An Evaluation of Separating Combined Sewers and Its Effect on Water Quality in the Greater Milwaukee, Wisconsin Area

In an effort to reduce combined sewer overflows (CSOs) and improve surface water quality in the greater Milwaukee area, construction of a 19.5-mile (31.4 km) inline storage system (ISS) was completed in 1994 to capture and convey for treatment, sanitary sewage and stormwater from the combined sewer service area (CSSA). Since the ISS has been in operation, a significant improvement in the area's water quality has been observed due to the capture and treatment of over 66.4 billion gallons (251 million cubic meters) of sanitary sewage and stormwater. Despite the water quality improvement in the CSSA area, the waterways are still not meeting Wisconsin Water Quality Standards. The continued occurrence of overflows (2.50 CSO's/year) perpetuates the public view that sewer separation is the solution to improve water quality.The application of the Chemical Mass Balance (CMB) model in this study has been successful because the relative contributions of sanitary sewage and stormwater in CSOs were well determined. The relative contributions of pollutants such as metals, TSS, and E. coli were found to be higher in stormwater than in sanitary sewage. This result confirms that stormwater, not sanitary sewage, is the dominant pollutant source of these specific pollutants in the CSSA during storm events. Based on these findings, the impact of sewer separation in the CSSA would be a significant detriment to the water quality in the greater Milwaukee area because of the large pollutant contribution of uncaptured and untreated stormwater directly discharging to the local waterways instead of being captured by the ISS and treated at the treatment plants.In the practice of sewer separation, the treatment of pollutants such as metals, TSS and E. coli is recommended prior to any discharges to waterways. Sewer separation only should be considered a viable alternative if the treatment of stormwater is supported through the use of effective stormwater best management practices (BMPs) that target the removal or reduction of metals, TSS and E. coli before stormwater is discharged to surface waters.