Combined Sewer Overflows (CSOs)
Each year, more than 24 billion gallons of combined untreated sewage and storm water is dumped into the Great Lakes. Although these discharges are less than 5% of the total amount of treated sewage discharged into the Great Lakes, bacteria and viruses are present in much greater concentrations in untreated sewage and so pose a significant health risk.
Sewage overflows from aging sanitary and combined sewer systems, leaking sewage pipes, and malfunctioning sewage treatment plants and pump stations are a major cause of Great Lakes beach closings and swimming advisories. Combined sewer systems carry both raw sewage from residences and industrial sites and stormwater runoff from streets to sewage treatment plants. Unfortunately, in heavy rainfall, the volume of the combined wastewater becomes too great for the treatment plant to handle. In such circumstances, the flow is diverted to outfall points (called CSOs) that discharge pollutants—including raw sewage; floatables such as trash, syringes, and tampon applicators; toxic industrial waste; and contaminated stormwater—into the nearest stream or coastal waterway. Many cities routinely have CSO events during wet weather. We are working with EPA to monitor progress on addressing these discharges around the Great Lakes. In September 2007, EPA released a report on CSO discharges to Lake Michigan. The largest source of combined sewer overflow pollution into the Great Lakes by volume comes from the wastewater treatment plant in Detroit, Michigan. Other major sources of combined sewer overflows into the Great Lakes include sewage plants in Cleveland and Milwaukee. The Alliance has been working to ensure that these major sources of pollution are addressed promptly. In August 2010, we sent suggestions to EPA on how to improve rules regulating Sanitary Sewer Overflows.
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