are impacted by the operation of the power plant. The previous permit was issued in June 1989.

The Mirant Canal Station is a 1120 megawatt fossil fuel electrical generation facility that withdraws cooling water from, and discharges pollutants (including heat), to the Cape Cod Canal. The plant currently uses a “once-through” cooling water system to discharge steam turbine condenser waste heat directly into Canal waters. Water for the existing cooling system is withdrawn from the Canal by the Station through two cooling water intake structures.

Each day when operating at capacity, the Station withdraws approximately 518 million gallons of water from the Cape Cod Canal and circulates it through the facility to condense the steam used to produce electricity. Water taken from the Cape Cod Canal contains millions of organisms including fish eggs and larvae that are killed by severe physical and chemical impacts and extreme water temperatures. 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.

Under the new permit, EPA and MassDEP call for Mirant Canal to install cooling towers or alternative technologies and/or operational methods that may result in comparable reductions in fish egg and larvae mortality.

In comments on the draft permit, Mirant Canal estimated that the cost of retrofitting the facility with closed cycle cooling would be $122 million. Careful assessments were conducted by both EPA and Mirant Canal of various potential technologies to reduce environmental impacts. EPA has determined that there is no other available technology as effective as closed cycle cooling that could be implemented at a lower cost. However, the Final Permit does allow Mirant Canal the flexibility to utilize any technology that can achieve reductions in fish mortalities comparable to what would be achieved with closed cycle cooling.

To reduce injury and death for juvenile and mature fish, the final permit requires that the cooling water intake structures be modified to include operational controls to reduce chlorine exposure, the use of fish holding buckets and low pressure spray washes, and the reconfiguration of the return system such that returned fish are transported away from the intake structures in order to reduce the probability of fish being re-impinged.

EPA is also concerned that the biological integrity of the receiving water be maintained, and has developed a thermal discharge limit that will be monitored in the discharge canal and also in the Cape Cod Canal. Compliance with the limits is required at both locations.

The permit also seeks to ensure that metal cleaning waste water will been segregated from other internal waste streams. To gather more information about the entrainment and impingement impacts from the power plant, and to gauge the effectiveness of protective measures, an extensive bio-monitoring program will be required.


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