Investigations

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

Report on drinking water quality incident:
Throughout Scotland in October 2006

Scottish Water Report

On Thursday 26th October extreme rainfall and heavy wind caused significant operational problems to the North East of Scotland, mainly Easter Ross, Grampian and Orkney due to flooding and power supply outages caused by high winds. Across Grampian and Easter Ross some 32 water treatment works experienced loss of supply. Most sites have stand-by generation back up, though these had to be checked due to loss of telemetry caused by the power outages. Stand-by generators were mobilised for two of the smaller works. At two sites, Bonar Bridge and Ballater, power surges caused faults in the control systems and required Instrumentation Technicians to attend. At Ballater, associated damage to the PLC resulted in the plant being operated manually and chlorine residuals boosted until the repair was carried out by ICAT. Although the need for tankering was assessed, supplies did not require to be augmented. At Braemar WTW, the control system was also affected but was recovered by resetting.

At Turriff WTW, following a power failure, the standby power changeover mechanism failed and required repair before the power could be restored. Only a small number of customers had been affected by the time full power was restored. At Forehill WTW, treatment problems due to poor raw water quality (as reported in Event Notice 197), resulted in rezoning of the supply area to limit the throughput of the works. However, this situation had recovered sufficiently for the rezoning to be reversed to help towards alleviating the problems at Turriff WTW. The heavy rain also resulted in poor raw water conditions in the River Dee, which feeds Invercannie and Mannofield WTWs. Although Mannofield WTW has full coagulation designed to remove colour, the slow sand and membrane system at Invercannie WTW does not and could not deal with the increased load. As a result, colours leaving Invercannie WTW breached the PCV. This mainly affected the Banchory area as this is supplied directly from Invercannie WTW rather than mixing with water from Mannofield WTW.

At Cannich WTW (200 properties), chlorination was lost overnight due to power failure but was restored in the morning. There was sufficient residual chlorine in the CWT and a boil notice was not required.

Approximately 50 metres of an 18 inch outlet main from Assynt WTW, that serves the Black Isle, was washed away with a landslide. Fortunately a duplicate main maintained supplies and only a few customers in the Black Isle were affected for a short period.

In Thurso, a 9inch main was swept away with a footbridge. Due to extensive flooding there was difficulty in re-valving supplies to the 370 properties that were affected. Customers were put back into supply through an alternative feed after approx 6 hrs. There was extensive general flooding and landslides in Dingwall. Scottish Water offered assistance to Highland Council, though this was not taken up to any significant extent. All available Treatment and network staff worked through the night, and in places into Saturday morning to ensure there was minimal impact on customers supply and that quality was maintained.

The most significant incident however occurred in Orkney. Torrential rai resulted in Kirbister WTW being flooded by up to 2 metres in depth on Thursday afternoon. Kirbister supplies some 4300 properties on the East side of Orkney mainland including Kirkwall.

Fortunately the distribution system supplied by the works has significant storage reserves and initial estimates indicated that customers would not be impacted until Monday afternoon. Unfortunately initial estimates to restore the works to production ranged from one to five weeks.

Emergency Teams were established in Fairmilehead, Inverness and Kirwall to manage the incident. By early Friday the water level had receded sufficiently to allow a proper assessment to be made. The main treatment process was relatively unscathed but electrical equipment, motors and control equipment for the high lift pumping equipment that pumps treated water into supply, and the backwash pumping system were severely waterlogged. Estimates of restoration tended towards six to seven days. This would result in supplies being lost to customers for up to four days.

A team were set up to restore the treatment works, supplemented by electrical contractors from Orkney and Inverness. This team worked around the clock to replace damaged equipment and dry out other equipment. Additional heaters and dryers being brought up from Aberdeen and Inverness.

A further team was established to maximise the flow from the other treatment works on the Orkney mainland, Boardhouse, that serves the West of th mainland and transfer as much spare capacity into the Kirbister system on the East. In addition press releases were issued and door to door delivery of leaflets carried out urging customers across the Orkney mainland to conserve as much water as possible. Larger users, such as food producers, were contacted individually.

In addition manpower, vehicles and equipment were mobilised and dispatched to Orkney to enable alternative supplies to be provided if necessary. This included five curtain-sided lorry loads of bottled water, 8 tankers, three curtain-sided lorry loads of emergency tanks, pumps, hoses and couplings, ten vans, six rapid response trailers loaded with emergency tanks and some 20 staff.

Up until the time of writing this, Monday 30th October at 4:00pm, the works carried out to maximise Boardhouse, transfer water across the system and reduction in demand has meant that practically no customers have had their supply interrupted. Only a handful of properties have been affected by the West to East transfer operation and even then only intermittently.

There is still sufficient water in the system to last until Tuesday afternoon at least. The high lift pumps have been operational for a short while and it is anticipated that the backwash pumps will be installed and working by this (Monday) evening and before a filter backwash is required.

The main agencies had established an Orkney Local Emergency Coordination Group to manage the impact of the flooding on Orkney. Scottish Water liaised closely with this group through out the incident and kept them, and the community and political leaders fully informed.

DWQR Assessment on the incident

The DWQR has assessed these water treatment failures that were caused by extreme weather. This included major flooding of the works at Kirbister on Orkney, a major trunk water main form Assynt water treatment works being washed away, power problems at Bonar Bridge, Ballater, Braemar and Turriff water treatment works, treatment problems at Forehill and Invercannie works and a loss of disinfection at Cannich due to a power failure.

DWQR accepts that the weather experienced over this period was extreme and that Scottish Water worked hard to mitigate the effects. However, DWQR recommends that SW should carry out a thorough review of its works, through the drinking water safety plan (DWSP) process, to ensure that the effects of challenging weather situations are minimised. This review should consider issues like - adequate protection against power dips and surges; treated water storage versus the provision of permanent or mobile standby generation in the event of power failure; and the ability of treatment processes to cope with the challenges of extreme weather.