Invercannie Water Treatment Works

Water Quality Incident Report - December 2009

DWQR’s assessment of this incident is that, at 08:30hrs on 2^nd December 2009 Invercannie Water Treatment Works (WTW) was shut down due to planned upgrade work, part of which was to enable one of the Programmable Logic Controllers (PLC) to be swapped in a fault finding exercise. At 14:30hrs the WTW was re-started, however at 16:00hrs the PLC failed and the coagulation section of the works shut down automatically, but the inlet valve, which should have shut at this time remained open. The contractor, who was still working on the site, restarted the WTW. At this time the operators left the site, however the standby operator contacted the site at 16:30hrs when the contractor confirmed that the plant was running normally and that work associated with the shutdown was still on-going at Invercannie. The standby operator received a low and a low-low final water chlorine alarm at 17:41hrs and he contacted the works and spoke with the contractor who was still on site. The contractor confirmed that the alarm was due to the on-going work at the WTW. At 18:40hrs the contractor confirmed that the work was now complete and that the works was operating normally. At this point the standby operative did not confirm with the contractor that all alarms had cleared.

At 00:00hrs on 3^rd December the PLC failed again causing loss of coagulation. The inlet valve again did not shut allowing un-coagulated water to pass through the works. Because the PLC installation lacked the “Watchdog” alarm element, the PLC fail did not alarm out and it was not until 08:00hrs, when the operators arrived on-site, that the failure was detected. As the water was not being fully treated, the chlorine demand of the water increased and the final water chlorine residual fell to around 0.2mg/l overnight.

The operatives, once aware of the situation, took effective corrective action. While the final water chlorine residual fell, and only partially treated water entered supply, a chlorine residual was maintained in the final water throughout the incident.

There were two root causes for this incident. Firstly, a faulty PLC unit was installed with a key piece of software missing and, secondly, there were no operatives on-site and the operation of the works was left to the contractors. Had the “Watchdog” software been in place it would have alarmed out and the incident could have been prevented in spite of the faulty PLC. Equally, if trained operatives had remained on-site until the contractors work was complete they should have responded more appropriately to the alarms that were generated.

DWQR endorses the actions taken by Scottish Water to prevent a repeat of this type of incident, particularly the replacement of the faulty PLC and all associated hardware and the recognition of the need for adequate operational cover during capital works which could impact upon the water treatment process.