Investigations

Report on drinking water quality incident at:
Scourie Water Treatment Works in March 2007

Scottish Water Executive Summary

The water treatment works at Scourie consists of upward flow sand/carbon filters feeding into a clear water tank. Disinfection is achieved by flow proportionally dosing sodium hypochlorite to the outlet main from the works.

On Saturday 17th February 2007 the sodium hypochlorite dosing pump tripped, telemetry alarm initiated a call out at 17:30hrs. The treatment operative reacted and replaced the faulty pump at the works. An event notice was raised by Public Health Team. Disinfection was restored (residual chlorine at works 0.58mg/l). Works checked on Sunday 18th February and all process parameters were in compliance. All samples taken passed.

On Friday 2nd March 2007 sampler reported to Treatment Team Leader that whilst taking a regulatory sample at Scourie WTW he found a low/no chlorine. Treatment operative went to site and found that the chorine dosing pump had failed, he then manually dosed the final water to restore disinfection. The operator tried to fix the pump but failed, he then left site to source a temporary replacement pump which was fitted on the Friday evening and was working well when the operator left site.

On Saturday 3rd March the works was revisited and chlorine residual was 0.51mg/l and all process parameters were in compliance. All samples taken passed.

Investigations revealed a faulty telemetry cable on the chlorine dosing pump. Consequently no alarm was received for the pump failure on the 2nd March.

The site is at present visited daily and will continue to be until the new duty/ standby chorine installation is completed in July 2007.

Scourie WTW is due to be replaced by a nano filtration plant as part of the Early Start Q&S3 investment programme. Commencement on site is forecast for July 2007 with completion estimated July 2008.

DWQR Assessment on the incident

The DWQR's assessment of this incident is that it was caused by a failure of the disinfection dosing pump. The pump failure should have been picked up by the telemetry system but it would appear that the telemetry cable may have been damaged when the pump was replaced due to an earlier failure and replacement of the pump on 17 February 2007. The DWQR assessment of this earlier event was that prompt action was taken and that it did not therefore warrant an incident report.

DWQR is pleased to note the Scottish Water's Public Heath Team was promptly informed in this incident to aid appropriate decisions to be taken in relation to sampling and the protection of public health.

DWQR notes the lessons learned and actions being taken to prevent a recurrence, namely to:-

1. Review the chlorine dosing philosophy by 16 April 2007
2. Operators to check telemetry connections when pumps are replaced. Pump fail alarm to be manually simulated to check telemetry system picks it up and all Ness Team Managers to ensure all site operators are aware of the new procedure. By 16 April 2007
3. Install second dosing pump and establish a duty/standby dosing system by 16 July 2007.
4. Ensure new works is in place by July 2008.
5. Circulate copy of report and learning points to all Team Leaders / Managers in the Ness area by 30 April 2007.
6. Regional Manager to visit site by 30 April 2007.

In relation to items 2 and 5 above, Scottish Water should ensure that lessons are learned across the authority's area and not just in the Ness area.

DWQR notes that the site is at present visited daily and that this will continue until the new duty/standby dosing pump installation is complete in July 2007. DWQR also notes that a replacement works should be in place by July 2008. DWQR also notes that the existing telemetry monitors level of raw and clear water tanks, outlet flow and pump status. The reason given as to why there is no residual chlorine monitor which is connected to telemetry is that there is no power. However, DWQR would suggest that the power requirements for a chlorine monitor are not large and has asked SW to review this situation with a view to installing a chlorine monitor with appropriate alarms connected to the telemetry system by end June 2007.