Investigations

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

Report on drinking water quality incident at:
Kenmore Borehole in February 2007

Scottish Water Executive Summary

Following routine checks undertaken at Kenmore WTW on Wednesday 7th February 2007 which confirmed that the plant was operating satisfactorily, sometime later, the plant is believed to have developed a fault with a relay in the Motor Control Centre (MCC), causing an intermittent problem with the Sodium Hypochlorite dosing pumps. This is considered to be the root cause of the disinfection problem. Such an event would not have instigated the auto-shutdown of the treatment works, therefore the works continued to operate with intermittent chemical disinfection for five days until the next routine visit on Monday 12th February.

The auto-shutdown sequence of the Borehole pump did not occur. This was due to the on-site control system not recognising the intermittent stopping of the dosing pump (due to the relay switch error) as an actual fault. This meant that the dosing pump alarms were not generated which would have signalled the Borehole pump to stop. The control system for the auto-shutdown process is due to be amended by the time this report is issued. Similar systems at Kinloch Rannoch and Kirkmichael WTWs will also be checked and amended as necessary.

Although alarms were not generated as a result of the dosing pump working intermittently, had alarms been triggered the signals would have been relayed to Scottish Water's telemetry system as a P2 callout.

A further warning system in the form of chlorine residual alarms also failed to highlight the disinfection problem, as the monitor failed to respond to the low level of Sodium Hypochlorite dose. In the meantime the chlorine monitor has been cleaned, calibrated and the operation checked to ensure it responds to low chemical dose in the future.

DWQR Assessment on the incident

The DWQR's assessment of this disinfection failure incident is that it was caused by an intermittent fault with a relay switch in the control panel for the disinfection system. The system should have been configured to automatically shut down the works with such a fault. Furthermore, the treated water chlorine residual monitor was not responding to the low dose and as such did not generate an alarm on the telemetry system.

DWQR notes that Scottish Water's Public Health Team were notified promptly following discovery of the problem and that samples were taken to demonstrate that the water was bacteriologically satisfactory. DWQR notes that not all samples were totally clear of bacteria as suggested in the report, but accepts that where found, the levels were low. DWQR also notes that the samples were taken fairly promptly but would prefer to see some improvement in the speediness of sample taking in incidents. DWQR recommends that the taking of samples should be one of the first things done in an incident.

DWQR notes the actions being carried out by Scottish water (SW) as a result of this incident namely:-

1. Replace faulty relay switch and check operation (action complete).
2. Calibrate and check electrode in chlorine monitor (action complete).
3. Check operation of chlorine monitor and alarm (action complete).
4. Reconfigure auto-shutdown procedure to include faulty relay switches (by 12/03/07).
5. Check and amend auto-shutdown procedures at Kenmore and Kirkmichael to include relays (by 19/03/07).
6. Operators to be trained to clean electrode during calibration procedure (by 31/03/07).
7. Site visits to be increased to three times a week (Monday/Wednesday/Friday) (action complete).

DWQR notes the actions being taken. Whilst generally content with these, DWQR expects that the cleaning of chlorine monitors as well as their electrodes and their calibration should be a matter of routine maintenance which should be included in task schedules at all water treatment works.