Investigations

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

Report on drinking water quality incident at:
Carron Valley Water Treatment Works in December 2007

Scottish Water Executive Summary

Carron Valley Water Treatment works produces an output of between 60 and 125 Ml/day and predominantly supplies the Falkirk area as well as parts of Stirlingshire. The coagulated lime dosing pump failed at 05.28 on 8 December 2008. This caused a treated water aluminium alarm to be generated which was received at the OMC Flight Desk, Balmore Road at 05:50. The alarm was dispatched at 07:10 and the operator attended site at 08:00. The apparent delay in the alarm being passed out is under investigation.

Initial fault finding and analysis was carried out by the operator between 08:00 and 10:00. At 10:00 the treatment works was shut down to prevent high levels of aluminium being passed forward to the 3 Clear Water Tanks. (CWTs)

At the beginning of the event, the final water aluminium monitor was choked and not reading and this was rectified later during the day.

The available storage and aluminium levels in the CWTs were monitored from 10:00 hrs until the problem with the lime dosing pump was resolved and the treatment works was re-started at about 12:30.

Further testing was carried out using hand-held alum monitors and these readings were within the prescribed limit for aluminium. However, once repaired the on-line final instrument indicated Alum levels above the PCV. An investigation is ongoing to determine why there was a difference in the readings.

The operator remained on site until 18:00 carrying out minor adjustments to the process and by 18:00 treated water quality had returned to normal.

DWQR Assessment on the incident

DWQR's assessment of this incident is that it was due to the malfunction of the lime pump. The site has a standby lime pump, but at the time of the incident this had to be brought on line manually. A delay in Scottish Water's Operational Management Centre passing out the alarm resulted in an operator being called to site too late to prevent the final water quality being affected. Once on site, DWQR considers that appropriate actions were taken by Scottish Water to minimise the impact of the failure. These included shutting down the flow through the plant temporarily until water quality was restored. A discrepancy between readings on hand-held and on-line water quality instrumentation caused initial confusion as to the actual effect of the failure on treated water quality, although laboratory analysis of a sample taken from the clear water tank showed that a failure of the aluminium standard did, in fact, occur.

In response to the incident, Scottish Water has improved the arrangement at the site so that the standby lime dosing pump will start automatically. It has also committed to investigating why there was a delay in passing out the alarm and investigating the reason behind the discrepancy between instrumentation readings.