Investigations

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

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

Scottish Water Executive Summary

Dornie water treatment works is a simple disinfection works and serves a population of approximately 188 people, producing an average 60 - 120m3/day of treated water dependent influx of tourists.

Water is abstracted from Allt Mhor Burn and is injected with chlorine which is controlled flow proportionately with residual control, there is no filtration process. There is both telemetry and power at the site. The mains power provides a trickle feed charge to a battery system which powers the dosing pump and telemetry remote terminal unit (RTU).

On the Tue 20/03/07 at 07:30 a P5 telemetry alarm was generated for a RTU battery volts low, at 08:36 on the same day a P2 comms fail alarm was generated at the OMC flight desk and Acknowledge but not passed out to operations.

On the Wed 21/03/07 the ICAT Team Leader, during a routine check of generated telemetry alarms, informed the Operations Team Leader of the failed comms alarm for Dornie water treatment works. An operator was dispatched to site to investigate the reason for the comms fail. On arrival the operator found the 24volt Deakin Davenset battery charging system had failed resulting in a loss of power to the disinfection process/ instrumentation and telemetry system.

The operator immediately checked the chlorine residual at the regulatory sample tap and found it to be Free 0.02mg/l. The operator then shock dosed the CWT which increased the residual at the regulatory sample tap to 0.80mg/l.

An E&M Engineer was called to site and carried out temporary repairs to the 24volt system allowing all disinfection process/ instrumentation and telemetry to be fully operational.

The cause of the disinfection fail was due to a fault with the 24volt battery charging system, which resulted in a loss of power to the chlorine dosing pumps, as a result untreated water entered the clear water tank. Whilst a telemetry alarm for the 'RTU battery volts low' would have highlighted a problem it was wrongly categorised as a P5 and was therefore not passed out to operational staff. Although the OMC Flight Desk acknowledged a P2 telemetry alarm for the loss of comms, it was not passed to operational staff for action. Analysis of the telemetry alarm and residual chlorine trends would indicate that the disinfection was not operational for approximately 33 hours.

Dornie water treatment works was visited on Mon 19/03/07 and task schedules show the plant to be compliant and there was no indication of a problem.

Dornie water treatment works is due to be linked into the Kyle Regional Scheme work has commenced on site and completion estimated end of July 2007.

DWQR Assessment on the incident

Evaluation of the Incident Report suggests that had the telemetry alarm for the loss of battery power been correctly assigned as a Priority 2 (P2) alarm then the incident may have been avoided. However, a further opportunity to prevent the event escalating to an incident was lost when the Priority 2 communications failure alarm (telling staff that the communications signal from the plant had been lost) was not passed to the appropriate operational staff upon receipt at the control centre. This specific point is still under investigation by Scottish Water with the staff concerned and so only the general point about the need for Priority 1 (highest priority) and Priority 2 alarms to be passed timeously to the appropriate staff can properly be made at this time. Given the reliance by Scottish Water on such battery back-up systems in these remote locations it is disappointing to note that only following this incident is a weekly battery check test to be included in the schedule of work required to be undertaken by operators. This is not the first time a battery system has failed with the resulting loss of disinfection and it would have been reasonable to assume that Scottish Water would have taken appropriate steps to prevent a recurrence. The incorporation, albeit belatedly, of the risk based maintenance scheduled task for battery condition checks at sites where battery powered dosing systems are in place along with the inclusion of weekly battery tests on the scheduled tasks for operators is welcomed by DWQR.