Investigations

This section contains summaries of reports on drinking water
quality incidents and investigations.

Report on drinking water quality incident at:
Lumsden Water Treatment Works in February 2008

Scottish Water Executive Summary

Lumsden Water Treatment Works is located to the West Side of the Aberdeenshire village of Lumsden, approximately 30 miles west of Aberdeen and 10 miles south of Huntly. Lumsden WTW supplies water to a population of approximately 400 domestic customers and rural businesses in the villages of Lumsden, Strathdon and Glenkindie.

The Treatment Works treats raw water from a borehole supply and consists of an aeration column for Carbon Dioxide removal and chlorination.

At 08:25 hours on Sunday 17 February 2008, a control system failure at Lumsden WTW resulted in a loss of disinfection to the public water supply. Automatic disinfection was interrupted for a period of approximately 7.5 half hours. It is understood that emergency disinfection of the tank was initiated at approximately 10:15 hours. The chlorine residual in the treated water storage tanks was continuously monitored by the treatment operative on site.

Fully automatic disinfection dosing and control was reinstated at approx 16:00 hours on Sunday 17th February 2008.

Investigations have concluded that the control system for the works failed, allowing the Borehole pump to continue pumping raw water into the works when the disinfection system had shut itself down. The failure of a surge protection barrier within the old control panel and the new control panel is thought to be the root cause of the problem and we are working with Scottish Water Solutions to address this.

There was a delay in the Public Health Team being notified of the initial event and also in samples being taken. We will be ensuring that staff are aware of the need to report events and take samples in line with existing procedures.

DWQR Assessment on the incident

DWQR's assessment of this incident is that, following the shut down of the chlorine dosing pumps at the borehole, a fault in the control system at the site allowed the borehole pump to continue to run, transferring undisinfected water into the clear water tank. An alarm was generated and responded to promptly by Scottish Water operations. Remedial actions taken at the site seem appropriate, although it is of concern that the operator was apparently unable to significantly raise the chlorine residual in the clear water tank by the addition of calcium hypochlorite tablets. At no time did the concentration of chlorine in the tank fall below 0.2mg/l, so a level of disinfection was maintained at all times in water supplied to the public.

The disinfection system at the site has recently been upgraded, with a new control panel having been installed. A number of control issues have been experienced by Scottish Water since commissioning, and DWQR looks to Scottish Water and Scottish Water Solutions to work together in order to ensure that any outstanding problems are resolved and the necessary safeguards are in place. It is apparent that the control philosophy of the plant only enables it to be operated in automatic mode. Whilst it is clearly essential that it is not possible to put undisinfected water into supply, Scottish Water are of the opinion that this aspect of the design is overly restrictive and significantly delayed start-up of the disinfection system following the incident. Scottish Water have identified resolution of this as an action point arising from the incident. DWQR is of the opinion that any such issues should have been resolved at the commissioning stage and that operational staff should have been fully involved at an early stage in the design and construction of the works.

DWQR is critical of the delay in notifying the Public Health Team and the fact that no-one from Scottish Water was available to take appropriate samples on the day of the incident. Public Health Team involvement and water quality sampling during and following an incident is essential in order to establish the potential risk to consumers and verify that quality has been restored. Scottish Water has committed to improving operational support and awareness of the vital role played by Public Health Team.