Drinking water standards

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The quality of drinking water in England and Wales is the subject of legally enforceable standards regulated by the Drinking Water Inspectorate (DWI). We take thousands of water samples each year, and put them through a large number of tests to ensure supplies are safe to drink.

The water samples we take are tested against several parameters to ensure supplies are safe to drink.

There are standards for over 50 parameters that are monitored to determine the microbiological, chemical and aesthetic qualities of drinking water.

These standards are outlined in legislation to protect public health, and compliance with these standards is monitored by the Drinking Water Inspectorate.

Take a look at the key for units used

Microbiological standards

Water is monitored for organisms that may indicate the presence of harmful bacteria or viruses.

The presence of an indicator organism does not necessarily mean that the water is unsafe to drink, however any exceedance is investigated immediately.

Bacteria found in the gut of humans and warm blooded animals. Their detection in water is an indication of faecal contamination. They are readily killed by disinfection but the occasional detection in drinking water can occur. 
Standard 0 per 100 ml

Group of organisms widely found in the environment including soil, water and vegetation. Their presence in water may indicate faecal pollution has occurred. They are readily killed by disinfection but the occasional detection in drinking water can occur. 
Standard 0 per 100 ml

Bacterium that can produce spores that can persist in the environment for long periods of time. Their presence in water can indicate historic contamination. 
Standard 0 per 100 ml

General measure of the bacterial population within drinking water. 
No abnormal change

Microscopic parasite that can cause gastroenteritis. It produces oocysts that can find their way into water. Careful control of treatment processes are required to protect public health. Continuous monitoring is undertaken at treatment works that have been identified as being at risk. 
No standard

Chemical standards

These standards are normally based on a life time's consumption and also take into account the intake from food and other sources.

Many of the chemicals listed below are not normally found in drinking water, or only occur in local circumstances.

Found in industrial solvents and can be detected in trace amounts in some source waters. They are removed by water treatment. 
Standard 3µg/l

Monomer is found in polyacrylamide which can be used in the treatment of water to remove impurities. Use of polyacrylamide is tightly controlled. 
Standard 0.1µg/l

Occurs as a natural constituent of many waters. At some treatment works aluminium salts are used to remove impurities. 
Standard 200µg/l

Ions are found naturally in most waters and removed by treatment. At some treatment works ammonia is added as part of the disinfection process. 
Standard 0.5mg/l

Rarely found in water and when this does occur it is likely to be due to the water being in contact with brass fittings or lead free solder. 
Standard 5µg/l

Occurs naturally in a small number of ground water sources. Specific treatments can be used but it is not normally found in the Thames Water area. 
Standard 10µg/l

Used in the petrochemical and plastics industry. Occasionally it is found in source water but is removed by treatment. 
Standard 1µg/l

One of several compounds known as polycyclic aromatic hydrocarbons. Trace levels can be found in drinking water where coal tar lining of mains was historically practiced to prevent corrosion. 
Standard 0.01µg/l

 

Occurs naturally at low levels in all waters. Some industrial discharge and detergents can increase the concentrations in river water. Boron is not removed by normal water treatment. 
Standard 1mg/l

 

Formed during the disinfection of drinking water though the reaction with natural bromide. It can also occasionally be detected in water through industrial pollution. 
Standard 10µg/l

Occurs in a small number of ground water sources. Specific treatments can be used but it is not normally found in the Thames Water area. 
Standard 5µg/l

Found as natural salts in all waters. It is not removed during normal treatment. Concentrations depend on the local water source. 
Standard 250mg/l

Very rarely found in drinking water. 
Standard 50µg/l

Occurs naturally in many water sources. It is readily removed by treatment. 
Standard 20mg/l Pt/Co

Measure of the dissolved inorganic substances. 
Standard 2500µS/cm at 20°C

Rarely found in source waters but can leach into drinking water as it passes through domestic copper pipes and fittings. 
Standard 2mg/1

Very rarely found in drinking water. 
Standard 50µg/l

Found in polyamines which can be used in the treatment of water to remove impurities. The use of polyamines is tightly controlled. 
Standard 0.1µg/l

Occurs naturally is many water sources. Some water companies add fluoride to the water supply at the request of health authorities to protect against tooth decay. This is not undertaken in Thames Water. Standard 1.5mg/l

