What are combined sewers?  How do they work?  Why do we have them?  How can they be eliminated?

What is a Combined Sewer?

When we dispose of household wastewater, it travels through sanitary sewers to a sewage treatment plant. In some cities (usually older ones), stormwater runoff from house roofs, parking lots, and streets empties into the same sewer system that carries household wastewater to sewage treatment plants. This system is referred to as a combined sewer.

Are Combined Sewers a problem? If so, why do we have them?

Sewer systems built before the mid-20th century disposed of household wastewater simply by discharging it directly into rivers and streams. These sewer lines also carried stormwater runoff to the river. Because of concern for water quality and public health, cities built sewage treatment plants where wastewater is treated before it is discharged. New sewer lines were constructed to carry household wastewater to these treatment plants. Diversion dams were built in the old sewer lines to divert sewage into the new system and to prevent sewage from discharging into the river except during rain.

However, the old river discharge openings, or outfalls, were left in place to act as "relief valves" to prevent sewage from backing up into people's homes during storms. Since combined sewers also carry stormwater, rain increases the volume of water in the system, which then overflows the diversion dams allowing raw sewage to flow into the rivers. These overflows are referred to as Combined Sewer Overflows, or CSOs.

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, etc. Sewage 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.

Why are cities and towns allowed to have Combined Sewers?

In 1989, the U. S. Environmental Protection Agency (EPA) adopted a Combined Sewer Overflow Control Policy designed to reduce and eliminate combined sewer overflows nationwide. States were required to develop strategies to reduce pollution from combined sewers and bring discharges into compliance with the federal Clean Water Act. More than 1,000 cities in the U.S. had combined sewer systems at that time. In Kentucky, combined sewers are present in several systems, the majority of which are along the Ohio River. For this reason, Kentucky's strategy incorporates not only the federal EPA strategy, but also that of the Ohio River Valley Water Sanitation Commission (ORSANCO) .  See information about ORSANCO's Combined Sewer Overflow Studies.

Will it be expensive to reduce and eliminate combined sewer overflows?

Preventing stormwater runoff from entering sewers is an enormous and expensive project. Solutions and their expense depend on what percentage of a city's sewers are combined sewers. In some cases, only a portion of the sewer system is combined; while in other cities, the entire sewer system receives stormwater runoff.

The solutions a city chooses will depend on a variety of factors unique to the individual situation. However, any solution is probably going to be expensive and will disrupt the routines of citizens. To pay for major construction, the city may have to raise large amounts of money through the sale of bonds, loans and grants from government agencies, increases in residential sewer bills, and expenditures of general fund money. In some cases, a city may have the added expense of upgrading its sewage treatment plant.

Since sewer lines are often located below city streets, construction will often inconvenience citizens. Yet, even though the project is expensive and is often disruptive, the outcome is cleaner water for all of us.