Publication Summary

Part I: Effects of Renton Wastewater Treatment Plant Effluent on Water Quality of the Lower Green/Duwamish River. Part II: The Impact of Effluent from the Renton Wastewater Treatment Plant on the Dissolved Oxygen Regimen of the Lower Green/Duwamish River.

Part I: Effects of Renton Wastewater Treatment Plant Effluent on Water Quality of the Lower Green/Duwamish River. Two time-of-travel surveys were conducted on the lower Green-Duwamish River to evaluate impacts of secondary-treated wastewaters discharged from Renton treatment plant during summer low flow. Drogues were placed in the river near the outfall on September 18 and October 2, 1979 and followed by boat until reaching the mouth at Elliott Bay. Samples were collected for the determination of: temperature; pH; conductivity; dissolved oxygen; fecal coliforms; BOD; COD; nutrients; and other indicators of stream quality. Flow and other physical data important for accessing stream quality also were obtained. The drouges nearly reached the river mouth in 24 hours during both surveys, reversing direction twice during this period and moving upriver in response to the influx of estuarine waters at high tide. Stream quality was generally poor and this condition was associated mainly with Renton WTP. At 4:1 and 4.4:1, the stream-to-effluent dilution ratios were far below the 20:1 recommended minimum. Violations of state standards for temperature and dissolved oxygen were observed. EPA red book criteria for total residual chlorine and ammonia (un-ionized) were exceeded while nitrite-nitrogen reached borderline levels. These problems are accentuated when waters near the outfall reverse direction at high tide forming poorly diluted blocks of effluent/river water. The ability of the Green-Duwamish River to assimilate wastewaters discharged by RWTP appears to be exceeded at the current 36 MGD rate of discharge. Problems associated with the plant will continue to become more acute as the plant approaches the 144 MGD ultimate site capacity.

Part II: The Impact of Effluent from the Renton Wastewater Treatment Plant on the Dissolved Oxygen Regimen of the Lower Green/Duwamish River. The impact of Renton wastewater treatment plant effluent on dissolved oxygen concentrations in the lower Green/Duwamish River was modelled using a modification of the Streeter-Phelps equation which accounts for longitudinal dispersion. Rate constants and other variables were determined from data collected during two 24-hour drift surveys conducted during September and October of 1979. Dissolved oxygen concentrations below the plant discharge were depressed 2 to 3 mg/1. Based on model results and interpretation of survey data, nitrification of effluent ammonia was found to be responsible for about 95 percent of the observed dissolved oxygen depression. Model extrapolation to 1985 suggests further substantial degradation of the lower Green/Duwamish river as plant flow increases and instream dilution ratios decrease.