Scottish Water has come under attack from a government safety watchdog because of the number of contamination problems and incidents doubling across the drinking water network in a year.

There had been a “deterioration throughout the water supply system” on bacterial quality, according to the Drinking Water Quality Regulator, Sue Petch. She has also launched legal enforcement action because of an “unacceptable” level of faecal parasites and a “very disappointing” number of incidents at one water treatment works in Aberdeenshire.

The regulator has released her report for 2014. It reveals there were 53 incidents across Scotland rated as significant, serious and major, some of them contaminating drinking water delivered to hundreds of consumers.

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Her report does not include the major incident last month when more than 6000 residents in North Lanarkshire were left without tap water for two days because of contamination by an “oily-based substance”. The cause of the incident is still being investigated.

Petch points out that water quality standards for coliform bacteria, used as an indicator of contamination, were breached 40 times in 2014, compared to 17 times in 2013. “I am disappointed to have to report this position and have requested that Scottish Water examine disinfection processes, in detail, at all of its treatment works,” she says.

Some water treatment works recorded multiple failures of the coliform standard, including Mannonfield in Aberdeen, Perth and Tobermory on the Isle of Mull. The E. coli bug, known to cause infections, was also found in single samples at Mannonfield and Perth.

Neither of the E. coli failures “could be explained by Scottish Water,” says Petch. She “will be monitoring performance very closely and if necessary will consider the need for regulatory intervention.”

She also expresses “great concern” about the discovery of a protozoan faecal parasite that can cause infections called cryptosporidium at 17 plants in 2014. The problem was particularly acute at Invercannie waste treatment works near Banchory in Aberdeenshire, where 17 failed samples were recorded.

“This is unacceptable,” says Petch, who has taken legal action against Scottish Water to force it to make improvements. Invercannie suffered nine further problems in 2014 with cryptosporidium, aluminium, ammonium, as well as with taste and odour of water.

Of the 53 incidents logged in 2014, 43 were classified as significant, eight as serious and two as major. They included 33 incidents at 20 water treatment works rated as significant or serious. In 2013 there were only 24 incidents in total.

Petch also highlights a disinfection failure at Lochenkit water treatment works in Dumfries and Galloway following a power cut in June 2014. “Undisinfected water was supplied to the public for over nine hours,” she says.

At Rosemarkie near Fortrose on the Moray Firth, 550 residents had to boil their water for two days in May 2014 after a botched repair. At Savalbeg at Lairg in Sutherland there were breaches in aluminium standards.

Scottish Water says that it has already upgraded equipment at Invercannie. “We have a thorough and comprehensive sampling programme,” says the public company’s customer planning director, Simon Parsons.

“We undertake detailed investigations into all water quality issues to understand the cause – as well as working proactively and taking steps to avoid such issues occurring. We will continue working with the [Regulator] to help us focus on the areas we need to improve on.”

Parsons points out that 99.89 per cent of all samples taken at customers’ taps in 2014 met strict regulatory standards. “This means the quality of drinking water received by our customers has never been higher,” he says.

“In the next six years, we will invest more than £300 million to ensure everyone in Scotland enjoys the look and taste of their tap water. This includes building increased resilience and reliability into our service.”

The Scottish Government accepts that “there is no room for complacency on water quality issues”. But it is confident that Scottish Water’s planned investments will deliver significant improvements.