Investigations

Report on drinking water quality incident at:
Lemreway Water Treatment Works in March 2007

Scottish Water Executive Summary

The water treatment works at Lemreway consists of upward flow sand/carbon filters feeding into a clear water tank. Disinfection is achieved by flow proportionally dosing sodium hypochlorite between the filter outlet and CWT. The dosing pumps are powered from a wind turbine/solar power source. There is no mains power on site.

On Tuesday 13th March 2007 at 00.53 a low chlorine alarm was generated by the telemetry system and at 02:00 was received by the standby supervisor. Given that the site had falsely alarmed on 2 separate occasions, the duty supervisor took the decision to pass the alarm to the operative first thing next morning. The standby supervisor contacted the senior operator at 08:00, however neither passed on the alarm to the site operator, who was not scheduled to visit the works until the next day.

On Wednesday 14th March at 08:00 the operator attended the site for the scheduled weekly visit. He noticed the chlorine level had fallen to zero. On further investigation the operator found the flow meter which controls the chlorine dosing had failed. The operator unblocked the meter and had the system restored with a free chlorine of 0.49 by 09:00. He checked the residual at the end of the system in Orinsay, which was found to be 0.15mg/l, and scoured the system until he had a residual of 0.26mg/l. This was completed by 09:30.

Scheduled bacteriological samples were taken on Tuesday 13th March, which were clear for coliforms, however plate counts were high and chlorine residuals were low at 0.07mg/l free. Further samples taken on the 15th were within operational target levels for this works.

The site was refurbished in 2004 in the Q&S2 programme, however is due to be abandoned in December 2007 as a main out to North Lochs Zone is the most efficient option to meet Q&S3 quality drivers.

DWQR Assessment on the incident

The DWQR assessment of this incident is that it was caused by a failure of the flow meter which controls the chlorine dosing. A low chlorine alarm did go through to Scottish Water's Operational Management Centre and this was passed on to the standby supervisor. However, due to previous false alarms, the standby supervisor decided not to pass the alarm on to the operator until the next day. He spoke to the senior operator the next morning but neither passed the alarm on to the site operator. The low chlorine was only therefore discovered during a routine visit by the operator some 31 hours after the initial alarm. A routine sample was taken early the next morning after the alarm had been sent but the sampler appears not to have informed anyone about the low chlorine residual, so this was a further missed opportunity and Scottish Water's Public Health Team were only informed after the problem had been fixed.

DWQR notes and is content in general with the actions being taken, for example interviewing and training staff, ensuring that all alarms are attended to and ensuring that samplers report low chlorine results to the Public Health Team. DWQR also notes the telemetry alarms will be reviewed to consider the potential to provide a flow meter flow alarm. However, given that the alarm was not reacted to in this instance due to previous false alarms, DWQR has asked SW to investigate this specific issue further with a view to reducing the potential for false alarms.