Investigations

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

Report on drinking water quality incident at:
Assynt Water Treatment Works in April 2007

Scottish Water Executive Summary

Assynt WTW is situated North of the village of Evanton, Ross-shire. Raw water is abstracted from Loch Glass is conditioned with polyelectrolyte dosing controlled by an automatic coagulation control system (CCS). The water then goes onwards to rapid gravity pressure filters. There is disinfection with chlorine gas, and ammonium sulphate dosing for Chloramination purposes. Final pH correction is done using lime. Phosphate dosing with Orthophosphoric Acid is added for plumbosolvency control. The treated water then goes to 16 hours of treated water storage in clear water tanks prior to distribution. The Water Treatment Works produces approximately 14-18 ML of treated water per day supplying a total of 28,663 properties.

During a routine sampling visit by a Scottish Water sampler on Saturday 28th April it was noted that the final water chlorine residual was well below the target of 0.9 mg/l total chlorine. Subsequent investigation revealed that the cause of the low chlorine residual was due to a loss of process. The loss of process was due to three things, namely:

1. Loss of flow to the CCS / Streaming Current Detector;
2. Ammonium sulphate vat being empty of chemical during the previous 24 hours;
3. No telemetry alarms available to pick up the problem with the CCS instrument.

Actions to resolve this problem include:

• Adding the CCS/Streaming current detector to the telemetry signals.
• Training will be done with the site Operators, this training will include Scottish Water
• Procedures, Importance of completing all Task Schedules with a specific emphasis on filling of chemical vats.
• A full telemetry review to be undertaken at the works to ensure that all set points comply with EAL's and alarm priorities are correctly set. Full end to end testing to be done between site alarms and Telemetry Support Group.

DWQR Assessment on the incident

The DWQR assessment of this disinfection failure incident is that it was caused by the operator not filling the ammonium sulphate vat, which is an essential chemical in the chloramination process. A low chlorine dose alarm was received by the Operations Management Centre but the operator was not called out to investigate. It would appear that the final water chlorine alarm at Assynt water treatment works was not on the telemetry system.

In addition to the above, the sample line to the automatic coagulation unit had been switched off. This resulted in too high a dose of polymer being added which affected the coagulation process resulting in highly coloured water which had a high chlorine demand. It would appear that there are no alarms on the automatic coagulation unit which would have given an early warning of a problem with it.

DWQR notes SW's actions to prevent a recurrence, namely:-

1. Ensuring operators are aware of the importance of completing all tasks in the works schedule
2. Carrying out a full review of the telemetry including alarms, priorities and end to end testing.
3. Ensuring that water quality bench test kits are available at all times.
4. Ensuring that operators fully understand the procedures to follow in the event of a breach of Emergency Action Levels.
5. Adding the automatic coagulation unit to telemetry.
6. Adding treated and final water chlorine alarms to telemetry.
7. Ensuring that all operators receive training in event and incident awareness.

DWQR will monitor SW's actions to ensure they are all completed as planned.