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Combined Sewer System

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Combined Sewer System

Evanston recently completed its long range sewer improvement program. The program was designed to mitigate sewage from backing up into basements. The entire City is served by the combined sewer system, a system that carries both sanitary waste and storm water.

In Event of Sewer System Emergency

Call the Evanston Utilities Department, Sewer Division. An employee will come out to analyze the cause of the problem as soon as possible. Between 7:00 a.m. to 7:00 p.m., call 311 (847-448-4311 outside Evanston). After hours, call the Utilities Department emergency number, 847-475-6880.

Do not use toilets and sinks unless absolutely necessary, as water sent down your drain will end up in the basement.

Sewer System History

The Evanston combined sewer system was constructed as the city was developed, and the majority of the system is 80 to over 100 years old. Typical of sewer systems built at that time, the sewer conveyed sanitary waste as well as storm water during rain events. Amazingly, over 60% of the 144 miles of the combined sewer system is comprised of 12-inch diameter or smaller pipes. While this size is adequate for sanitary flow, it is not at all suited for storm water drainage. As long ago as 1902, the city’s combined sewer system was declared by the Commissioner of Public Works to be “inadequate in size and depth to serve the demands of the city.”


In 1990 the City Council approved a Long Range Sewer Improvement Program to mitigate property damage caused by sewage backing up into basements. The program consisted of the following components.

The installation of restrictors in drainage structures connected to the combined sewer to prevent the combined sewer from becoming overloaded even during extreme rainfall events including, but not limited to the 100 year design storm. This provided basement backup protection under nearly all situations.

The installation of larger diameter relief combined sewers at a depth that could convey combined sewer overflows to the Metropolitan Water Reclamation District (MWRD) deep tunnel. The storm water that is forced to stay on the street due to the restrictors flows downhill for up to two blocks where it is intercepted by high capacity inlets that flow into the relief sewer system. (For more information about the MWRD Tunnel and Reservoir Plan (TARP) visit their website at

In a few areas separate storm sewers were installed to convey surface drainage directly to the North Shore Channel.

This innovative approach of partial sewer separation combined with overland street flow was utilized because it was much more cost effective than installing a completely separate storm sewer system throughout the entire city.

The Long Range Sewer Improvement Program was constructed between 1991 and 2008 and cost approximately $210 million to complete. Initially the relief sewer system was installed prior to the installation of the restrictors. The relief sewer system consists of 9 miles of shallow tunnels ranging in size from 60 to 120 inches in diameter installed approximately 60 feet deep, and 43 miles of sewer mains ranging in size from 6 to 54 inches in diameter. The relief sewers are designed so that no street flooding should occur during a 5-year storm and only minimal flooding in isolated areas during a 10-year storm. After the relief sewers were installed, the installation of the restrictors in the drainage structures began. Drainage structures are restricted on most streets and alleys and will cause water to stay on the street surface. After a rain event ends, the storm water will slowly drain away and should be entirely gone within two hours after the rain stops.

Sewage Backup and Flooding

Causes of Sewage Backups:

  1. The City’s combined sewer system has become overwhelmed
    • More storm water has entered the combined sewer system than it can handle causing water to back-up into basements. Unfortunately there isn’t anything that can be done to relieve this type of flooding until after the rain event subsides and sewer flow returns to normal. If the water doesn’t drain away after the rain stops, contact the Water & Sewer Division.
  2. Blockage in the City sewer
    • Debris, roots or a sewer collapse blocks or hinders the flow of the sewage in the City’s combined sewer system. The Water & Sewer Division will correct the problem so that flow can be re-established.
  3. Blockage in the homeowners sewer lateral - pipe installed between the building and the City’s combined sewer system
    • Debris, roots, or a sewer collapse blocks or hinders the flow of sewage in the property owner’s sewer lateral, or the sewer lateral becomes overwhelmed by storm water. When notified of a basement backup, a Water & Sewer Division employee will be dispatched to investigate the cause of the back-up. They will inspect the City’s combined sewer system, and if the City’s system is functioning properly, they will advise the property owner that the basement flooding is a private problem.
    • The property owner will need to contact a plumber to clear the blockage or repair a collapsed pipe in the sewer lateral. We recommend that the homeowner seek three quotes for any type of sewer work. Property owners are welcome to contact the Water & Sewer Division to review the plumber’s recommendations, but ultimately the property owner must make the decision on how to proceed.
    • Frequently, private sewer back-ups are a result of the sewer lateral becoming overwhelmed by the amount of storm water entering it from roof gutters and downspouts. The Water & Sewer Division highly recommends that all downspouts discharge to the ground rather than being connected to the sewer lateral whenever feasible. This can be prevented by having a plumber inspect your sewer lateral every few years for any damage or blockage.



Protection During a Combined Sewer Overflow

  • Wash hands frequently!
  • Keep contaminated objects, water, and hands away from mucous membranes (eyes, mouth, and nose).
  • Avoid skin contact with sewer water. Be especially careful with cuts and sores. Keep them clean and covered.
  • If you receive a cut while working in flood or sewer water, contact your physician or Health Department about receiving a tetanus vaccine or booster.
  • Before entering the affected area, assess for potential electrical shock hazards and gas leaks. Gas leaks can occur in hot water tanks and furnaces.
  • Disinfect all areas, equipment, toys and objects that came into contact with sewer water with a solution of 8 tablespoons of liquid chlorine bleach to a gallon of water. Bleach is the most effective method for removing bacteria and odor, but it can cause discoloration of many materials.
  • Do not mix bleach with ammonia. This produces chlorine gas, a dangerous and toxic substance.
  • Machine wash contaminated clothes in hot water and soap. Add one cup of chlorine bleach to wash water.
  • Ventilate the affected area with floor fans and dehumidifier to properly dry the area.
  • Do not use heat to dry closed building interiors. Mildew and expanded water damage may result.
  • Small, loose rugs and wall-to-wall carpet installed on tacks can be cleaned professionally. In-plant cleaning is the best option. In-plant cleaning for wall-to-wall carpets that are glued down may not be practical or economical. All padding should be discarded.

What happens if the storm water doesn’t drain away after the rain stops?

Street flooding is cause by leaves or other debris can clog the grate of the drainage structures at the street level. Residents can help prevent this from occurring by cleaning the debris off the grate when the rain event starts. There is also the possibility the restrictor is blocked. In order to reduce the amount of flow entering the combined sewer system, the restrictors have a small 2 ½ inch diameter opening. If the restrictor becomes clogged, a city employee is needed to clear this blockage.

If the storm water doesn’t drain away one hour after a rain event or is damaging vehicles, please call the Utilities Department, Sewer Division seven days a week between 7:00 a.m. and 7:00 p.m. at 847-448-4311. After hours call the emergency number, 847-475-6880.

Alley Flooding

Alleys can flood as well. If you observe an alley flooding, consider the type of alley:

  • Gravel alleys do not have sewers. Unfortunately the Water & Sewer Division is not able to help relieve flooding in these types of alleys. Consider petitioning to get the alley paved. Refer to the Alley Improvements page for the Special Assessment Paving process.
  • Paved alleys with storm drainage structures can flood as well. The causes, correction, and prevention of flooding in paved alleys are the same as street flooding. Please refer to the street flooding section to find out what causes paved alleys to flood and what can be done to correct the problem.


Sewage Water in Basement: 311 or 847-448-4311. After Hours: 847-475-6880

Senior Handyman Program: 311 or 847-448-4311

Street Flooding: 311 or 847-448-4311. After Hours: 847-475-6880