Investigations

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

Report on drinking water quality incident at:
Alligin Water Treatment Works in February 2008

Scottish Water Executive Summary

The water treatment works (WTW) at Alligin consists of a colour removal membrane filtration plant. The burn source water receives prefiltration via a basket strainer before membrane filtration. Following membrane filtration the water is disinfected and pH corrected. The water gravitates to 2 No. Clear Water tanks (CWT) situated near the works. The CWT then feed into distribution supplying a total of 60 properties in the village between Alligin Shuas.- Inveralligin - Rechullin.

The membrane plant went into supply in December 2007 replacing a simple disinfection process only. A new 6m3 CWT was installed at this time and was built in parallel to the old CWT to allow for either of the tanks to be taken off line for cleaning. The old CWT was programmed to be emptied and cleaned on the week commencing 4th February 2008.

On 1st and 2nd of February severe winds and heavy snowfall hit the Wester Ross area. This resulted in power outages and Alligin WTW was one of the plants affected. Low level CWT alarms were received by the standby operative at 19:12hrs on the evening of the 1st Feb. These alarms could not be reacted to because all roads were impassable due to the severe weather conditions. The standby operative managed to respond to the low CWT levels at 10:30am on the 2nd. but site access was restricted to small 4X4 vehicles. Power was restored at this time but the CWT was very low indeed. Further outages/shut downs during the day hindered CWT recovery. Weather conditions were still very bad. High houses on the network complained of low pressures. Bottled water was delivered to affected customers. It was not possible to transport a generator to site because of the road and weather conditions.

By early evening 2nd February WTW was back producing water, but the CWT was not gaining as demand was outstripping production. It was decided to tanker water to site, but as access was still restricted to site this would have to be done next day if conditions improved. The level of CWT was so low at this time that settled particulate matter on the floor of the tank was carried into supply.

The duty officer had mobilised the tanker which is sited at Henderson Drive depot Inverness. This tanker is routinely used for transportation of disinfected potable water only and although it is not disinfected before each and every use it is disinfected and checked on a regular basis by competent staff based at Henderson Drive depot. The contractor employed to haul the tanker unit is Seafield Parks and their driver /operator has been DOMS trained. A generic method statement for their tankering procedures is attached in the Appendices.

The tanker was filled at Henderson Drive depot and chlorine residuals checked at site prior to discharge. The tankered water was discharged into the old CWT at Alligin using disinfected delivery hoses. A second load of water was taken from nearby Torridon water supply. Again chlorine residuals were checked prior to discharge (results contained in tanker log in Section 8). The Scottish Water operatives Involved in this incident are both DOMS trained. Water tankering logs were completed for this operation (See Section 8).

The supply was restored at this stage.

Samples taken on Monday 4th February resulted in PCV breaches for Turbidity, Aluminium, Iron and Manganese

Actions being undertaken as a result of this incident include;

• Cleaning of the old CWT.
• Review of the end to end procedures for tankering operations.

DWQR Assessment on the incident

DWQR accepts Scottish Water's assessment that the adverse weather that caused the loss of power to the water treatment plant was the root cause of the turbidity failure at Alligin WTW on 1st/2nd February 2008 but notes there were significant deficiencies in the deployment of the tankered water used to restore the supply. Scottish Water have recognised this deficiency and DWQR welcomes Scottish Water's revision of their procedures covering tankering operations. DWQR also notes that a contributory factor was the inappropriate linking of the turbidity monitor to the operation of the plant and welcomes the review of similar sites to ensure such a circumstance does not occur in future. DWQR also welcomes the review of provision of stand-by generation capacity in the Wester Ross area and would support moves to provide fixed supply generation capacity at such water treatment plants in order to allow drinking water production to be maintained in the event of the loss of mains power.