Investigations

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

Report on drinking water quality incident at:
Acharacle Water Treatment Works in April 2007

Scottish Water Executive Summary

Acharacle water treatment works is a gravity system - Inverness type filtration works and serves approximately 297 properties, producing an average of 388 m3/day of treated water.

Water is abstracted from Allt Beithe, and gravitates through a GAC filtration plant employing 4 Inverness sand filters (run in parallel), followed by 4 Inverness GAC filters (run in parallel). Sodium hypochlorite (residual control) is injected into the main prior to the clear water tank (CWT) and monitored at this point. The free chlorine residual is also monitored on the outlet main of the CWT. There is both telemetry and power at this site.

On the evening of Friday 20/04/07 the duty standby person received a 'P2 treated water chlorine (dosed) low' alarm which he correctly identified as an alarm generated by a plant start up and therefore took no further action. Again the duty standby person received the same alarm at 12:33 on the Saturday afternoon, and believing it to be the same start up alarm, records show, that he requested this chlorine alarm to be suppressed.

Numerous low chlorine alarms were generated but due to suppression they were neither acknowledged by OMC Flight desk nor actioned by Operational Standby staff.

At 20:30 on Sunday 22/04/07 a P2 alarm was generated for 'low CWT level'. This was acknowledged and passed to Operational standby staff to action. On arrival at site the standby Operational staff found the plant had shut down, they started the plant on manual and left site early due to worsening weather conditions. No process checks were undertaken.

On returning to the works the following morning, Monday 23/04/07, the site operator found that the chlorine dosing pump no1 had failed due to a burst diaphragm and was not dosing any sodium hypochlorite into supply. At approx 08:30 the site operator informed the Treatment Team Leader that there was 0 mg/l chorine residual leaving the works. The site operator then switched to chlorine dosing pump no2 and shock dosed the clear water tank.

The site operator then remained on site until a satisfactory stable residual was achieved into supply and left site at c. 15:30.

Diary entries show that the site operator attended site on a further 2 occasions and made adjustments to the disinfection control, however he did not inform his Team Leader of the ongoing problems he was experiencing with establishing disinfection control. Following the fact finding interviews it was established that the site operator was unclear on the remedial actions required in response to a disinfection failure.

The route cause of the Acharacle WTW disinfection failure was initially caused by a chlorine dosing pump failure. The works is designed to deal with this type of failure, and shuts down within 30mins. However this safeguard was over ridden due to the following actions:

• Inappropriate response to P2 alarms by operational standby staff.
• Lack of communication between operational staff.
• Failure of Scottish Water Staff to comply with Scottish Waters policies and procedures.

Actions have been put in place to address these issues. The site came back into compliance on Tuesday 24/04/07.

DWQR Assessment on the incident

The DWQR assessment of this incident is that it was caused by a catalogue of errors but basically it was caused by a failure of one of the disinfection pumps. A low chlorine alarm was initiated at around midday on Sunday 22 April 2007, but this was not acknowledged due to the operator having suppressed the alarm the previous day. However, the plant did shut down automatically as designed to do on a low chlorine later that day. A low level clear water tank alarm later in the day resulted in SW visiting the site and putting the plant into a manual operation state. Further low chlorine alarms were initiated but were not acted on properly due to the suppression. This suppression came off early the following day on Monday 23 April when another low chlorine alarm followed on from this. The site was visited, the disinfection pumps switched over from the failed one to the other working one and the two clear water tanks dosed with additional chlorine, but this latter operation was not in accordance with SW's procedures. There were further low chlorine alarms during the course of Monday 23 April and the plant continued to be run on manual until 10.00 on Thursday 3 May.

DWQR notes that SW's actions include a review of the operation of the works, a review of the telemetry system (including all alarms), checking the plant to ensure it is fit for purpose (it is still under warranty), communicating the severity of this incident to all operational staff and stressingthe importance of complying with procedures, providing additional training for staff and placing the operator concerned on a formal performance review which will be monitored monthly. The procedure for alarm suppression is also to be reviewed. DWQR will be monitoring these actions to ensure that SW completes them as planned.

There is nothing to suggest that the water supplied in this case was unfit for human consumption. However, Scottish Water must ensure that it acts with all due diligence to ensure that the water it supplies is fit for human consumption.