The investment at the Fulwell groundwater station, which provides drinking water to Sunderland and the surrounding areas, has received an overhaul, after work started six months ago.
The modernisation of Northumbrian Water's site - involving the installation of new filters, pipework and more efficient pumping arrangements - was carried out without any interruption to customers' water supplies.
Northumbrian Water's Project Manager, Paul Wood, said: "We're delighted to deliver this investment in partnership with our delivery partner, Interserve Construction Limited, and for the refurbishment work to be completed - while keeping customers' water supplies flowing.
"The investment will not only bring water quality improvements and ensure resilient water supplies for generations to come - but it will also benefit the environment by improving the efficiency of water treatment methods.
"Some customers may notice a slight change to their water once the source of their supply is switched back to being supplied from our Fulwell site.
"Any noticeable change is normal, as the raw water we treat and supply comes from different sources - like rivers, reservoirs and groundwater stations - so it can vary in hardness, and may taste or smell a little different from area to area across the North East.
"The water we provide complies with the strict regulations expected of us and we pride ourselves on providing clear, great tasting tap water."
The groundwater station in Fulwell plays an important part of the Sunderland and South Tyneside water supply network - with the site pumps planned to be switched on from Monday, September 7, 2020 - which is when the site will come back into water supply operation.
For frequently asked questions and information about water quality, customers can visit www.nwl.co.uk and to keep up to date with the latest project work www.nwlcommunityportal.co.uk