Investigations

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

Report on drinking water quality incident at:
Acharacle Water Treatment Works in January 2008

Scottish Water Executive Summary

Acharacle water treatment works is a gravity system - Inverness type filtration works and serves approximately 297 properties, producing an average of 388 m3/day of treated water.

Water is abstracted from Allt Beithe, and gravitates through a GAC filtration plant employing 4 of Inverness sand filters (run in parallel), followed by 4 of Inverness GAC filters (run in parallel). Sodium hypochlorite (residual control) is injected into the main prior to the clear water tank (CWT) and monitored at this point. The free chlorine residual is also monitored on the outlet main of the CWT. There is both telemetry and power at this site.

At 01:39hrs - on the morning of Monday 21st Jan 08 , duty standby person received a 'P2 distribution water chlorine (supply) Hi alarm - 1.82mg/l. Treated water value was 1.95mg/l, which is not breaching the EAL Hi hi value of 3.00mg/, and the Treated Water Chlorine alarm had not been triggered.

The EAL - Hi Hi distribution water chlorine value for the site is 1.80mg/l and therefore took no further action, until 07:50hrs that same morning , as it was just breaching the Hi hi level EAL.

07:50hrs - Treated water chlorine residual for the site was 1.84mg/l, all aspects of plant appeared to be functioning correctly, distribution water free chlorine had now increased to 1.99mg/l.

07:50hrs - Monday 21st Jan 08, duty standby person passed the only P2 alarm that was generated at (01:39hrs ) for 'high distribution water chlorine level', 1.82mg/l. This was acknowledged and passed to Operational standby staff to action. On route to site the site Operator checked the sample tap chlorine residual (Regulatory sample tap water chlorine) - this was found to be >2.20mg/l at 07:50hrs. The difference between the two chlorine residuals is due to one being measured at the regulatory sample tap approx 300M from WTW, and the actual distribution water being measured on-line immediately after exiting the CWT.

The site operator contacted the Treatment Team Leader to advise of high level chlorine residuals at site, the site operator was advised to 'lower the CWT and Dechlor' the tank. The miscommunication between site operator and Treatment Team Leader - site operator took this as an order from the Treatment Team Leader to add Dechlor tablets into the CWT to reduce the chlorine levels going into the supply.

Site operator was carrying the Dechlor tablets in his van, for the purpose of dechlorinating scours to water courses. He was not aware of the label on the original Dechlor container stating that they were 'Only to be used in the De-Chlorination of Waste Water Effluent'.

Site operator was unaware that the Dechlor tablets (Sodium Sulfite) he had access to, were not regulation 27 approved for use in contact with potable water supply. (10 number Dechlor tablets were added in total). Using the tablets for lowering chlorine residuals in CWTs is not an authorised process.

Water quality in WSZ was constantly monitored throughout the event, chemical samples were completed after Dechlor tablets being added to the CWT. The samples taken passed all parameters measured, results attached in this report.

The route cause of the Acharacle WTW high chlorine incident was initially caused by a chlorine dosing probe fault. The chlorine probe did not detect the true chlorine residual being dosed into the works, the plant is designed to deal with low chlorine residuals and initiates a shutdown, in the event of a sustained process failure, but there is not on high chlorine.

The Acharacle WTW came back into compliance on Tuesday 22/01/08.

DWQR Assessment on the incident

DWQR has reviewed the report made by Scottish Water into the circumstances surrounding the high chlorine levels occurring at Acharacle WTW between 21 and 22 January 2008. While the incident itself resulted from several failures of the chlorine dosing system resulting in a higher that required chlorine dose being applied to the system, the principle concern for DWQR was the inappropriate response made by Scottish Water staff while trying to recover the situation. DWQR notes with concern the use of unapproved chemicals to reduce the chlorine levels and the lack of awareness of emergency procedures to be followed in such circumstances. DWQR, while not wishing to prejudice any internal actions taken by Scottish Water, is seeking reassurances from Scottish Water regarding the level of training and authorisation given to staff involved in work of this nature. DWQR requires Scottish Water to ensure that robust, auditable training for staff is put in place and maintained such that only appropriately trained and authorised staff are placed in such positions in future.