A Two-Step Optimization Approach for Retrofit of Water Network System

Many industrial plants have been using an enormous amount of water to run their
processes. The scarcities of water supply, increasing cost of providing freshwater as well
as treating wastewater, and the stricter environmental regulations have driven more
efficient water usage. In this regard, the most common way is the application of water
reuse, regeneration, and recycle (W3R concept). Even though the W3R concept is not
new, the systematic approach to select the optimum combination of the W3R is relatively
new. Thus, the latter has drawn more attention of many researchers.
Researchers have addressed common practical constraints such as forbidden/compulsory
piping connections, minimum permissible water flowrate, and geographical constraints in
developing their water network. However, most engineers challenge the researchers to
provide a tool that can be controlled manually (user-interactive tool) during the designing
process, especially in retrofit scenarios. This ability will be used to direct the solutions to
become more realistic and practical featuring achievable targets. The tool must also able
to produce multiple options simultaneously to give a large picture of what the solutions
are going to be when more modifications are taken. These challenges were missed by
previous works that are commonly based on graphical and automated mathematical
programming methods, especially when their grassroots methods are used in retrofit
designs.
A two-step optimization approach is presented in this work to provide a new procedure to
design multiple options of water network. The presented approach incorporates manual
control (user-interactive tool) over the optimization process to consider the
aforementioned practical constraints. The approach consists of structural targeting step
utilizing Mixed Integer Linear Programming (MILP) and parametric optimization step
using Non Linear Programming (NLP), which can be used to grassroots and retrofit
design of water network.
The considered water networks are water-using operations with water reuse, regeneration,
and recycle (W3Rs) concept. wastewater treatment systems, and total water systems.
Several water problems are solved and compared with their corresponding solutions obtained by previous work. The comparison shows that the presented approach can
provide the same optimum solutions for each water problems. The approach can also
provide multiple realistic solutions incorporated in "a class of good solutions" and the
ability to see large picture of the developments of water system design ("project road map
of achievable target and design"). These good solutions are to be further chosen by
engineers based on their considerations or their company preferences. In the retrofit case
study, one of the solutions has achieved a reduction of 59% of freshwater consumption.
The total annual cost for the solution of retrofit water network system is reduced until
29%.