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 May 2006

Scottish 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.

A soda ash dosing trial, previously halted due to loss of disinfection was recommenced on 10 May 2006. On 15 May 2006 it was discovered that the soda ash and chlorine dosing pumps had tripped. From the amount of chemicals left in the tanks it was estimated that dosing had ceased some time on the evening of the 12 May 2006.

The CWT and one S.R. in distribution were shock dosed with chlorous and the chlorine dosing was re-started. Samples were taken for Coliforms and E.Coli, from distribution on the day of the incident, and from the Final Water and in distribution the day following the incident. All samples passed.

The cause of failure of the chlorine dosing pump was found to be due to a wiring anomaly. The pump condition (available or tripped) was linked to the condition of the soda ash pump, which had tripped due to crystallisation of soda in the dosing line. Previous investigations by the Electrical and Mechanical (E&M) team had identified the stirrer overload and the distribution board as the cause of previous pump failures, and these had been replaced.

Poor communication within the E&M team, and between them and the Treatment team caused a delay in the identification of the root cause of pump trips, as the focus of investigations had been on the soda ash dosing tank stirrer rather than the cause of the chlorine pump tripping.

The lessons learnt from this incident are that:

More effective communication between the E&M and Treatment Teams, and especially within the E&M team is required, To address communication between the teams a mechanism in the form of Quality Improvement Teams , which meet monthly to discuss issues effecting the maintenance of water and waste water quality is already operating in Scottish Water. The importance of attendance at these meetings will be re-iterated to staff. Improvements in communication within the E&M team will be addressed by review of repeat visits at their fortnightly team meetings and by improved dissemination of additional information from the Team Leader.

Soda ash dosing will be recommenced at Tomatin WTW, but only when the following provisions are in place:

• The link in the condition of the chlorine and soda pumps have been separated and end to end tested.
• Telemetry for the condition of the chlorine pump is in place and end to end tested.Daily Operator visits have been arranged for the first 2 weeks after dosing commences.

DWQR Assessment on the incident

DWQR agrees with Scottish Water's proposed actions in its report to resolve the issues at this works. However, in addition Scottish Water has agreed with DWQR's request to install a final water chlorine residual monitor at the works which will be connected to its telemetry system and appropriately alarmed to provide an early enough warning of low disinfection to prevent undisinfected water from going into supply. This will be done by 1 November 2006. Until this is in place, Scottish Water will undertake daily visits to ensure the disinfection system is working as it should do.