The optimization of an integrated network of water-using (WU) and treatment (TU) units is addressed in this paper, by targeting for maximum treated water reuse as a considerably better alternative to fresh water consumption. An optimal integrated water network (WN) is an oriented graph, starting with inlet contaminant free WUs and then the other WUs ranked by a certain criteria. The outlet wastewater streams are either split to be reused by the next WUs in sequence or sent to treatment. The mathematical model of the integrated WN is based upon total and contaminant species mass balances for each and every WU/TU which is restricted with respect to the inlet and outlet contaminant concentrations. The performance of the approach is tested on a synthetic example. Several scenarios were used, the results being analyzed in connection with the reduction of fresh water consumption and the increase of internal and treated water reuse.