Investigations

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

Report on drinking water quality incident at:
Muirdykes Water Treatment Works in May 2006

Scottish Executive Summary

On Thursday 18th May at 2350 Hrs, a power failure at Muirdykes WTW caused the shutdown of all of the treatment processes. The raw water inlet valve failed to close, which resulted in raw water passing through the works into the on-site Service Reservoir (SR).

The Water Management System (Telemetry) for the site was out of service due to a problem which occurred with the signal from the Langbank radio mast earlier on that day, 18th May. A contingency plan had been put in place whereby the standby operator was carrying out 4 hourly callbacks to the site, until the telemetry was restored. The first callback at Muirdykes WTW occurred between 2100 Hrs and 2200Hrs. The standby operator arrived on site at 0200 Hrs on Friday 19th May to find that the power failure had initiated a plant shutdown involving all aspects of the process, including chemical dosing. The standby generator had started as expected, restarting a number of key processes including disinfection and ran until mains power was available. A power loss normally causes an automatic callout, but because this site was served by the Langbank radio scanner (which was out of service), this was not possible. Therefore, raw water was flowing through the plant for a period of 2 hours. When the standby operator arrived on site, the raw water inlet valve was shut manually. The Operational Team Leader was informed of the situation. The chemical dosing pumps were then reset, and the plant re-started to normal operation.

Samples were taken from the onsite SR's, and examined both in the works laboratory for a range of water quality parameters and analysed in Scottish Water's laboratory for bacteriological parameters. The Public Health team were informed of the situation by the team leader who in turn informed the area CPHM and EHO. The Chlorine dose leaving the works was increased from 1mg/l to 1.5mg/l. The raw water inlet valve automatic control has been repaired so that the valve automatically closes in the event of a power loss and therefore prevents untreated water from passing through the works.

DWQR Assessment on the incident

The DWQR notes the circumstances of this incident and that the site was being visited by Scottish Water on a four hourly basis through the night in order to monitor the works whilst telemetry was not functioning. Raw water flowed through the works for a period of two hours, although this water mixed with previously treated water in the on-site service reservoirs, preventing the chlorine residual from dropping to zero. DWQR considers that quality was quickly restored once the problem was discovered and no microbiological failures were recorded following this incident. DWQR notes that Scottish Water has repaired the automatic inlet valve to prevent a similar incident recurring.