Investigations

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

Report on drinking water quality incident at:
BradanWater Treatment Works in May 2006

Scottish Executive Summary

On the evening of Thursday 4th May, an electrical storm resulted in a power dip at Bradan water treatment works. This affected the ultrasonic level probes in the lime slurry make-up tanks, resulting in an increasing lime concentration being dosed throughout the works.

This affected the coagulation dosing the most since this was under manual control and could not adjust to the stronger lime slurry. The increase in pH meant that the alum dosed raw didn't coagulate properly, resulting in high aluminium concentrations throughout the works (max = 3000 ugAl/l, and later into supply (max = 1000 ugAl/l.) The Operative was paged at 1915 Hrs on the 4th May for the high coagulation pH alarm. He was on site by 1945 Hrs. Realizing the source of the problem he re-set the make-up system, diluted the lime slurry in the tank and manually kept adjusting the pump stroke to maintain the correct coagulation pH. He also cleaned and re-instated a back-up system of electrode level sensors in the tank. At 2130 Hrs the clear water tank (CWT) was spiked with chlorine since the residual had dropped.

By 0040 Hrs the next day, the coagulation was operating normally and the clarified water quality was improving (500 ugAl/l). By 0340 Hrs the treated aluminium residual was down to 900 ugAl/l but the CWT (final) was increasing (peaked at 0830 Hrs Al = 1000 ugAl/l).

The Scottish Water Public Health Team was notified first thing on the 5th May. They notified the various health bodies and dialysis patients. Throughout the day the aluminium and chlorine levels were monitored to ensure that the situation did not deteriorate. By 1510 Hrs, the filtered Al = 280 ugAl/l, the final was 600 ugAl/l. Bacteriological samples were also collected. These all passed.

Aluminium levels were below 100 ugAl/l by the 7th May and back to normal (30 ugAl/l) by the 8th May. The coagulation lime dosing is now back under automatic control. (New pH probes have been installed).

DWQR Assessment on the incident

In the DWQR assessment of the incident, it is acknowledged that Scottish Water responded quickly and appropriately to the water quality alarm, although it took some time before aluminium concentrations returned to normal. Scottish Water's Public Health Team were not notified of the incident by operational staff until the morning of 5 May, despite the highest concentrations being recorded overnight. Correspondingly, the team were unable to provide the required notification to NHS health professionals and local authority environmental health teams to enable them to provide advice to protect consumers, particularly those on dialysis. DWQR expects that in future, Scottish Water will ensure its Public Health Team are alerted to any water quality incident as soon as possible after it commences in order that the appropriate action may be taken. It is noted that Scottish Water have made improvements to the pH monitoring and lime dosing system to prevent such an incident occurring in the future.