KPDES Permit Compliance: Metals in Wastewater Effluent at Fort Knox

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Courtesy of Weston Solutions, Inc

ABSTRACT

Prior to constructing a new FOTW, Fort Knox had a history of exceeding KPDES effluent limitations for ammonia and BOD. The new 6 MGD FOTW solved the problem, but metals exceedances became an issue. The exceedances could be linked to several factors: low water-quality based effluent limitations (WQBELs) on the new KPDES permit, high background concentrations of some metals, and high metals in some waste streams. Fort Knox adjusted its operating practices and negotiated with the agency to achieve compliance.
The key elements include the following: The post implemented a P2 program that includes: prohibiting certain practices, training, inspections, banning detergents, installing silver recovery units, plugging floor drains, and cleaning sewer lines and traps. In addition, a voluntary wastewater pretreatment program and regulation that includes user limits and compliance monitoring is in place. The FOTW outfall discharged into a zero-flow stream, resulting in the most stringent WQBELs. The effluent enters an aeration ladder that flows through a natural conveyance into the intermittent stream bed at the outfall. The intermittent stream flows 150 yards into Mill Creek, a 2.5 MGD perennial stream. By diverting the effluent directly into Mill Creek, the new permit reflected greater assimilative capacity at the outfall and an increase in WQBELs. The proposed new permit included a monthly mercury limit of 0.021 mg/l. This is lower than the 0.1 mg/l MDL, and the mercury concentration in Fort Knox’ groundwater and surface water. In lieu of the limit, Kentucky agreed to a study of methylmercury in fish tissue in Mill Creek to demonstrate a reasonable potential for mercury in the FOTW effluent. The first-year results in this ongoing study show that fish tissue meets applicable mercury standards.

Through these efforts, Fort Knox reduced metals concentrations and negotiated more favorable permit terms, eliminating violations at the FOTW.

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