Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Unst Water Treatment Works in May 2006
Scottish Water Executive Summary
Unst WTW supplies 0.45 ML/day of water to approximately 700 customers living on the most northerly inhabited island of the Shetland isles. Treatment consists of 6 upward flow Inverness type sand filters operating in parallel. Post filtration disinfection is by sodium hypochlorite dosing from a single battery powered dosing pump.
There is no mains electricity at the works, and no telemetry. There is one service reservoir in distribution with secondary chlorination, Midfield/Heog SR. The Unst WTW is attended seven days per week and the Midfield/Heog SR three times per week.
On Mon 1st May the operator attended the Unst WTW to carry out routine maintenance and recorded a free chlorine residual of 0.6mg/l . He left the site at 3.30pm.
On Tues 2nd May the operator attended the Unst WTW to carry out routine maintenance and found that the chlorine pump had air-locked. He recorded a free and total chlorine residual of <0.2 mg/l. The line was bled and dosing was resumed. A Statutory Sample was taken of the final water and sent to the Kirkwall laboratory for bacteriological analysis. A Statutory Sample was also taken from Midfield/Heog SR where residuals were <0.02 mg/l free chlorine and 0.13 mg/l total chlorine
On Wed 3rd May notification was received that the Unst WTW sample had failed with a result of 16 total coli forms and 0 E.coli per 100/ml. The Midfield/Heog SR result was 0 total coliforms and 0 E.coli. The laboratory results were available too late to allow resamples to be taken in Unst in time to be transported to Kirkwall for analysis on the same day. The operator carried out routine maintenance at the works and recorded a free chlorine residual of 0.6mg/l.
On Thurs 4th May resample number 3231875 was taken at the works and residuals of 0.7mg/l free chlorine and 0.93mg/l total chlorine were recorded. A sample was also taken from Heog SR, with residuals of 0.2mg/l free chlorine and 0.34mg/l total chlorine.
These samples passed with 0 total coliforms and 0 E.coli.
A Q&S3 early start project is underway to provide improved disinfection control, including mains power and telemetry.
DWQR Assessment on the incident
This incident was basically a disinfection failure but due to the absence of on an on-line residual chlorine monitor and any telemetry with appropriate alarms, this failure was only discovered during a routine visit by the local operator. Scottish Water has agreed however to reduce the risk of a recurrence by providing improved disinfection control, connect the works to mains power and telemetry and install a final water residual chlorine monitor that will be connected to telemetry with an appropriate alarm. At time of the report Scottish Water's forecast completion date for this work is March 2008 and the treatment works will therefore be visited daily in the interim to check on the process.