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Ky.gov An Official Website of the Commonwealth of Kentucky

Energy and Environment Cabinet

Division of Water

Division of Water
Combined Sewer Overflows

Municipalities with combined sewer systems must include all wet weather discharge points in the discharge permit applications for their respective publicly owned treatment works (POTWs). This information is reported on Part G of Form A as part of the application process.

Combined sewer systems were the norm for 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. Originally, combined sewer systems discharged continuously into the surface waters at various outfalls along the system to minimize the contact between people and waste. The majority of the combined sewer systems in Kentucky are located along the Ohio River because most of these river towns are older. Also, some larger cities opted for combined sewers because they were the cheapest option. If you have walked the streets of Philadelphia, New York or Paris, you have most likely walked over large, complex combined sewer systems without realizing it.

The other sewer design option is the construction of sanitary sewers. Sanitary sewers are sewer systems designed to transport only domestic, commercial and industrial wastewater. In most cases, stormwater runoff is collected and directed to the nearest surface waterbody through the use of storm sewers. Sanitary sewers are now the standard for transport of wastewater to a centralized wastewater treatment facility. Construction of new combined sewers is prohibited under 401 KAR 5:005, Section 8.

 wet and dry conditions for combined and separated sewers

After centralized wastewater treatment became available for cities with combined sewer systems, adjustments were made to the system that directed the dry weather flow to the treatment plant.  However, the surface water outfalls were left in place with diversion structures such as weirs and dams to create “relief” points during times of increased precipitation or heavy rain. If the wastewater flow is too much for the capacity of the combined sewers, then the wastewater will spill over the diversion structure and out an outfall into the surface water. These types of discharges are called combined sewer overflows, or CSOs (see the graphic above).  Without the combined sewer outfalls, the wastewater and extra stormwater would overwhelm the capacity of the combined sewers during major wet weather events and cause basement backups in private residences or discharge through manhole structures. These types of overflows can occur in separate sanitary sewer systems and are commonly called sanitary sewer overflows, or SSOs.