Investigations
This section contains summaries of reports on drinking water
quality incidents and investigations.
Report on drinking water quality incident at:
Laggan Bridge Water Treatment Works in February 2008
Scottish Water Executive Summary
Laggan Bridge WTW supplies an average of 20m3/day to 59 customers. The plant was commissioned in December 2007. Raw water is abstracted from three boreholes. Water is pumped from the boreholes to a blending tank. Treatment process consists of blending of borehole water which is then passed through 3 cartridge filters (3 per borehole), followed b chlorination, chlorine contact tank and then pumping to the Clear Water Tank which contains a storage capacity of 24 hours.
The root cause of the disinfection failure was due to failure within the chlorine dosing system, it is suspected that a blockage of dosing lines led to loss of primary disinfection for a two hour period. Alarms were generated, but an inexperienced standby operator, along with confusion over work done on site that day by Scottish Water Solutions resulted in a temporary loss of disinfection not being actioned on at the time of the event. Chlorine trends were monitored by the Scottish Water Capital liaison Co-ordinator and it was confirmed that neither treated water chlorine, nor final water chlorine into supply fell below 0.6mg/l free chlorine at any stage of the event.
An additional issue was that the plant failed to shut down on low dosed chlorine, the pass forward pumps continued to transfer water from the chlorine contact tank forward to the Clear Water Tank for a two hour period, disinfection then commenced again until the CWT reached capacity and a controlled shut down occurred.
Actions being taken in response to this incident include:
- Full review of control philosophy of the WTW process
- Additional training and competency checks for all Standby Operatives concerned
- Review of communication between Scottish Water Solutions and Scottish Water for commissioning of new assets
DWQR Assessment on the incident
While DWQR accepts the root cause of the incident was most likely to have been caused by a blocked chlorine dosing line, the incident was certainly exacerbated by the lack of effective communication between Scottish Water Solutions (SWS) and Scottish Water and the inadequate level of training provided to the standby operative.
The level of communication between SWS and Scottish Water was poor and was a contributing factor in the chain of events that caused this incident. Had this incident occurred with a new contractor, then some consideration could reasonably be given to the issue of effective communication. However, SWS and Scottish Water have been working together for a number of years and the fact that poor communication between the two organisations still results in incidents suggests that something more fundamental than a "review of communication between Scottish Water Solutions and Scottish Water for commissioning new assets in the Ness Area" is required. DWQR is concerned that effective communications between SWS and Scottish Water had not been established prior to this incident.
Any repetition of the circumstances that occurred at Laggan Bridge will be pursued by DWQR with a view to bringing appropriate sanctions against Scottish Water.
On the issue of training, it is for Scottish Water's management to ensure that staff are properly trained and equipped. On the evening of the incident the standby operative, quite correctly, assessed the situation to be out with their level of competence and attempted to escalate the matter to their line manager. Unfortunately, the Team Leader/Duty Escalation Officer was not available that evening, something that has not been adequately explained by Scottish Water, nor have the actions of the Area Team Manager who effectively passed the buck to another operator at Inverness WTW. Further, the standby operator was unable to perform simple diagnostic tests to verify that the on-site chlorine monitors were reading correctly, as he had neither the apparatus nor training to undertake this basic confirmation. The lack of equipment and training on such a basic piece of apparatus as a hand-held chlorine monitor suggests that the level of training provided to the standby operator were wholly inadequate for the tasks he would be expected to undertake as part of his normal duties. As a result of this incident, DWQR will be investigating the level of training given to standby operators during the course of the scheduled audit activity to be undertaken in 2008.