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Energy and Environment Cabinet

Division of Water

Division of Water
What are combined sewers and combined sewer overflows?

What are combined sewers?

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

 What is a combined Sewer Overflow?

Example of Combined Sewer Overflow after RainfallAfter 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.


What are the risks associated with the discharge of raw sewage?

Raw sewage can carry a variety of human bacteria and viruses.  Depending on the amount and concentration of the sewage and on how people are exposed to it, these bacteria and viruses can cause illness. Combined sewer overflows also contain a variety of chemicals, oils and other wastes picked up by stormwater as it flows across parking lots, roads, lawns and other surfaces. Most combined sewer overflows occur during rain and are therefore diluted by rain and river water, but the potential for health and environmental hazards is still present. People most likely to be affected are water skiers, swimmers and others involved in water sports.

There are water quality standards in Kentucky that limit the amount of certain bacteria in water. When these standards are exceeded, the Division of Water, in conjunction with the Department for Health Services, issues advisories to alert citizens to potential health hazards.

 Combined Sewer Overflow

Some factors to take into account when determining the level of risk or characteristics of the discharge are time of day that the combined sewer system begins to discharge, ability of the publicly owned treatment works to capture the first flush, size of receiving surface waters and duration of the precipitation event.

Combined sewer overflows that occur in the absence of wet weather are called dry weather overflows. Dry weather overflows are usually a result of line blockages, power failures at pump stations, bottlenecks in the system or some other operation and maintenance issue. Adjusting the regulator settings of combined sewer systems for peak water usage during dry conditions will help ensure that dry weather discharges do not occur. Dry weather CSO discharges are prohibited under the National Pollutant Discharge Elimination System program. Dry weather overflows consist of concentrated wastewater, and the risks associated with exposure are therefore greater than wet weather combined sewer overflows.

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