A Water Quality Modeling Approach to Negotiating NPDES Wastewater Permits

Johnson County Wastewater (JCW) of Overland Park, Kansas operates two wastewater treatment plants on Indian Creek, the Douglas L. Smith Middle Basin WWTP (MB) and the Tomahawk Creek WWTP (TC), which discharges to Indian Creek about 7 creek miles downstream of the MB discharge location. In 2004, the Kansas Department of Health and Environment (KDHE) issued draft revisions to the NPDES permit for each plant. JCW appealed draft permit limits and conducted a study to determine if less stringent limits would meet water quality standards in Indian Creek. This study involved use of a water quality model to determine allowable carbonaceous biochemical oxygen demand (CBOD5) and ammonia limits for each plant. The model results were used by JCW to recommend to KDHE appropriate changes to the draft permits. The recommended CBOD5 effluent limits for each plant were less stringent or equal to the draft permit limits. The recommended ammonia limits for MB were more stringent than the draft limits during July and August when the limits were determined by meeting the creek DO criterion of 5 mg/L rather than meeting the ammonia toxicity criterion. The recommended ammonia limits for TC were all less stringent than the draft permit because the model recognized the significant decay of ammonia between the two plants. Revisions to the draft permit were successfully negotiated with KDHE.