Sanitary sewer overflow (SSO) is a condition whereby untreated sewage is discharged into the environment prior to reaching treatment facilities thereby escaping wastewater treatment. When caused by rainfall it is also known as wet weather overflow. It is primarily meaningful in developed countries, which have extensive sewage treatment facilities. The main causes of SSO are:

SSO is distinct from a combined sewer overflow (CSO), a similar problem of untreated sewage discharges, but which occurs only in a municipal system specifically designed with a combined sewer.


Wonga wetlands sewage plant

Sewage treatment plant used to create the Wonga Wetlands, Australia

Since medieval times rulers have been aware of the impact of raw sewage improperly discharged to the environment. Before treatment systems existed in 16th century England, King Henry VIII decreed that sewage troughs should be kept flowing so that they would not stagnate in London prior to reaching the River Thames. (London sewer system) It was not until the 19th century when America and parts of Europe developed wastewater treatment, that the concept of SSO materialized; however, communities were merely happy to have wastewater treatment systems, and did not complain greatly about overflows until the dawning of 20th century environmental awareness in the 1960s. At that time the USA began recognizing locations and frequencies of SSOs in a systematic way. Local governments heard complaints of citizens, beach closure protocols were systematised and the U.S. Environmental Protection Agency (EPA) began detailed engineering analyses for major cities across the country. After passage of the U.S. Clean Water Act in 1972, the 1980s saw spending of billions of dollars on system upgrading (although most funds during this period were spent on upgrades to secondary treatment rather than addressing SSO problems). In the 1990s Japan, the UK and a number of other European countries began earnest investigation of some of their countries’ overflow issues.

Magnitude of the problemEdit

EPA estimates that about 40,000 SSO events occur in the United States each year.[1] The Agency estimated that to upgrade every municipal treatment and collection system to reduce overflow events to no more frequently than every five years, would cost about 88 billion dollars as of 2004.[2] This cost would be in addition to approximately ten billion dollars already invested. Although the volume of untreated sewage discharged to the environment is less than .01 percent of all treated sewage in the US, the total volume amounts to several billion US gallons per annum and accounts for thousands of cases of gastrointestinal illness each year.[2]:Ch. 6 Advanced European countries and Japan have similar or somewhat larger percentages of SSO events.[citation needed]

Engineering aspectsEdit


Sanitary sewer line in Paris more than two meters in diameter.

By far the most prevalent cause of Sanitary Sewer Overflow stems from heavy rainfall events which can cause massive infiltration of stormwater into sewerage lines. The combined flow of wastewater and stormwater exceeds the capacity of the sewer system and is released into local waterways to prevent flooding in homes, businesses and streets. This circumstance is most prevalent in older cities whose subsurface infrastructure is quite old; Paris, London, Stockholm[3], New York, Washington, DC, and Oakland, California[4] are typical examples of such locations. Inflow into the sanitary lines can be caused by tree root rupture of subsurface lines or by mechanical fracture due to age and overpressure from trucks and buildings above.

Other modes of system failure can include power outage which may disable lift station pumps or parts of the treatment plant operations themselves; in fact, any mechanical system failure within a treatment plant can create a circumstance leading to overflow: breakdown of rotating arms of trickling filters, jamming of line gates, clogging of filters or grates etc. Furthermore, some forms of human error can infrequently lead to diversion of sewage and result in an overflow event.

Decentralized failures in dry weather mainly occur from collection sewer line blockages, which can arise from a debris clog, line rupture or tree root intrusion into the line itself. One of the main problems of a decentralized line failure is the difficulty of defining the location of overflow, since a typical urban system contains thousands of miles of collection pipage, and the central treatment plant has no way of communicating with all the lines, unless expensive monitoring equipment has been installed.

Human health and ecological consequencesEdit


Human health impacts include significant numbers of gastrointestinal illness each year, although death from one overflow event is uncommon. Additional human impacts include beach closures, swimming restrictions and prohibition against consumption of certain aquatic animals (particularly certain molluscs) after overflow events. Ecological consequences include fish kills, harm to plankton and other aquatic microflora and microfauna. Turbidity increase and dissolved oxygen decrease in receiving waters can lead to accentuated effects beyond the obvious pathogenic induced damage to aquatic ecosystems. It is possible that higher life forms such as marine mammals can be affected since certain seals and sea lions are known to experience peaks in pathogenic harm.[5]

Worldwide perspectiveEdit

It is difficult to visualize the issue of SSO in perspective, since underdeveloped countries discharge most of the sewage they create as effluent into the environment. Even a highly industrialized country such as the People's Republic of China discharges about 55 percent of all sewage without treatment of any type.[6] In a relatively developed Middle Eastern country such as Iran, the majority of Tehran's population has totally untreated sewage injected to the city’s groundwater.[7] In Venezuela, a below average country in South America with respect to wastewater treatment, 97 percent of the country’s sewage is discharged raw into the environment.[8] Most of sub-Saharan Africa is without wastewater treatment, contributing to the excessive infant death rates in that region.

While developed countries such as the US, Canada, most Western European states, Australia and Japan are considered to be struggling with a public health problem of SSO prevention, the underdeveloped countries of the world discharge approximately 20,000 times the amount of raw sewage into the environment as those advanced countries collectively, approximately (the equivalent of) 100 trillion US gallons (380 km³) of untreated sewage per annum. This dichotomy of expenditure and public health benefit is arguably the greatest disparity between developed and underdeveloped nations as of the year 2006.[citation needed]

See alsoEdit


  1. U.S. Environmental Protection Agency (EPA). Washington, DC. "Sanitary Sewer Overflows." Accessed March 28, 2008.
  2. 2.0 2.1 EPA. "Report to Congress: Impacts and Control of CSOs and SSOs." Executive Summary. August 2004. Document No. EPA-833-R-04-001.
  3. Impeller Magazine, published by Monica Spendilow, ITT Flygt AB, SE-174 87 Sundbyberg, Sweden (2004)
  4. Environmental Impact Statement for the East Bay Municipal Utility District Wet Weather Overflow Project, Earth Metrics Incorporated, prepared for the U.S. EPA and East Bay Municipal Utility District, Oakland, Ca. (1985)
  5. Johnson, S.P., Jang, S., Gulland, F.M.D., Miller, M., Casper, D., Lawrence, J., Herrera, J., "Characterization and clinical manifestations of Arcanobacterium phocae infections in marine mammals stranded along the central California coast." Journal of Wildlife Diseases, 39:136-144 (2003).
  6. World Bank Supports China's Wastewater Treatment, The People’s Daily, Friday, November 30, 2001, Beijing
  7. Massoud Tajrishy and Ahmad Abrishamchi, Integrated Approach to Water and Wastewater Management for Tehran, Iran, Water Conservation, Reuse, and Recycling: Proceedings of the Iranian-American Workshop, National Academies Press (2005)
  8. Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region, Caribbean Environment Programme Technical Report #40 1998

External linksEdit

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