Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Inverasdale Water Treatment Works in June 2007
Scottish Water Executive Summary
The water treatment works (WTW) at Inverasdale consists of a colour removal membrane filtration plant. The river source water receives prefiltration before the membranes, post membrane pH correction and disinfection. The water is pumped to a twin compartment clear water tank situated on the hillside above the works. The clear water tank (CWT) then feeds into distribution supplying a total of 411 people in the villages from Poolewe to Cove.
On 12th and 13th June 07 Scottish Water had a requirement to tanker treated water from Inverasdale WTW to Aultbea WTW to augment supply during a membrane cleaning operation at Aultbea while, simultaneously, ongoing structural maintenance left one compartment of the Aultbea CWT offline. A total of 5 tanker loads, each of 15m3, were drawn from a distribution hydrant at Poolewe, on the eastern extremity of the WSZ.
The tanker draw off caused the instantaneous flow leaving the Inverasdale CWT to increase from 7.5m3/hr to 41m3/hr for a duration of approximately 75 minutes on each occasion.
On 14th June 07 Inverasdale final water statutory sample failed for turbidity at a level of 3.01NTU, the subsequent sample analysis for cryptosporidium detected no oocysts.
The root cause of elevated turbidity within the final water was the high instantaneous flow in distribution, as a result of tankering operations, causing excessive dropping of the treated water levels within the CWT at Inverasdale WTW and disturbance of settled particulate matter on the floor of the CWT.
Elevated pH levels in the water leaving the CWT on 12th and 14th June are attributed to routine maintenance work on a remineralisation vessel, carried out the week before. It is unclear whether this was a contributory factor to the elevated turbidity level recorded in the statutory sample taken on 14th June and is subject to further investigation.
The online final water turbidity instrument would normally have picked up the rise in turbidity immediately if it had been operational, and would have alarmed out with a priority 2 alarm via Scottish Water's telemetry system. However, the instrument was offline awaiting repair following identification of a fault the previous week resulting in manual turbidity measurements being taken.
Actions being undertaken as a result of this incident include:
- Cleaning of Inverasdale CWT
- Review of tankering procedures to include control/limitation on draw-off rate.
- Review of procedure for expediting repair of critical process instrumentation.
- Assessment of routine maintenance work on the remineralisation vessel.
DWQR Assessment on the incident
The Incident Report from Scottish Water identified the elevation in turbidity experienced during this incident was due to the draw down of the system by tanker operations in the area. The loss of the on-line turbidity meter occurred 4 days prior to the tanker operations (on 8 June) but the tanker log (Appendix III) shows that no problems were highlighted at the water treatment works or the clear water tank associated with the works. The length of time taken to deal with the inoperative turbidity meter is a cause for concern. Scottish Water has noted in additional information provided to DWQR that there were 41 High Priority jobs and 13 Urgent jobs in the area at the time of the incident. DWQR will discuss with Scottish Water what measures need to be put in place to monitor Urgent and High Priority job requests along with the provision of appropriate resources to ensure that inappropriate delays or backlogs do not occur.