A waste-load-assimilation study of a reach of Koshkonong Creek below the Sun Prairie, Wisconsin, sewage-treatment-plant outfall indicated that a high level of treatment would be required to meet Wisconsin water-quality standards. To maintain a minimum dissolved-oxygen concentration of 5 mg/liter during the critical summer low-flow period, 5-day carbonaceous biochemical-oxygen demand in waste discharges should not exceed 5 mg/liter and ammonium nitrogen should not exceed 1.5 mg/liter. Advanced treatment with denitrification is required because stream-reaeration coefficients are not high enough to offset deoxygenation caused by an abundance of attached biological slimes. The slimes apparently consumed dissolved oxygen at a rate of about 110 mg/liter per day at the time of the stream survey. During the critical summer low-flow period, natural stream discharge is very small compared to waste-water discharge , so benefits of dilution are insignificant. An evaluation of two proposed alternative waste-water discharge sites indicated that the present discharge site is hydraulically superior to these sites. Stream-reaeration coefficients used in the study were based on measurements using the radioactive-tracer method. (Woodard-USGS)
