Energy Savings vs. Freshwater Consumption when Optimizing Total Wastewater Networks Raluca Tudor*, Vasile Lavric University Politehnica of Bucharest, Chemical Engineering Department; RO-011061, Polizu 1-7, Bucharest, r_tudor@chim.upb.ro The dual-objective optimization of an integrated water/wastewater network (IWWN) is addressed in this paper, by targeting for simultaneous minimum fresh water consump- tion and investment and operating costs reduction of the pipeline system; the latter is a main component in energy savings opportunities. An IWWN is a recycle system com- posed of two oriented graphs, the first encoding the water-using units (WUs) and the second, the treatment units (TUs). Internal wastewater recycling is forbidden ab initio for the WUs graph, external recycles from the proper TU to the WU whose inlet restric- tions are fulfilled by the partially treated water are favoured. In order to encourage this kind of recycling the critical contaminants for each wastewater internal flow are identi- fied and assigned to the local regeneration unit, as a better alternative to complete de- contamination. The corresponding mathematical model was written and assessed on a synthetic example. A thorough comparison is made highlighting the differences be- tween the network’s performances with respect to the features aforementioned for some points of the Pareto front. Keywords: Integrated wastewater network, Fresh water minimization, Costs optimiza- tion, Regeneration, Multi-objective optimization, Pareto front 1. Introduction Process integration of total water/wastewater networks enables the efficient use of freshwater taking into consideration the opportunities for wastewater reuse either direct- ly or throughout treatment (end-pipe removal of contaminants to dispose the water in environment) or regeneration (partial removal of pollutants to increase local reuse). Since there are many ways of reusing, treating or regenerating wastewater to reuse it, thus decreasing freshwater consumption, the development of design targets for the min- imum freshwater demand is crucial to assess alternatives. In the implementation of wastewater reuse, the beneficiary WUs will govern the degree of wastewater treatment required. A high degree of treatment would allow more wastewater reuse, which would in turn reduce costs associated with freshwater consumption at the expense of increased treatment costs and network complexity, which raises investment and operating costs. The right balance of these costs and freshwater minimization to achieve an optimal net- work is the objective of this study, taking into account their dichotomic nature. Two main categories of methods are used to obtain good designs of IWWNs: pinch technology with all its derivatives and mathematical programming. The most recent and