Investigations

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

Report on drinking water quality incident at:
Ballachulish Water Treatment Works in May 2006

Scottish Water Executive Summary

The Ballachulish WTW supplies approximately 400m3/day into the Ballachulish water supply zone and serves a population of 606 in the village of Ballachulish. The treatment works consists of pressure sand filters followed by GAC filters, pH correction, a duty/stand-by disinfection system dosing sodium hypochlorite, quality monitoring and telemetry. The water going into supply from the CWT is monitored for free chlorine.

On Thursday 18th May SW staff on site at Ballachulish WTW became aware that the chlorine demand of the treated water had increased and the free chlorine residual of the water entering into supply was low. Consequently the chlorine dosing set point was increased to try and ensure an adequate the free chlorine residual was achieved into supply. Two set point changes were made that day, one in the morning from 1mg/l to 1.2 mg/l and another change at about 15:00 to 1.4 mg/l. It was felt by the staff that a set point of 1.4mg/l should be sufficient to maintain a residual over night.

During the night the works shut down. This was due to low chlorine in the dosed water which is thought to have been caused by gassing in the sodium hypochlorite dosing system. At just after 08:00 the SWS commissioning engineer arrived at site. He found that the works had shutdown on low chlorine and restarted the works. The free chlorine residual going into supply was recorded as 0.19mg/l and therefore the decision was made to shock dose the tank. The senior operator increased the chlorine set-point to 2 mg/l. The Public Health Team were notified and they advised on the samples to be taken in response to the low residual.

At 17:30 the area process scientist visited site and as the free chlorine residual into supply was still low (0.28mg/l) shock dosed the tanks again with 300mls of sodium hypochlorite.

Saturday 20th May a SWS subcontractor visited site and confirmed to Asset Operations the works was operating within spec and the free chlorine going into supply was 0.85mg/l (hand held test kit measurement). Laboratory report bacteriological samples passing. Incident closed.

SWS are committed to working with SW in monitoring the filter performance at Ballachulish WTW. It has been agreed that on-line raw and final turbidity meters are required for this site. The process for funding to install this equipment has been initiated and approval is expected by 12th July.

SWS have been investigating the problems with the chlorine dosing system. Larger pumps are being supplied by Friday 16th June and fitted and commissioned by Thursday 22nd June. Modifications to the control loop have been made and some operational changes are being trialled to in an attempt to minimise the gassing of the sodium hypochlorite. SWS are optimistic that this work will be complete and a final solution will be in place by the 7th July 2006.

DWQR Assessment on the incident

This low disinfection incident caused by a problem in the disinfection system could have been discovered earlier than it was if the telemetry alarm had been set up properly in the first place as it should have been at this new works. However, the automatic shut down facility in the works should have prevented undisinfected water from entering the supply and once the problem was discovered, Scottish Water reacted quickly to the situation. The larger pumps and other work on the disinfection system should help to prevent a recurrence. The repair to the chlorine monitor and the correct setting of the alarm on it should provide an early warning of any further problems.