Investigations

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

Report on drinking water quality incident at:
Dores Water Treatment Works in October 2006

Scottish Water Executive Summary

Dores WTW consists of a gravity fed up flow sand and granular activated carbon filter with flow proportional sodium hypochlorite dosing. There is no mains power at the site, so the dosing is powered by two 12volt batteries which are changed weekly, and there is no telemetry in place at the works. The works supplies approximately 120 customers, mainly residing in the village of Dores.

On Sunday 1st October there was heavy rain for a period of over 24hrs. This caused an increase in raw water colour to such a degree that the chlorine dose was insufficient to disinfect the water. On Monday 2nd October the Sampler visited site and discovered a low free and total chlorine residual of 0.05mg/l and 0.24mg/l respectively. The water was also noted to be highly coloured. In response to a call from the sampler, the Operator came immediately to site and increased the chlorine dosing pumps to a level which he believed would provide an adequate final water free chlorine residual. He also backwashed the filter. The sample taken on October 2nd was reported on the 3rd to have failed with 280 Coliforms/100ml and 270 E.coli/100ml. Operators were sent to site to further check the works and it was found that the chlorine residual was again very low, with a free chlorine of 0.02mg/l (no total chlorine residual was recorded). The Operators increased the chlorine dosing pump incrementally until a residual of 0.97mg/l was obtained at the statutory sample tap. During the time allowed between each increase, for the additional chlorine residual to reach the statutory sample point (approximately 45 minutes), the filters were backwashed.

After discussions with CPHM, a Boil Water notice was issued at 14:00hrs on the 3rd October and bottled water was also provided to customers. The resamples taken on the 3rd and 4th passed, and the Boil Water notice was lifted on the 5th.

During this incident, the period of time that inadequately disinfected water entered into supply and hence the boil water notice being in place was prolonged by the poor management of the situation on 2nd October. The Operator miss-judged the increase in chlorine dose required to ensure adequate disinfection on Monday October 2nd and because he did not check the free chlorine residual prior to leaving site he was not aware of this until the next day. The Treatment Co-ordinator should also have instructed the Operator to stay on site until an adequate residual was achieved. The level of response on Tuesday October 3rd was appropriate.

The following actions will be taken as a result of this incident:

1. Visits to Dores WTW will be increased to three times a week.
2. Treatment operators will attend toolbox talks on the Emergency Action Levels (EALs) and disinfection awareness.
3. Treatment Operators, Team Leaders and Co-ordinators should be instructed that treatment works, without telemetry, are to be monitored on site until an adequate level of disinfection has been verified and to inform the appropriate personnel as soon as an EAL breach has been confirmed.
4. Investigate the feasibility of installing telemetry and chlorine monitoring at Dores WTW as a temporary solution until the capital investment project has been delivered.
5. Dores WTW to be mained out from Inverness WTW as part of the capital investment "early start" programme.

DWQR Assessment on the incident

Bad weather caused an increase in demand for chlorine at the works but since there is no mains power or telemetry the lack of sufficient disinfection was only noticed through a routine sample being taken the following day. The operator increased the dose but did not check this was sufficient by taking a measurement of the residual chlorine into supply as he should have done. The correct action was taken the next day but at the same time the supply was then subject to a boil water notice.

I am content with the actions being taken by Scottish Water to minimise the possibility of a recurrence of this type of incident but have asked them to undertake daily visits of the works until either telemetry and chlorine monitoring are in place or until the works has been mained out by Inverness WTW (currently expected in autumn 2007), whichever is the sooner.