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Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Strontian Water Treatment Works in September 2006
Scottish Water Executive Summary
Strontian WTW supplies a maximum of 225m3/d into the Strontian water supply zone, there is a single treated water storage tank of 70m3. The population served is approximately 600. The works consists of upflow sand and GAC sandwich filters followed by manual sodium hypochlorite dosing prior to the clear water tank (CWT). The works is visited twice a week and there is no telemetry on this site.
On 11th September a SW Sampler notified Customer Operations of a low chlorine residual of 0.02mg/l free chlorine at the statutory sample point. The sodium hypochlorite dosing pump is powered by a battery and on visiting site the works operator found the battery was flat, this was replaced and the CWT was shock dosed. Following corrective action sampling was carried out at the WTW, service reservoir and in distribution on 11th September. Further resamples were taken at the WTW, service reservoir and 4 properties in distribution on 12th September.
The bacteriological samples taken from the CWT and the service reservoir by the sampler who had reported the low chlorine residuals were reported as failed the next day.. All samples taken after corrective action on 11th Sept and further resamples taken on 12th Sept passed.
The batteries at this site are replaced weekly on the Tuesday visit. The Batteries normally last two weeks so should have easily lasted until 12th September. The incident was therefore most likely to have been caused by a faulty battery. Changes to the management of batteries have been put into place to avoid a reoccurrence. New batteries and an amp meter have been delivered to site to avoid a faulty battery being used. The operator will now change the battery on a Friday and check it again on a Tuesday.
Under the Q&SII investment programme a new Membrane plant will be installed at Strontian WTW's. The water supply zone for Strontian has an authorised departure granted for Total Trihalomethanes, pH, iron and Colour. The departure will expire on 30/11/2007 and the new WTW will be installed, commissioned and into supply by this date.
DWQR Assessment on the incident
This incident resulted from a faulty battery failing to power the sodium hypochlorite dosing pump. The faulty battery was not detected when it was entered into service. Scottish Water have undertaken to change the batteries twice a week and to check their condition with a amp meter to ensure they are fit for purpose. While there was no telemetry on site at the time of the incident work is currently underway to install and commission a new membrane treatment plant which will have telemetry system incorporated within it. It is anticipated that the new plant will be operational by early January 2007 at the latest. Until the new plant is operational Scottish Water are visiting the site 5 times a week. DWQR will review the situation once the new plant is operational to ensure appropriate telemetry systems are in place.