The Loch source water comes into the works and is screened through a basket strainer. The water treatment works (WTW) at Melness is an ultrafiltration membrane plant utilising ceramic monolith membranes. Post membrane pH correction is achieved via a limestone chip contactor and Sodium Hypochlorite is dosed as a disinfectant before the treated water is stored in a chlorine contact Tank at the WTW. The treated water is then pumped to a twin compartment service reservoir situated on the hillside above the works. The service reservoir (SR) then feeds into distribution supplying a total of 153 people in the Melness/Talmine area.

On the evening of 24th January, prior to the colour sample failure, the membrane recirculation pumps stopped causing the WTW to shut down. The service reservoir levels and outlet flows were monitored and were in a healthy situation, so there was no urgency to restart the plant and it was thought prudent to leave it to first thing next working day.

The plant was restarted 08:00 hrs on the 25th January and the statutory and operational samples were taken at 09:15hrs.

On 25th January 2008 Melness WTW final water operational sample failed for colour at a level of 21mg/l Pt/Co. The subsequent resample taken on the 4th February passed for colour, but failed for manganese (58.7µg/l) and iron (267ug/l).

Water quality failure investigations have indicated that the colour failure from a final water sample taken on 25th January was not as a result of a membrane integrity issue. The indications are that the elevated colour, and subsequent samples with higher than expected manganese and iron levels, were as a result of carry over from the limestone contactor.

The limestone media was last changed on 6th September 2006. Under normal conditions the blend valve on the contactor vessel is set at 50% to achieve a pH of 8.0. As the media dissolves the blend valve is opened further. At the time of the sample fails the blend valve was open to 100% to achieve a pH of 7.5. The last backwash of the contactor prior to the high colour sample was done on the 10th Jan 2008.

Actions being taken as a result of this incident include:-

Investigate the backwash efficiency and optimum backwash regime for the works
Ensure that the limestone contactor media is topped up on a more frequent basis
Initiate a sampling programme to monitor the performance of the membrane filters

DWQR Assessment on the incident

Review of the incident report highlights several areas where the operation of Melness WTW requires more effective control. The initial cause for concern regarding drinking water quality was over the colour failure on 25 January 2008. Subsequent investigations by Scottish Water highlight a number of areas where the control and operation of the works should have been under tighter control, this included; the operation of the limestone contactor, follow-up of Cryptosporidium failures and the operation of the turbidity monitor. While DWQR accepts that the colour failure of 25 January 2008 was not as a result of any breach of membrane integrity, the current weakness in Scottish Water's membrane integrity testing protocols have been highlighted by this investigation and these protocols need to be addressed across the business as a matter of urgency. While DWQR notes that Melness is due to be mained out from the North Coast Scheme in September 2009, the required improvements in both the water treatment plant and operational practices must be completed and maintained such that drinking water quality is maintained at an appropriate standard until this further investment in changing the source of the water is completed.