Energy Savings vs. Freshwater Consumption when Optimizing Total Wastewater Networks

The dual-objective optimization of an integrated water/wastewater network (IWWN) is addressed in this paper, by targeting for simultaneous minimum fresh water consumption 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 composed 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 restrictions 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 identified and assigned to the local regeneration unit, as a better alternative to complete decontamination. The corresponding mathematical model was written and assessed on a synthetic example. A thorough comparison is made highlighting the differences between the network’s performances with respect to the features aforementioned for some points of the Pareto front.