Investigations

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

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

Scottish Water Executive Summary

Blackpark water treatment works supplies an average of 5.7 Mld of treated water per day to a population of approximately 10518. The raw water for the works gravitates from Loch Einich and the only treatment carried out at the works is sodium hypochlorite dosing to provide disinfection. The site is fed by mains power, and there is on-line water quality monitoring and telemetry. The site is visited three times a week.

On Thursday 15 February 2007, the chlorine dosing pumps airlocked in the early hours of the morning because of modifications made the previous day to try and improve the dosing system. The plant alarmed on low chlorine at 04:48hrs and the Operator was called in response to the alarm at 06:19hrs. On reaching site, the Operator was unable to solve the problem himself, so the assistance of the Senior Operator and the Electrician, who had made the modifications the previous day, was provided. The bottom level probe in the sodium hypochlorite tank was raised to prevent the problem recurring. The pipework was bled of air and the dosing recommenced at 08:50hrs. A chlorine residual of 0.7 mg/l was established at the WTW sample tap.

Samples taken from the WTW before disinfection was re-established, and from the works and in distribution an hour after disinfection had been re-established, were reported as having passed on the 16th February.

Investigations into this incident highlighted a 11/2 hour delay in the low chlorine alarm being acknowledged by the Operations Management Centre (OMC) Flight Desk and the alarm being dispatched to an Operator. By giving the alarm time to clear, the Flight Desk operated with best intention to avoid a potentially unnecessary callout, however the lack of treated water storage at this site resulted in an immediate risk of undisinfected water going into supply. Instructions have been given to the Flight Desk staff that all Chlorine alarms have to be dispatched within 15 minutes wherever possible.

Improvements will continue to be made to the sodium hypochlorite dosing system to reduce gassing, and funding will be sought to try and provide a more robust chlorine dosing system until a new works, with adequate storage, is built under Q&SIII.

DWQR Assessment on the incident

DWQR's assessment of this disinfection failure incident is that it was caused by an airlock in the disinfection dosing pumps due to modifications the day before the incident.

DWQR notes that a low chlorine alarm was initiated on the telemetry system but that there was a one and a half hour delay between the control room picking this up and relaying it to the local operator. SW's report suggests this was partly to "prevent unnecessary call out" and also that the duty controller "would have assumed that the site would have treated water storage available".

DWQR notes that there was a one and three quarter hour delay between the operator arriving on the site and a sample of the final water being taken for analysis. Whilst not excessive, DWQR would like to see any such delay of this type being reduced to an absolute minimum. Taking of samples should be one of the first actions carried out in incidents of this nature.

DWQR notes the actions being taken by Scottish water (SW) as a result of this incident, namely -

1. Further modifications to reduce gassing will be made to the system (by 01/04/07).
2. Funding is currently being sought to provide a duty standby dosing arrangement of the works (by 31/07/07).
3. Electrical and mechanical personnel will undergo Scotland-wide "Operator Event Awareness Training" to make them more aware of the possible consequences of their actions (by 30/04/07 for Blackpark initially)
4. Instructions have been given to the control centre itself that all chlorine alarms have to be dispatched within 15 minutes whenever possible. This includes habitual chlorine alarms and chlorine alarms that result as a consequence of planned/automatic plant shutdown (e.g. membrane plants). While this will cause unnecessary call-outs and disturbance to SW operational staff until work on RTUs [Remote Telemetry Units] can be implemented to increase their intelligence, it is seen as being the most secure response to preventing a similar incident (action complete).

Carry out a review of how much "intelligence" and decision making Control Centre staff are allowed to apply to alarms, and clear guidelines provided (by 31/05/07).

DWQR is content with these actions and will be following them up with SW to ensure they are completed as planned.