Dual-objective optimization of integrated water-wastewater networks (original) (raw)
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Energy savings vs.freshwater consumption when optimizing total wastewater networks
Chemical Engineering Transactions, 2011
The dual-objective optimization of an integrated water/wastewater network (IWWN) is addressed in this paper, by targeting for simultaneous minimum fresh water consumption and investment and operating costs reduction of the pipeline system; the latter is a main component in energy savings opportunities. An IWWN is a recycle system composed of two oriented graphs, the first encoding the water-using units (WUs) and the second, the treatment units (TUs). Internal wastewater recycling is forbidden ab initio for the WUs graph, external recycles from the proper TU to the WU whose inlet restrictions are fulfilled by the partially treated water are favoured. In order to encourage this kind of recycling the critical contaminants for each wastewater internal flow are identified and assigned to the local regeneration unit, as a better alternative to complete decontamination. The corresponding mathematical model was written and assessed on a synthetic example. A thorough comparison is made highlighting the differences between the network's performances with respect to the features aforementioned for some points of the Pareto front.
Water Network Optimization with Wastewater Regeneration Models
Industrial & Engineering Chemistry Research, 2014
The conventional water network synthesis approach greatly simplifies wastewater treatment units by using fixed recoveries, creating a gap for their applicability to industrial processes. This work describes a unifying approach combining various technologies capable of removing all the major types of contaminants through the use of more realistic models. The following improvements are made over the typical superstructure-based water network models. First, unit-specific shortcut models are developed in place of the fixed contaminant removal model to describe contaminant mass transfer in wastewater treatment units. Shortcut wastewater treatment cost functions are also incorporated into the model. In addition, uncertainty in mass load of contaminant is considered to account for the range of operating conditions. Furthermore, the superstructure is modified to accommodate realistic potential structures. We present a modified Lagrangean-based decomposition algorithm in order to solve the resulting nonconvex Mixed-integer Nonlinear Programming (MINLP) problem efficiently. Several examples are presented to illustrate the effectiveness and limitations of the algorithm for obtaining the global optimal solutions. 1 Introduction With increasing costs, diminishing quality of supplies, and stricter environmental effluent standards set forth by the Environmental Protection Agency (EPA), water is playing an increasingly important role in the process industries. The primary water uses are process water, cooling water, and boiler feed water, with each use being emphasized by different industries. For example, the chemicals, petroleum refining, and metal sectors primarily use water for cooling, while paper and pulp and food processing mostly use water for process use. In a study by Carbon Disclosure Projects of 137 companies with total assets over $16 trillion, it has been reported that water has risen high on the corporate agenda[1]. Eighty nine percent of responding companies have developed specific water policies, strategies, and plans. Specifically, in the chemical sector, all ten companies surveyed recognize that there is a high growth potential for processes and products that support more efficient water use and water recycling. Consequently, it is essential to incorporate reuse schemes at the process design level for optimal water use. 1
General Superstructure and Global Optimization for the Design of Integrated Process Water Networks
egon.cheme.cmu.edu
In this paper, we propose a general superstructure and a model for the global optimization for the design of integrated process water networks. The superstructure consists of multiple sources of water, water-using processes, wastewater treatment and pre-treatment operations. The unique features are first, that all feasible interconnections are considered between them, including water re-use, water regeneration and re-use, water regeneration recycling, local recycling around process and treatment units. Second, multiple sources of water of different quality that can be used in the various operations are included. Third, the superstructure incorporates both mass transfer and non-mass transfer operations. The proposed model of the integrated water network is formulated as a Nonlinear Programming (NLP) and as a Mixed Integer Nonlinear Programming (MINLP) problem for the case when 0-1 variables are included to model the cost of piping and/or selection of technologies for treatment. The MINLP model can be used to find optimal network designs with different number of streams in the piping network. In this work, we propose to represent the bounds on the variables as general equations obtained by physical inspection of the superstructure and using logic specifications needed for solving the model. We also incorporate the cut proposed by Karuppiah and Grossmann (2006) to significantly improve the strength of the lower bound for the global optimum. The proposed model is tested on the several illustrative examples, including large-scale problems.
Bi-objective optimization of a water network via benchmarking
Journal of Cleaner Production, 2013
This paper presents an approach to water system retrofitting by estimating both the economic and environmental impacts of a water network structure, using bi-objective optimization. The environmental impact is evaluated via benchmarking. By using benchmarking, the decision maker can have an insight, not only into the environmental impacts of certain designs belonging to Pareto optimal solutions, but also into the competitiveness of the design within a particular production sector. The economic criterion used is the total cost of the water network involving the freshwater cost, wastewater treatment cost, and the annual investment costs of storage tank, piping, and local treatment unit installation. A mixedinteger nonlinear programming (MINLP) model is used for water re-use and regeneration re-use within batch and semi-continuous processes. The Pareto fronts are generated using the classic and adaptive weighted-sum methods. The proposed approach was applied to an industrial case study within a brewery. The results obtained show that the benchmark could not be reached by process integration within the packaging area, therefore investment is needed regarding new technologies that lower freshwater consumption. Within the production area, however, the freshwater consumption could be reduced below the benchmark by water re-use and regeneration re-use, meaning that the brewery could achieve better performance than its competitors.
