Investigations

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

Report on drinking water quality incident at:
Amlaird Water Treatment Works in July 2007

Scottish Water Executive Summary

On the 16th of July at 03:40 the ferric sulphate dosing system at Amlaird WTW failed causing loss of the coagulation process. The system has a duty and standby pump facility.

The duty ferric sulphate dosing pump had stopped dosing causing loss of coagulation. The diaphragm within the duty pump had burst causing the pumping failure; this failure did not generate an alarm on SCADA or Telemetry since the pump continued to run. As the process gradually failed with little or no coagulant being injected, a number of process alarms such as clarified and filtrate turbidity were then activated at the works and received via telemetry by staff within Scottish Waters Operational Management Centre (OMC). Staff within OMC passed out these alarms to the standby operative.

On arrival at the works the standby operator experienced difficulties in identifying the exact cause of the process failure, since the SCADA system was indicating the coagulant pump was running. Advice was sought and received from colleagues, and the failed dosing pump was identified. The standby pump was then selected as duty via SCADA. However, no physical or pump calibration checks were completed and it was later established that the standby pump was also faulty.

The resultant impact of the dosing system failure was loss of coagulation and clarification processes causing higher colour, filtrate turbidity and iron residual. Although disinfection was maintained the poor quality water resulted in the Clear Water Tanks (CWT) chlorine residual levels being suppressed. A decision was taken to isolate the CWT from the works and the tanks were spiked with sodium hypochlorite on two occasions during the day to increase the chlorine residuals.

The Public Health team were notified of the incident who worked in liaison with the CPHM keeping them informed.

Final water was being analysed on site for iron at regular intervals and samples were collected in distribution for iron and bacteriological analysis by the laboratory.

Water quality and production were back to normal by the 17th July. Only one customer complaint regarding water quality was received in the course of the incident, this was at the end of the system and was regarded as unrelated.

DWQR Assessment on the incident

The DWQR assessment of this incident is that it was caused by a failure of the ferric dosing system. The dosing pump failed but an alarm was not generated immediately since the pump continued to run. Turbidity alarms were generated about an hour after the initial failure and this was followed by a high iron alarm about half an hour after that. The alarm monitoring centre called out the operator at this time who arrived on site about an hour later at 06:00. In the three hours that followed a number of different actions were taken to find the cause of the problem and fix it. Finally, a decision was taken to shut the works down and run the water to waste while the problem was fixed. The water stored in the two clear water tanks was used to maintain supply. It is noted that there was only one customer complaint during the time of the incident, which came from a customer at the end of the system and is regarded as unrelated. High iron levels (i.e. 360 and 436 microgrammes per litre) were recorded at a few points in the distribution system but the action of shutting the works down probably reduced the number of failures and the number of customer complaints. The decision to shut the works down could possibly have been taken a little earlier thereby reducing the volume of water with high iron levels going into the clear water tanks and the subsequent impact on customers.

It is noted that the site staff failed to realise the impact of the failure of the dosing system, acting reactively rather than stepping back to consider the impact on the works processes as a whole and water quality generally. It is also noted that changing over of the ferric dosing pumps is a scheduled monthly task but that this task had not been carried out at least two months prior to the incident.

DWQR notes the actions being taken as a result of this incident, namely:-

1. Review competence and knowledge of operators and agree improvement plan.
2. Review content and frequency of scheduled maintenance tasks for dosing pumps and associated components.
3. Set up a schedule for cleaning out injection points.
4. Procure new dosing and carrier water pumps.
5. Distribute report to all Regions for learning points.