Investigations

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

Report on drinking water quality incident at:
Lochinver Water Treatment Works in February 2007

Scottish Water Executive Summary

Lochinver water treatment works is situated in the hills to the East of the village of Lochinver and supplies an average of 330 m3 of treated water per day. The distribution area supplies 323 properties and a population of approximately 506. The water is abstracted from Loch Bad Na Goibhre and is disinfected before passing into supply.

At approximately 22:30hrs on the 11/02/07, the stand-by officer received a telemetry alarm from the Operations Management Centre (OMC) Flight Desk for low treated water chlorine at Lochinver WTW of 0.27mg/l. The low chlorine alarm was immediately reported to the Network Service Operator (NSO) who was on stand-by and he went to site to investigate. On arrival at site, at 23:15hrs, the free chlorine residual at final water sample tap was 0.40mg/l. The operator checked that the sodium hypochlorite dosing pumps were operational, which they appeared to be, however failed to notice that there was no flow to the online chlorine analyser and that it was therefore reading zero. The operator left site at approximately 01:15am.

The occurrence of a low chlorine alarm was reported by the stand-by officer to the treatment team leader at 08:00 the following day. On receiving this information the treatment team leader checked the telemetry and found that the live treated water chlorine residual trend was showing 0.00 mg/l and had been since the operator attended site and therefore no further alarm had been received. A treatment operator was immediately sent to site and found the residual in final water was 0.13mg/l. The operator discovered that there was no flow to the chlorine analyser and this was the reason that it was displaying 0.00 mg/l. The operator changed the sample line filters and reset the flow.

The operator checked the treated water free chlorine residual and found it to be zero. The treatment operator shock dosed the clear water tank (CWT) with sodium hypochlorite solution to achieve a residual of approximately 0.60mg/l at the outlet. The operator then removed and replaced the suction side of the sodium hypochlorite dosing pump and restored the dosing by 10:30am. By approximately 11:00 hrs the free chlorine residual to supply was 0.59mg/l. The operator was then instructed to secondary chlorinate the Service Reservoirs (SR) in distribution, where required, and take bacteriological samples.

On investigation it became apparent that there had been a blockage in the suction line from the hypochlorite storage dosing drum. This blockage was caused by small crystals from the hypochlorite solution. The operator, after informing his team leader of his findings, was instructed to flush all the suction lines from the hypochlorite dosing drum to the dosing pumps and also to empty and clean out the hypochlorite storage dosing drum.

Lochinver water treatment works is programmed for substantial capital investment; however the project has been considerably delayed due to significant problems with land acquisition and is currently forecast to be completion by 31st Dec 2008. (This site has an authorised departure until 31st Dec 2008).

DWQR Assessment on the incident

The DWQR's assessment of this disinfection failure incident is that it was caused by a blockage (believed to be crystallisation of the disinfectant) in the disinfection dosing line.

The initial low chlorine alarm was investigated at 22.30 on 11/02/07 by a Network Service Operator (NSO) on standby, not a treatment operator. The NSO did not fully investigate the problem and did not notice that there was no flow to the chlorine monitor. It was only after discussing the matter with the treatment team leader early the next morning that the issue was fully investigated at 09.30 on 12/02/07.

Scottish Water's (SW) Public Health Team (PHT) were only informed via email at 13.18 on 12/02/07. This is far too late in the proceedings. SW's PHT should be informed immediately a problem such as this is discovered and this should be by telephone in the first instance.

DWQR notes the actions being taken by SW as a result of this incident, namely:-

1. Formal interviews to be carried out with the Network Service Operators who are on the stand-by rota in the Ullapool area to assess the treatment training requirements to ensure they have the skills and knowledge required to carry out the treatment stand-by role (by 6/4/07).
2. Review the scheduled cleaning frequency of sample line filters to prevent loss of flow to the chlorine analyser at Lochinver (by 31/3/07).
3. Ensure that all sodium hypochlorite dosing systems in the Ullapool area are fitted with the appropriate filters on the suction lines and that lines are lifted from the bottom of sodium hypochlorite dosing drums to prevent any crystals entering the dosing lines and pump (action complete).
4. Temperature monitoring to be carried out at Lochinver WTW to assess whether low temperatures (<minus 5 degrees Celsius) are causing crystallisation of the sodium hypochlorite (action ongoing).
5. Scotland wide 'Operator Event Awareness Training' will be rolled out from 26th March 2007. This training, based upon previous incidents and events, will include the appropriate actions to be taken in the event of a low chlorine alarm (by 31/5/07).

DWQR is, in general, content with these actions. However, in relation to actions 1 and 3, DWQR would ask SW to make these Scotland-wide, rather than limited to the Ullapool area. DWQR will be monitoring these actions to ensure that they are completed as planned.