Last edited 12 May 2020

Water quality

Water is a chemical compound that is essential for all known forms of life. It is a transparent, tasteless, odourless and almost colour-less substance that covers 71% of the Earth’s surface and is the main constituent of rivers, lakes oceans and so on.

The term ‘water quality’ refers to the chemical, physical and biological composition of water in relation to the use that it is intended for. For example, water used for flushing toilets need not be of the same quality as water that is used for cooking or drinking, and the quality required of water in water bodies such as lakes will be different again.

Access to safe water is essential for good health. The 2030 Agenda for Sustainable Development recognised access to safe drinking water and sanitation as a human right. However, nearly a third of the global population do not use safely-managed drinking water and only two fifths have access to safely-managed sanitation services.

In the UK water is generally supplied to consumers by utilities companies using a pump and pipe system. Typically, the first stage of the water supply process is the collection of rainwater in reservoirs, either from rivers or streams or from groundwater. This is pumped to water treatment works where it is processed to make it suitable for use. The purity of this water is tested several times and then a very small amount of chlorine is added to kill any remaining organisms or bacteria. Water is then stored in covered reservoirs before being pumped out via the distribution network.

In 2017, 99.96% of drinking water in England and Wales complied with UK and European quality standards. Ref https://www.water.org.uk/policy-topics/improving-services-for-customers/world-class-drinking-water/

Water qualities issues that do occur can be caused by odour or taste or by the presence of iron, manganese, pesticides, bacteria, aluminium, lead, nickel, clostridium perfringens, benzo(a)pyrene and so on. The highest risk failure is considered to be due to coliforms at treatment works and reservoirs as this can affect a very large number of consumers.

Water that is supplied to consumers does not all have exactly the same composition. For example, hard water is found in areas which have a chalk and limestone geology and refers to water with a high level of naturally-occurring calcium and magnesium compounds. This is not associated with any health risk.

In addition, there may be issues with water quality after it has reached consumers. Properties built before 1970 may contain lead pipework. This lead can leach into the water and can be hazardous to health. The UK standard for the concentrations of lead in drinking water is a maximum of 10 micrograms per litre (µg/l) or parts per billion (ppb). It is recommended that lead pipework should be replaced.

Very broadly, qualities of water can be categorised as:

Potable water: water that is suitable for drinking (this may be referred to as white water, clean water or fresh water although these terms are ambiguous, for example, water in a stream might be considered 'fresh' although it may not be drinkable).
Wholesome water, that is, water complying with the requirements of regulations made under Section 67 (Standards of wholesomeness) of the Water Industry Act 1991. Water supplied through the public water main should be wholesome water and should be fit to use for drinking, cooking, food preparation or washing.
Softened wholesome water: Water which would be regarded as wholesome but for the presence of excess sodium caused by a water softener or water softening process which reduces the concentration of calcium and magnesium.
Non-potable water or non-wholesome water: Water that is not of drinking water quality.
Greywater: domestic wastewater excluding faecal matter and urine. When appropriately treated this may replace the use of wholesome water in wcs, urinals, irrigation or washing machines.
Black water, brown water or sewage: Water that has come into contact with faecal matter or urine. Black water can be harmful to health and may contain bacteria, viruses, protozoa and parasites.

For more information see: Types of water.

Non-wholesome water must not be inadvertently supplied in place of wholesome water and it should not be permitted to contaminate wholesome water systems, for example, by mis-connection. Pipes carrying non-wholesome water should be marked clearly, using different types of pipework to avoid mis-connection.