Investigations

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

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

Scottish Water Executive Summary

Badcaul Water Treatment Works is situated on the hillside to the south of the A832 at Ardessie. The works supplies an average of 65 cubic metres per day to a total population of approximately 136.

Raw water is abstracted from Allt Airdeasiadh which is a burn intake. The raw water is disinfected only before passing into supply. The site has mains power, however the sodium hypochlorite dosing pumps are powered by 12 volt batteries which are continuously charged by a mains powered battery charger. The site has telemetry and is visited twice per week as required by the task schedules for the works.

At approximately 2200 hrs on 27/10/07 the stand-by officer was called by the Operational Management Centre (OMC) Flight Desk with a telemetry alarm for loss of chlorine dosing at Badcaul WTW. This alarm was reacted to immediately and once the operator was on site at 2300 hrs it was found that neither of the sodium hypochlorite dosing pumps were running. Preliminary checks were carried out, but there was no apparent reason for failure. The electrical system was reset and pumps started dosing again. Operators carried out maintenance checks on dosing equipment and altered chlorine dosing rates to achieve required residuals. Samples were taken before and after the incident and the Public Health Team were notified.

The OMC received a second low chlorine alarm at 04:11hrs on 28/10/07 and this was passed to the standby officer at 04:35. When he arrived on site he found the same fault as before to the dosing system. He followed the same procedure as before and the plant operated satisfactorily and no further alarms were received. The telemetry trend (see appendix 2) shows the chlorine falling off at 02:00hrs. The second chlorine alarm was only triggered after the scheduled 4 hourly poll at 04:00 and there were no failings by the OMC or operational staff reacting to this alarm. The Instrumentation Control Automation and Telemetry (ICAT) team will investigate why this problem occurred.

A work order was raised with Electrical and Mechanical Team and the battery charger, batteries and dosing pumps were checked by the E&M craftsman on the 30/10/07. It was found that one battery had burst and one was only giving 5 volts. The supply voltage from the charger was found to be too high and this could have caused the damage to the cells or may have been a result of the faulty cells.

As a result of this incident an end to end test of the telemetry system for the site will be carried out, new batteries have been installed and dosing equipment power supply will be rechecked.

Badcaul WTW is on the investment programme. The project is due to deliver a new membrane filtration plant including full disinfection control and a new clear water tank is also to be constructed. The project should be completed by the end of 2008.

DWQR Assessment on the incident

The DWQR's assessment of this disinfection failure incident is that it was caused by a fault in the power supply to the disinfection pumps. There is mains power at the site but this only charges two 12 volt batteries that operate in series to provide the 24 volts required to power the pumps. The site is visited twice a week and the power / charging system for the pumps is checked once a week according to Scottish Water's (SW) task scheduling for the works.

When the first failure occurred on 27 October, a low chlorine alarm was initiated, the operator was called out and found the pumps were off but the power system appeared to be operating effectively. Samples were taken which subsequently failed as they contained coliforms.

The system failed again very early on 28 October but this time the low chlorine alarm failed to initiate and only came through two hours later. The operator followed a similar procedure as the earlier incident. Again the samples failed.

Following an urgent request issued on 29 October to check for an electrical fault on the system, electrical and mechanical staff went to the site on 30 October to find that one battery had burst and only 5 volts was being produced by the other. The standby set of batteries was found to be fully charged so the system was set to operate on these.

DWQR notes the actions being taken by SW as a result of this incident. These include a full end to end check on the telemetry system to determine the reason for the failure of the alarm on 28 October. DWQR notes that a "sensor to screen" test was subsequently carried out at Badcaul as part of SW's "telemetry project". This project is essentially checking all alarms at each and every site to ensure that they are set up correctly and operate at the correct level.

Another key SW action is to install four new batteries for the disinfection pumps. Although SW's task scheduling allows for weekly checks on the 24 volt chlorine dosing pumps and battery systems, the fault that occurred does not appear to have been identified during these tasks. DWQR has therefore asked SW to make this task more specific such that the condition of all 4 batteries is checked together with the charging system to ensure that it is operating at the correct voltage for sufficient but not excessive charge.