Private Water Supplies - Case Study 2010/06

Failure of a borehole supply serving a public building

The subject of this case study is a borehole supply serving a public building which is a visitor attraction for children and families (an open farm with a café) and residential properties. When the local authority carried out check and audit monitoring under the new private water supply regulations, a sample of drinking water from the café was found to contain 14µg/l of bromate. This parameter was not looked for under the previous 1991 regulations. The health-based standard for this disinfection by-product, which is a carcinogen, is 10µg/l. The local authority carried out an investigation (under Regulation 15) and found that sodium hypochlorite was being overdosed in response to a previous problem when E.coli had been found following a breakdown of the chlorinator which froze in cold weather. It was suspected that the product being dosed was a swimming pool grade of sodium hypochlorite. Unless a drinking water grade conforming to BS EN 901:2007 is used there is a likelihood of bromate being present in the product as an impurity. Although it is important to ensure that drinking water is disinfected to ensure it is microbiologically safe, it is a requirement of the regulations that the method of disinfection employed minimises disinfection by-products (in this case bromate) because long-term exposure to elevated levels of disinfection by-products poses an increased cancer risk.

After considering all the evidence, and consulting with the Inspectorate and the Health Protection Agency, the local authority concluded that without improvements to the management of the supply there was a potential danger to human health due to the likelihood of either bromate or bacteriological failures. A Regulation 18 Notice was served and users informed of the situation. The supply was restricted by means of requiring the owner to immediately lower the dose of chlorine. This was the most effective short-term safeguard; boiling water would not have been an effective safeguard and using only bottled water was disruptive. However, it was made clear that bottled water should be resorted to if there were further instances of elevated bromate. The Notice imposed the following requirements for improved management of the supply:

  • Use of a product conforming to BS EN 901:2007
  • Storage of the product not to exceed the maximum period recommended by the supplier.
  • A free chlorine residual of no less than 0.2mg/l and no more than 0.5mg/l to be maintained and dosing to be in line with the supplier’s recommendations.
  • Monitoring of free chlorine residual sufficiently frequently to validate that levels are consistently within the control criteria set and no less often than every four days (i.e. twice a week) until good control has been demonstrated and thereafter to be monitored weekly.
  • Keeping records of chlorine residual measurements including the date of monitoring, the name of the person carrying out the test and the result. Such records to be kept for at least 12 months and made available to the local authority.

There was no appeal by the owner in relation to the Notice and the management of the supply has improved.

This case study illustrates how the Notice provisions in the new regulations can be used to encourage owners to understand that safety may be compromised by lack of knowledge about how to operate and maintain water treatment. The Inspectorate is pleased to note how this case was identified because the local authority applied a risk-based approach to selecting audit monitoring parameters which included consideration of the likelihood of disinfection by-products being present.

The Inspectorate notes that this case is one of many where there is evidence that private water supply owners are reliant on private consultants/contractors whose competence and advice is questionable.

It also illustrates alternative approaches to restricting a supply, namely lowering the dose of hypochlorite to prevent the formation of by-products was more appropriate than boiling water or using bottled water.