Investigations

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

Report on drinking water quality incident at:
Invermoriston Water Treatment Works in September 2007

Scottish Water Executive Summary

Invermoriston Water Treatment Works (WTW) is a carbon filtration plant with disinfection. The raw water is pumped to the site from a loch source. The raw water enters the WTW and goes upward through the carbon filter media. The filtered water is then pH corrected using limestone chips prior to final disinfection with sodium hypochlorite. There are two clear water tanks, each with a capacity of 12 hours storage.

Due to low throughput of the works, one CWT has been isolated to reduce chlorine decay and promote treated water turnover. Invermoriston WTW is designed to operate with a maximum flow of 0.14 ML/day, supplying a population of approximately 110.

On Tuesday 11th September 2007 customers in the Invermoriston supply zone reported that they had no water. After investigations by Scottish Water Customer Operations it was established that the WTW was not producing any water and that the Clear Water. Tank was empty with no flow going into supply.

There were no alarms generated from either the raw water pumping station or the water treatment plant to alert Scottish Water of the problems on site. On inspection it was found that both duty and standby raw water pumps had been shutdown, due to a faulty inlet air valve on the raw water main. Water passed out of this air valve and starved the treatment process and sample pumps of water. With limited water entering the chlorine sample lines the plant controls believed there to be low chlorine levels and automatically shutdown the raw water pumps to prevent any un-chlorinated water entering the CWT. This control mechanism is all as per the design parameters of the plant and is a safeguard. The telemetry outstation appeared to have frozen and no alarms were generated as the CWT emptied and some customers were without water for 4 hours.

Actions being taken as a result of this incident include:

1. End to end test all telemetry signals from the site.
2. Air valve on raw water main to be inspected and serviced.
3. Chlorine sample pump checking to be added to routine task schedules.

DWQR Assessment on the incident

The DWQR assessment of this "no water" incident is that it was probably caused by a failure of an air valve on the raw water inlet main. It would appear that water came through the air valve rather than being passed forward to the water treatment works. This loss of water meant that there was no water going to the chlorine monitors resulting in the plant believing that there was no chlorine and thus shutting down the raw water pumps as designed to do to prevent undisinfected water from going unto supply. Water was however still being supplied from the clear water tank and when it reached its low level an alarm should have been transmitted to Scottish Water's (SW) Operational Management Centre (OMC) which should, in turn, have been reacted to. However, no such alarm was generated and as such, customers were out of water for longer than they would have been if the telemetry alarm had worked. In fact the alarm was raised by a customer calling in to say that they had no water which prompted the investigation.

DWQR is aware of SW's ongoing work on its telemetry system and associated alarms to ensure that they are all appropriate and working as they should. DWQR notes from the end of October report on the telemetry project that testing of the signals at Invermoriston from the works to the OMC was still to be carried out.

DWQR notes SW's actions as a result of this incident and that these include end to end testing of all telemetry signals from the site, inspecting and servicing of the air valve, adding a check on the chlorine sample pumps to the day to day tasks for the site and reviewing the works control to see if anything more can be done to provide an earlier indication of a raw water system failure.