Combined Sewer Overflow - CSO

What is CSO?

Why Our Sewers Overflow When It Rains

More than 100 years ago, South Bend and many other cities across the country, built storm sewers to carry rainwater and melting snow away from homes, businesses and streets. In those horse-and-buggy days, these cities didn't have sewage treatment or even indoor plumbing. When indoor plumbing came later, homeowners and business owners hooked their sewage lines to the existing storm sewers, combining storm water and raw sewage into one pipe. The pipes emptied directly into the river, until the 1950's when sewage treatment plants were built. This was common practice in many U.S. cities, especially in the Northeast and Midwest.

During dry weather, a "combined" sewer system works much like a separated sewer -- carrying all sewage to the treatment plant for treatment; however, when it rains or snow melts, the sewers can be overloaded with incoming stormwater. When this happens, the sewers are designed to flow over internal dams in the underground pipes and into nearby streams and rivers. This is referred to as a Combined Sewer Overflow (CSO) system. Without this release valve, raw sewage would back up into people's basements and streets.

Millions of gallons of untreated sewage and rainwater enter the St. Joseph River each year. State and federal regulations require cities to develop long term plans to reduce these overflows. The City of South Bend is just one of almost 800 CSO communities across the country dealing with Environmental Protection Agency (EPA) mandated CSO Long Term Control Plans (LTCP) that are costly and pose multiple social and economic impacts to our residents. Our motivation is driven by the fact that South Bend has 36 CSOs which typically overflow 60-70 times each year, resulting in approximately 1 billion gallons of overflows into the St. Joseph River. In addition, when the CSO system’s capacity is exceeded, basement backups can occur.

What is a Long Term Control Plan and Consent Decree?

Historically, the City invested in wastewater infrastructure to reduce CSOs prior to the formalization of a LTCP or signing of the consent decree. During the period between 1990 and 2004, the City spent over $87M on CSO infrastructure improvements in order to reduce overflows, increase system capacity, and improve efficiencies; thus, the City began implementation of its LTCP prior to 2011, when the consent decree was signed.

Motivated by the idea of doing more with less, and focusing on getting as much capacity from the current collection and wastewater system as possible, the City deployed approximately 140 real-time monitoring sensors throughout the sewer collection system in 2007. This was done in order to answer the question: Is the City making the most of the existing infrastructure? In other words, the City desired to know if there was unused capacity within the existing collection system, such as within the sewers and at the Wastewater Treatment Plant (WWTP), and then develop solutions to use the existing system to its full capacity and identify areas to prioritize improvements.

In December 2011, the US EPA approved the City of South Bend’s Long Term Control Plan document, which was created in an effort to record the historical context of the evolution of the plan. After a considerable amount of negotiations, the City of South Bend, Indiana and the U.S. Department of Justice entered into a Consent Decree relating to the Clean Water Act CSO Control Policy in early 2012.

The 2012 consent decree requires the City to spend approximately $600M (2012 Dollars) on wastewater treatment plant (WWTP) upgrades and CSO control methods, separated into two phases of work within the sewer collection system, to reduce both the volume and number of overflows to the river. Phase 1, includes source control technologies to prevent storm water from entering the collection system both upstream and downstream of the river. Phase 2 consists of the ultimate conveyance of excess flows from the existing CSOs to nine separate storage facilities.

As allowed for in the consent decree language, the City has continued to look for ways to improve upon and reduce LTCP costs. The City recently investigated the feasibility and potential cost savings of using a combination of low impact development, real time control, and conventional methods for CSO control. The resultant Optimatics Study was a feasibility and planning document that provided useful insight for optimization of the 2012 LTCP.

In addition, since 2010 several improvements have been made to the collection system as a result of real-time monitoring and remote throttle activation by CSONet. Improvements included reducing the total amount of combined sewer area via separation projects, raising weir heights, and throttling more flow to the main interceptor sewer to reduce overflows.

Reassessment of the LTCP

In 2014, the City initiated a reassessment of the 2012 LTCP in order to find additional cost savings. The reassessment of the 2012 LTCP will review system optimization and evaluate several options based on real-time technology. For example, a preliminary review of real-time data has identified that potential trunk line interconnections throughout the collection system may move flows from satellite areas of the system to the WTTP, eliminating the need for possible storage as planned in the 2012 LTCP. Opportunities for inline storage and regulating flows at other CSOs to the interceptor will also be evaluated. The reassessment of the LTCP will follow a comprehensive approach to include integrated and cost-effective solutions that also consider elements of green infrastructure solutions (GSI).

The City will provide updates on the reassessment of the LTCP throughout the process. An initial meeting was held in December of 2014 as an update to the Common Council, with a follow-up meeting held recently in February 2015 with the Common Council Utilities Committee. The next scheduled meeting with the Common Council Utilities Committee is scheduled for July 21, 2015. A history of the LTCP development is shown below.

Image depicting the development process of the Long Term Control Plan