This work presents the water cascade analysis (WCA) as a new

Water network synthesis has been an active area of research for the past one and a half decades. Many think that the technology reached a mature stage in the late 1990s, especially for the insight-based technique based on pinch analysis. The only review for the field dates back to 2000. However, many new papers published in this century reveal that new research gaps are found and more works were carried out to address the limitations of the "old" techniques. The main objective of this review is to provide a state-of-the-art overview of the insight-based techniques developed in the 21st century, particularly those developed for single impurity network of the fixed flow rate problems. Comparisons were also made between these recent techniques and those developed for the fixed load problems in the past century. Various flow rate targeting techniques developed for water reuse/recycle, regeneration, and wastewater treatment are reviewed in detail, along with the network design techniques that achieve the established targets. Finally, future research directions are outlined at the end of the review.

This work describes the gas cascade analysis (GCA) as an extension to cascade analysis technique for targeting the minimum flowrates for water and property-based networks. The GCA technique enables quick and accurate identification of the minimum flowrate targets, pinch-point location(s), and resource allocation targets for a utility gas network. Multiple-pinch problems and appropriate selection of gas purification techniques were systematically assessed via the GCA. The GCA technique was successfully used to determine the minimum flowrate targets for nitrogen, oxygen, and hydrogen utility gas networks prior to detailed design.

This pair of articles presents an optimization-based, automated procedure to determine the minimum resource consumption/target(s) for a single-impurity resource conservation network (RCN). This optimization-based targeting technique provides the same benefits as conventional insight-based pinch analysis, in yielding various targets for an RCN prior to detailed design. In addition, flexibility in setting the objective function is the major advantage of the automated targeting approach over a conventional pinch analysis technique. The model formulation is linear, which ensures that a global optimum can be found if one exists. In part 1 of this pair of articles, the model for direct material reuse/recycle is presented. Its application is then demonstrated for single, multiple, and impure external resources using several literature examples. Part 2 of this pair of articles extends the automated targeting technique for RCNs with waste-interception (regeneration) placement.

Part 1 of this pair of articles presents an automated targeting technique to identify minimum fresh resource flow rate/cost targets in a resource conservation network (RCN) with material reuse/recycle. After the potential for conservation through direct reuse/recycle is exhausted, fresh resource consumption can be further reduced by incorporating waste-interception (regeneration) processes. Hence, the proposed automated targeting technique in part 1 of this pair of articles is extended to determine the targets for RCNs with interception placement. The waste-interception systems are modeled as treatment processes with either fixed outlet concentrations or fixed impurity load removal ratios. The approach also distinguishes between single-pass and partitioning regenerators, which have different implications for RCNs. Literature examples and industrial cases are solved to illustrate the proposed approach.

To date, most work on water network synthesis has been focusing on a single water network. The increase of public awareness toward industrial ecology has inspired new research into interplant water integration (IPWI). In this context, each water network may be grouped according to the geographical location of the water-using processes or as different plants operated by different business entities. Water source(s) from one network may be reused/recycled to sink(s) in another network. In this work, two different IPWI schemes, that is, "direct" and "indirect" integration are analyzed using mathematical optimization techniques. In the former, water from different networks is integrated directly via cross-plant pipeline(s). A mixed integer linear program (MILP) model is formulated and solved to achieve a globally optimal solution. In the latter, water from different networks is integrated indirectly via a centralized utility hub. The centralized utility hub serves to collect and redistribute water to the individual plants, and may even function as a shared water regeneration unit. For the indirect integration scheme, a mixed integer nonlinear program (MINLP) is formulated and solved using a relaxation linearization technique to obtain an optimal solution.

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Part 1 of this series of papers presented graphical and algebraic approaches that are used to identify waste streams in a total water network. In this part of the series, the interaction between waste treatment and water regeneration is explored. Appropriate selection of waste streams for regeneration or waste treatment will lead to the minimum impurity load to be processed in the regeneration and waste treatment units; hence, leads to the reduction of the network capital and operation costs. In addition, a novel wastewater composite curve for targeting the minimum impurity load removal is presented in this work. Targeting for the minimum treatment flow rate and the minimum number of treatment units is presented for the treatment system of the fixed outlet concentration and removal ratio type, respectively. Literature examples are solved to illustrate the proposed approaches.