Investigations

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

Report on drinking water quality incident at:
Kinloch Rannoch Water Treatment Works in July 2006

Scottish Water Executive Summary

Following routine checks undertaken at Kinloch Rannoch WTW on 6th July 2006 which confirmed that all plant was operating satisfactorily, the sodium hypochlorite dosing pumps are believed to have been signalled to stop during the next routine automated operation of the treatment works. Suspected air locks in the sodium hypochlorite dosing lines triggered flow switches which in turn signalled the sodium hypochlorite dosing pumps to stop. Although such an event should have instigated the auto-shutdown of the treatment works, the works continued to operate without chemical disinfection for 5 days until the next routine visit on 11th July.

The auto-shutdown procedure did not occur due to the control system on site not recognising the dosing pumps stopping as an actual fault. This unforeseen configuration error also prevented dosing pump alarms from being generated, again due to the control system seeing the pumps stopping rather than a fault being generated. The control system for the auto- shutdown process will be amended by the time this report is issued. Similar systems at Kenmore and Kirkmichael WTWs will also be checked and amended as necessary.

Although alarms were not generated as a result of the pumps stopping, had alarms been triggered the signals would not have been relayed to Scottish Water's telemetry system as a common link cable was missing from one of the telemetry cards on site. This is still subject to an investigation.

A third warning system in the form of chlorine residual alarms also failed to highlight the disinfection failure as new chlorine residual monitors had not been configured on telemetry by the time of the incident. This has now been completed. Daily site visits which were initiated after the event will continue until the monitors have proven to be reliable over a number of days. The revised auto-shutdown procedure will provide the initial response to any future disinfection failures.

DWQR Assessment on the incident

Drinking water produced by Kinloch Rannoch WTW was not disinfected for a period of 5 days from 6 July 2006. The primary cause of the incident was an error in the system controlling the disinfectant dosing pumps; secondary issues contributing to the incident were the failure to properly integrate chlorine monitors to the telemetry system. In subsequent investigations it was found that even if appropriate alarms had been generated, no telemetry signal would have been generated as there was a hardware fault with the telemetry installation. During SW's investigation it was found that in addition to Kinloch Rannoch, Kenmore and Kirkmichael WTWs have similar disinfectant dosing systems and the errors highlighted from this incident have been addressed at all three sites. DWQR is satisfied that the actions taken by Scottish Water as a result of this incident are appropriate to the conditions found at the time of the incident and that the actions taken at the other sites identified as results of the investigation are apposite. One area of concern that remains is the failure to identify who was responsible for the hardware fault found in the telemetry system; this lack of clarity over responsibility for work carried out on the treatment works undermines DWQR's confidence in Scottish Water's procedures.