Investigations

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

Report on drinking water quality incident at:
Glassford Water Treatment Works in April 2006

Scottish Executive Summary

On Thursday the 20th April 2006, a low level chlorine gas escape alarm was generated at site and received by the Operational Management Centre (OMC), following a routine chlorine cylinder change. The chlorinators were inspected and the gas escape was rectified at 1230 hrs. At this time the chlorinators were found to be at their normal dosing limit of 1.2mg/l. The works SCADA was also inspected and found to have no low chlorine alarms. Prior to leaving the works for the day at 1600 hrs, site staff contacted the OMC who confirmed that no alarm conditions were present for the site.

At 0800 Hrs on Fri 21st April the duty operator arrived and noticed that the combined filter chlorine residual was at zero. Both site chlorinators were reset and dosing restored at a higher than normal dose rate. To help improve chlorine residuals leaving site a temporary chlorine dosing unit was installed and operational by 1230 hrs. The unit was dosing Sodium Hypochlorite into the clear water tank (CWT) outlet mains at a dose of 0.8 to 1.0 Mg/l. The scour valves for the tank were opened to increase the turn around of the tank and the Public Health team were informed of the situation. At 2030 hrs the chlorine residual ex CWT was increasing and at a level of 0.4mg/l. At 2100 hrs the temporary dosing unit was switched off and the chlorine residual ex CWT had returned to its normal level of 0.6 mg/l.

Further investigations found that during the recent upgrade works, a software interlock had been put in place to stop the chlorine motive water pumps during a gas leak. A problem with the chlorinator flow tube meant that the inspection of the chlorinators following the gas leak did not indicate an issue. The interlock has since been removed and software returned to its normal operation, thus preventing this type of failure re-occurring. The chlorination flow tube issue has also been resolved. The OMC had received low residual alarms as well as gas escape alarms during initial problems on the Thursday and had accepted them following making contact with site staff. No further alarms were received as the low residual alarm remained at the works although not shown on the works SCADA.

The works SCADA was investigated and a fault on the low chlorine alarm was found on the SCADA. This has now been repaired and tested thoroughly. The current SCADA system was not user friendly but has now been modified to show all critical process parameters on the works overview page.

Report on Drinking Water Quality Incident Involving Disinfection Failure at Glassford Water Treatment Works

This incident concerned a failure of the chlorination system at Glassford WTW. DWQR concluded that the primary cause of the failure of the chlorination system to restart after the alarm was the control software at the site which had recently been changed during an upgrade to stop the chlorine motive water in the event of a gas leak alarm. Although the operators correctly conducted checks on attending to the alarm, the chlorinator flow tube, which would normally have shown that no chlorine was being added, was stuck in position due to dirt in the tube.

Additionally, the site control system (SCADA) was configured in a way which did not make it immediately apparent that chlorination had not restarted. The numerous samples which were taken from the works and distribution system, were satisfactory for microbiological parameters. DWQR notes that Scottish Water has put measures in place to ensure any changes to the control system are discussed with site staff. Further changes to the system at the site have been made to ensure any problems are apparent to operators at a glance. Additionally, DWQR has recommended that the maintenance for all chlorinator flow tubes is reviewed to ensure that they are kept in working order.