Language > Cymraeg

Search the site:

Notice a leak

Freephone: 0800 2987112

Standards Explained

Drinking Water Quality standards explained

  • Directive standards are from the European Drinking Water Directive (DWD), this is European Union legislation that sets out limits for drinking water standards based on World Health Organisation guidelines.
  • National standards are tighter limits applied to parameters from the DWD in the UK
  • Indicator standards are not mandatory requirements but instead are used as an indicator of water quality.
  • 1,2-Dicholoroethane is a solvent that may occur in groundwater from industrial activity, a health based directive standard of 3ug/l applies.
  • Aluminium can occur naturally in some waters but is also used as a coagulant during water treatment. The aluminium used in this way is later removed during the treatment process, so that aluminium levels leaving the works are kept to a minimum. A national standard of 200ug/l is used.
  • Ammonium is naturally present in some source waters, but high amounts of ammonia can be an indicator of faecal contamination. Ammonia is an indicator parameter with a specification value of 0.5mg/l.
  • Antimony is not present in drinking water but can occur due to impurities from brass fittings. A directive health-based standard of 5μg/l is in used.
  • Arsenic can occur naturally in some groundwater, a European health based standard of 10ug/l applies.
  • Benzene is not found in drinking water but can leach through plastic pipes on contact with petrol. A directive health based limit of 1μg/l applies.
  • Benzo(a)pyrene is a polycyclic aromatic hydrocarbon that can occur in water mains that have been lined with coal tar. A directive health based limit of 0.01μg/l applies.
  • Boron can occur in surface water due to industrial discharges, levels in drinking water are usually low. A directive health based standard of 1mg/l applies.
  • Bromate is formed from the oxidation of natural bromide during chlorination and can also be a present in sodium hypochlorite. A directive health based limit of 10μg/l applies.
  • Cadmium is very rarely detect in drinking water but can be found as an impurity from water fittings. A directive health based limit of 5μg/l applies.
  • Chloride occurs naturally in water but is also present from de-icing of roads and a possible indicator of saline intrusion. An indicator parameter with a taste based specification of 250mg/l.
  • Chlorine residual is a measure of the small amount of chlorine present in the water to maintain disinfection through the distribution system and customers pipework. The indicator specification is for no abnormal change.
  • Chromium should not be present in drinking water. A directive health based standard of 50ug/l applies.
  • Clostridium perfringens are bacterial spores that may be present in the digestive system of warm blooded animals, the spores are quite resistant to chlorine and can be an indication of a historic contamination event. An indicator specification of 0 cfu/100ml is in used.
  • Coliform bacteria are commonly found in the environment and can be associated with plants, soil, animal and human activity. The presence of these organisms in potable water leads to an investigation into disinfection and integrity of the supply system. An indicator specification of 0 cfu/100ml is in used.
  • Colony counts cover a broad spectrum of bacteria that have no sanitary significance but are measured to give indications in changes of water quality. An indicator specification of no abnormal change applies.
  • Colour is naturally present in most source waters but is removed by water treatment, colour can reoccur in the distribution system due to an action such as a mains burst. A national standard of 20mg/l (Pt/Co) is in used.
  • Conductivity measures the amount of dissolved inorganic substances present in the water. An indicator specification of 2,500μS/cm applies.
  • Copper in drinking water comes from copper pipes and fittings in customerís premises. A directive health based limit of 2mg/l applies.
  • Cryptosporidium is a protozoa that can cause gastro-enteritis and is resistant to chlorination. Current treatment works are designed and operated in accordance with the Badenoch and Bouchier report designed to limit the risk from Cryptosporidium. Continuous monitoring is performed at all sites classified as high risk. There is no longer a national standard for Cryptosporidium as it is almost impossible to set a health based limit on the current methodology. Dee Valley Water uses a level 0f 0.2 oocysts per 10 litres to represent a significant increase in Cryptosporidium levels.
  • Cyanide is not present in drinking water. A directive health based limit of 50μg/l applies.
  • E.coli and Enterococci are contained in the digestive tract of warm blooded animals and are unlikely to grow in the environment, due to this they make ideal indicator organisms for faecal pollution. Although the organisms themselves are not a human health concern they are used to initiate thorough investigation into the disinfection system and integrity of distribution system. The directive standard is 0cfu/100ml.
