The water company is starting a major upgrade at its Altcar Lane works, with the project expected to take 10 months to complete. The work will be contained within the current site.
Four new screens - which act like a sieve on wastewater or sewage entering the plant - will be installed, alongside a new pumping station and associated works. The work will mean treated wastewater leaving the works and going into the Alt will be of a better quality, meeting strict European standards on levels of ammonia.
The River Alt begins in Huyton and runs through Formby before flowing into the Irish Sea at Hightown.
United Utilities project manager Mark Graham said:
"In the last 15 years the quality of the water in the River Alt has improved remarkably and it is now a popular fishing spot, particularly for Flounder.
"This work will further improve the water in the Alt, while also ensuring the works are equipped to meet anticipated future increases in demand from the local area."