Investigations

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

Report on drinking water quality incident at:
Kirkmichael Water Treatment Works in September 2006

Scottish Water Executive Summary

On Monday 11th September/Tuesday 12th September 06 the chloramination disinfection process at Kirkmichael Water Treatment Works (WTW) was compromised due to a Sodium Hypochlorite dosing plant failure. Although chlorine dosing at the treatment works was interrupted, the supply to customers served from his site maintained a good although slightly reduced residual disinfectant level.

At 19:05 hours on Monday 11th September 2006 the standby operative was alerted by the Operational Management Centre (OMC) that Sodium Hypochlorite dosing pump No1 at Kirkmichael Water Treatment Works (WTW) had failed. The standby operative requested the OMC to interrogate the site to determine if the standby Sodium Hypochlorite pump No2 had become operational. The OMC confirmed that Sodium Hypochlorite pump No2 was operational. The standby operative therefore decided, that as the Treatment Works was running and the standby Sodium Hypochlorite pump was operational, that he would not visit the site that evening but would arrange for the fault to be investigated the following day. This is normal operational practice.

At 10:00 hours on Tuesday 12 September 06 the site was visited by an operative to investigate the fault reported in Sodium Hypochlorite dosing pump No 1. This was an unscheduled visit and had been initiated purely as a result of the previous evening's alarm. At this visit the operative discovered that although Sodium Hypochlorite dosing pump No 2 was on and available for duty, it was not dosing any chemical. It also became apparent that the auto-plant shutdown unfortunately did not operate due to faulty wiring which has since been rectified.

The residuals taken immediately following discovery of the problem with Sodium Hypochlorite dosing indicated 0.04 Free Chlorine and 0.20 mg/l Total Chlorine leaving the clear water tank. The operator immediately advised his Team Leader and corrective action was undertaken by the operator: draining the pumping main and dosing the clear water tank with 1 litre of sodium hypochlorite to restore the target chlorine residuals of 0.05mg/l free Chlorine and 0.50mg/l total chlorine.

As this was a near miss event with a residual being maintained into distribution, additional bacteriological sampling was not considered necessary.

Investigations have highlighted that whilst a low chlorine alarm was generated this was not passed out since it had been suppressed. As chloramination is carried out on-site the team leader had requested that low free chlorine alarm was suppressed. Unfortunately, due to human error all the chlorine alarms were suppressed although this has since been rectified.

DWQR Assessment on the incident

DWQR has reviewed the circumstances surrounding the near miss event at Kirkmichael WTW and is satisfied that the actions taken by Scottish Water should ensure that the near miss event will not be repeated in the future.