Sanitary sewer systems collect and transport domestic, commercial and industrial wastewater and limited amounts of stormwater and infiltrated groundwater to treatment facilities for appropriate treatment. Sanitary sewers are different than combined sewers, which are designed to collect large volumes of stormwater, in addition to sewage and industrial wastewater. Occasionally, sanitary sewers will release untreated or partially treated sewage from a municipal sanitary sewer into the environment before reaching sewage treatment facilities. These are called sanitary sewer overflows (SSO). 

All permitted wastewater treatment facilities must report SSOs or bypasses from their treatment works or collection system to the department within 24 hours of discovering the event. The department developed a convenient online system for reporting SSO and bypass events, which the facilities can access through the Missouri Gateway for Environmental Management (MoGEM) online portal system.

Why do sewers overflow?

SSOs occasionally occur in almost every sewer system, even though systems are intended to collect and contain all the sewage that flows into them. When SSOs happen frequently, it means something is wrong with the system. Problems that can cause SSOs include:

  • Inappropriate materials sent to the sewers 
  • Tree roots entering through defects or openings in a sewer line 
  • Leaky sewers
  • Inappropriate connections 
  • Improper or inadequate maintenance and cleaning of sewers
  • Inadequate pump maintenance and lack of backup power
  • Undersized sewers and/or pumps
  • Equipment failures and breaks

What health risks do SSOs present?

Because SSOs contain raw sewage they carry bacteria, viruses, protozoa (parasitic organisms), helminths (intestinal worms) and inhaled molds and fungi. Individuals may be exposed through:

  • Sewage in drinking water sources
  • Direct contact in areas of high public access such as basements, lawns, streets or waters used for recreation
  • Shellfish harvested from areas contaminated by raw sewage 
  • Inhalation and skin absorption 

Exposure may cause diseases ranging in severity from mild gastroenteritis (causing stomach cramps and diarrhea) to life-threatening ailments such as cholera, dysentery, infections, hepatitis and severe gastroenteritis.

What other damage can SSOs cause?

  • Property damage
  • Environmental issues

How can SSOs be reduced or eliminated?

  • Cleaning and maintaining the sewer system
  • Reducing infiltration and inflow through system rehabilitation and repairing broken or leaking service lines
  • Enlarging or upgrading sewer, pump station or sewage treatment plant capacity and reliability
  • Limiting fats, oils and grease that go into the sewer system
  • Educating the public on how fats, oils, grease and certain household products can clog sewers
  • Constructing wet weather storage facilities
  • Expanding the capacity of the treatment works

Communities also should address SSOs during sewer system master planning and facilities planning, or while extending the sewer system into previously unsewered areas.

What costs are involved with reducing or eliminating SSOs?

  • Sanitary sewer collection systems are a valuable part of the nation's infrastructure, with an estimated worth of more than $1 trillion. Sewer rehabilitation to reduce or eliminate SSOs can be expensive, but the cost must be weighed against the value of the collection system asset and the added costs if this asset is allowed to further deteriorate. Ongoing maintenance and rehabilitation adds value to the original investment by maintaining the system's capacity and extending its life.
  • The costs of rehabilitation and other measures to correct SSOs can vary widely by community size and sewer system type. Those being equal, however, costs will be highest and ratepayers will pay more in communities that have not put together regular preventive maintenance or asset protection programs in place.
  • Assistance is available through the Clean Water Act State Revolving Fund (CWSRF) for capital projects to control SSOs.