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​​​​What are ​Combined Sewer Systems and CSOs?

Centralized sanitary sewer systems collect and convey sanitary sewage (consisting of domestic, commercial, and industrial wastewater) in pipes, pump stations, storage tanks, and other infrastructure to the wastewater treatment plant (WWTP) for treatment and discharge to a receiving water. Many cities also collect and convey stormwater runoff to receiving waters in centralized storm sewer systems.

Separate sewer systems collect and convey only sanitary sewage to the WWTP while municipal separate storm sewers (MS4s) collect and convey stormwater runoff directly to receiving waters.

Combined sewer systems are designed to collect sanitary sewage and stormwater runoff in a single pipe system for conveyance to a POTW treatment plant. These were common in the mid- to late-19th century before centralized wastewater treatment became the solution to protecting public health of the increasing populations located in urban areas. The majority of the combined sewer systems in Kentucky are located along the Ohio River because most of these river towns are older. Originally, combined sewer systems discharged continuously into receiving waters from outfalls throughout the sewer system to minimize the contact between people and sanitary sewage. After centralized wastewater treatment became available, regulators (also called diversions), such as orifices and weir dams, and interceptor sewers were added to the combined sewer system that directed the dry weather sanitary sewage to the WWTP. During precipitation events such as rainfall or snowmelt, the regulators convey some combined sewage to the WWTP but allow flows that exceed downstream capacity to go to combined sewer overflow (CSO) outfalls to discharge directly to receiving waters.  Without the regulators and CSO outfalls, the combined sanitary sewage and stormwater would overwhelm the capacity of the conveyance and treatment plant, potentially causing releases of untreated sewage in buildings, in streets or yards, or partially treated sewage from the treatment plant.

Illustration og diffrent types of wet weather conditions affecting sewer drainage.

Separate sanitary sewers are now the standard for conveyance of wastewater to a centralized wastewater treatment facility and construction of new combined sewers is prohibited except as allowed under 401 KAR 5:005, Section 8.

As of January 1, 2019, Kentucky has the following combined sewer systems and permitted CSO outfalls:
​KPDES Permit
​# of Permitted CSO Outfalls
City of Ashland
​City of Catlettsburg
​City of Frankfort
​City of Harlan
​Henderson Water Utility
​Louisville/Jefferson County Metropolitan Sewer District
​City of loyall
​Maysville Utility Commission
​City of Morganfield
​Paducah-McCracken County Joint Sewer Agency
City of Pineville
Regional Water Resource Agency
​Sanitation District No. 1 of Northern Kentucky
​City of Worthington

​For more information contact:​

Mark Absher, PE 
Municipal Section Supervisor

The U.S. Environmental Protection Agency (EPA) issued a National CSO Control Strategy [54 FR 37370, September 8, 1989] that requested that states develop statewide CSO permitting strategies and recommended that permits for municipal combined sewer systems include, at a minimum, BAT/BCT technology-based controls established according to the best professional judgment of the permitting authority. The Kentucky Combined Sewer Overflow Strategy was approved by EPA on January 29, 1991. 

EPA then published the CSO Control Policy on April 19, 1994, which represents a comprehensive national strategy to ensure that municipalities, permitting authorities, water quality standards authorities, and the public engage in a comprehensive and coordinated planning effort to achieve cost-effective CSO controls that ultimately meet appropriate health and environmental objectives. The CSO Control Policy recognizes the site-specific nature of CSOs and their impacts and provides the necessary flexibility to tailor controls to local situations. The Cabinet updated the Kentucky Combined Sewer Overflow Strategy to ensure consistency with the CSO Control Policy in 1997.

The Wet Weather Water Quality Act of 2000 [Consolidated Appropriations Act for Fiscal Year 2001, P.L. 106-554 ] amended the Clean Water Act to add section 402(q), which requires that permits issued after December 21, 2000, for a discharge from a municipal combined sewer system shall conform to the CSO Control Policy. 

Construction of combined sewers and separate sewers is regulated by 401 KAR 5:005

The U.S. EPA website  has information about CSOs and related programs.

The permitting provisions of the CSO Control Policy apply to all combined sewer systems that overflow as a result of stormwater flow from precipitation events, which includes stormwater runoff, snow melt runoff, and surface runoff and drainage. Dry weather flow is the flow in a combined sewer system that results from domestic sewage, groundwater infiltration, commercial and industrial wastewaters, and any other non-precipitation related flows (e.g., tidal infiltration). Discharges from combined sewer systems during dry weather are prohibited by the CWA, CSO Control Policy, and the KPDES permit. 

CSO outfalls are point sources subject to KPDES permit requirements including both technology-based and water quality-based requirements of the CWA. Information about CSO outfalls is included on Part G of Form A as part of the KPDES permit application for POTWs.

Nine Minimum Controls

Permittees with CSOs are required by Section II.B of the CSO Control Policy to immediately implement the Nine Minimum Controls (NMC). The activities used for the NMC are particularly applicable for combined sewer systems because the types, concentrations, and quantities of pollutants expected from CSO discharges are highly variable for each precipitation event. The Combined Sewer Overflows Guidance for Nine Minimum Controls [U.S. EPA, May 1995, EPA 832-B-95-003] provides examples of activities that may be used to address each control measure and guidance to evaluate and document implementation of the NMC, which are:

  • Proper operation and regular maintenance programs for the sewer system and the CSO outfalls
  • Maximum use of the collection system for storage
  • Review and modification of pretreatment requirements to assure CSO impacts are minimized
  • Maximization of flow to the POTW for treatment
  • Prohibition of CSOs during dry weather
  • Control of solid and floatable materials in CSOs
  • Pollution prevention
  • Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts
  • Monitoring to effectively characterize CSO impacts and efficacy of CSO controls
  • Combined sewer systems in Kentucky have all submitted a NMC Compliance Report documenting implementation of the NMC, including proposed schedules for completing any minor construction activities.

