A Water Quality Modeling Approach to Negotiating NPDES Wastewater Permits

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ABSTRACT
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.

INTRODUCTION
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. Both permits included more stringent limits on Carbonaceous Biochemical Oxygen Demand (CBOD) and ammonia. In reviewing the permits, JCW noticed that in developing the allowable CBOD and ammonia limits for TC, KDHE did not recognize potential assimilation of MB effluent CBOD and ammonia in Indian Creek between the two plants. KDHE approved JCW’s appeal of the draft permit limits, allowing JCW the opportunity JCW to review KDHE’s water quality analysis used to develop the permit limits.

JCW contracted with the Black & Veatch Corporation (B&V) to conduct a study to review KDHE’s water quality modeling of Indian Creek to determine if the proposed CBOD and ammonia effluent limits were modeled and calculated correctly.

In 1994, B&V conducted the Indian Creek Water Quality Impacts Study for JCW. KDHE required JCW to conduct the study to determine the effect of MB and TC effluents and their stormwater lagoons on dissolved oxygen (DO), ammonia, fecal coliform, and suspended solids in Indian Creek. The primary analytical tool for the study was BVSP, a water quality modeling program, based on a modified version of the Streeter Phelps DO sag equation, which was coded in an Excel spreadsheet.

For the study described in this paper, the BVSP Indian Creek Model was re-calibrated using more recent hydraulic and water quality data. The BVSP Indian Creek Model begins just upstream of MB at the Highway 69 bridge in Overland Park, Kansas, and the study reach extends approximately 12 miles downstream to the confluence of the Blue River. The most downstream three miles of the creek is in Kansas City, Missouri. The study reach is shown in the following figure. At the time the study began the permitted flow rates for MB and TC were 12 mgd and 10 mgd, respectively.

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