Combined sewers were built to collect sanitary and industrial wastewater, as well as storm water runoff, and transport this combined wastewater to treatment facilities. Flows conveyed to the treatment plant are then treated and discharged to a nearby river or stream.

Ohio EPA no longer permits the installation of combined sewers. For newly installed sewers, sewage and storm water are collected in separate pipes with sewage being transported to a wastewater treatment facility and storm water being transported to nearby surface waters. This type of system is called a separate sewer system.

View an illustration depicting both a combined and a separate sewer system.

During dry weather and small wet weather events (i.e., rainfall and snowmelt), combined sewers are designed to transport all flows to a treatment plant. During larger wet weather events the volume of storm water entering the combined sewer system may exceed the capacity of the combined sewers or the treatment plant. When this happens, combined sewers are designed to allow a portion of the untreated combined wastewater to overflow into the nearest ditch, stream, river or lake. This prevents the rupturing of pipes, backing up of sewage into basements, and/or flooding of streets. The locations where these discharges of untreated combined wastewater occur, as well as the discharge events themselves, are known as Combined Sewer Overflows (CSOs). CSOs are located at various locations along combined sewers, and are unique to each system.

CSOs contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. This is a major water pollution concern for cities with combined sewer systems. CSOs are among the major sources responsible for beach closings, shellfishing restrictions, aesthetic impairments and other water body impairments. Additionally, contact with discharges from CSOs can have adverse effects on human health. A December 2001 report to Congress cited the Center for Marine Conservation as stating that, "[s]ome of the common diseases include hepatitis, gastric disorders, dysentery, and swimmer's ear. Other forms of bacteria found in untreated waters can cause typhoid, cholera, and dysentery." (Report to Congress on Implementation and Enforcement of the CSO Control Policy, December 2001, EPA 833-R-01-003)

Ohio has approximately 1,233 permitted CSOs in 81 remaining communities (August 2011), ranging from small, rural villages to large metropolitan areas.

View an inventory of Ohio's CSO communities.

CSOs are located throughout Ohio. View maps of individual CSO outfall locations organized by community.

CSO outfalls and their receiving waters should be avoided during and immediately after any wet weather event (i.e., rainfall and snowmelt). Additionally, these locations should be avoided any time a discharge is observed from the outfall pipe regardless of weather conditions.

Ohio EPA continues to implement CSO controls through provisions included in NPDES permits and using orders and consent agreements when appropriate. The NPDES permits for our CSO communities require them to implement the nine minimum control measures. Requirements to develop and implement Long Term Control Plans (LTCPs) are also included where appropriate.

In 2010, U.S. EPA modified their goal for the Water Safe for Swimming Measure, which seeks to address the water quality and human health impacts of CSOs. The goal includes incorporating an implementation schedule of approved projects into an appropriate enforceable mechanism, including a permit or enforcement order, with specific dates and milestones for 86% of the nation's CSO communities by the end of September 2011. As of August 2011, 77 of Ohio's 89 CSO communities meet this definition which equates to 86%.

Community-specific Solutions

• City of Columbus. Ohio EPA has issued final approval of the City of Columbus' wet weather management plan and modifications to two of the City's wastewater discharge permits. Go here for more information.

• More LTCP summaries.

There are multiple ways in which the general public can help reduce the impacts of CSOs. An initial step is to contact your local wastewater utility to notify them of your interest and obtain information specific to your city or village's program. Some municipalities even implement cost-sharing programs to lower the financial aspect of some measures. Topics to discuss include: 

• Water conservation techniques

• Disconnection of gutter downspouts and other sources of clean water that may be connected to your combined/sanitary sewer system

• Proper disposal of grease and/or hazardous materials

• Litter prevention

• Proper disposal of yard wastes

• Composting

• Green Infrastructure (e.g., rain gardens, rain barrels, green roofs, etc.)

The following links will take you to web sites maintained by the given municipality:

• Columbus Sewer Overflow Discharge Information - An interactive map shows the locations of the known CSOs and SSOs (sanitary sewer overflows) on the Columbus sewer system. Overflow information is available. Information on the City’s "Wet Weather Management Plan" is available through the "Frequently Asked Questions" link.

• Metropolitan Sewer District of Greater Cincinnati - CSO advisories are available by phone or by e-mail through the CSO Public Notification link. Other CSO-related information is available though the Consent Decree, Wet Weather Improvement Plan, and Sewer Overflows links.

• Northeast Ohio Regional Sewer District (NEORSD) - See the Notification tab on the left for information on CSO locations and on the dates and locations of overflow occurrences.

• Toledo Sewer Overflow Report - Information is available on the location, date and duration of CSOs from the Toledo sewer system. Information on the City’s long-term control plan for CSOs is available through the "LTCP" link.

• U.S. EPA Combined Sewer Overflow Control Policy, April 1994

• U.S. EPA CSO web page - Links to principal guidance documents, CSO control fact sheets and outreach materials, reports to Congress, and other information.

• U.S. EPA Green Infrastructure web page - Links with basic information, technical information, case studies, funding and other information.

• The Wet Weather Partnership - Formerly The CSO Partnership, this is a nationwide coalition of communities promoting environmentally sound, cost effective and affordable controls for CSOs and urban storm water. Ohio EPA has co-sponsored several CSO workshops with the Partnership.

• Wet Weather Operating Practices for POTWs With Combined Sewers  and Appendices  - New York State DEC Technology Transfer Document.

• Urban Wet Weather Flow graphic