Sanitary sewer overflow

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A manhole cover unable to contain a sanitary sewer overflow

Sanitary sewer overflow (SSO) is a condition in which untreated sewage is discharged into the environment prior to reaching sewage treatment facilities. In Europe the term Combined Sewer Overflow (CSO) is often used. When caused by rainfall it is also known as wet weather overflow. It is primarily meaningful in developed countries, which have extensive treatment facilities. The main causes of SSO and CSO dry spills are:

Whatever the name SSO and CSO the net effect on the environment is the same and in many countries there are obligations to measure and report the occurrence using real-time telemetry to warn the public, bathers and shellfishery operators. In Europe the revised Bathing Water Directive call upon water companies to actively monitor CSOs and to publish league tables on bathing water quality, often referred to as the Blue Flag system. The process of monitoring is called event duration monitoring (EDM), which on critical CSO is set to two-minute sampling and live reporting. On less critical CSOs the sampling rate is 15 minute.

The concept of CSO / SSO containment valves is something that has been pioneered in the UK and they are installed to mitigate what called dry spills, by correlating rainfall data with CSO / SSO spill activity.

In all cases both EDM monitoring and in particular CSO spill containment requires real-time data and this usually is delivered either using the cell phone network or radio telemetry, where the occurrence of a spill is detected using a form of flow detection transducer (FDT), rather than a level transducer that is used for blockages.

Magnitude of the problem[edit]

EPA estimates that about 40,000 SSO events occur in the United States each year.[1] The Agency estimated that upgrading every municipal treatment and collection system to reduce the frequency of overflow events to no more than once 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 0.01% of all treated sewage in the United States, the total volume amounts to several billion U.S. 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.[3][not in citation given]

Engineering aspects[edit]

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

By far the most common cause of Sanitary Sewer Overflow are heavy rainfall events, which can cause massive influx of stormwater into sewerage lines. The combined flow of wastewater and stormwater exceeds the capacity of the sewer system and sewage 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,[4] New York, Washington, DC, and Oakland, California[5] 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 outages, 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.

Historically, one of the biggest causes of CSO / SSO operation is the overloading of the sewer during storms and there has been a call to reduce the amount of water entering sewers by introducing SuDS.

It has been mentioned that the other primary cause of overflow is due to blockages. Companies in the UK have widely deployed bulk dielectric transducers suspended in the sewers to detect high levels and to report the events back over fixed wireless data networks. In certain locations it has been said that this practice has permitted the reduction of pollution events by up to 60%.

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 consequences[edit]

Decentralized wet weather overflow event

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 of the 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.[6]

Worldwide perspective[edit]

Man unclogging sewer overflow, Chennai, India

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 developing country such as the People's Republic of China discharges about 55 percent of all sewage without treatment of any type.[7] 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.[8] 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.[9] 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 United States, 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 U.S. gallons (380 km³) of untreated sewage per year. 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]


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.

See also[edit]


  1. ^ U.S. Environmental Protection Agency (EPA). Washington, DC. "Sanitary Sewer Overflows and Peak Flows", Updated February 2012.
  2. ^ a b EPA. "Report to Congress: Impacts and Control of CSOs and SSOs", Executive Summary. August 2004. Document No. EPA-833-R-04-001.
  3. ^ "A Worldwide View Of Sanitary Sewer Overflow". Retrieved 19 July 2009. 
  4. ^ Monica Spendilow, Impeller Magazine, ITT Flygt AB, SE-174 87 Sundbyberg, Sweden (2004)
  5. ^ 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)
  6. ^ 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).
  7. ^ "World Bank Supports China's Wastewater Treatment", The People’s Daily, November 30, 2001, Beijing
  8. ^ 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)
  9. ^ Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region, Caribbean Environment Programme Technical Report #40 1998

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