Sanitary sewer overflow
Sanitary sewer overflow (SSO)  is a condition whereby untreated sewage is discharged into the environment, escaping wastewater treatment.  This situation, also known as Wet Weather Overflow,  is primarily meaningful in developed countries, which have extensive sewage treatment facilities.  The main causes of SSO are:
- Infiltration of excessive stormwater into sewer lines during heavy rainfall
- Rupture or blockage of sewerage lines
- Malfunction of pumping station lifts or electrical power failure
- Human operator error at treatment plant facilities
Since a number of countries have essentially 100 percent treatment of domestic wastewater, an SSO episode is viewed as a significant breakdown in environmental control of water resources; for example, the USA, Japan, the United Kingdom and some other European countries strive for complete secondary treatment of all effluent and pursue vigorously shortcomings in the sewerage systems.

# History
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.  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 began detailed engineering analyses for major cities across the country.   After passage of the U.S. Clean Water Act, the 1980s saw spending of billions of dollars on system upgrading.  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 problem
The United States Government has set a goal of upgrading every municipal treatment and collection system such that an overflow event would be expected in any given community no more frequently than every five years.  Costs to achieve this goal approximate 88 billion dollars.  This cost is in addition to approximately ten billion dollars already invested in the USA.  Although the volume of untreated sewage discharged to the environment is less than .01 percent of all treated sewage in the USA, the total volume amounts to several billion US gallons per annum and accounts for thousands of cases of gastrointestinal illness each year.  Advanced European countries and Japan have similar or somewhat larger percentages of SSO events.

# Engineering aspects
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.  This circumstance is most prevalent in older cities whose subsurface infrastructure is quite old; Paris, London, Stockholm, New York and Oakland, California 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 consequences
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.

# Worldwide perspective
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.  In a relatively developed Middle Eastern country such as Iran, Tehran's majority of population has totally untreated sewage injected to the citys groundwater.  In Venezuela, a below average country in South America with respect to wastewater treatment, 97 percent of the countrys sewage is discharged raw into the environment.  Most of sub-Saharan Africa is without wastewater treatment, leading to the excessive infant death rates in that region.
Thus the USA, Canada, Western Europe, Australia and Japan are considered struggling with a public health problem of SSO prevention, while the underdeveloped countries of the world discharge approximately 20,000 times the amount of raw sewage into the environment as those advanced countries collectively; thus, lesser developed countries discharge 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.