Investigations

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

Report on drinking water quality incident at:
Dalwhinnie Water Treatment Works in February 2007

Scottish Water Executive Summary

Dalwhinnie water treatment works is a small works supplying an average of 100 cubic meters of treated water per day to a population of approximately 120. The raw water for the works is abstracted from a shallow spring close to the River Truim and the only treatment carried out at the works is sodium hypochlorite dosing to provide disinfection. The site is fed by mains power, however there is no on-line water quality monitoring or telemetry at the works and thesite is visited twice weekly.

On Monday 5th February, during a routine site visit, the works operator discovered a low chlorine residual from the water leaving the clear water tank (CWT) and at the works. Investigations found that although the sodium hypochlorite dosing pumps were pumping, no chemical was being delivered due an air locking in the pipework. The pipework was bled of air and dosing recommenced; a chlorine residual of 0.91mg/l was established at the WTW treated sample tap. The CWT was then shocked dosed to achieve a chlorine residual into supply of 0.5mg/l.

Incident samples taken from the WTW and in supply on the evening of the 5th and the morning of the 6th February were reported as having passed on the 7th February. As there is no on-line chlorine monitoring, chart recorder or telemetry at the works it is not possible to accurately ascertain when sodium hypochlorite dosing failed, however as there was no chlorine residual in the CWT, which has 24hours storage, dosing must have failed at least 24hrs before the loss of disinfection was discovered.

Had telemetry been in place, and a timely response to a sodium hypochlorite dosing pump failure alarm occurred, the period of the disinfection failure would have been reduced. A Q&SIII project which will install a more robust chlorination system, chlorine monitoring and telemetry, is forecast to be complete by June 2008. The Area Team Manager is currently pushing the project delivery team to fast track the installation of telemetry and chlorine monitoring part of the project.

To reduce the risk of disinfection failures at this site in the interim, the Dalwhinnie works Operator has been instructed to purge the sodium hypochlorite dosing line at each visit and signage has been put on site to remind the Operator of this duty. This it the first occasion that the dosing pumps have air locked at Dalwhinnie water treatment works.

DWQR Assessment on the incident

The DWQR assessment of this disinfection failure incident is that it was caused by an air lock in the disinfection pump. Due to the lack of any telemetry on the site, this was only noticed during a routine visit which normally takes place twice a week. DWQR is content with Scottish Water's response following discovery of the problem but would expect SW's Public Health Team (PHT) to be informed immediately, not waiting for 5 hours before doing so as in this case.

There was then a further delay of about 3 hours before samples were taken from the works outlet and from the distribution system. These delays are a recurring theme and although SW says it is informing operational staff of the importance to engage with the PHT immediately, the message is obviously not getting through and SW management must do more about this issue.

DWQR is also concerned that the reason for the delay in sampling was that "the operator had to travel to Inverness to get bacteriological bottles". This is unacceptable and SW should have these available closer to hand in case of incidents.

DWQR notes the actions being taken by SW namely:-

1. Operators instructed to purge the disinfection dosing line at each visit (action complete).
2. Sign to be displayed to remind operators of above task (complete).
3. Team leader to be reminded that loss of disinfection should be reported to PHT immediately (complete).
4. Scotland-wide "Operator Awareness Training" to start on 26/03/07. Based on previous incidents, this will include appropriate actions to be taken in the event of a disinfection failure (complete by 31/5/07).
5. Ensure all staff taking incident samples are recording chlorine residuals and so that information is available for incident reporting purposes (ongoing).
6. Site visits to be increased to three times a week until telemetry and chlorine monitoring is in place (by 12/3/07).
7. Install more robust chlorination system, chlorine monitoring and telemetry (by June 2008).