Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Elphin Knockan Water Treatment Works in July 2007
Scottish Water Executive Summary
The water treatment works at Elphin Knockan is a simple disinfection site with residual controlled sodium hypochlorite dosing. The raw water is from a natural spring (Knockan Spring) and is fed directly to the works through a 3mm strainer. The treated water gravitates to a Clear Water Tank (CWT) and this supplies approximately 40 customers in the Elphin Water Supply Zone.
On Saturday 7/7/07 the West Coast Standby Inspector received a high treated water turbidity alarm at approx 18:34hrs, the Inspector decided to monitor the turbidity trend to evaluate if this was a spike due to a sudden increase in flow At 20:00hrs the Inspector interrogated the site via telemetry and found the turbidity to be at 0.995 ntu. Following discussion with the standby Operator it was decided to defer fitting a crypto filter until the following morning.
At approx 23:15hrs on 7/7/07 the Standby Inspector received a low treated water chlorine alarm and immediately sent the Operator to investigate.
On Sun 8/7/07 at 00:40 hrs the Operator arrived on site and found the chlorine dosing pump failed and he restarted in manual mode to achieve a satisfactory chlorine residual. The turbidity level was at 1.00ntu, however, the Operator's focus on resolving the disinfection problem resulted in him omitting to fit a crypto filter.
On Sun 8/7/07 a follow up site visit highlighted that the sample flow to the analyser was too low. This caused the analyser to go into fault and sodium hypochlorite dosing pumps to stop to prevent the gross over dosing. The sample line was found to be blocked by organic matter and once this was cleared the residual controlled dosing could be reinstated. During this visit a crypto filter was fitted at approx 11:30hrs.
The turbidity remained above the site EAL level of 0.4 NTU until approx 02:45 on 9/7/07. The root cause of the disinfection failure was the drop in flow to the chlorine analyser. As the sodium hypochlorite dosing is residually controlled at Elphin WTW this resulted in the sodium hypochlorite dosing pumps stopping. A multifunctional standby rota is in operation and the Network Supervising Officer (NSO) and operative on call at time of incident were Networks personnel. Although both employees have experience in the treatment function , it must be noted that the operative has not completed his Crypto Training Workbook.
As a result of the incident a number of actions have been generated including:
- Formal fact finding interviews to take place with NSO and Operative
- Event reporting /Crypto Regs refresher training for all staff in the North Coast Wester Ross area will take place.
- Investigation into security of source water to Elphin Knockan WTW
DWQR Assessment on the incident
Following review of the incident report provided by Scottish Water, the DWQR accepts that the incident was primarily caused by the blockage of the chlorine sample line by organic matter derived from the source which had a cascade effect of shutting down the dosing system and hence triggering a low chlorine alarm condition. DWQR is aware of the emerging issues with respect to Cryptosporidium and is in discussion with Scottish Water and the Water Industry Commission for Scotland regarding the installation of appropriate treatment at this and other similar sites.