Reliable method for water reuse in multicontaminant batch plants: NIS policy

This paper presents a mathematical technique for minimisation of freshwater and wastewater in operations characterised by multiple contaminants. The minimisation of wastewater is achieved through the exploitation of recycle and reuse opportunities using a superstructure that entails all possible recycle and reuse possibilities. This eventually forms the basis for the mathematical formulation. The existence of an optimal production schedule, which provides starting and finishing times for water using operations, is not necessary in this formulation. The paper addresses the case where central reusable water storage is nonexistent, which implies that recycle and reuse opportunities are effected by direct stream routing. The contaminants in any given water stream need not be limiting at the same time which is characteristic of practical situations. This condition enforces the overall model to be cast as a mixed integer nonlinear programming (MINLP) problem which global optimality cannot be guaranteed for complex problems. A solution procedure to overcome structural problems and guarantee global optimality is also presented.