Water consumption and emission can be decreased by optimal allocation of water re-use, regeneration and re-use. regeneration and recycle in water consuming mass exchange networks and by applying optimally distributed effluent treatment processes in case of several emission sources of different contamination levels. Several example problems taken from the literature have been reproduced and solved by GAMS/MINOS/CONOPT package. In most cases our optimal solution is identical to those of others; in some other cases, however, we found better solutions. We also found our method more flexible than those based on targeting and conceptual design. Some example problems involve constraints and/or specifications of those kinds the referred methods cannot deal with. Namely, the constant mass load specification is substituted by either mass load proportional to the;water flow rate (involving constant concentration shift) or some other, even more realistic, non-linear functions. The optimal systems have different structures according to the applied mass load relations.