Today's situation

The last three decades have brought increasing national and local efforts to clean up our streams.  Large sewage treatment plants have been built and upgraded; small treatment plants are being upgraded or eliminated; new sewer lines are eliminating septic tanks.

Now, the pollution from stormwater — and especially from CSOs — is getting increasing attention.  The U.S. Clean Water Act requires cities and states to come up with plans to reduce this pollution.

MSD's program

MSD has been grappling with the problem since the early 1980s and began a detailed engineering study of its CSOs in 1989.   MSD engineers and consultants identified and mapped more than 120 CSOs.  From this inventory, they developed a computer model of the combined sewer system, to help them understand and predict the conditions that would cause the sewers to overflow.

The next step was to "calibrate" this computer model by installing water flow meters in the sewers and recording what actually happens during rainstorms.  At the same time, the engineers began studying ways to modify the system to reduce the overflows without major construction projects.

The task will be difficult, because the size of the combined sewers can be much larger than the interceptors they flow into.   The Trevilian Way combined sewer, for instance, can carry 35 times as much water as the Beargrass Interceptor into which it discharges — and the interceptor already is carrying some water when it reaches Trevilian Way.  The rest overflows into the creek.  The Beals Branch Sewer can carry 64 times as much water as it can discharge into the Middle Fork Trunk interceptor; the rest overflows into the Middle Fork of Beargrass Creek just south of Lexington Road.  And the Northwestern Trunk sewer can carry nearly 26 times as much water as the Western Interceptor; the rest overflows into the Ohio River.

There are three basic ways to reduce the overflows:

• Construct separate storm and sanitary sewers throughout the combined sewer area (an extremely expensive and disruptive option);
• Find ways to treat the overflow water before it is discharged into the streams;

• Find ways to store the overflow water and release it into the sewer lines later, when the rain is over and the sewers have room for it.

One possibility is to use the large sewer lines to store rainwater until after the storm, then release it slowly into the interceptors.  This can be done by raising the height of the weirs (or dams) inside the combined sewer lines.  In many areas, however, this might increase the possibility of basement backups, and cannot be considered.

Another possible solution is to build large underground drainage reservoirs and pipes to store the excess water until it can be treated.  This would be expensive — but not as expensive as building a separate drainage system.

Another possibility is to change the way the flood pumping stations are operated, to reduce the amount of raw sewage pumped into the river.

By the end of 1993, MSD hopes to accomplish three major steps in its CSO abatement program:

• Complete the engineering studies that will define the extent of the problem, based on the computer model and actual experience;

• Make as many cost effective changes as possible to reduce the overflows; and

• Complete the master plan for future action to meet the federal and state water pollution standards for CSOs.

At that time, the estimated cost to the community will also be known.

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