Dual-objective optimization of integrated water/wastewater networks

The dual-objective optimization of an integrated water/wastewater network (IWWN) is addressed by targeting for minimum fresh water consumption at the same time with operating costs reduction. 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). Although internal recycles are forbidden ab initio for the WUs graph, external recycles from the appropriate TU to the WU whose inlet restrictions are met by the partially treated water are encouraged. The corresponding mathematical model was written. A synthetic example is proposed and analyzed under several scenarios with respect to the fresh water consumption, the magnitude of internal and treated water reuse and the investment/operating costs related to the active pipes network. A comparison is made regarding the differences in network topology and fresh water consumption implied by different points from the Pareto front (PF).

► Simultaneous water/wastewater and treatment networks optimization as two subsystems linked by external recycles.
► Concomitant freshwater consumption and costs minimization revealing the degeneracy of the integrated network.
► Optimal topology depends upon the ordering criterion of the WU and treatment subsystems, with respect to operating conditions.
► Local and overall mean availability analysis pointing out the critical contaminants and bottleneck islands.
► Pareto front revealing equally optimal solutions of internal/external flows accommodating the same pipe network topology.