  • Fluoride can occur naturally in some water types especially in groundwater. Some water companies artificially fluoridate the water supply if required by the local health authority. Dee Valley does not fluoridate any of our water. A directive health based limit of 1.5mg/l is in used
  • (pH) Hydrogen Ion is a measure of the acidity of water. A pH below 7 is acidic, above 7 is alkaline. An indicator specification range of 6.5 to 9.5 applies.
  • Iron is naturally present in most source waters, especially in surface waters. Iron is removed during water treatment, however subsequent iron levels can rise due to corrosion of iron mains. Dee Valley has relined or replaced iron mains that were shown to cause discoloured water. A national standard of 200ug/l based on aesthetic quality of water is in use.
  • Lead is primarily from the use of lead plumbing in older properties, Dee Valley have a plumbosolvency treatment scheme where high risk areas were identified and phosphate dosing put in place at the treatment works supplying them to reduce the lead concentration. The only real solution to high lead levels is for the removal of lead pipes and fittings. A directive health based limit of 25ug/l applies, although this will reduce to 10ug/l in 2013.
  • Manganese is naturally present in some source waters, especially in surface water. Some manganese is removed in treatment however unless specific treatment is in place it is hard to remove it all. A national level of 50ug/l is used based on aesthetic quality of water.
  • Mercury is not found in drinking water. A directive standard of 1ug/l is used.
  • Nickel can occur in some groundwaterís but is usually found in water due to coatings on modern taps and fittings. A directive health based limit of 20 ug/l is used.
  • Nitrate is found in all source waters but can occur in higher concentrations where fertilisers are used, The Dee protection zone helps to limit the levels of nitrate present in Dee Valleys water. A directive health based standard of 50ug/l applies.
  • Nitrite can be formed from the reduction of nitrate in the distribution system as well as being a by-product in the formation of chloramines. When chloramination is practised careful operation of the process ensures that by-product formation is kept to a minimum. A directive health based standard of 0.5mg/l applies for customerís taps with a standard of 0.1mg/l at the treatment works.
  • Odour can occur due to a wide variety of reasons such as natural organic compounds found in water or the reaction of chlorine with customerís pipes and fittings. A national standard of acceptable to customer and no abnormal change applies.
  • Aldrin, dieldrin, heptachlor, heptachlor epoxide are banned substances in the UK and are not found in drinking water. A directive based limit of 0.03ug/l for each parameter is used.
  • Other Pesticides includes herbicides, insecticides and fungicides that are chosen using a risk assessment looking at usage and raw water concentration. All of Dee Valleys river derived treatment works have granular activated carbon filters that will help to remove pesticides. Each pesticide has a directive limit of 0.1ug/l with a separate standard of 0.5ug/l for the sum of pesticides found.
  • Polycyclic aromatic hydrocarbons (PAHs) can occur in water mains that have been lined with coal tar. A directive health based standard of 0.1 ug/l for the sum of all PAHís is in use.
  • Selenium is an essential element that is found in low levels in drinking water. A directive health based standard of 10ug/l applies.
  • Sodium is present in seawater and brackish ground water, concentrations in drinking water are usually very low. Chemical water softeners can add considerable amounts of sodium when they are installed in customerís premises. A national limit of 200mg/l applies.
  • Sulphate is naturally present in water and has an indicator specification of 250mg/l.
  • Taste can occur due to a wide variety of reasons such as natural organic compounds found in water or the reaction of chlorine with customerís pipes and fittings. A national standard of acceptable to customer and no abnormal change applies.
  • Tetrachloroethane and Trichloroethene are solvents that may occur in groundwater from industrial activity. A directive health based standard of 10ug/l for the sum of both substances is in use.
  • Tetrachloromethane is a solvent that may occur in groundwater from industrial activity. A national standard of 3ug/l is used.
  • Total organic carbon is an indication of the total amount of organic matter in the water. This is an indicator parameter with a specification of no abnormal change.
  • Trihalomethanes can be created with the reaction between chlorine and natural organic substances in the water. The production of these THMís is controlled by using the treatment process to remove as many organic compounds from the water as possible before chlorination. A directive health based limit of 100ug/l applies.
  • Turbidity measures the clarity of the water, at treatment works this is useful to monitor performance of the works against an indicator specification of 1 NTU. At customers taps it can be elevated due to re-suspension of mains deposits and has a national standard of 4 NTU.