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 February 2007

Scottish Executive Summary

Sallachy WTW is a simple disinfection works, with no filtration. The sodium hypochlorite is dosed flow proportionally into the raw water main prior to the clear water tank (CWT).

A sampler arrived on-site on morning of 14th February to take a cryptosporidium sample and checked the free chlorine residual, at the regulatory sample tap. It was apparent that the residual was less than it should have been for the site and the operator was called immediately.

The operator was in Broadford at the time of the call, on other duties. He left immediately and went to Sallachy WTW to investigate why the residual was lower than expected. He

checked the dosing system was operating correctly, which it was. The flow proportional pump was dosing chlorine, at the level set on a prior visit, by the same operator.

Commitment had been given to install residual chlorine control dosing after the disinfection failure in August 2006. However a residual control system needs a reliable and consistent power source and early indications for the timescale and cost of provision of mains power to did not fit in with the short term option of maining out this site from Kyle WTW. As a result residual control was not included in this small value project which consisted of battery powered final water chlorine monitoring and telemetry using a mobile phone link. An additional requirement the August 2006 incident was to increase the site checks from two to three times a week. On 5th March the number of operator visits per week was reduced back to two. This follows the telemetry going live on 28th Feb.

There had been excessive rainfall prior to this incident, which typically causes increased organics in the raw water. As a result the free chlorine residual is reduced significantly as treated water passes through the CWT. A residual chlorine control dosing system would not take account of chlorine decay through the CWT with 24 hour storage and therefore would not have prevented a loss of chlorine residual in the final water. It was not a reduction or loss in the inlet chlorine dose that caused the incident.

Sallachy is due to be linked into the Kyle Regional Scheme. This work is progressing and providing the commissioning of the link main goes smoothly Sallachy WTW is currently forecast to be mained out by end of July 2007.

DWQR Assessment on the incident

The DWQR's assessment of this disinfection failure incident is that it was caused by heavy rain causing a deterioration in the quality of the raw water which increased the chlorine demand at the works. Due to the lack of a robust residual chlorine dosing system, the disinfection dose would not have been increased automatically in line with the demand. This therefore resulted in a loss of the "normal" disinfection residual.

DWQR notes that residual chlorine dosing should have been installed as a result of the previous incident in August 2006 but that such a system needs a reliable and consistent power source. DWQR notes that the Scottish Water's (SW's) timescale and cost for provision of mains power did not fit in with the short term option of maining out this site from Kyle WTW.

DWQR notes that the telemetry system went live on 28 February 2007, some 2 weeks after this latest incident.

DWQR notes the actions being taken by SW as a result of this incident, namely:-

1. Emphasise importance of Kyle scheme at regional management meetings to ensure the connection of Sallachy WTW to the regional scheme as soon as practicable (by 31/03/07 and on-going).
2. Connect wind generator/solar panel to 24 volt battery for dosing system (by 31/03/2007).
3. There were no samples taken during the period of this event. Regional manager to meet with logistics manager and ensure satisfactory arrangements are in place to ensure adequate sampling is taken following an incident/event (by 31/03/07).
4. Sallachy is due to be linked into the Kyle Regional Scheme. This work is progressing and providing the commissioning of the link main goes smoothly and the successful use of the addition source of Loch na Smeoraich is achieved, Sallachy will be mained out by the end of July 2007 (by 31/07/07).

DWQR is content with the above actions and will be following them up with SW to ensure they are completed as planned but would stress the need to:-

1. concentrate on maining out Sallachy with the Kyle works by end July,
2. ensure prompt reaction to any low chlorine alarms and
3. take samples for analysis as soon as practicable after the discovery of such an event .