Investigations

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

Report on drinking water quality incident at:
Windyfield Rhynie Water Treatment Works in December 2007

Scottish Water Executive Summary

Windyfield Rhynie WTW (also known as Rhynie WTW) is located to the north of the Aberdeenshire village of Rhynie and supplies water to a population of approximately 400. At the time of the incident, the treatment works provided minimal treatment to a spring supply source in the form of chlorination by a sodium hypochlorite drip-feed system.

This was replaced by a more robust disinfection system with pH correction as part of the Q&S 3 programme of improvements. This new works was commissioned in January 08 and was inspected by the DWQR on 18/01/08.

The disinfected and pH corrected water passes through 2 storage tanks which supply the water to the distribution network.

Cold weather on 17/12/07 caused the drip tube from the aspirator bottle to freeze leading to aloss of disinfection for approximately three to four hours. The telemetry system detected a low chlorine residual from the site at 04:49 hours on the morning of 17/12/07 and an alarm was sent to the OMC Flight Desk. Although three separate Control Engineers investigated this alarm, there was a delay of just over an hour before the alarm was despatched to the standby operative on the site.

At 05 52 hours the alarm was despatched to the standby operative who attended site and disinfection was restored at 07:00 hours.

The delay in despatching the alarm from the OMC Flight desk of just over an hour is out with the agreed target of 15 minutes for a low chlorine alarm to be despatched. All staff are to be reminded of the importance of getting P1 chlorine alarms passed out with the agreed timescales.

No bacteriological samples were taken to determine the effect of this loss of disinfection and an investigation into why this occurred is under way.

DWQR Assessment on the incident

DWQR accepts that the cause of the incident was the freezing of the drip aspirator bottle supplying sodium hypochlorite disinfectant at Rhynie water treatment works. The failure of the disinfection system was picked up by the on-site telemetry systems and an alarm was passed to the Operational Monitoring Centre (OMC) "flight desk". From this point on Scottish Water did not perform against their agreed targets or procedures. There was a delay in responding to the low chlorine alarm of over an hour and a failure to take bacteriological samples to investigate the impact of the loss of disinfection. Both the delay and the failure to act are considered unacceptable by DWQR.

The DWQR considers that all water treatment works operated by Scottish Water must comply fully with the requirements set out in statute to supply clean, wholesome drinking water. Failures to meet target response times and follow procedures, such as those that occurred at Rhynie, are unacceptable. Any repetition will be viewed by DWQR as evidence of systemic failure on the part of Scottish Water and result in appropriate action being taken by DWQR against those responsible.