Investigations

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

Report on drinking water quality incident at:
Invermoriston Water Treatment Works in February 2007

Scottish Water Executive Summary

Invermoriston Water Treatment Works is situated on the shores of the West Bank of Loch Ness. The works supplies treated water to a population of approximately 110. The only treatment at the works is sodium hypochlorite dosing for disinfection. There is mains power and telemetry at the site.

On Wednesday 31st January a sampler, taking the weekly statutory sample from Invermoriston Water Treatment Works reported a low chlorine residual to Scottish Water's Public Health Team and a local treatment operator, who in turn notified the area treatment team leader. The chlorine residuals that were reported were 0.03 mg/l free and 0.05 mg/l total. The area team leader immediately sent an operator to site to investigate the low chlorine residual. On arrival at the works the operator observed that the sodium hypochlorite dosing pumps had stopped. He established that there was no flow proportional signal going to the pumps and took the decision to put them into manual operation along with shock dosing the raw water inlet chamber. After further investigation, it was then noticed that there was a break in the flow proportional signal cable from the final water meter to the chlorine pumps, this was immediately repaired by a contractor that was working on the telemetry system at that time. After the repair was complete, the flow proportional chlorine dosing was then returned to normal operation. Bacteriological samples were taken from the final water and the distribution system.

The statutory bacteriological sample taken by the sampler on the 31st January from Invermoriston WTW final water was reported as having failed with a count of 14 total Coliforms and 14 E. Coli per 100 mls. The sample taken by operations from Invermoriston WTW final water also failed with a count of 12 total Coliforms and 3 E. Coli per 100 mls. All the samples taken from Wood SR passed bacteriological compliance. After receiving notification about the failures from the day before, the area team leader took further re-samples from the final water sample point and two from the distribution zone. Notification was received on the 2nd February that the second set of re-samples had all passed.

A low chlorine alarm was not generated via telemetry as the low alarm had been suppressed, as result of a software problem. It appears from telemetry that the final water chlorine residual started to drop off at 22:00 hours on the 30th January and dropped to low level by 00:15 hours on the 31st. Following subsequent investigation into the alarm suppression was immediately removed.

On Monday the 5th March the new Invermoriston works, comprising of sand and granular activated carbon filters and residually controlled chlorine dosing, went live

DWQR Assessment on the incident

The DWQR's assessment of this disinfection failure incident is that it was caused by a breach in the signal cable between the flow meter and the disinfection pumps.

The works is on telemetry but the low chlorine alarm had been suppressed, so the failure was only discovered through a sampler taking a routine sample from the works.

The samples taken from the works outlet at 18.00 on 31/1/07 and 09.25 on 31/1/07 failed bacteriologically. Had the alarm not been suppressed, this incident could easily have been averted.

DWQR notes Scottish Water's actions being taken as a result of this incident, namely:-

1. Suppressed alarm reports are currently available on Scottish Water's Telemetry reporting system, available on SW's intranet. Team Leaders are to be reminded of the existence of this report and to check it regularly to identify unexpected suppressed alarms. (action complete but ongoing)
2. As part of the Telemetry Data Quality initiative Scottish Water are moving towards a standard naming convention for all signals by asset type. This will allow the introduction of a top end proactive report and monitoring service focussing on signal suppressions by name (by 31/12/07 for the WTW part of the project).
3. Scotland wide 'Operator Event Awareness Training' will be rolled out from the 26th March 2007. This training, based on previous incidents and events, will include the appropriate actions to be taken in the event of a disinfection failure (by 31/5/07).
4. Treatment operator and Team Leader will be reminded of the importance of contacting the area process scientist when an Emergency Action Level has been breached to ensure that appropriate remedial action is carried out (by 16/3/07).
5. Ensure compliance with Task Scheduling and carry out formal interviews with any operators who are failing to comply (by 16/3/07 but ongoing).
6. Whilst DWQR is in general content with these actions, DWQR expects the suppressed alarm reports to be checked on a Scotland-wide basis and not just in the Ness Region as indicated in the actions.

DWQR notes the other actions and will be following these up to ensure they are completed as planned.