Investigations

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

Report on drinking water quality incident at:
Carrick Castle Water Treatment Works in July 2007

Scottish Water Executive Summary

Following a fault with the Programmable Logic Controller (PLC) on 6th July 2007 (estimated time 0300hrs) non-chlorinated water was passed forward to the Clear Water Tank (CWT) for approximately 9 hours. This fault was identified during a routine check of the plant at 1215hrs on 6th July 2007.

The plant was immediately shut down and the CWT emergency dosed with calcium hypochlorite tablets, the network system flushed and a sample regime set up. The PLC fault was identified and a temporary fix implemented within two hours of finding the fault.

The investigation has determined that an alarm was not raised because an inhibit was placed on the treated and supply chlorine analysers. This had previously been highlighted in a Process Water Quality Audit carried out on the 6th March 2006, when a high priority work request line was raised to investigate a fault with the chlorine analyser. The work had not been actioned. The investigation into this will be concluded once the Team Leader for the site has returned from annual leave.

All samples taken in response to the incident were satisfactory. The main recommendations from the investigation were to repair the PLC, repair/ replace the chlorine analysers, remove the inhibit on the signals, and test the system to ensure that all alarms are set correctly and that telemetry is working.

Key learning points :-

1. Review status of urgent and high priority Electrical & Mechanical (E&M) task requests at monthly Regional Water Quality meeting.
2. Complete the current exercise within Scottish Water involving a number of initiatives to reduce water quality incidents e.g. Operator Awareness Training, Telemetry Data Quality Project, review of Risk Based and Reactive Maintenance.

DWQR Assessment on the incident

This incident occurred when a fault with the PLC controlling the works meant that un-disinfected water was passed into supply until the problem was discovered during a routine site visit approximately nine hours later. The investigation in response to this incident discovered that the alarm from the treated water chlorine monitor that should have alerted operational staff to a problem had, in fact, been inhibited three years earlier due to a fault. Despite several work orders having been raised to repair the monitor, they were never actioned. Scottish Water is unable to say why this was, and accepts that several opportunities to rectify the fault were missed.

All chlorine analysers and alarms at the site are now fully operational. Scottish Water has committed to reviewing risk based and reactive maintenance and producing regular reports on all suppressed alarms across the company to enable them to be investigated. Operational staff in the area have been reminded of the importance of ensuring that alarms and monitors are operating correctly.

DWQR considers that, although the occurrence of the PLC fault was probably unavoidable, the ongoing suppression of both chlorine monitor alarms at the site should have been rectified by Scottish Water long ago. DWQR accepts that actions taken by operational staff in response to the incident were appropriate, and that the subsequent actions to prevent a recurrence here or elsewhere are reasonable.