Carron Valley WTW - Loss of Treatment - November 2009

Summary of Incident

This incident resulted in untreated water passing through the treatment works, which serves a large area of central Scotland. This was the result of a power failure, which meant that all chemical dosing and treatment processes requiring power ceased. As with all treatment works, Carron Valley has a number of back-up systems that should have protected the supply. Due to a number of coinciding circumstances, none of these was effective on this occasion, with the result that untreated water passed un-noticed into supply for approximately 4 hours.

DWQR Assessment of Cause of Incident

A power failure at a water treatment works is not uncommon, and mitigating measures should be in place to enable treatment to continue, or at the very least to notify operational staff and ensure no untreated water can pass into supply. An extremely unfortunate combination of events meant that none of these measures was effective at CarronValley on 22 November.

The works should have a dual power supply, however one of these had inadvertently been disconnected following a previous problem in July 2009. The back-up generator, which should have operated in the event of both power supplies failing, did not do so because it perceived the disconnected mains power supply as being faulty. The works inlet valve that should have closed to shutdown the works when the treatment process was compromised did not do so because its Uninterrupted Power Supply (UPS) batteries were old and did not hold sufficient charge to operate the valve. The telemetry system that should have alarmed and alerted operational staff to the problem did not do so because there was no Uninterrupted Power Supply (UPS) connected to it to enable it to operate in the event of a complete power failure. Finally, when operational staff attended site at 07:45 the morning of 22 November, they assumed that the inlet valve to the works had shut, preventing water from flowing through the works. In fact it did not close until mains power was restored at 08:41. Operational Staff did not check this until 11:50 that morning when it was realised that the clear water tank contained far more water than would be expected had the works shutdown immediately.

It is Scottish Water’s responsibility to ensure that its treatment processes are resilient and that its contingency measures are robust. This incident calls into question the adequacy of the provision of these contingency measures, the maintenance of measures, and the operational practices at the site.

DWQR Assessment of Actions Taken by Scottish Water

Once Scottish Water realised that the treatment process had been compromised, it reacted promptly to restore quality. Fortunately, although treatment ceased for a time, chlorine residuals leaving the works did not fall to zero as the untreated water mixed with previously chlorinated water in the large clear water tank – although this is no substitute for full treatment. Scottish Water arranged for the clear water tank and service reservoirs in the distribution system to be dosed with extra chlorine. Additional samples for microbiology were taken over the following two days, although only three of these were actually taken on 22^nd November. It is DWQR’s opinion that more samples should have been taken on the day of the treatment failure itself. All samples taken during the incident passed, with the exception of a single coliform failure that may or may not be related to the incident. Whilst microbiological safety was rightly given priority, DWQR considers that, given the absence of any treatment process for four hours and the inevitable initial disruption to sensitive treatment processes once they were restarted, it would have been prudent for Scottish Water to have sampled for additional parameters such as pH, aluminium and colour. No mention of Cryptosporidium is made in Scottish Water’s report on the incident - samples should also have been taken to verify that this was not present in the water supplied. DWQR notes that residual chlorine levels were not recorded by some of the Network Service Operators when taking samples from the distribution system. This information is vital in the interpretation of microbiological results and the impact of the event on consumers, and microbiological samples should always be accompanied by chlorine residuals.

It was not clear from Scottish Water’s report into the incident when health boards and local authorities were notified. As it is ultimately the responsibility of the Health Board to decide whether or not additional precautions to protect consumers are appropriate, timely notification is vital. DWQR has since been informed that the Health Board was, in fact, notified on the day, however this information was not presented in the incident report. Liaison with Health Boards and Local Authorities is an important part of the incident management process, and such details must be included in reports to DWQR.

Scottish Water has identified a number of actions from this incident. DWQR accepts that these are appropriate and will be monitoring to ensure they are completed prior to signing off the incident:

Action Number	Action Description	Completion Date
1	Check the standby generator to ensure correct operation.	Complete
2	Repair fault on one of the incoming mains power supplies to the works.	Complete
3	Install Uninterrupted Power Supply (UPS) on telemetry dial out unit.	Complete
4	Test inlet valve to ensure automatic shutdown occurs following replacement of batteries.	Complete
5	Ensure regular maintenance checks are carried out on incoming power supplies.	Complete
6	Remind site operators of procedures for responding to loss of site power and ensure that all necessary written procedures are available on site.	Complete
7	Review maintenance frequency for back-up power systems for the site.	Complete
8	Distribute incident report to other managers in Scottish Water to ensure lessons are learned.	Complete
9	Formally investigate sequence of events causing the original loss of one of the power supplies in July 2009	Complete

Additionally, DWQR has made a number of recommendations following this incident:

Recommendation Number	Recommendation	Completion Date
DWQR 1	Investigate number of telemetry units across Scotland without independent back-up power in the event of a power failure and agree measures with DWQR to address this.	31/8/10
DWQR 2	Ensure testing of UPS battery function is included on maintenance scheduled tasks at all sites.	Phased approach - First 20 sites due 29 Oct 2010
DWQR 3	Review Scotland-wide procedures for actions to be taken in the event of a full power failure to ensure operators are required to check that works is shut down as a priority if treatment is compromised. Ensure operators are aware of this requirement.	31/8/10
DWQR 4	Investigate why no Cryptosporidium sampling was undertaken in response to the incident, reviewing training and procedures if necessary.	Complete
DWQR 5	Review procedures for planning and conducting sample surveys in response to serious treatment failures, ensuring adequate guidance and training is available to staff.	30/6/10
DWQR 6	Remind staff including details of notification of external stakeholders of serious water quality issues in DWQR incident reports.	Complete
DWQR 7	Remind operational staff taking water quality samples that all microbiological samples must be accompanied by free and total chlorine residuals.	Complete