above_stlMuch of MSD’s jurisdiction, including most of St. Louis County, is serviced by a separate sewer system with two sets of pipes – one for wastewater and one for stormwater. In the separate system, wastewater sewers are designed to carry wastewater from, for example, sinks, dishwashers, showers, and toilets. Stormwater should be absorbed back into the ground or carried to a nearby creek or stream through stormwater sewers or natural drainage paths.

In the city of St. Louis and the near St. Louis County area, homes and businesses are served by a combined sewer system. When combined sewers were built, they were built with a finite capacity to handle wastewater and rainwater at the same time. (This is true of separate sewers as well – all sewers have a capacity limit.) The elimination of green space and natural areas has caused combined sewers to handle more and more rainwater and has put additional capacity demands on the system. During rainfalls, so much rainwater can sometimes enter a combined sewer that it becomes overcharged.

The wastewater portion of the separate sewer system was not designed to handle stormwater flows. However improper connections can allow stormwater to enter portions of the separate sewer system designed to handle wastewater only.

Click on the images below to view “Before MSD Project Clear” and “After MSD Project Clear” diagrams of connections to the public sewer system.

Consider a moderate to heavy rainfall. When a home or building contributes to stormwater entering the wastewater system, the carrying capacity of the wastewater pipe can be exceeded; there is just not enough room for both wastewater and stormwater in that pipe. As a result, basement or building backups and sewer overflows could cause untreated sewage to enter area basements and local waterways.

stomwater_street_001Remember, the backup may happen not only in the home or building with the improper connection; that connection can contribute to overflows and backups elsewhere through the web of pipes that make up the wastewater sewer system. Every connection matters.

MSD seeks to reduce the number of sewer overflows and basement backups in the region by helping the wastewater sewer do what it was designed to do – transport and treat wastewater only.

Keeping stormwater out of the wastewater sewer system will:

  • Reduce sewage backing up into homes and businesses.
  • Reduce untreated sewage overflowing into ponds, lakes, streams, and rivers.
  • Save ratepayers money, because treatment costs and unnecessary system expansions can be kept to a minimum.

Q: What is a wastewater sewer?
A: Wastewater sewers can be found in the St. Louis County service area, and are designed to carry wastewater from residences and businesses to a sewer treatment plant. Wastewater originates from toilets, sinks, dishwashers, and washing machines, and needs to be treated by MSD.

Q: Where should stormwater go, if not into the sewers?
A: Unlike wastewater, stormwater does not need to be treated so it can bypass MSD’s wastewater treatment plants. Stormwater is handled in different ways depending on where you live in MSD’s service area. Most customers rely on two systems: wastewater sewers to move wastewater to a treatment plant and separate storm sewers to convey rain and snowmelt to natural streams and creeks. Natural streams and creeks are a vital part of the stormwater removal system. Regardless of how stormwater is handled where you live, it should not go into the wastewater sewer.

Q: How does MSD identify a stormwater connection?
A: As a matter of routine system maintenance, MSD conducts sewer evaluation studies to assess the condition of the sewer system and identify areas in need of improvement. Common methods include:

  • Closed-circuit TV: uses electronic equipment to televise sewer pipes from the inside.
  • Smoke testing: harmless smoke is forced into manholes and traced to identify leaks in sewer pipes and places where stormwater can enter.
  • Dye testing: non-toxic dyed water is introduced into downspouts, driveway drains, or area drains and tracked to locate the pipe where the water flows.

MSD also encourages property owners to review their yard, driveways and structures to ensure all stormwater connections are identified. Sample photos of these connections can be found here.


Like to learn more?