An approximate mixed integer linear programming (MILP) model for the design of water reuse/recycle networks with minimum emergy

Recent trends in process engineering have placed increased emphasis on the design of inherently clean and efficient processes. For example, a wide range of pinch analysis and mathematical programming methods have been developed for designing schemes for water reuse/recycle in industrial plants for both grassroot design and plant retrofit. In the latter case, the conventional approach is to maximize water recovery and thereby minimize fresh water demand and effluent volume. However, it is possible that with such an approach the reductions in environmental impact brought about by saving water can be offset by other impacts arising from increased use of energy and materials in the plant after retrofit. This work presents a model for minimizing the total resource consumption impact of a water reuse/recycle network. The total impact is expressed in terms of emergy - a measure of cumulative solar energy inputs into a life cycle system. A simplified model is proposed that focuses on the impact contributions of water, electrical power and material for capital goods. Two case studies illustrate the approach. Results show that the network with the lowest total impact can be found by sacrificing water recovery for savings in energy and material use.