Investigations

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

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

Scottish Water Executive Summary

Invercannie WTW is located to the west of the Aberdeenshire village of Banchory and supplies water to a population of approx 250,000 to 300,000 domestic customers and businesses in the village of Banchory and also in the city of Aberdeen and surrounding area. The Treatment Works provides colour removal using ozone, filtration via slow sand and membrane filters with chloramination as disinfection and pH correction.

On Tuesday 12th February 2008 at 12:50 hours there was a sudden loss of vacuum on the chlorine dosing system resulting in a loss of disinfection to the public water supply for a period of 1 hour 55 minutes. The loss of disinfection was discovered by the treatment plant operators who were initially alerted to the fact there was no ammonia sulphate being dosed as a result of an alarm from the on site SCADA system. On further investigation it was found that this was as a direct result of loss of chlorine dosing at the site, the ammonia dosing being directly linked to the chlorine dosing system.

Corrective action was taken immediately with a manual changeover of the chlorine drum with vacuum and full disinfection being restored by 14:45 hours. The chlorine residual at both Invercannie WTW was increased to 1.4ppm immediately following restoration of disinfection with the chlorine residual increased at Mannofield WTW to the same level the following day.

On Wednesday 13th February 2008, the auto changeover system at Invercannie was completely stripped down to investigate the case of this occurrence by SW Mechanical and Electrical fitters. The equipment was found to be in full working order and no explanation for the sudden loss of vacuum being found.

The original chlorine drum was returned to service and lasted a further two weeks with fully automatic changeover successful on Sunday 24th February 2008. Therefore the original chlorine drum had sufficient chlorine gas.

DWQR Assessment on the incident

The DWQR assessment of this incident is that, on 12 February 2008 Scottish Water supplied un-disinfected water from its Invercannie water treatment works for a period of approximately two and a half hours (13:30 to 16:00) due to an unexplained loss of vacuum to the chlorine dosing system. Fortunately, this was discovered promptly, as the site was manned at the time the incident occurred. The chlorine drum was changed manually, which seemed to correct the problem about 1 hour 45 minutes after it occurred, although it took longer than this for a measurable chlorine residual in the final water to appear. Scottish Water has investigated the incident, but has not been able to identify a cause, although a blockage is suspected. Two new chlorinators are to be installed at the site in July.

Had the loss of chlorination occurred out of hours, it may not have been discovered until the next day as it has subsequently emerged that the telemetry system was not operational for this plant between 12 February and 15 February.

Operational staff were not made aware of this at the time, and this is being investigated by Scottish Water. DWQR is highly critical of this, and looks to Scottish Water to ensure that both its telemetry systems and its internal procedures for reporting telemetry failures are robust and regularly tested.

No additional samples were taken by Scottish Water at the time of the incident to determine the microbiological quality of the water being supplied to consumers. Scottish Water's explanation for this is that the works is sampled daily for microbiological quality anyway. However, the sample for 12 February was taken from Invercannie at 09:40, whilst the disinfection failure was at 12:50. Correspondingly, Scottish Water does not know the quality of the water it was supplying to consumers at the time of the incident. DWQR believes that Scottish Water should have requested a final water microbiology sample at the time of the incident and is critical of them for not doing so.