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Scotland's water supplier was today criticised for an incident in which water with twenty times the legal standard of aluminium was supplied to thousands of homes.

In March this year Scottish Water consumers in parts of North and West Glasgow were told not to use their water supply for drinking or cooking for 24 hours because of the high aluminium levels.

Nearly 12,000 consumers in Strathblane, Bearsden and parts of Milngavie were unable to use their tap water for drinking or cooking until Scottish Water lifted the restriction on the morning of March 19.

A report by the Drinking Water Quality Regulator (DWQR) found Scottish Water had used scientific evidence with "deficiencies" when it made the case for lifting the restrictions.

And it found aluminium levels "remained well above normal" even after the restrictions had been lifted.

It happened after a problem at Burncrooks Water Treatment Works, North of Bearsden, on the night of March 17 2011.

Aluminium sulphate is used to clarify water during the treatment process.

The report by the Drinking Water Quality Regulator (DWQR) found the higher aluminium concentrations occurred because Scottish Water allowed the pH of the water to fall "significantly", dissolving any solid aluminium accumulations throughout the treatment system.

The report found there were "deficiencies" in the scientific evidence used to request a lifting of the restrictions and a "lack of rigour in a number of aspects of the management of the incident".

It found that aluminium levels "remained well above normal" even after the restrictions had been lifted.

It said: "On the basis of on-site tests undertaken overnight, Scottish Water contacted the Consultant in Public Health Medicine (CPHM) at NHS Greater Glasgow at 7am on March 19 to report that water quality had returned to normal.

"On this basis the CPHM agreed that the restrictions could be lifted, and Scottish Water did so without delay.

"Subsequently, results from laboratory analysed samples taken on the morning of March 19 contradicted the on-site test results and suggested that concentrations of aluminium remained well above normal".

Scottish Water also failed to spot the potential for an incident involving aluminium, which meant they were less able to respond quickly and reduce its impact.

There were also delays in telling consumers about the restrictions, and an inability to make accurate measurements of aluminium concentrations at the treatment works and in the distribution system.

But the regulator said there is "little indication that the substance is acutely toxic to humans" and the restrictions had been imposed as a precaution.

The DWQR had considered preparing a case for prosecution against the company for supplying water unfit for human consumption under Section 76C of the Water (Scotland) Act 1980, but decided not to proceed due to the lack of health-based evidence that the water supplied actually was unfit for human consumption.

A DWQR spokesman said: "This incident is extremely disappointing and should not have been allowed to occur.

"Although it is likely that any risk to health was minimal, a large number of people were greatly inconvenienced by the restrictions on water use that were put in place.

"We know Scottish Water can do much better than this and we are now looking to the company to make sure that it does whatever is necessary to prevent a repeat of this incident here or anywhere else in Scotland. We will be watching very closely to ensure this happens."

DWQR has made 17 recommendations to the company, including a review of the availability and scope of provision of comprehensive scientific support staff.

Margaret McGuinness, Scottish Water's public health manager, said: "Scottish Water apologises for the inconvenience caused to customers by the incident at Burncrooks Water Treatment Works in March 2011.

"We take our responsibility to provide high quality drinking water to our customers very seriously and, across Scotland, drinking water quality has never been higher, with 99.86% of samples in 2010-11 having met strict standards.

"Water quality incidents are rare but Scottish Water is committed to learning from the incident at Burncrooks.

"We carried out our own investigation into the incident and this identified a number of actions, including the installation and/or replacement of monitoring equipment. These actions have been completed.

"We will study the DWQR's report and recommendations and will do everything possible to reduce the risk of a similar event occurring at Burncrooks or elsewhere."