Private Water Supplies - Case Study 2011/09

Use of a Notice to secure the safety of a private supply through improved management arrangements

This private supply is located on a small estate and serves a domestic dwelling, rented accommodation and an office. The source is a borehole from which water is pumped to a reservoir where treatment is by filtration followed by UV disinfection before water is distributed to all the properties. The estate employs a contractor to carry out monitoring, monthly checks and annual maintenance, including UV lamp replacement. The estate owners had a good awareness of source-water quality risks as demonstrated by them installing controls in the form of fencing off the source from livestock, restricting the application of pesticides on land near the source and ensuring the estate workers were aware of the private water supply.

In July, Enterococci was detected in a routine sample and the local authority put in place a Regulation 18 Notice to require water to be boiled before use because the result was suggestive of faecal contamination of the supply and thus a potential danger to human health.

The estate manager immediately asked the engineer to investigate. It was found that the UV lamp was nearing the end of its recommended life, and although it was switched on, it had ceased to be functional, resulting in inadequate disinfection. Based on these findings the need for more frequent checks on the UV lamp status and the keeping of auditable records were specified in the Notice. There was no appeal against these conditions set out in the Notice. The Notice was revoked once the improved management procedures had been put in place and verified by two subsequent satisfactory samples.

This case study illustrates why owners should not rely on generic or standard maintenance regimes for UV systems. Every private supply source and distribution system is unique and the effective life of UV lamps will vary according to a range of water quality characteristics. Each system needs to be regularly checked and the UV lamp cleaned or a new UV lamp installed as soon as function (transmissivity) declines. The performance of UV lamps will be influenced also by the design and maintenance of any upstream filter. The manufacturer’s recommended shelf-life should only serve as a guide to the maximum time between UV lamp or filter changes. When maintenance is contracted out, local authorities should advise owners to require the service provider to check and record functionality on every visit and to clean or change UV lamps and filters on the basis of these checks, not at standard set annual intervals. The Inspectorate would welcome being informed by local authorities of the details of water treatment and maintenance contractors providing services to private supply owners in their area, with examples of good and poor practice. This information will enable the Inspectorate to develop guidance for these service providers.