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National Pollutant Discharge Elimination System (NPDES)

Sanitary Sewer Overflow (SSO) Frequent Questions

On this page:

  • What are sanitary sewer overflows?
    • Sanitary Sewer Overflows (SSOs) are a release of untreated or partially treated sewage from a municipal sanitary sewer.
  • 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 – materials such as fats, oils and grease (FOG), and some household products (including some marked ‘flushable’) such as baby wipes, facial wipes, sanitary pads, and tampons. All of these may create blockages,
      • Tree roots entering through defects or openings in a sewer line may cause blockages,
      • Leaky sewers – stormwater, ground water and snowmelt entering the sanitary sewer from cracks and faults in the sewer or leaky sewer joints can overload a sanitary sewer,
      • Inappropriate connections – Connections of sources of water such as sump pumps, roof leaders, foundation drains and area drains can overload a sanitary sewer,
      • Improper or inadequate maintenance and cleaning of sewers,
      • Inadequate pump maintenance and lack of backup power,
      • Undersized sewers and/or pumps, and
      • 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. As a result, they 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.
    • People may be exposed through:
      • Sewage in drinking water sources.
      • Direct contact in areas of high public access such as basements, lawns or streets, or waters used for recreation. At least one study has estimated a direct relationship between gastrointestinal illness contracted while swimming and bacteria levels in the water.
      • Shellfish harvested from areas contaminated by raw sewage. One study indicates that an average of nearly 700 cases of illness per year were reported in the 1980s from eating shellfish contaminated by sewage and other sources. The number of unreported cases is estimated to be 20 times that.
      • Some cases of disease contracted through inhalation and skin absorption have also been documented.
    • More information on the extent of environmental and human health impacts caused by SSOs can be found in EPA’s 2004 Report to Congress: Impacts and Control of CSOs and SSOs.
  • What other damage can SSOs do?
    • SSOs also damage property and the environment. When basements flood, the damaged area must be thoroughly cleaned and disinfected to reduce the risk of disease. Cleanup can be expensive for homeowners and municipalities. Rugs, curtains, flooring, wallboard panels and upholstered furniture that come into contact with sewage usually must be replaced.
    • A key concern with SSOs that enter oceans, bays, estuaries, rivers, lakes, streams, or brackish waters is their effect on water quality. When bodies of water cannot be used for drinking water, shellfish harvesting, fishing, or recreation, society experiences an economic loss. SSOs can close beaches. Tourism and waterfront home values may fall. Fishing and shellfish harvesting may be restricted or halted.
  • How can SSOs be reduced or eliminated?
    • Many avoidable SSOs are caused by inadequate or improper operation or maintenance, inadequate system capacity, and improper system design and construction.
    • SSOs can be reduced by:
      • Sewer system cleaning and maintenance.
      • 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/or reliability.
      • Controls limiting fats, oils and grease (FOG) into the sewer systems.
      • Construction wet weather storage facilities.
      • Expanding the capacity of the treatment works.
      • Educating the public on how FOG and certain household products can clog sewers.
    • 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. EPA estimates that our nation's sewers are worth a total of more than $1 trillion. The collection system of a single large municipality is an asset worth billions of dollars and that of a smaller city could cost many millions to replace. 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. State Revolving Funds in each state and Puerto Rico can help arrange low-interest loans.

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