Investigations

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

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

Scottish Water Executive Summary

Kilchoan WTW supplies a maximum of 158m3/d into the Kilchoan water supply zone, there is a single treated water storage tank of 73m3. The population served is approximately 300. The works consists of 2 pressure sand filters, followed by 5 pressure GAC filters and then one pressure limestone contactor for pH correction. The water is then pumped back up to the CWT where sodium hypochlorite is dosed for disinfection. Telemetry and a new automatic chlorine dosing and control system are still to be commissioned under the Q&SII investment programme.

On 9th August 2006 the SW Operator found a 0.02mg/l free chlorine residual at the statutory sample point and the sodium hypochlorite stock tank was found to be empty. The stock tank was refilled, dosing was re-instigated immediately and the CWT was then shock dosed. Bacteriological samples were taken but were later found to be invalid due to out-of-date bottles. Re-samples were taken the next day and were reported as passed on 11th August.

The commissioning of the new water treatment works at Kilchoan is on going. Until it has been completed and SW operations are confident with the new telemetry for the site and the functionality of the new works, a SW operator will visit the site on a daily basis. Once the new works is fully commissioned there will be sodium hypochlorite dosing with flow proportional and residual control and automatic plant shut down in the event of a low treated water chlorine residual reading and/or disinfection failure.

DWQR Assessment on the incident

DWQR's assessment of this incident is that it was caused primarily due to a decision being made by Scottish Water (SW) to delay a routine visit to the works which is still undergoing commissioning for its disinfection and telemetry systems. Had the works been visited as planned, the failure would not have occurred. However, SW has agreed to carry out daily visits to the works until the disinfection and telemetry systems are fully operational. SW is also taking action to ensure that operational staff are fully aware of the sampling procedure which should ensure that out of date bottles are not used for taking samples. Finally, SW is taking action to ensure that those unfamiliar with the site do not deviate from the standard schedule visits unless authorised by a more senior member of staff.