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Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Achmelvich Water Treatment Works in January 2007
Scottish Water Executive Summary
Achmelvich water treatment works is a small sand and granular activated carbon filter workssupplying the village of Achmelvich and the surrounding area. Raw water is abstracted from Loch an Sgeireach and gravitates to the works where it is filtered and disinfected. There is no mains power at the works so the disinfection system is powered by batteries which are trickle charged by a wind generator. There is no telemetry at this site.
The Achmelvich works is currently being upgraded and the programme is for the site to have both mains power and telemetry by the end of March 2007 (was due February 07 however landowner issues have prevented installation of mains cable). There will be further investment in the Q&S3 programme to upgrade the chlorination system and install pH correction. This is due to be completed by early 2008, subject to no delays being encountered with project approvals and land acquisition.
Scottish water has undertaken to visit this site on a daily basis until telemetry has been installed and commissioned.
On Friday 12th January the treatment operator attended the site and found that the sodium hypochlorite dosing pump was not running. An initial inspection could not ascertain what had caused the dosing to fail. The operator switched the pump from flow proportional to a manual setting to restore disinfection. Once an adequate free chlorine residual was achieved at the statutory sample tap, the operator opened a scour valve in the distribution network to draw the disinfected water through the system. An incident sample was taken from the works regulatory sample tap along with a sample from a property in distribution. These samples were reported as having passed the following day.
The pump was left on the manual setting until a full inspection was carried out onsite on Tuesday 16th January. This investigation found that the pulse cable from the flow meter to the dosing pump had shorted out on the edge of the meter chamber causing a loss of the dose controlling pulse. This was probably due to high winds on January 11th and 12th moving the dosing kiosk. Repairs were effected to the pulse cable and the works was returned to flow proportional dosing.
DWQR Assessment on the incident
The DWQR's assessment of this disinfection failure incident is that it was caused by the dosing pump failing to receive a signal from the flow meter due to the cable having shorted out due to adverse weather conditions. Due to the lack of any telemetry on the site, this was only discovered during a routine visit but Scottish Water (SW) had already undertaken to visit this site on a daily basis (including weekends) until mains power and telemetry has been installed and commissioned by 28/03/07.
DWQR also notes that SW will protect the cable between the flow meter and the dosing pump by 28/03/07, that Operational Team leaders are made aware of the need to communicate incidents/events directly to the Public Health Team (by 14/3/07), and that task schedules will be reviewed to ensure all appropriate water quality tests are completed by (21/03/07)."