Facing the challenge of combined sewer overflows

Combined sewer overflows (CSOs) are a major source of stream pollution in most older cities.  Ironically, they are also a heritage of society's early efforts to reduce stream pollution.

Combined sewers carry both sanitary waste and stormwater drainage.  CSOs are outlets that dump excess water from the sewers into streams and rivers, keeping the sewers from backing up into homes, business and streets when it rains.

While combined sewers have fallen into disfavor since World War II, they were once considered a major advance in improving public health and safety.  Louisville's story is typical of the nation's older cities.

Early sewers

For its first 40 years, Louisville had no sewers and no drainage system.  Sanitary waste went into outhouses; washwater was dumped into yards, streets and alleys; rainwater eventually found its way into streams and the Ohio River.  In the flat areas of the city, much of the water simply stayed, forming large, stagnant ponds that bred mosquitoes that spread disease.

In the 1820s, the city started digging ditches to drain the ponds; over the years, underground drainage pipes were added.   This first drainage system took the water directly to small streams and to the Ohio River.

The Louisville Water Company opened for business in 1860, bringing running water to the city.  Running water brought "indoor plumbing," and the need for a way to dispose of the water after it was used.  The solution seemed simple: build wastewater lines and connect them to the city's drainage system.

It wasn't long before the streams stank with sewage.  The situation was especially bad in dry weather, when sewage was often the only water flowing in the small streams.

How a CSO works:

In this simplified illustration, the combined sewer line is blocked by a low weir, or dam, before it reaches the stream.   The weir diverts the flow into the interceptor sewer, which takes it to a sewage treatment plant.  In dry weather, all of the flow is sanitary sewage, and the interceptor line can handle it.  In wet weather, stormwater mixes with the sanitary sewage, increasing the flow.  If the flow is large enough, part of the water may flow over the weir and through the CSO into the stream.  Most overflows are equipped with valves that allow overflow water to enter the stream, but prevent stream water from entering the sewers.  The simplest is the flap valve, shown here.  Some valves may be inside the sewer lines, where they cannot be seen from the creek.

Early cleanup efforts

The early solution to this problem was the interceptor sewer, designed to "intercept" the sanitary sewage before it could reach the streams.  The interceptors were designed to carry the normal load of sanitary sewage — but not the large amounts of additional water from rainfall.   CSOs were installed along the interceptors to allow excess water to flow into the streams when it rained.

This was a great improvement.   In dry weather, the creek looked and smelled much better.  When it rained, the rainwater diluted the sanitary sewage and helped carry the overflow away.  The streams were still polluted, but not nearly as much as before.  And they didn't spread as much disease.

Louisville's first interceptor sewer was built in the early 1900s along the South Fork of Beargrass Creek.  It carried the sewage directly to the Ohio River.  Later, an interceptor was built along the Middle Fork of Beargrass Creek, also emptying into the river.  The Ohio River Interceptor wasn't built until the 1950s, after the state ordered the city to construct a sewage treatment plant and stop dumping raw sewage into the river.

All of the city's major combined sewer interceptors have combined sewer overflows.  There are more than 50 overflows along the South and Middle Forks of Beargrass Creek.  There are some very large ones along the Ohio River, serving the central, western, southern and southwestern parts of the city.  In places where there were severe drainage problems and no nearby creeks, large underground relief drains were constructed to take the water from combined sewer overflows directly to Beargrass Creek or the river.

Several of the large pumping stations that are part of the Ohio River floodwall system also handle sanitary sewage.   They were designed before the city had a sewage treatment plant.  When the river is at its normal level, they pump the sewage into the Ohio River Interceptor.   But when the river rises a few feet, they pump raw sewage along with stormwater directly into the river.

Most of the CSOs in Jefferson County are in the city of Louisville, in the area served by the Morris Forman Wastewater Treatment Plant.  The Middle and South Forks of Beargrass Creek have so many overflows that they overlap and obscure the creeks on this map.  The Ohio River has fewer overflows, but they're much larger.

Two of the major relief drains — Central and Fourth Street — serve the downtown area and Old Louisville and flow directly into the Ohio River.  The Sneads Branch Relief serves an area along Shelby Street from near Eastern Parkway to Kentucky Street, and flows directly into the South Fork of Beargrass Creek.  Much of the stormwater from the western, southwestern and southern parts of the city flows through large sewer lines toward the Ohio River, then overflows into the river.

Combined Sewer Overflows continued

For more information, please contact:  Angela Akridge