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 August 2006

Scottish Water Executive Summary

The water treatment works at Inverasdale consists of a colour removal membrane filtration plant. The river source water receives pre-filtration before the membranes, post membrane pH correction and disinfection. The water is pumped to a clear water tank next to the works. This feeds 405 people in the villages from Poolewe to Cove.

On the 16th of August it was reported that there was a low free chlorine residual at Inverasdale WTW. The chlorine first dropped to low level on Sunday 13th but was not discovered by operational staff until Tuesday 15th. The site had been experiencing chlorine dosing control and instrument calibration problems prior to this and the disinfection system eventually lost control of the chlorine dose and was unable to achieve the residual set-point.

This resulted in the chlorine residual in the clear water tank dropping considerably and shock dosing of the tank was required on the 15th. The pumps have been set to manual chlorine dosing only and monitoring of the system is on going, as is remedial works to resolve the issues for return to fully automatic chlorine dosing control.

DWQR Assessment on the incident

DWQR's assessment of this disinfection failure is that it was caused by a failure of the disinfection control system. Due to a history of continual low chlorine alarms and some apparent confusion between Scottish Water's alarm control centre and their Customer Operations, the alarm in this case was not acted on as quickly as it should have been.

However, DWQR notes the actions being taken by Scottish Water to ensure that this situation does not recur. In particular, DWQR notes that the emergency action levels have been reviewed to allow an adequate response to safeguard against a disinfection failure, that a new disinfection system monitor and controller has been installed and that this should now be fully operational in automatic mode. A project to further improve the disinfection control arrangements should hopefully be complete by early 2008.