Investigations

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

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

Scottish Water Executive Summary

Blackpark WTW supplies 10,518 customers with disinfected water. Telemetry is in place at the works. On 22 August 2006, a burst in the sodium hypochlorite dosing line led to a disinfection failure. The burst line allowed high pressure mains water into the plant room and water entered the electrics. This caused a total power failure at the plant that morning and also later the same evening despite repairs having been carried out. As a result a second disinfection failure also occurred that evening.

In both cases telemetry alarms were notified to Customer Operations, however because of the frequent occurrence of nuisance low chlorine alarms at the site, and the regular occurrence of "comms failures" in general, the judgement was taken by the standby team that the alarms would clear and therefore were not urgent and immediate attendance at the site was not required. As a result disinfection failed for a period of 10 hours.

The dosing line failure was discovered mid morning and the second power failure the following morning. In both cases emergency chlorination was carried out whilst repairs were effected, and samples were taken from the works and in distribution for Coliforms and E.Coli. All samples passed.

To prevent the re-occurrence of a similar incident dosing control has been investigated and improved at the plant to prevent continual low and high residual chlorine alarms occurring at low flows. Comms failures have been investigated and found not to be a regular occurrence at this site specifically. A Tool box talk is also to be rolled out at the next Team Leader meeting which will remind Team Leaders and Operators of their responsibilities whilst on standby.

DWQR Assessment on the incident

DWQR's assessment of this incident is that the disinfection system failed due to a burst on the delivery line for the pump. A low chlorine alarm was initiated but there appeared to be some confusion initially on the need for the operator to attend the site. Also the situation was further exacerbated by an apparent high frequency of alarms at this site with the result that not all alarms were investigated. Better disinfection control coupled with relevant staff being reminded of their responsibilities in relation to reacting to alarms should resolve these issues.