A water authority has taken over ownership of four north-east waste water treatment sites.

Scottish Water is now in charge of the plants, which were run under a private finance initiative (PFI).

The Aberdeen PFI includes the sites at Nigg, Peterhead, Persley and Fraserburgh.

The Nigg plant has come under criticism over the last 17 years with residents in Torry complaining of a foul stench since the £63 million plant opened.

An independent expert was commissioned by Scottish Water to investigate the smell – branded the Nigg Niff – and £12m was invested in the plant in an effort to stem the stench.

A report by Professor Rob Jackson in 2015 found seawater had been allowed to enter at some points in the sewage network causing the foul odour.

A total of 46 staff across the four waste water treatment planst will continue to work on the sites following the change of ownership.

Scottish Water said the agreement will enable opportunities for the water authority to deliver value for customers.

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The plans will now share purchasing with Scottish Water’s waste water operations and optimisation of the works with waste water networks.

Scottish Water chief executive Douglas Millican said: “As part of our ongoing commitment to deliver value for our customers, I am pleased to have reached agreement with Kelda Water Services Ltd for the return of the Aberdeen PFI.

“Work is already under way to ensure that these facilities continue to provide a resilient and reliable waste water service to communities in the north-east for many years to come.”

Maureen Watt, MSP for Aberdeen South and North Kincardine said she was looking forward to Scottish Water being in charge.

She said: “With regard to the takeover of the wastewater treatment plants, this is welcome news.

“As chair of the Nigg Waste Water Treatment Works Liaison Group, I have been aware for some time of the uncertainly that the previous arrangement caused for Scottish Water and other stakeholders, particularly over the last year, and look forward to seeing the plants operate under their expertise.”