Private Water Supplies - Case Study 2010/07

Surface water supply to a public building with accommodation – Risk assessment

This large supply serves a small group of residential educational centres, holiday cottages and an office, and therefore it is classed as a large supply (Regulation 9). There is one land owner, but several different owners of premises. Up to 160 people may be using the accommodation at any one time. The supply originates from an upland surface water source. The off take to the private supply is located on a stream further down the hillside. The local authority carried out a risk assessment which identified a number of uncontrolled hazards as follows:

  • wildlife and livestock, particularly sheep faeces or carcasses in the surface water;
  • intermediate storage tanks not adequately protected;
  • no maintenance of the distribution network (i.e. tanks, covers, fencing, etc);
  • no vermin proof covers;
  • ineffective disinfection.

Disinfection was by means of UV light. The treatment unit had been installed so that the raw water passed into two parallel pipes, each then feeding water through a filter followed by the UV unit. The disinfected water from each pipe then passed into a common pipe and on to taps in the premises. The design was such that if one of the two treatment streams failed then untreated water would mix with, and contaminate, treated water from the other unit and, as a consequence, all water passing to taps in each of the premises would be unsafe.

The local authority concluded from the risk assessment that the supply was a potential danger to human health because untreated water could potentially enter the supply. The evidence from the catchment and the distribution network showed the risk was high due to the likelihood of pathogens being present either in the raw water or gaining access to the supply as a consequence of inadequate maintenance. Historic monitoring under the previous 1991 regulations recorded the presence of E.coli in 25% of samples.

Arrangements before improvement (Note the combined treatment streams).

combined treatment streams

Collection chamber prior to improvements.

Collection chamber prior to improvements.

A Regulation 18(a) Notice was served on all the owners restricting the supply (requiring the owner to make all users aware of how to safeguard their health by boiling water before use or by using bottled water) and a Regulation 18(c) Notice set out the specific improvement measures needed to make the supply safe. This invited the owners to provide the local authority with a proposed method of adequately treating the raw water, including a fail-safe mechanism to ensure that action is taken in the event of a power failure or an equipment failure to prevent the supply of undisinfected water. In addition, the owners were asked to provide a written procedure for the maintenance and monitoring of the disinfection equipment and the supply as a whole, particularly checks of the source for animal carcasses and general repairs. There was no appeal by the owner in relation to the Notices.

This case study illustrates how the risk assessment approach in the new private water supply regulations can be used as an effective tool for identifying ineffective treatment arrangements, enabling local authorities to bring this to the attention of owners by directing them towards the particular defects and the most appropriate multi-barrier action and control measures that would make the supply safe. Each water supply is different and owners are often entirely reliant on the technical competence of the sellers and installers of equipment. This case also serves to demonstrate how risk assessment introduced through the new regulations, benefits owners who are able to receive independent and impartial technical advice with a focus on what is essential to safeguard public health. This case study shows how testing alone (under the previous 1991 regulations) without risk assessment would not have identified this supply as unsafe.