Investigations

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

Report on drinking water quality incident at:
Kilmuir Water Treatment Works in January 2007

Scottish Water Executive Summary

Kilmuir water treatment works is a small remote works situated north of Uig on the Isle of Skye and supplies an average of 170 cubic meters of treated water per day to a population of approximately 250. The raw water for the works is abstracted from the Kilmuir springs an dthe only treatment at the works is sodium hypochlorite dosing to provide disinfection. There is no mains power supply to the site so the disinfection system is powered by two 12 volt batteries which are trickle charged by a wind turbine and a solar panel. There is no on-line water quality monitoring or telemetry at the works and the site is currently being visited three times a week.

At approximately 09:40 hours on Monday 29th January 2007, during a routine site visit, the treatment works operator for Kilmuir WTW discovered that the sodium hypochlorite dosing system had stopped. On investigation, he found that there was no power being supplied to the dosing pump due to a corroded battery connection. The operator restored the battery connection to re-established the power supply to the dosing pump and sodium hypochlorite dosing was reinstated. The operator shock dosed the clear water tank and informed his team leader of the situation. By approximately 10:35 the free chlorine residual at the regulatory sample tap was 0.64mg/l and the operator went to check the chlorine residuals at the outlets of the service reservoirs (SRs) to establish if these would need to be shock dosed. The free chlorine residuals at Herbusta and Bornaskitaig SRs were 0.30mg/l and 0.22mg/l respectively, so further dosing was not required.

The situation was reported to the Public Health Team by the team leader and although incident samples were requested they were not taken due to the limited availability of operations and sampling staff at this remote location. Statutory bacteriological samples were taken from the SRs, distribution and the treatment works on Thursday 1st February and were reported as passing the next day.

Kilmuir WTW is being visited on a daily basis until the works is upgraded and a robust disinfection system is in place.

The installation of duty/stand-by auto changeover system for the sodium hypochlorite dosing pumps is being progressed and the feasibility of Installing mains power, telemetry and chlorine monitoring to the site is being investigated as actions from the previous incident. A meeting was held on site with the contractor on 22nd February and although we are awaiting their proposals and a programme for the work, based upon delivery times for key equipment, it is likely that the changes for the sodium hypochlorite dosing system will be completed by 30th April, 2007

DWQR Assessment on the incident

The DWQR's assessment of this disinfection failure incident is that it was caused by a corroded connection on the battery which provides power for the dosing pump. Due to the lack of any telemetry on the site, this was only noticed during a routine visit, which normally takes place three times a week.

Having discovered the problem, DWQR is content with the actions taken, which include informing SW's Public Health Team. However, DWQR would expect SW's PHT to be informed immediately, not after one hour, and is concerned about the fact that incident samples were not taken "due to the remote location of the works and the limited availability of operations and sampling staff". Of the samples that were taken later, it is of no surprise that these all passed since they were taken after the problem had been fixed.

DWQR notes the actions being taken by SW, namely:-

1. To visit the site on a daily basis until the works is upgraded (visit daily from 26/02/07).
2. Review whether visual checking of battery terminals for evidence of corrosion needs to be added to task schedule for battery sites (by 16/03/07).
3. Install duty/standby auto-changeover system for the disinfection dosing pumps (to be complete by 30/04/07).
4. Investigate the feasibility of installing mains power, telemetry and chlorine monitoring at the works (by 16/03/07).
5. egional manager to meet with logistics manager and ensure satisfactory arrangements are in place to ensure adequate sampling is taken following an incident/event (by 22/03/07).