20 - Approaches to water network design

Abstract

In view of costly nature of water treatment processes, minimization of freshwater usage as well as wastewater generation in process systems is of great environmental and economic importance. Significant effects are achieved by optimization of water network, which is a network involving both water-using processes and regeneration/treatment units. Ultimately, zero discharge, i.e., closed water circuit, can be reached. The means to reach substantial reduction of freshwater consumption by water network optimization are formulated as: (1) water reuse in water-using processes and (2) application of regeneration processes by using redistributed water treatment. Traditional water network is illustrated where freshwater is supplied to processes in parallel with central water treatment station. Example of water reuse network is given. The concept of water network in a single industrial site is extended to industrial complexes of industrial parks and urban water networks. Mathematical models are formulated for water network problem. An overview of approaches in the literature of water network design is presented. The approaches are classified into two broad groups: (1) insight-based methods and (2) optimization-based methods. Similarly to HEN design methods, existing approaches to water networks include pinch concept as the most useful. The water pinch targeting method for water-using processes of mass transfer type are described by a simple example. The wastewater composite curve plot gives the minimum treatment flow rate and concentrations of the contaminant. It also shows the designer which streams require treatment and which streams can be left untreated. Examples are presented that concern design of water network with reuse and regeneration. They are able to cope with various goal functions, multiple contaminants, and various technological constraints.