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 April 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 the site was being visited three time a week at the time of this incident.

On Friday 6th April, a low chlorine residual in water leaving the clear water tank (CWT) was discovered by the sampler. Investigations found that although the sodium hypochlorite dosing pumps were pumping, no chemical was being delivered due an air locking of the pumps. 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 at the time of the event from both the works and the supply zone were reported as having passed on the 7th April.

On the 16th of April the chlorine pipe worked choked again. In order to mitigate the risk of disinfection failure whilst waiting for process upgrades, Dalwhinnie WTW is currently visited daily by an Operator, and this will continue until a chlorine monitor and telemetry has been installed at the works. This is expected to be completed by the end of June 2007. To address the issue of air locking, new pumps with de-gassing heads have been installed.

DWQR Assessment on the incident

While DWQR accepts the conclusions of the report into the incident by Scottish Water has having been caused by air-locked dosing lines in the disinfection system it is concerning that following the first occurrence of the problem on 6th April 2007 that Scottish Water waited until the second occurrence on 16th April 2007 before instigating daily checks by the operator on the treatment plant. Given that the works at the time of the two incidents did not have telemetry, the level of risk posed by a failure of the disinfection system was substantially higher than for a works where an appropriate telemetry system was installed due to the long time delays between visits by an operator. While DWQR welcomes the daily site visits by an operator until a telemetry system is installed and operating it is unfortunate that such a risk-sensitive approach was not operating prior to these incidents. In order to prevent any future occurrence of this type of treatment failure, DWQR will require Scottish Water to immediately review the frequency of operator checks on all treatment works where there is no installed and/or operating telemetry systems and report on the outcome of that review.