Combined Sewer Overflow

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Combined sewers are a method for conveying both stormwater and sewage in one combined system. Historically these systems were built in hundreds of communities across the U.S. before indoor plumbing became commonplace.
During periods of normal rainfall, the systems function properly by conveying both stormwater and sewage to wastewater treatment facilities. However, during floods or periods of heavy rainfall, this combined system can cause raw sewage to overflow, called Combined Sewer Overflows (CSOs), into our streams and rivers causing a threat to public health.

Under the EPA's Clean Water Act, Indianapolis and other communities must develop plans to reduce these overflows to protect human health and the environment. Ignoring these problems make it more difficult to attract new businesses, jobs and residents to our world-class city.

The Problem

The White River and many of our neighborhood streams are polluted by sewer overflows during rain and snow storms. When it rains as little as a quarter-inch, these sewers overflow into nearby streams, including White River, Fall Creek, Eagle Creek, Pleasant Run, Bean Creek and Pogues Run.

Raw sewage overflowing in our streams is a health hazard, smells and looks disgusting, hurts the environment and harms the quality of life in our neighborhoods.
Click here to see how the CSO works.

The Plan

The Combined Sewer Overflow (CSO) Consent Decree is an agreement between the City, the EPA and the Indiana Department of Environmental Management, under which Indianapolis developed and is executing a long term control plan to curb the overflow of raw sewage from combined sewers into local waterways. At Mayor Ballard's direction, the Department of Public Works assessed cost overruns and through value engineering, achieved the modification plan. The plan allows the City to meet the required Consent Decree performance criteria and 2025 timeline, but in a more cost-effective manner that removes 3.5 billion gallons of sewage ahead of schedule.

By utilizing value engineering and implementing sustainable approaches, the City has achieved $740 million in savings. The new Consent Decree program is slated to reduce sewage overflows from the current average of 7.8 billion gallons per year to 414 million gallons at the end of the program. That reduction is an even greater reduction than the original Consent Decree required.

Not only will the new Consent Decree remove billions of gallons of sewage from entering the stormwater system, but its projects will now cost less and involve less risk to groundwater contamination. The Deep Rock Tunnel Connector (DRTC), scheduled to bid this spring, is an integral component. The DRTC is more than six miles long and will extend from the Southport Advanced Wastewater Treatment Plant to north of the Belmont Advanced Wastewater Treatment Plant. Because the DRTC is larger than original designed, it eliminates the need for a $30 million pumping station to assist in controlling excess wastewater. DPW also adjusted the design of the Belmont treatment plant expansion, which saved approximately $90 million.

Costs and Benefits

When the 20-year federally mandated plan is complete at the end of 2025, residents and businesses will benefit from the following:

  • Improved public health and quality of life in many neighborhoods now suffering from the sight and stench of raw sewage
  • The capture of up to 95-97 percent of sewage during wet weather on White River and Fall Creek.
  • Reduced overflow frequency from 45 to 80 events per year to two to four events per year—actual overflows will depend on the weather each year
  • Cleaner streams for fish and other aquatic life
  • Reduced odors, sanitary waste and other unsightly materials found overflowing from sewers into streams
  • Minimized impacts on neighborhoods and businesses by constructing most overflow storage facilities deep underground.

Because water pollution has many causes, we need an integrated, watershed-wide effort to achieve our water quality goals. State and regional cooperation will be needed to resolve stormwater runoff and other difficult-to-control water pollution sources. Indianapolis wants to ensure that affordable investments in water pollution control will yield the greatest benefit possible for human health, the environment and the citizens who live in and downstream of Marion County.

For more information on this plan, view the Long Term Control Plan.