Multiobjective Optimization of Water-Using Networks with Multiple Contaminants
Industrial & Engineering Chemistry Research, 2011
A systematic multiobjective optimization procedure is developed in this study to produce realistic multicontaminant water-using network designs. In every given design problem, the most appropriate structure is identified by solving three mathematical programming models (that is, one nonlinear program and two mixed-integer nonlinear programs) sequentially so as to satisfy different criteria in order of decreasing importance. Freshwater conservation is given the top priority in the present work because of its scarcity in environment and the pollution problems caused by wastewater effluents. Minimization of the total number of interconnections in the water network is treated as the next design goal since network complexity is directly related to operability, controllability and safety. The final step in the sequential procedure is to minimize the total throughput and, consequently, the operating and capital costs of all water using units as well. Five examples are presented in this paper to demonstrate the effectiveness of proposed procedure. When compared with the network designs obtained with available methods reported in literature, it can be observed that better solutions can usually be generated with our approach.
Integrated Optimization of a Dual Quality Water and Wastewater System
Journal of Water Resources Planning and Management, 2010
When addressing urban water problems, it is no longer adequate to consider issues of water supply, demand, disposal, and reuse independently. Innovative water management strategies and opportunities for water reuse can only be properly evaluated in the context of their interactions with the broader water system. An integrated linear deterministic optimization model is applied to Beirut, Lebanon, to determine the minimum cost configuration of future water supply, wastewater disposal, and reuse options for a semiarid coastal city. Previous urban water system optimization models considered only a single quality of potable water and were thus unable to demonstrate the cost-effectiveness of reclaimed water among all viable options for water supply. Two innovations of our work include incorporation of the entire anthropogenic water cycle including interconnections between supply, demand, disposal, and reuse and modeling of the suitability of nonpotable and potable qualities of water for each demand sector. The optimization model yields surprising insights. For example, after full use of inexpensive conventional sources, nonpotable direct reuse appears to be Beirut's most costeffective option for supply of its urban nonpotable and irrigation demands. Our work highlights the importance of modeling the utility of multiple qualities of water in modern water supply planning. Downloaded 19 Jan 2010 to 130.64.2.235. Redistribution subject to ASCE license or copyright; see http://pubs.asce.org/copyright Fraction of water from each sector, u, of each grade, gu, that can be recycled in each season, ts fcl u,gu,ts Fraction of each grade, gu, of water consumed at each use sector, u, in each season, ts cs s,gs,u,gu Cost to supply water from each grade, gs, of each source, s, to each grade, gu, of each use sector, u ͑$ / m 3 ͒ cw u,gu,w Cost to dispose of wastewater from each grade, gu, of each use, u, at each waste site, w ͑$ / m 3 ͒ cr u,gu,u,gu Cost to recycle water of each grade, gu, from each sector, u, to each grade, gu, of itself, u ͑$ / m 3 ͒ cx s Cost to augment conventional water sources, s, with water from CWWTP ͑$ / m 3 ͒ Variables (capital letters) Qs s,gs,u,gu,ts Flow from each grade, gs, of each source, s, to each grade, gu, of each use sector, u, in each season, ts ͑m 3 / day͒ Qw u,gu,w,ts Wastewater flow from each grade, gu, of each use sector, u, to each waste site, w, in each season, ts ͑m 3 / day͒ Qr u,gu,u,gu,ts Recycle flow from each grade, gu, of each use sector, u, to each grade, gu, of itself, u, in each season, ts ͑m 3 / day͒ Qx s,ts Water treated by WWTP to augment conventional water sources, s, in each season, ts ͑m 3 / day͒ Qs max s,gs Maximum capacity of each grade, gs, of each proposed supply source, s ͑m 3 / day͒ Qw max w Maximum capacity of proposed new secondary wastewater treatment facility, w ͑m 3 / day͒ Qr max u,gu Maximum capacity of each grade, gu, of all on-site treatment facilities in each use sector, u ͑m 3 / day͒ JOURNAL OF WATER RESOURCES PLANNING AND MANAGEMENT © ASCE / JANUARY/FEBRUARY 2010 / 41
Optimization of Inter-Plant Water Network Design Involving Multiple Contaminants
IOP Conference Series: Materials Science and Engineering, 2020
Minimization of freshwater consumption and wastewater generation are being critical concerns in the process industry due to the scarcity in freshwater supply, the increase of freshwater and effluent treatment costs and stricter regulations. One of the efficient ways to reduce freshwater consumption in the process is by recycling and/or reusing wastewater that is generated by the process or utility after being treated to acceptable limits. This work presents the development of a systematic approach for synthesizing indirect inter-plant water network integration with centralized regeneration system involving multiple contaminants. In this approach, water reuse prospects were analysed within individual plants and between different plants via inter-plant water integration possibilities. The water network problem is formulated as a mixed integer nonlinear programming (MINLP) based on the water network superstructure and is solved using GAMS optimization software. The applicability of the...
Industrial & Engineering Chemistry Research, 2005
A design procedure to generate practical structures for the water-usage and-treatment networks is presented in this paper. The optimization strategies used in the proposed procedure are developed on the basis of a modified version of the existing nonlinear programming model. In particular, a systematic method is used to incorporate additional design options and a fixed number of repeated treatment units into the superstructure. Also, to account for the possible existence of unrecoverable solutes, the inequality constraints on their concentrations are added in the revised model formulation. To enhance convergence efficiency, a reliable method is developed in this work to produce a good initial guess. The advantages of this initialization technique are demonstrated with several examples adopted from the literature. Finally, several useful solution techniques to manipulate the structural properties of water networks are provided at the end of this paper. The effectiveness of our approach for creating favorable network structures is shown in the results of case studies.