The new facility will be sunk into the redundant Wellington Dock and retain some original features such as coping stones.
Together with the existing Sandon Dock plant, it will treat 11,000 litres of waste a second and serve a million people.
Mark Walker, United Utilities’ Principal Project Manager, said: “Our existing plant has played a key role in the clean up of the River Mersey and the new works will ensure we can continue to build upon that legacy.
“The expansion of Liverpool as a city down the years meant the current works were beginning to reach the end of their useful life. The new works will ensure we can continue to provide one of the city’s most essential services for many years to come.”