What if Industrial Flows Don't Make It to the Plant? Determining and Addressing Potential Water Quality Impacts of Non-Domestic Dischargers in a Combined Sewer System

The Sanitation District No. 1 of Northern Kentucky (SD1) entered into a Consent Decree (CD) with the USEPA, Region 4 and the State of Kentucky (EPPC) on April 18, 2007. The CD required SD1 to submit documentation to support compliance with the Nine Minimum Controls (NMCs) for combined sewer overflows, including NMC No. 3, the review and modification of pretreatment requirements to minimize the impacts of discharges into the combined sewer system (CSS) from non-domestic sources during wet weather and to minimize CSO occurrences by modifying the pretreatment program. To address this NMC, a thorough review of SD1's Pretreatment Program was performed leading to the development of several programs to better understand the effects of non-domestic dischargers (NDDs) on the CSS.Within its pretreatment program, SD1 permits non-domestic dischargers (NDDs) classified as Significant Industrial Users (SIUs). NDDs are classified an SIU when they meet the guidelines in the Code of Federal Regulations and all other NDDs are classified as Non-Significant Users (NSUs). There are currently 5 SIUs and 740 NSUs in the CSS. Two separate programs were developed to analyze the potential water quality impacts from the NSUs and the SIUs within the combined sewer system.A NSU is an industrial or commercial user that generally does not pose an obvious threat, potential harm, or a measurable impact to SD1's wastewater collection system or employees. However, groups or clusters of these industries that discharge like pollutants tributary to a particular CSO may contribute to higher than expected concentrations and therefore additional analyses were undertaken to determine the potential for impacts related to discharges into the CSS during wet weather events. An analysis identified six clusters of NSUs that discharge like pollutants. The pollutants of concern within each cluster, their process sources, and proposed action levels were determined and written guidelines developed outlining further evaluation by the SD1 Industrial Monitoring Department that may lead to permitting of additional industries.For SIUs, a separate comprehensive analysis procedure was conducted that includes six different sequential steps incorporating eight different evaluations for each outfall/SIU combination. The analysis determined:If WQS are being exceeded at the outfallIf the SIU permit limits are protective of WQSIf there is a risk of a particular SIU causing a water quality violation at the outfallIf a particular SIU is a major contributor to the pollutant load at the outfallRecommendations for permit or process modificationsThis paper details this innovative evaluation and the resulting modifications.