There are estimated to be more than 20,000 CSOs in the UK
In Britain's Dirty Beaches, Panorama looks at combined sewer overflow pipes, known as CSOs, and the problems they pose to the UK's coastline.
The Marine Conservation Society (MCS) estimate that there are at least 500 CSOs that discharge directly onto or near beaches in its annual publication, the Good Beach Guide.
What is combined sewer overflow?
Combined sewer overflow (CSO) is the discharge of a combination of storm water and domestic sewage waste water caused by sewer capacity being exceeded during heavy storms.
Much of the UK network of CSOs dates back to Victorian times.
When waste water exceeds the sewage system's capacity, storm and sewage water is directed into CSOs which flow into streams, rivers and the sea, through outfall pipes.
CSOs provide a failsafe during heavy storms to stop household domestic sewage waste and storm water backing up through the system and flooding peoples' homes and streets.
CSOs are the responsibility of the UK's 12 water companies and regulated by the Environment Agency (EA) in England and Wales, The Scottish Environment Protection Agency and the Northern Ireland Environment Agency.
Millions of pounds have been invested in sewer improvements
How many CSOs are there?
In response to questions from Panorama the 12 UK water companies said they have a total of 20,233 CSOs between them.
Figures for how many are monitored are incomplete, but the responses we got suggest that roughly 25% of CSOs are monitored.
The MCS estimates there are 22,000 CSOs discharging into rivers and the sea around the UK, including at least 500 beaches.
What are the issues?
It is generally accepted that underground sewer systems have a finite capacity and that CSOs are necessary. It is believed by campaign groups, such as the MCS, that some CSOs are spilling routinely rather than providing emergency relief.
The wetter weather of recent summers has periodically overwhelmed the sewerage system in some parts of the country.
The waste water from CSOs pose a possible danger to public health, wildlife and the environment.
Campaigners are concerned about the standards of monitoring, licence regulation and operation of CSOs by the water companies.
How do you know if there is a CSO on a beach near you?
A CSO is basically a pipe and often will have a hinged lid or grille on the front to prevent access by people or animals.
The attached Panorama map provides locations for a beach or surrounding area which is known to have a CSO.
What can I do if I am concerned over a spilling CSO?
Report it. You can raise your concern with your local council or local water company, the EA or a group like the MCS.
The MCS advises bathers not to swim for at least 24 hours after very heavy rainfall.
Domestically, basic water conservation by households can reduce the strain on the sewer system. Not flushing sanitary items, cotton buds and other debris down the toilet can also help stop sewage-related litter on beaches.