Investigations

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

Report on drinking water quality incident at:
Raasay Water Treatment Works in August 2006

Scottish Water Executive Summary

Raasay WTW is a simple disinfection works. The sodium hypochlorite is dosed flow proportionally, with residual trim into the raw water main prior to the CWT. There is power and telemetry available at this location.

Following a routine visit by an operator on the 21/08/06 the free chlorine residual was found to be zero entering supply. On investigation it was found that the injector was cracked and no disinfectant was being dosed. A replacement injector was installed, chlorination reestablished and the CWT shocked dosed. The situation was reported by the Operator to the Portree Office, who emailed the Public Health Team advising them of the situation. Due to the fact that Raasay is an island off Skye, logistically it was not possible to arrange for samples to be taken to the analysing laboratory in Inverness until the following day. Samples were taken at the WTW and one property in distribution on the 22nd August. The Samples taken were reported clear on the 23rd August. The free chlorine residual taken at WTW at the time of sampling was 1.27mg/l and 0.14mg/l in distribution. The results were reported by the Public Health Team to the Highland Health Board and Environmental Health.

There was a delay in responding to the initial disinfection failure alarm on the morning of Sunday 20th August resulting in the chlorination system being down for more than 12 hours. Due to poor chlorine control at Rasaay WTW, there is a history of rapid fluctuations in and out of low chlorine alarm status at the control point and into supply. In this case however the cause of the disinfection failure was mechanical and the delay in responding to the low chlorine alarm was due to an oversight by the duty officer in not contacting the operator to visit the site.

In order to deal with shortcomings with the standby provision for Raasay WTW a revised procedure is to be drawn up and implemented for dealing with critical alarms from such a remote site.

In addition to this, chlorine control will improve as a new WTW has been constructed, is currently being commissioned and is due to be in supply by 31/12/06.

DWQR Assessment on the incident

DWQR's assessment of this incident is that it was caused primarily by a failure of the disinfection system. However, remedial action could have been taken much earlier had the low chlorine alarm been passed on to the operator. It is noted that the only operator on this remote island is not on a formal standby system but that Scottish Water is putting steps in place to resolve that issue. Although water samples were taken from the works and distribution system, these were after the situation had been resolved so do not demonstrate the quality of water going into supply during the incident. DWQR notes that the existing works is being replaced by a new works which should hopefully be fully commissioned by 31 December 2006.