Water Quality - Urban Waste Water Treatment Directive
Sewage Treatment in the UK: Implementation of the
EC Urban Waste Water Treatment Directive
Background on urban waste water
Urban waste water, commonly referred to as sewage, is generally a mixture of domestic waste water from baths, sinks, washing machines and toilets, waste water from industry and rainwater run-off from roads and other surfaced areas.
Every day in the UK about 347,000 kilometres of sewers collect over 11 billion litres of waste water. This is treated at about 9,000 sewage treatment works before the treated effluent is discharged to inland waters, estuaries and the sea.
Without
suitable treatment, the waste water we produce every day would damage
the water environment and create public health problems. Untreated sewage
contains organic matter (carbohydrates, fats, proteins), bacteria and
chemicals. Bacteria naturally present in environmental waters do break
these substances down, but in doing so they use the oxygen dissolved in
the water. If there were large or continuous untreated discharges of urban
waste water the result could be too little oxygen for fish and other aquatic
life to survive. So the purpose of waste water treatment is to remove
organic substances to protect the environment from these effects. Sewage
works therefore reproduce what would occur in the environment, settling
out much of the solid matter (PRIMARY TREATMENT),
and using bacteria that 'digest' and break down the organic substances
(SECONDARY TREATMENT).
Sometimes, further treatment (TERTIARY) is required to protect sensitive water environments. Tertiary treatment can involve disinfecting the treated effluent to protect bathing or shellfish waters. It can also involve the removal of phosphorus or nitrates (nutrients present in sewage) to protect waters that are threatened by EUTROPHICATION. An illustration of the different treatment processes is below.
Diagram of sewage treatment process cycle
Level of treatment for areas indicated
1 There are no Less Sensitive Areas in England and Wales.
2 Secondary treatment will be applied to coastal discharges down to 2,000pe in England and Wales (rather than the 10,000pe specified in the Directive). This will enhance protection of the environment.
3 There are no mainland Scotland Less Sensitive Areas - it has three in the Islands.
4 Northern Ireland has two Less Sensitive Areas.
5 Higher levels of treatment than those specified under the UWWTD may be required to meet relevant quality objectives and other Community Directives, such as the Bathing Water Directive or Shellfish Waters Directive.
Even untreated waste water is mostly water (less than 0.1% is solid material). Treatment allows this water to be returned to the environment. In this way, it can help to maintain river flows which is important for use of water, for example for abstraction, conservation and fishery interests. The re-use and disposal of the removed solid material, sewage sludge, is discussed later in this report.
Page published 25 April
2002;
Page last modified
29 September, 2002