The chemical has been added to the water supplied to Cornhow and Ennerdale water treatment works since the late 1960s.
However, the process was halted in April 2006 in Cornhow and in November 2011 in Ennerdale, to allow the treatment equipment to be upgraded.
Public Health England – which took over from the abolished North West Strategic Health Authority on April 1 – has announced that the service is expected to resume in the coming weeks.
United Utilities, which provides the service, has installed new equipment at both sites and extensive checks are now being made before it becomes fully operational.