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Water Resources Investigations Report 93-4188

Relations of Changes in Wastewater-Treatment Practices to Changes in Stream-Water Quality During 1978-88 in the Chicago Area, Illinois, and Implications for Regional and National Water-Quality Assessments

By Paul J. Terrio


A study was performed in the upper Illinois River Basin to determine ( 1 ) relations among changes in wastewater-treatment practices and stream-water quality on the basis of available information and (2) the limitations of available information for this purpose. Five large waste water-treatment plants operated by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) were studied because of the potential effect of these treatment plants on stream-water quality and because of the availability of extensive computerized wastewater quality and stream-water-quality data for 1978-88. Major changes in treatment practices were identified, and statistical tests were used to determine significant differences in effluent quality and stream-water quality from periods before and after the changes were implemented.

Two major changes in wastewater-treatment practices-the cessation of chlorination and the implementation of MWRDGC's Tunnel and Reservoir Plan-were identified at the three largest treatment plants. Chlorination was discontinued at these three treatment plants because the receiving streams are designated as secondary contact waters and the benefits of chlorination were limited. The Tunnel and Reservoir Plan was designed to capture and route overflows from combined sewers through the treatment plants, thereby decreasing the quantity of untreated over flows discharged to streams. Other changes and upgrades, such as improved aeration and the construction of additional treatment units, also were made at some of these treatment plants.

After the cessation of chlorination, increases in densities of fecal coliform bacteria were found in the effluents and at stream-monitoring sites as far as 6.8 miles downstream. At the Calumet Water Reclamation Plant, median densities of fecal coliform bacteria increased from 3,100 to 1,200,000 colonies per 100 milliliters.

After the implementation of the Tunnel and Reservoir Plan, various changes in effluent and stream-water quality were noted, but few similarities were found between treatment plants. For example, at the Calumet Water Reclamation Plant, decreases in concentrations of most constituents were identified. At the Stickney Water Reclamation Plant, however, increases in concentrations of biochemical oxygen demand, ammonia, and cyanide and decreases in concentrations of dissolved oxygen were observed.

Effects from other types of changes were analyzed at two of the treatment plants. The James C. Kirie Water Reclamation Plant, which began operation after stream-water-quality data had been collected for several years, provided an opportunity to evaluate the effects of the addition of effluent on the water quality of the receiving stream. No major changes in treatment practices were made at the John E. Egan Water Reclamation Plant, but a trend test was used to determine relations between trends in effluent and stream-water quality.

The available water-quality data were generally suitable for this study and were more comprehensive than data from most monitoring programs. The results of this study, however, identified some needed enhancements to increase the usefulness of the monitoring data for additional purposes. Paramount is the need to identify the evaluation of changes in wastewater treatment practices on stream-water quality as a clearly defined monitoring goal. Useful enhancements identified from this study apply to monitoring programs nationwide and include collecting streamflow data at all stream-monitoring sites, improving records of changes in waste water-treatment practices, establishing comparable sample-collection and analytical methods at all monitoring sites and between monitoring agencies, and designing monitoring programs to specifically evaluate effects of wastewater-treatment practices on stream-water quality. Stream flow data associated with stream-water-quality data would facilitate mass-loading analyses, which is required for understanding causal relations and for identifying contributions from point and nonpoint sources. Designing applicable monitoring programs may involve converting from time-composited effluent monitoring to discharge-composited effluent monitoring, collecting influent-quality data, and implementing a more appropriate and (or) efficient list of constituents to be sampled.

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