The paper addresses some important results as to optimal design of water usage networks. The aim was to minimize freshwater consumption by water re-use at minimum total flow-rate of water streams among processes and small number of process interconnections. The design approach was based on mixed-integer nonlinear multi-objective programming problem of superstructure optimization. The superstructure includes all possible structures and consists of mass balances of mixers, splitters, water users and partial treatment processes. The general solution algorithm of optimization was outlined. The optimal network design procedure is described. The examples presented show that the approach is robust technique for the synthesis of water usage networks.