Measure of the acidity of the water. pH values above 7 indicate alkaline conditions, pH 7 is neutral and below pH 7 is acidic. 
Standard 6.5 - 9.5 pH

Naturally present in most water sources. Iron salts can also be used in water treatment to remove impurities. Iron can also be present in drinking water through corrosion of iron water mains. 
Standard 200µg/l

Rarely found in source waters but can be found in drinking water due to pick up from lead pipes and fittings in domestic properties. Where required Thames Water treats supplies to minimise this happening. 
Standard 10µg/l

Occurs naturally in many water sources. It is readily removed by treatment. 
Standard 50µg/l

Very rarely found in drinking water. 
Standard 1µg/l

Rarely found in source water. In the Thames Water area, nickel found in drinking water is normally associated with nickel coatings used on some domestic taps and fittings. 
Standard 20µg/l

Occurs naturally in most source waters but concentrations can be increased as a result of fertiliser use. Where necessary concentrations in drinking water can be reduced by diluting with sources where nitrate levels are low or through specific treatment. 
Standard 50mg/l

Occurs naturally at low levels in some waters but is removed by treatment. It is sometime produced as a by-product when chloramine is used as a disinfectant. 
Standard 0.5mg/l at consumers' tap. 
0.1mg/l at water treatment works

Measure of the combined concentrations of these two compounds in drinking water. Concentration of nitrate divided by 50 + concentration of nitrite divided by 3. 
Standard <= 1

Diverse group of organic compounds that include herbicides, insecticides, fungicides. Many water sources contain traces of one or more pesticides as a result of both agricultural and non-agricultural uses. Thames Water is actively working with users and manufacturers to reduce pesticides in water sources. Where required treatment is in place to remove pesticides from drinking water. 
Standard 0.03µg/l for aldrin, dieldrin, heptachlor and heptachlor epoxide. 
0.1µg/l for other individual pesticides.
0.5µg/l for the total of all pesticides detected

Found in drinking water where coal tar lining of mains was historically practiced to prevent corrosion. The standard is a measure of benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(ghi)perylene and indeno(1,2,3-cd)pyrene. 
Standard 0.1µg/l

Found naturally in water at very low concentrations. Elevated levels may indicate the presence of other artificial radionuclides. 
Standard 100Bq/l

Measure of radiation exposure through drinking water. Radioactivity is naturally present in all water sources. Levels of radioactivity are normally monitored by measuring gross alpha or beta activities. 
Standard 0.1 mSv/year

Treated water contains a small amount of residual disinfectant to ensure microbiological quality throughout the distribution system. 
No standard

Very rarely found in drinking water. 
Standard 10µg/l

Naturally present in many water sources. Concentrations in Thames Water's supplies are well below the standard. Domestic water softeners can increase the sodium concentration. 
Standard 200mg/1

Naturally present in many water sources. Concentrations in Thames Water's supplies are well below the standard. 
Standard 250mg/1

Measure of the aesthetic quality of drinking water. Unusual tastes or odours may indicate a problem that requires investigation. 
Standard acceptable to customers, no abnormal change.

These are solvents which are very occasionally found in water sources. Specialist treatment is needed to remove these compounds. The standard is a measure of the combined concentrations. 
Standard 10µg/l

This is a solvent which is very occasionally found in water sources. Specialist treatment is needed to remove this compound. 
Standard 3µg/l

Measure of the amount of organic material in the water, most of which comes from natural sources. Standard no abnormal change (mg/1)

Formed by the reaction of chlorine with natural organic substances in water. The standard is a measure of chloroform, bromoform, dibromochloromethane and bromodichloromethane. 
Standard 100µg/l

Measure of the clarity of water. 
Standard 4 NTU at consumers' tap. 1 NTU at water treatment works

Found in water pipes containing polyvinyl chloride (PVC). Concentrations are strictly controlled by product specification. 
Standard 0.50µg/l

Key

Code/Units Description
Pt/Co Platinum-Cobalt Scale
NTU Nephelometric Turbidity Units
°C Degrees Centigrade
µS/cm Micro Siemens per Centimetre
Bq/l Becquerels per Litre
mSv/year Millisieverts per Year
mg/l Milligrammes per Litre (one part per million)
µg/l Microgrammes per Litre (one part per billion)