Investigations

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

Report on drinking water quality incident at:
Touch Water Treatment Works in October 2007

Scottish Water Executive Summary

On the morning of 7 October, at 0932hrs, a digital alarm for low Treated Water chlorine residual was triggered, at which time the standby Operator was already on site to undertake routine plant checks. The standby Operator had already identified the problem and contacted another operator for advice. As the situation did not improve, the standby Operator then called the Senior Operator for assistance.

Site investigations at the time highlighted that both banks of chlorine cylinders were empty. These were changed over and the pressure in the system reinstated. The chlorinator was then put back on and gas returned to the dosing system. Although the operators waited until the treated water residual started to increase towards the Emergency Action Level (EAL) od 0.2mg/l, they left the site before the required level was achieved so the initial alarm had not cleared by this time. No further action was taken at this stage in response to the disinfection failure.

Later the same day, the standby Operator was called back to site following a p H related alarm. On attending the site at approximately 2100hrs, the standby Operator established that the chlorination system had failed again although the reason for the fault could not be established. The Senior Operator attended site at 2345hrs to assist and once it became clear that the fault couldn't be established quickly, the decision was taken to shut down the plant. After notifying the Team Leader, further investigation established by 0115hrs on 8th October that the fault was due to a blockage in the pipe leading from the ejector. Upon reinstatement of the chlorination system, the chlorine dose set-point was increased, although no additional action was taken to boost the chlorine residual leaving the clear water tank.

The initial failure was due to the emptying of both banks of chlorine cylinders which wasn't detected or indicated by the control panel.

No alarm was received for the second chlorination failure due to the initial treated water chlorine alarm not clearing, which suggests that the chlorination failed shortly after the Operators had left site after rectifying the original problem. No alarms were received for the final water chlorine residual leaving the clear water tank (CWT) which fell below the Emergency Action Level (EAL) of 0.2mg/l at 15.20hours on 7 October 2007. This is under investigation.

Although there were no failures detected in the distribution system following this incident, additional sampling wasn't carried out until Monday 8th October following notification to the Public Health Team.

DWQR Assessment on the incident

This incident was caused by an unfortunate combination of events. The warning system at Touch Water Treatment Works that indicates which bank of chlorine cylinders is operational had reset itself following a power failure. Failure to spot this resulted in both banks of cylinders running empty, although chlorine stock records should have made it obvious to site operators that the second bank of cylinders was not full. A low level chlorine alarm alerted operators to the situation and following resolution of the problem the operators left the site once chlorine residuals were increasing, but crucially, before they were sufficient to reset the original low chlorine alarm. Consequently, when a blockage of the chlorination pipework occurred, no further alarm was raised. Scottish Water was fortunate that this problem was discovered when an operator attended to deal with a different alarm. Although no water quality failures were recorded following this incident, it is noted that the sampling that took place may not have been truly representative of water directly supplied by Touch Water Treatment Works.

In response to the incident, Scottish Water has implemented a number of actions. These include improvements to chlorine stock control at the site and inclusion of the chlorination pipework on the maintenance schedule. Additionally, the shortcomings in the operational response to the incident have been dealt with through training and awareness sessions. Further actions addressing the appropriate checks and sampling following a disinfection failure are also to be taken, although those are not yet complete. The Environmental Health section of Stirling Council noted their concerns to DWQR over the actions taken by Scottish Water that led to this incident. DWQR visited the site on 4 February to talk to staff and verify that the relevant actions were complete. The operational area has adopted new working practices to improve ownership and knowledge of individual sites, and it was evident that staff were well aware that their actions during the incident fell short of what is expected and that lessons had been learned.