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Metropolitan St. Louis Sewer District requirements to reduce sewer overflows


A new federal consent decree imposes the Metropolitan St. Louis Sewer District (MSD) to make more efforts to reduce sewer overflows.

The MSD operates and maintains the sewer system for the City of St. Louis and most of St. Louis County.  It is the fourth-largest sewer system in the United States. The collection system delivers flow to seven wastewater treatment plants. A combined sewer system serves approximately 75 sq mi of the MSD’s service area and discharges 13 billion gal of overflows per year.

The MSD has spent $2.3 billion during the past 20 years to reduce overflows; however they will have to spend twice as much over the next 20 years.  In 2007, the MSD was sued by the US Department of Justice and the State of Missouri for violating the Clean Water Act. Portions of the system are over 150 years old, and infiltration and inflow from old pipes lead to water quality problems.

The Missouri Coalition for the Environment tried to find a solution to resolve the Clean Water Act violations and came to an agreement on August 4. This agreement was a consent decree that imposes the MSD to spend $4.7 billion over the next 23 years to upgrade its sewer infrastructure to reduce overflows. The MSD has some requirements to observe and some measures to take:

  • Reduce overflows and eliminate sanitary sewer overflows. The MSD must complete a ‘’master plan’’ before 2013 for eliminating approximately 200 overflows from its sanitary sewer system.
  • Address Combined Sewer Overflows (CSOs): this measure starts with the submitting of a long-term control plan to the State of Missouri. The MSD has to increase the storage capacity of the combined sewer system and has to find a way to send stored flows to wastewater treatment plants after peak volumes. To reduce CSOs, the MSD will separate the sanitary and storm sewer functions in some sections of the combined sewer systems.
  • To increase the storage capacity, the MSD must spend at least $20 million on efforts to mitigate the effects of wet-weather surcharging and overland flooding caused by the insufficient capacity of the combined sewer system.
  • Implement primary treatment and disinfection to overflow before they are discharged to the environment.
  • Expand secondary treatment capacity.
  • Implement a mix of ‘’green’’ infrastructure (rain gardens, bioswales) with traditional ‘’gray’’ infrastructure. The consent decree requires spending at least $ 100 million on projects intended to reduce stormwater runoff from impervious surfaces.  The MSD has created a five-year pilot program to implement green infrastructure on a large scale in its combined sewer area.

The consent decree and its cost will have consequences on the MSD’s rates. The District will increase its rates and will spend more than $ 1 billion on capital improvements.    

 If you have any questions about sewer overflows, please contact John McAllister at or at (508) 747 - 7900 x 117.

 Information in this article taken from November 2011, article ‘’St. Louis MSD to spend $4.7 billion over 23 years to reduce sewer overflows‘’ by Jay Landers, published in Civil Engineering. 

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