Based on graphical method, this paper proposed the necessary condition of zero liquid discharge (ZLD) and optimized the corresponding regenerated water flowrate and regeneration concentration for single contaminant water system with fixed mass load. As far as the water system without water loss is concerned, ZLD can be accomplished when the post-regeneration concentration is not higher than the minimum limiting inlet concentration of all the operations. Moreover, considering the water system with water loss, the formulations for calculating the optimal targets (the optimal regenerated water flowrate and the optimal regeneration concentration) for ZLD are deduced. The maximum post-regeneration concentration for ZLD is defined. If the post-regeneration concentration is not higher than it, ZLD is feasible. The optimal zero discharge water network is constructed with a given post-regeneration concentration. (c) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.