Optimal design for heat-integrated water-using and wastewater treatment networks

This work proposes a novel general superstructure and a simultaneous optimisation model for the designing of a heat-integrated water-using and wastewater treatment network (HIWTN) by combining a water-using network (WN), a wastewater treatment network (WTN), and a heat exchanger network (HEN). The proposed work is an extension of our previous studies that considered only heat-integrated water networks (HIWNs) or combined WN and HEN without WTN. The new proposed superstructure of this work combines water integration (water-usage, wastewater treatment, and recycling) and heat integration (direct and indirect heat exchanges) within an overall network. The simultaneous optimisation model of the proposed superstructure is formulated as a non-convex mixed integer non-linear programming (MINLP) problem for minimising the total annual network cost (TAC). This model enables appropriate trade-offs between freshwater usage, hot and cold utilities consumption, and capital cost of heat exchangers (HEs) and wastewater treatment units (TUs). Three literature examples are used to test the proposed model. The improved results of the first two examples are given whilst for the third modified example a novel network design is presented in order to include wastewater treatment.