Tobermory, Mull Exceedence of Aluminium Standard Following Process Failure – August 2009

This incident occurred when a Scottish Water operator was called to Tobermory Water Treatment Works on 2 August following a number of water quality alarms. On arrival, it was evident that a number of dosing pumps had tripped out. Having reset these, it was evident that there was a further, underlying problem and at this point aluminium concentrations in the water leaving the works were well in excess of the regulatory standard. The quantity of chlorine in the water was also being affected and was falling, despite measures taken by the operator to correct this.

It was eventually discovered that the root cause of the problems lay with the dissolved air flotation (DAF) process that removes particles and colour from the water prior to filtration. Once this problem was corrected, water quality began to respond rapidly and was soon back within acceptable limits.

A number of checks and actions have been proposed by Scottish Water in response to this incident. On reading the report on the incident, DWQR queried why water quality monitoring instrumentation that existed between the DAF and the filters hadn’t alarmed and alerted operational staff to the cause of the problem more quickly, perhaps before the fault had had such a major effect on water quality. Scottish Water responded that although the post-DAF monitors existed, it was thought that they only triggered an alarm within the works. This would not have resulted in an operator being called to the site, although it should have alerted him to the problem with the DAF more quickly once he was on site.

It appears that there is some uncertainty over the exact function of alarms at the works. DWQR has recommended that Scottish Water undertakes urgent work to better understand the operation of monitors and alarms at the site and to ensure these are adequate. Scottish Water has agreed to do this.