Long Term Control Plans

Permittees with CSOs are required by Section II.C of the CSO Control Policy to develop and implement long term CSO control plans (LTCP) that will ultimately result in compliance of CSO discharges with the requirements of the CWA. LTCPs consider the site-specific nature of CSOs and evaluate the cost-effectiveness and performance of a range of control options/strategies. Implementation of CSO controls may be phased based on the relative importance of and adverse impacts upon water quality standards and designated uses, as well as the permittee’s financial capability and its previous efforts to control CSOs. The Combined Sewer Overflows Guidance for Long-Term Control Plan [U.S. EPA, September 1995, EPA 832-B-95-002] provides guidance to assist permittees with CSOs in developing technically feasible, affordable, and comprehensive LTCPs consistent with the objectives of the CSO Control Policy. 

Combined sewer systems in Kentucky have all either submitted and are implementing an approved LTCP, or have separated their combined sewers and eliminated all CSO outfalls instead of submitting an LTCP.

Monitoring and Reporting

Permittees with CSOs should have a comprehensive, representative monitoring program that measures the appropriate frequency, duration, flow rate, volume or pollutant concentrations of CSO discharges in order to characterize CSO discharges and their water quality impacts. The monitoring program helps evaluate CSO controls alternatives during develop of the LTCP; evaluate impacts of CSOs on receiving waters to verify compliance with water quality standards and protection of designated uses; determine the effectiveness of NMC and CSO controls and that the CSO controls are constructed, operated, and maintained as designed in the LTCP; and determine compliance with permit limits. 

The KPDES permit requires permittees with CSOs to submit a comprehensive monitoring plan prior to completion of LTCP implementation that documents the locations, methods, and frequency of monitoring and sampling of the combined sewer system, and describes the criteria and calculations that will be used to evaluate the performance of the combined sewer system and overall success of the LTCP in meeting the requirements of the CSO Control Policy and the KPDES permit.

Discharge Monitoring Reports

KPDES permits for combined sewer systems include effluent limitations and monitoring requirements for the combined sewer system and for each permitted CSO outfall with monitoring results reported on Discharge Monitoring Reports similar to other outfalls at the POTW.  During implementation of the LTCP, the permit includes system-wide precipitation along with volume, duration, and number of occurrences of CSO discharges as a result of precipitation for each permitted CSO. Additional pollutant-specific and performance criteria monitoring and reporting requirements may be applied as appropriate. 

Dry Weather Overflows 

Dry weather overflows from CSO outfalls are prohibited and are reported to DOW, along with other non-compliance of the combined sewer system, in accordance with 401 KAR 5:065.

CSO Annual Reports

While some CSO discharge information is reported on DMRs, the KPDES permit requires additional information including compliance with NMC, progress of implementation of LTCP CSO controls, and annual summaries of monitoring results and combined sewer system performance to be provided in CSO Annual Reports. Submission of a CSO Annual Report is a requirement of the KPDES permit for permittees with CSOs. Annual reports cover an entire calendar year (January – December) and must be submitted to DOW by March 1 of the following year.  The CSO Annual Report for 2018 template is available and due March 1, 2019. All entries in the reports must be filled out completely or the report will be considered deficient. Failure to submit the report by the deadline may result in enforcement actions.

CSO Annual Reports may be submitted as a single .pdf file through the DEP/DOW ePortal at  Instructions on how to create an account and use the portal can be found in the “eDocument Module User Guide” found at:

These CSO Annual Reports are separate from progress reports that may be required to be submitted to the Division of Enforcement by enforcement orders, although some information may be similar and can be used in both types of reports.

A CSO outfall may be eliminated if it is made physically incapable of discharging sanitary sewage, such as by conversion of the combined sewer to a sanitary sewer and new separate storm sewers installed, conversion of the combined sewer to a storm sewer and new separate sanitary sewers installed, construction of new separate sanitary and storm sewers, or disabling and abandoning the regulator and overflow structures. The permittee may submit a CSO Outfall Elimination Certification form and supporting documentation that describes the actions and investigations done for the elimination of the CSO outfall.  If approved by DOW, the CSO outfall will be removed from the permit through a minor modification, with no further monitoring or reporting requirements for the CSO outfall.  Once removed from the permit, any discharge from an eliminated CSO outfall is not authorized and must be reported as non-compliance in accordance with 401 KAR 5:065. If separation of combined sewers results in all permitted CSO outfalls being eliminated, the sewer system is considered to be a separate sanitary sewer system and all permit conditions related to the CSO Control Policy are removed from the KPDES permit.

Once the LTCP is fully implemented, a permittee with CSOs may submit to DOW a completion report that describes what was approved in the LTCP and documents what was actually implemented for each CSO control and the resulting post-LCTP performance of the combined sewer system.  A sample format for the CSO LTCP Completion Report is available.

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