Investigations

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

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

Scottish Water Executive Summary

Sallachy WTW is a simple disinfection works. The sodium hypochlorite is dosed flow proportionally into the raw water main prior to the CWT. There is no power or telemetry available at this location.

Following a routine visit by an operator on the 08/08/06 the free chlorine residual was foun to be less (0.11mg/l) than the target value 0.5mg/l. On investigation it was found that the flow meter pulse-head needed to be 'reseated'. The sodium hypochlorite pump had been receiving an intermittent signal. In addition, the weather was extremely wet and the quality of the incoming raw water was poor. The flow meter pulse-head was repositioned, the flow paced dosing reset and the CWT dosed with 0.5l of 15% sodium hypochlorite.

The situation was reported by the Operator to the Portree Office, who then forwarded an e-mail to the Public Health Team advising them of the situation.

Later that same morning, a sampler took a statutory (bacteriological and chemical) sample at a customers tap within the Sallachy water supply zone. No routine bacteriological sample was taken at the WTW, as this location is scheduled for one sample every 4 weeks

The statutory zone sample taken on the 08/08/06 was reported as failing the bacteriological standards on the 09/09/08, showing a Coliform count of 17/100ml and an E.Coli count of 10/100ml. The free chlorine residual taken at the time of sampling was 0.06mg/l.

The result was immediately notified by the Public Health Team to the Highland Health Board and Environmental Health and following discussion with the HHB it was agreed to await resample results.

Following the report of the bacteriological failure, two operators were immediately sent to site to investigate. They found that the flow paced dosing was working correctly, but that there was simply insufficient sodium hypochlorite being dosed to deal with the increased chlorine demand of the incoming raw water. The pump rate was increased and the CWT was further dosed with 2 litres of sodium hypochlorite. The disinfected water was then flushed through the distribution system.

The resamples taken on the 09/08/2006 from the WTW and three properties in distribution were reported on the 10/08/2006. All four resamples passed. A further 4 resamples ( WTW and 3 properties in distribution) were taken on the 10/08/2006 and all were reported clear of Coliforms and E.Coli on the 11/08/2006.

Sallachy WTW is a small rudimentary works with no power or telemetry. The quality of the incoming water can vary greatly. Sallachy is due to be linked into the Kyle Regional Scheme by December 2008.

DWQR Assessment on the incident

DWQR's assessment of this incident is that the disinfection failure was caused by a fault in the system which, due to the absence of telemetry with appropriate alarms, went unnoticed until a routine visit by the operator discovered it. To add to the delay, this routine visit was postponed due to Scottish Water reacting to issues as a result of the bad weather at other sites which do have alarms connected to telemetry.

DWQR applied some pressure to Scottish Water and they subsequently agreed to provide a power supply, residual chlorine control dosing and telemetry link via a mobile or landline link. This should provide better chlorine control and an early warning of problems with the disinfection system at the works. DWQR is pleased that this work is being fast tracked with an estimated completion date of 30 November 2006. However, it is understood that there is a risk that if a BT landline is required it may take longer, so Scottish Water is investigating the ability to use a mobile link.