Investigations

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

Report on drinking water quality incident at:
Tomatin Water Treatment Works in January 2008

Scottish Water Executive Summary

Tomatin WTW supplies 360 customers with disinfected borehole water. pH correction is needed at the site in order to comply with the Water Quality Regulations for pH and Copper at the customers' taps.

The cause of failure of the chlorine dosing pump was found to be that an electrical power surge caused the circuit breakers in the control panel to trip. On the day of the event there was thunder and lightning in the area, it is suspected that this was the reason for the power surge. With these breakers tripped there was no power going to the chlorine pump or chlorine analysers thus disinfection failure occurred.

A delay of 4.45 hours with the OMC Flight Desk sending out the telemetry alarm highlighting the failure to Customer Operations resulted in chlorine levels out with emergency action level (EAL) limits entering supply. Initial investigation shows no justifiable reason for the delay in dispatching the low chlorine alarm.

The root cause of the disinfection failure was indeed the loss of power to the pump caused by the tripping of the circuit breakers, however with the unacceptable delay in alarm handling the chlorine levels did fall below EAL, although no water quality failures were recorded on the day of the incident on for the period after the incident.

Scottish Water Customer Operations reset the breakers on arrival to site and disinfection was immediately restored. Operations checked water quality parameters at the WTW and in distribution and these showed sufficient chlorine residuals, no secondary disinfection of shock dosing was carried out.

Actions being taken in response to this incident include:

• Telemetry Alarm Handlers at OMC Flight Desk to obtain additional training and reinforcement of importance of passing chlorine alarms out within the designated timescales.
• Operators to be assessed on diary entry procedures and reinforcement given to Operators to contact the Public Health Team immediately they are aware of a breach of EAL.

DWQR Assessment on the incident

Following consideration of the report prepared by Scottish Water into the circumstances surrounding the drinking water quality incident at Tomatin WTW between 23 and 25 January 2008, DWQR accepts Scottish Water's assessment that the failure was due to a fault with the chlorine dosing pump arising from a power surge causing the electrical protection system on the pump to trip and cut power to the pump. Such occurrences are commonplace and when responded to in an appropriate manner cause no threat to drinking water quality. However, on this occasion the response does not appear to have been appropriate. The telemetry system at the works operated correctly and alerted the Operations Management Centre (OMC) "Flight Desk" of the failure of the chlorine dosing at the works but the OMC appears to have not passed on the appropriate information for around four and three-quarter hours resulting in a delayed response from Scottish Water to the incident.

Without wishing to prejudice any enquiry or action taken by Scottish Water as a result of the delay, DWQR is extremely concerned over the apparent failure to pass on details of such alarms in a timely and appropriate manner. DWQR requires Scottish Water to ensure that all personnel operating in the OMC have the appropriate level of training and supervision to ensure that such circumstances are not repeated. DWQR requests Scottish Water to review the "human" element of their telemetry response chain with a view to further automation of priority alarms direct to those operational staff who are on duty and who have responsibility for maintaining the integrity of drinking water systems.