Investigations

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

Report on drinking water quality incident at:
Laggan Bridge Water Treatment Works in September 2006

Scottish Water Executive Summary

Laggan Bridge WTW supplies 59 customers with filtered and disinfected stream water. There is no mains power at the site. Dosing is powered by a battery which is trickle fed by a wind turbine and solar panel. There is no telemetry in place at the works. On September 4th, the Operator visited site and discovered a low chlorine residual of 0.04mg/l into supply. When he checked the battery it had a low voltage, and thinking this was the problem he changed it and started the chlorine dosing again.

A sample taken on 4th September from the supply zone had a free and total chlorine residual of 0.03 and 0.04 mg/l respectively; it was reported on 5th September to contain 13 Coliforms and 8 E.Coli. In reaction to this sample result, the operator attended site and discovered that the chlorine level into supply was again low at only 0.02mg/l free chlorine and that no chlorine was being dosed at the works. On investigation a fault was found in the flow meter that controls the chlorine dosing. Repairs were completed and chlorination resumed that day.

The CWT was shock dosed and samples were taken from the works and in distribution for Coliforms and E.Coli. on the day of the incident. The sample from distribution had no chlorine residual and was reported to contain 10 coliforms and 9 E.Coli. The sample from the works had a high chlorine residual of 2.2 mg/l and passed. Further samples taken on September 6th passed.

The flow meter continued to cause problems with chlorine dosing at low flows after this incident, and a new flowmeter was installed on September 12th to prevent further instances of chlorination failure.

To reduce the period of time that plant failures can remain undetected Operator visits will be increased to three times a week; Mondays, Wednesdays and Fridays. Guidance on the number of tablets to be added to the tank for emergency chlorination will also be provided at the works.

The permanent works for a new Laggan Bridge WTW are installed and wet tested but not process tested. Land acquisition and access servitude issues have delayed completion of this project. Land acquisition has been sought for both the sites of the new boreholes/treatment works compound and the clear water tank. In addition rights of servitude are being sought for both the access track to the boreholes and to the clear water tank.

Early entry to the sites was granted to allow construction work to be undertaken but an increasing number of conditions of acquisition have materialised during the course of negotiations. A legally binding agreement is being discussed to allow sufficient security to SW to complete commissioning and come into supply. If agreement cannot be reached there may be recourse to Compulsory Purchase with the resultant time delay. It is not possible to predict a completion date while negotiations continue.

DWQR Assessment on the incident

DWQR's assessment of this incident is that it was caused by a fault with the flow meter that controls chlorine dosing. A new flowmeter has now been installed. The lack of chlorine monitoring and telemetry provision at the site was a compounding factor in the incident as it meant that the problem was not detected until Scottish Water visited the site. A subsequent incident at the site occurred in October which also involved the disinfection system.

Scottish Water has agreed to increase the number of routine visits to the site and carry out additional checks. The works is due to be replaced by a new one which will have a robust treatment process and telemetry monitoring of water quality.