Investigations

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

Report on drinking water quality incident at:
Rhynie Water Treatment Works in August 2006

Scottish Water Executive Summary

On Saturday the 5th August 06, the disinfection process at Rhynie WTW was restricted, resulting in a gradual but continual decay in the chlorine residual. At around 20:00 hours on the 5th the chlorine residual passed through the low level alarm threshold and continued to decay until discovered during a routine scheduled visit to the site at 13:00 hours on Monday 7th August 06. There was low or no chlorine residual in the supply from the wtw from 04:15 hours on the 6th August until 14:00 hours on the 7th August 06.

Upon discovery of the situation, the chlorine residual in the tank was immediately boosted to reinstate disinfection using calcium hypochlorite tablets. Flushing of the distribution network was carried out to pull the chlorine residual through the system and additional samples were taken to determine the bacteriological compliance of the water on the afternoon of the 7th and on the 8th August 06. Normal Chlorine residual was achieved by 14:00 hours on the 7th August 06.

An initial communications failure alarm was generated on 5th August and passed out to the standby-operator who attended site at 06:00 to verify status. Chlorine residuals were checked using the on-line residual monitor and pocket meter and both were satisfactory. The stand-by operator left site assuming there to be no plant issues.

Investigations have highlighted that the LOW LOW chlorine alarm, received on the Open Enterprise Telemetry System on 6th August at 20:30, was actually generated on 5th August at 20:15. However, as this alarm was 24hrs old, it was not passed out to the standby-operator. The delay in relaying the alarm is due to a presumed problem with the network service provider. However, this is subject to further investigation.

Sodium Hypochlorite is dosed on-site site using an Aspirator. It is possible that the restriction was caused by scum formation on the surface and through the body of the liquid. Investigations are underway to ascertain whether this is connected with the switch to low bromate specification hypochlorite. A new chlorine dosing system is to be installed through the Q&S3 process with a target commissioning date of April 2007.

DWQR Assessment on the incident

DWQR has investigated further the events surrounding this incident at Rhynie WTW. The DWQR has noted that Scottish Water do not now consider power dips or the mobile 'phone network provider's service in the area to have been factors pertinent to the incident and so the delay in responding to the alarm originated within Scottish Water's Operations Management Centre. Formal training of the staff involved during the incident is welcomed by the DWQR as a suitable measure to ensure appropriate actions are taken by staff in such circumstances. Scottish Water are maintaining a 'watching brief' regarding the scum formation associated with the low-bromate sodium hypochlorite being used at this site. DWQR has noted that Scottish Water will upgrade the chlorination system at Rhynie by August 2007 and DWQR will monitor progress on this matter very closely.