An Integrated Approach for Industrial Water Systems Optimal Design G. F. Porzio, E. Alcamisi, I. Matino, V. Colla Scuola Superiore Sant’Anna, TeCIP Institute Via Alamanni 13D, 56010 Ghezzano, Pisa, Italy, g.porzio@sssup.it ABSTRACT An efficient exploitation of resources is fundamental in modern industries. Among these, an essential one is water. Two approaches have traditionally been adopted for the minimization of freshwater consumption, trying to maintain the compliance with environmental regulations (which are becoming more and more stringent) on wastewater discharge. The first approach is based on water pinch technology and is aimed at optimal water network design or retrofit analysis; the second one is aimed at optimizing the operating conditions of existing processes without changes in the network design. Both generally aim at minimizing freshwater consumption or total cost. This paper presents a possibility for integration of the two approaches, together with two case studies intended to assess the feasibility of such integration through simulation of a water network. Results of the case studies show an interesting potential for freshwater saving. Keywords: process integration, wastewater treatment, industrial water systems, water systems optimization 1 INTRODUCTION Minimization of freshwater consumption and wastewater generation are fundamental issues in industrial water systems. Such a problem has traditionally been tackled through Process Integration (PI) techniques such as water pinch analysis [1]. In such an approach, the limiting water profile defines the boundary between feasible and unfeasible water using regions. If more operations are involved in a process, the combination of different limiting water profiles can bring to the design of a process limiting composite curve [2]. By analyzing such a curve a water network can be designed that targets the maximum reuse of water and therefore the minimum freshwater consumption. From a computational point of view, the problem of water network design aimed at minimization of freshwater consumption can be solved through optimization of a superstructure in which all the possible connections between water using operations and wastewater treatment are assessed exploiting traditional or advanced optimization algorithms [3]. A drawback of such an approach lies in the fact that, even in its most recent evolutions [4], it does not take into account the influence of the water streams properties, or of the process operating conditions, on the process performances: only very simplified models are considered for optimal water network design. A second approach consists in the search of an improved management of industrial water system performances by considering optimal operating conditions of water using operations and wastewater treatment processes, assessed through exploitation of more detailed process models. In general, such an approach considers the connections between different operations and treatment as fixed: the existing plant layout is taken as a datum of the problem and the optimization study only considers variations in the operating conditions. This approach, that does not allow to carry out a water pinch analysis in order to assess the best design configuration, is less indicated for new design or system retrofit studies. The present article describes a possible extension and combination of the above described approaches, which is currently under investigation in a research project funded by the European Union. Section 2 presents the methodology that is being pursued within the project; in Sec. 3 a preliminary example describing different alternatives for the minimization of freshwater consumption in an integrated steelwork is presented, while final conclusions are drawn in Sec. 4. 2 METHOD The two approaches described in the introduction are graphically presented in Figure 1a. An evolution of the two approaches resides in their combination, considering both the optimal network design and the influence of operating conditions onto the treatment process performances. Such a combined approach is currently under investigation and development in an ongoing EU-funded Research Fund for Coal and Steel (RFCS) research project; a schematic representation is shown in Figure 1b. In order to develop such an integrated approach, a library of water using operations and wastewater treatment models has been created, which includes a set of common industrial processes. Among the modeled processes there are clarification, ammonia stripping, reverse osmosis-based desalination, gas scrubbing and biological oxidation (activated sludge). A detailed description of the models is beyond the scopes of the present paper; however, they have been developed exploiting available literature as well as process knowledge and data coming from the industrial partners of the project. For instance, the clarifier model has been derived from the description given in [5], and provides as output the removal efficiency for a specific contaminant (e.g. total suspended solids) as a function of the particle settling velocity, calculated through application of the Stokes’ sedimentation law. The influence of the CTSI-Cleantech 2014, www.ct-si.org, ISBN 978-1-4822-5819-6 216