Welsh Water says customers may notice a change in the taste and smell of tap water while a £14m development at a treatment works is under way.
The company said it is increasing its disinfection process using chlorine to ensure water quality while the upgrade continues.
A 20-year-old filtration system at Alwen works, near Cerrigydrudion, is being replaced.
The works serve customers in Conwy, Denbighshire and Flintshire.
The upgrade started on the site in the Clocaenog Forest in November 2009, two years earlier than planned, after "evidence that the old filtration system was not working effectively".
Precautions were introduced for a short time after the discovery of bacteria in water supplies serving 70,000 homes.
Then, people across Conwy, Denbighshire and Flintshire, were advised to boil their tap water. No one was affected.
In a letter to customers, managing director Nigel Annett said: "This disinfection boost will make sure that there is no risk to the quality of your tap water while we carry out commissioning of the new treatment works."
The work is expected to be completed by the summer.