The obtimisation of water networks in a deinking mill has been carried out using Mixed Integer Non Linear Programming. The objective function included the costs of water and chemical usage, and of piping retrofitting. The constraints were stream composition limits, forbidden connections and whitewater treatment unit capacities. Depending on whether paper machine whitewater clarification could be envisaged or not, the fresh water intake could be diminished by 4.9 or 4.3 m3/odt of pulp respectively.