Water Supply Industry Interpretations and Advice

  • Area of Interest : Water Quality
  • Topic : Chemical dosing and maintaining wholesomeness
  • Title : Chlorine dioxide dosing
  • Clause Reference : Regulation 3(2)
  • Reference Number : WQ03
  • Decision Date : October 2000 (Revised June 2013)

Where owners or occupiers propose to use equipment for dosing of chemicals, such as chlorine dioxide, to maintain water supply quality in premises (e.g. to protect against the growth of Legionella bacteria), dosed water must remain wholesome if used for drinking, bathing, cooking or food production purposes.

Where chlorine dioxide is to be used for this purpose, to ensure that dosed water is wholesome, the following conditions should be met by the operators of the equipment:

  1. there should be evidence of the quality of the chlorine dioxide and its concentration e.g. a current Secretary of State approval i.e. “approved under regulation 31 of the Water Supply (Water Quality) Regulations 2000” or the requirements and national conditions of use of the final chlorine dioxide generated shall meet the requirements of BS EN 12671 http://dwi.defra.gov.uk/drinking-water-products/approved-products/index.htm .
  2. there should be evidence of the effectiveness of the control of flow and chemical dosing device(s) i.e. evidence of the stability of the proportional control of chlorine dioxide against typical water flow rates that would be encountered in practice;
  3. there should be evidence demonstrating that the 0.5 mg/l* concentration of chlorine dioxide, chlorate and chlorite is not exceeded. This would require chemical analysis, for chlorine dioxide, chlorites and chlorates, of water samples from trial installations extending over several weeks.
  4. there should be a programme for regular chemical analysis of chlorine dioxide, and, where significant background concentrations in the mains water supply justified it, by the measurement of chlorites and chlorates in the proposed installation.

Where these conditions are met, the chlorine dioxide dosing equipment is considered to be fluid category three necessitating the installation of appropriate backflow protection in compliance with the Water Supply (Water Fittings) Regulations/Byelaws.

If these conditions are not met, it is recommended that the Water Supplier should inform the owner and occupier that drinking water taps supplied directly off the supply pipe should be installed in convenient locations with notices and clear instructions provided indicating that taps supplied with dosed water should not be used for drinking, bathing, cooking or food production purposes. In addition the local authority Environmental Health department should be advised for their information.

Where chlorine dioxide is generated in situ for subsequent dosing then the generation equipment must have appropriate backflow protection.

These conditions could be used as a model for acceptability of other types of chemical dosing equipment used for similar purposes”


* the 0.5 mg/l concentration refers to residual concentration that is to say the combined concentration of chlorine dioxide, chlorite and chlorate shall not exceed 0.5mg/l as chlorine dioxide in the water entering supply.