Investigations

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

Report on drinking water quality incident at:
Shiel Bridge Water Treatment Works in January 2006

Scottish Executive Summary

Shiel Bridge WTW is a small disinfection works. There is no physical filtration or any other form of treatment available at this location. Mains power is utilised on site, but there is no online chlorine monitoring or telemetry. There are no service reservoirs within this water supply zone. The works is supplied by a small burn; the Ault a Chruinn.

As a result of a routine visit for bacteriological sampling on the 10/01/06 the sampler recorded a low chlorine residual (0.03mg/l) at the WTW final sample tap. This was immediately reported to the operator covering that site who then attended site and found small stones lodged within the CWT ball valve, jamming it open. These stones were removed and the chlorine dose re-established to normal levels. In addition the CWT was shock dosed with sodium hypochlorite. The sodium hypochlorite pump doses into the inlet side of the CWT but is flow proportionally controlled from the outlet flow meter.

There is no telemetry currently installed at this location and due to the previous day's problems, the operator returned to site on the morning of the 11th January to find that the chlorine residual exiting the CWT had dropped to zero. The CWT was immediately shock dosed prior to investigating the problem. There were no stones within the ball valve and although there was no evident problem with the pump or injector these were replaced. In addition the contents of the chloros vat were replaced and dosing re-established.

While the operator was attending site on the 11th January a bacteriological failure was reported for the sample taken on the 10th January. The reported result of 12 Coliforms and 10 E.coli was immediately reported to the Highland Health Board (HHB) and Environmental Health. Following discussion with the HHB it was agreed to await resample results before making the decision to issue a boil notice.

The resamples taken on the 11/01/06 were reported on the 12/01/06. Three out of the four samples passed .The fourth sample, which was taken in distribution, failed, with a reported result of 5 E.Coli and 5 Coliforms. This sample had a recorded free chlorine residual of 0.05mg/l and a total chlorine residual of 0.12mg/l.

A further two resamples were taken on the 12/01/06 and another set of three taken on the 13/01/06, all five samples were reported the following days as clear.

Shiel Bridge is a small disinfection works with no online monitoring or telemetry. To ensure compliance with the Water Supply (Water Quality) Regulation 2001 this site requires a full upgrade or to be linked into the Kyle Regional Scheme.

The earliest forecast date for connection into the Kyle Regional Scheme is December 2007.

DWQR Assessment on the incident

This incident was basically a disinfection failure and that the existing treatment works was due to be shut down with existing customers transferred to the Kyle regional scheme at the end of 2007. DWQR noted the remedial measures Scottish Water was planning to put in place to reduce the risk of a recurrence. However, DWQR expressed concerns over the lack of disinfection control and the lack of a telemetry connection to Scottish Water's main control room for the Shiel Bridge works whilst it waits for the connection to the Kyle works. Scottish Water subsequently agreed to install chlorine dosing and measurement equipment which will be connected by telemetry to its control room by 7 April 2006.