Investigations

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

Report on drinking water quality incident at:
Ardrishaig Water Treatment Works in October 2007

Scottish Water Executive Summary

A telemetry alarm for first stage treatment fail was received by the OMC Flight Desk at 01:27 on 31 October and passed out to the Standby Operator at 01:50 who arrived on site at the water treatment works at 02:45. He carried out investigations, and determined the root cause of the problem which was elevated aluminium levels caused by failure of the dosed water pH correction pump. He shut down the duty pH correction pump at 04:20 and changed over to the standby pump. Following this action, front end chemistry started to recover.

With the plant fully operational and the front end chemistry back to normal, the Standby Operator left site to carry out routine duties at Tarbert WTW while the changes he had made took effect.

The Standby Operator received a further call from the Flight desk at 07:20 with a treated water high turbidity alarm for Ardrishaig WTW. When he arrived on site to check that alarm, it proved to be a spike as a result of the backwash process on the manganese contact tanks. In total, three high treated water turbidity alarms were received. All three were spikes as a result of backwashes. All three were accepted, reset and levels reverted to acceptable values within range.

The Standby Operator left site at 07:45, having discussed the matter with his Senior Operator and knowing that the Senior Operator would be on site at 08:00. The Senior Operator remained on site all day monitoring performance of the works.

The elevated aluminium levels caused as a result of the initial failure took some time to pass through the treatment process and although there was consideration given to utilising storage at the Clear Water Tanks (CWT), this was not taken forward.

Flushing had not been considered in the discussions between the local Team Leader and the Public Health Team. It is accepted that flushing would have helped to restore water quality to acceptable levels at an earlier stage.

Aluminium levels leaving the clear water tank (CWT) rose above 200 ug/l at around 8:30 on 31 October reaching a maximum level of 340 ug/l (as measured by bench tests) by 13:00 on the same day. Levels started to reduce from 13:00 reaching 200 ug/l by 08:00 on 1 November.

DWQR Assessment on the incident

DWQR's assessment of this incident is that it is likely that Scottish Water supplied water that exceeded the standard for Aluminium to the area served by Ardrishaig works, which includes Lochgilphead. It is not possible to be certain of the levels of aluminium in the distribution system because Scottish Water failed to take adequate samples to establish the impact on water quality, a shortcoming they have identified in their report into the incident.

The cause of the failure was a malfunction in the pH correction dosing pump that elevates the pH of the incoming water to enable it to be treated using aluminium sulphate. In response to the failure, an appropriate alarm was generated and an operator attended site. Scottish Water took appropriate action at the treatment works to resolve the problem, however by their own admission it is possible that more could have been done to mitigate the effects of high aluminium concentrations on consumers by utilising water stored on the site and by carrying out flushing in the distribution system. Scottish Water has appropriately identified this as a learning point from the incident, along with the timely notification of such events to Scottish Water's Public Health Team who are able to offer advice on water quality and protecting consumers.