The work at Warkworth and at Whittle Dene, near Horsley, will help to ensure the highest possible water quality for more than 290,000 homes across Northumberland.
Phased across two years, the upgrades at Whittle Dene WTW started at the end of 2016 and the work at Warkworth WTW commences this month.
At Whittle Dene, the project includes the refurbishment of old tanks, installation of a new tank and apparatus as well as the upgrade of existing equipment.
The work at Warkworth will see upgrades to tanker facilities and water treatment equipment, the introduction of new security fencing and CCTV, and the installation of new screens in the river intake, which serves the site.
Northumbrian Water partners Mott MacDonald Bentley (MMB) and Interserve Construction are carrying out the work.
Glyn Jenkins, project manager at Northumbrian Water, said: “We are focused on providing the best possible experience and service to our customers. This investment at our water treatment works at Warkworth and Whittle Dene will help us to deliver upon that aim.”