Investigations

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

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

Scottish Water Executive Summary

Waterstein is a simple disinfection works. The sodium hypochlorite dosing is flow initiated on plant startup and there after is manually controlled. The disinfectant is dosed immediately prior to the CWT. There is mains power constantly charging batteries and telemetry available at this location. The mains power supply is currently provided by means of a temporary arrangement from nearby privately owned outbuildings. Scottish Water Solutions are pursuing a wayleave for the installation of permanent mains power and until that is complete the WTW will continue to be powered using the temporary arrangement. The permanent power supply is unlikely to be installed before February 2007.

Following a routine sampling visit on the 13/08/06 a low residual chlorine measured at the WTW tap was reported to the operator. The residual recorded was 0.09mg/l Free Cl2 and 0.12mg/l Total Cl2. On investigation it was found that the battery charging system had failed due to a power outage within the locality and the batteries powering the chlorine dosing pumps, had eventually ran out of charge. The power outage had caused the miniature circuit breaker in the householder's outbuilding to 'trip', so even after the mains power had been reestablished to the area, the house and WTW remained isolated. Power to the WTW was reestablished when the householder returned to his home and reset the switch, late in the evening of the 14th September 2006. On discovery of the problem the CWT was dosed with a slow release chlorine tablet to ensure that disinfection would be maintained till replacement batteries could be brought to site. The situation was reported to the Team Leader. Due to the fact that the sampler had sampled the WTW and that the sampling courier had already left the island, resamples were not taken till the following day.

Bacteriological resamples were taken at the WTW and three properties in distribution on the 14th &15th September 2006. Both sets of samples were reported clear on the 15th & 16th September respectively.

The results were reported by the Public Health Team to the Highland Health Board and Environmental Health.

The primary cause of the disinfection failure was a power outage. If the WTW had its own mains power supply rather than the temporary system from the privately owned outbuildings, the mains would have come back on earlier, probably before the batteries would have failed and, it is unlikely that any problem would have occurred. In addition, had there been a 'WTW mains power failure' alarm installed, this would have resulted in an immediate response to site and it would have been highly unlikely that disinfection would have been lost.

The free chlorine meter has failed to produce reliable readings and the high and low chlorine alarms have been suppressed by Customer Operations. As part of the incident review the operation of this instrument will be investigated, resolved and the high and low alarms activated.

As part of the incident investigations, an end to end review of what should have been provided by the project against what was provided under the project will be carried out by SWS and SW personnel.

It is forecast that a permanent power supply will be installed by end of February 2007 and that a WTW mains power fail alarm be installed by the 23/10/06 . A request has already been made for the existing telemetry 'power fail' alarm priority to be altered from a P4 to a P2.

DWQR Assessment on the incident

This incident was a disinfection failure caused by a failure of the back-up battery power supply to the works when the mains power supply failed. The loss of disinfection was not detected by telemetry as there had been problems with the chlorine meter causing this alarm to be suppressed. The telemetry alarm for the loss of the mains power supply was set for response the next day and the loss of power from the battery back-up system was set to the lowest priority which did not require any response. These telemetry settings had been operating for some time prior to Scottish Water's introduction of their new telemetry system (Open Enterprise) and they had not been reconciled with the revised policy and procedures covering telemetry alarms. Scottish Water have given DWQR assurances that appropriate telemetry alarms settings in accordance with their current policies will be fully in place by end November 2006. Further, Scottish Water have undertaken to review all site signal workbooks for all asset types that may be in a similar situation to Waterstein prior to the incident with a view to rectify any deficiencies that may be identified.