Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Cannich Water Treatment Works in October 2006
Scottish Water Executive Summary
Cannich water treatment works supplies approximately 200 people, who mainly reside in the village of Cannich. The works consists of four gravity fed up flow sand and granular activated carbon filters. Sodium hypochlorite is dosed for final disinfection into the filtered water and is flow proportionally controlled. There is a clear water tank at the works which provides approximately 6 hours of treated water storage.
A new water treatment works is current being commissioned at Tomich and is going through performance tests. Depending on the results of these tests it is expected that Cannich WTW will be mained out by this works either before the end of this year or early in the next.
On Thursday 26th October at16:00hrs, the OMC reported a mains power supply failure at Cannich WTW during a period of severe weather. The treatment team leader immediately contacted the treatment works operator, who was on site already carrying out maintenance. The operator confirmed that the mains power had failed, however the battery back-up had automatically come on and was powering the sodium hypochlorite dosing pump and tank level indicator. The works operator left the works at 19:00hrs with the battery back-up maintaining the sodium hypochlorite dosing.
On Friday 27th October at 04:30hrs, the OMC reported a Low Battery Alarm at Cannich WTW to the Treatment team leader who was on stand-by. An operator was not asked to attend as a result of the alarm as the team leader had already arranged for the operator to go to site early next morning with replacement charged batteries. In addition during evening of 26th emergency services were advising that road to Cannich (A831) was unadvisable due to fallen trees and flooding.
On arrival at the site, the Operator checked the chlorine residual from the CWT. As the free chlorine was low (0.18mg/l) and out with the normal operating range for this site, which is 0.80 - 1.30mg/l, the Operator shock dosed the CWT and replaced the batteries. At 09:30hrs the mains power supply to the works was restored, however the sodium hypochlorite dosing was not operating, despite new batteries being put on, due to damage to the automatic battery charging unit which had been caused by a power surge when the power supply was restored. Contractors were called in to repair the damaged unit and they reported that it would take a number of hours to carry out the repair so a temporary dosing system was installed until the onsite batteries were charged and powering the dosing pumps. A statutory bacteriological sample was taken before the temporary dosing was installed.
On Saturday 28th October the area Stand-By Operative visited site at 12:00hrs and found everything to be okay, the final water chlorine residual was, Free 0.54 mg/l and Total 0.68 mg/l. The Public Health Team contacted the treatment team leader at 13:15hrs to report that the previous days sample from Cannich WTW had failed with a count of 5 Total Coliforms and 5 E-Coli per 100mls. The treatment team leader arranged for re-samples to be taken from Cannich WTW. As the team leader was unable to make contact with the bacteriological lab, the samples were refrigerated at the Inverness SW depot overnight.
Sunday 29th October. the area stand-by Operative returned to Cannich and collected another set of WTW and zone re-samples and checked the works. Good chlorine residuals were recorded, Free 0.72 mg/l and Total 0.88 mg/l. On return to Inverness all 6 re-samples are delivered to the bacteriological lab.
Monday 30th October. Lab confirms that all the re-samples had all passed.
The following actions will be taken as a result of this incident:
- Cannich WTW will be visited daily will until the fault with the telemetry has been resolved. The fault with the telemetry was also a burnt out circuit board. The board also took a power surge when the mains power was re-connected to the WTW. When the new circuit board was fitted, there seemed to be an incompatibility with the existing board, this issue was not fully resolved until the 27th November 2006.
- An incident debrief will be carried out with the operational personnel to cover the lessons learnt.
DWQR Assessment on the incident
DWQR's assessment of this incident is that it was caused by a series of events. A mains power fail resulted in in an automatic switch over to the battery back up as the system is designed to do. However, the batteries lost their charge resulting in an alarm but severe weather hampered access to the site overnight to investigate the problem. With mains power back on the following day, it was found that the battery charger had failed and that there were additional problems with the disinfection dosing pump and telemetry which had failed due to the power surge. However, by this stage the disinfection level had dropped below the minimum required which meant that Scottish Water (SW) had to manually dose the system to ensure adequate disinfection. The batteries had been charged and permanent dosing was resumed late on the second day.
DWQR is content with SW's actions which include visiting the site daily until the telemetry has been fixed and supplying Cannich with the new Tomich water treatment works by early January 2007.