Work at Whittle Dene Water Treatment Works (WTW) began at the end of 2016, while another project at Warkworth is set to start this month.
The work will contribute to a £15m investment to provide a better water quality supply to homes.
Phased across two years, the project at Whittle Dene includes the refurbishment of old tanks, installation of a new tank and apparatus and the upgrading of existing equipment.
Glyn Jenkins, project manager at Northumbrian Water, said: “We are focused on providing the best possible service to our customers.
“This investment will help us to deliver upon that aim.”
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