Optimization And Application For The Synthesis Of Region-wide Water Resource Allocation Networks

Water is one of the most important natural resources. Population growth coupled with economic development has put increased pressure on water resource around the world. Industries consume plenty of fresh water and discharge large amount of wastewater. These urgent problems become more and more serious restricting social development as well as economic construction. According to the theory of circular economy, it is crucial for industries to develop circular economy and promote sound cycle of water resource utility. The strategy, which is essential requirement for carrying out scientific outlook on development and constructing harmonious socialist society, has very important practical significance to promote sustainable development of society, economy and environment. Base on the theory of water circular economy, the thesis carry out research on the techniques and methods to minimize fresh water consumption and wastewater generation.The thesis introduces the present status of the water resource utilization in China. Through supply and demand investigation of water resources in China, the thesis analyzes the problems existing in water utilization, points out the value of the research on techniques and methods to minimize freshwater consume and wastewater discharge. After summarizing the roadmap for development of water resource allocation network, the development of research in water targeting and network design techniques using pinch analysis and mathematical programming are chiefly reviewed. Using mathematical programming method, the thesis carries out the research on techniques for optimization and synthesis of in-plant, inter-plant and region-wide water resource allocation networks. The main contents of this thesis are as follows:1. Study on the mathematical model for the synthesis of in-plant water allocation network.Minimization of freshwater consumption and wastewater discharge should first be achieved within individual plants. In order to solve the problems of water resource utilization in Chinese plants such as low reuse rate, lack of cascade use, serious water wasting, the thesis proposes superstructures that incorporate all feasible design alternatives for wastewater reuse, regeneration reuse and regeneration recycle. The superstructures are formulated into mixed inter non-linear programming models. The objective of the model is to minimize the total annual cost, which is crucial to the plant. The total annual cost of a water system consists of fresh water cost, effluent treatment cost, network construction and operating cost, energy cost, etc. The return on investment of networks construction is integrated into the model as topological constraints, making the model more in line with actual production. The models have the ability to optimize in-plant water resource allocation networks, solving the problems mentioned before. Case study 3-1 is used to proof the capacity of the models. The results are identical to the solutions of literatures. The case study 3-2 illustrates the application of the model to design the water resource allocation network in a Chinese pulp and paper mill. The results show that fresh water consumption is reduced by 25% and wastewater discharge is reduced by 27%.2. Study on inter-plant water resource allocation network optimization.Once the maximum potential for water resource conservation within independent company has bee reached, further improvements are only possible through symbiotic programs among companies. In order to optimize the inter-plant water allocation network, the thesis proposes direct and indirect superstructures for inter-plant water resource allocation networks optimization. In the direct integration scheme, water from different networks is integrated directly via cross-plant pipeline(s). In the indirect integration scheme, water from different networks is integrated indirectly via centralized water mains or regeneration units, which serve to collect and redistribute water to the individual plants. For the two inter-plant integration schemes, the superstructures are formulated into mixed integer nonlinear program (MINLP), which objective is to minimize the total annual cost. The results of case study 4-1 are identical to the solutions of literatures on the same limitation of interconnections. The case study 4-2 illustrates the application of the model to design inter-plant water resource allocation network between an alumina plant and a steam power plant, which belong to an alumina production company. Compared to the base case that the two plants are designed separately without exploiting inter-plant integration opportunities, the result for inter-plant water system integration corresponds to a 13.2% reduction in freshwater consumption and there is no wastewater discharge.3. Study on the multi-objective optimization of region-wide water resource allocation network.For a region, it is important to take into account the environmental performance of water resource allocation network construction and operation in order to be in line with circular economy and sustainable development. Traditional process design and optimization often pay much attention to the economic profits, such as fixed capital investment, operation cost, and pay back period. The environmental impacts of process design have been given a lower priority. However, in all life cycle stages of water allocation network, environmental impacts, which incurred during construction, operating stages, can't be ignored. Therefore, it is essential to take into account tradeoffs among the environmental impacts and annual costs, i.e., environmental and economic perspectives should be incorporated into the process design and optimization of regional water allocation network simultaneously.In order to optimize the regional water resource allocation network, multi-objective optimization method has been studied to minimize a total annualized cost and environmental impacts. The concept of life cycle assessment (LCA) is integrated into the objective function of the model to evaluate the environmental effect scores of potential environmental impacts incurred during the life cycle. A generalized superstructure model is used to develop the mathematical optimization model for an environmentally friendly water allocation network. The mathematical model treats environmental impacts as one objective together with annualized cost as another objective. The trade-off between the two objectives can make water allocation network minimize freshwater consumption and wastewater discharge while achieving simultaneous minimization of environmental impacts incurred during the network life cycle and annualized cost. Case study 5-1 demonstrates the effect of the multi-objective model on the configuration and environmental performance of the water allocation network and validates the mathematical optimization model. Case study 5-2 illustrates the application of the model to design inter-plant water resource allocation network in a development zone. The result corresponds to freshwater consumption and wastewater discharge reduction by 22% and 75.8%, respectively.4. Study on application of water allocation network optimization techniques and methods in an industrial park area.Compared to the studies on the theory of water allocation network optimization which are current research focuses, few case studies for inter-plant water system integration using mathematical programming have been reported. In the thesis, a case study for water allocation network optimization of an industrial park area, which consists of six plants, is undertaken with an aim to reduce freshwater flowrates and consequently the wastewater flowrate. A detailed survey of current water and wastewater streams and other options for water minimization such as contaminants, temperature was carried out. The problem is identified as a multi-contaminant, reuse and recycle problem. Firstly, the water allocation networks of the six plants are optimized using the model developed in Chapter 3 one by one without taking account of inter-plant water integration opportunities. The results show that the reductions of combined freshwater consumption and wastewater discharge are of the tune of 5.94% and 18.64%, respectively. Secondly, inter-plant water allocation network integration among the six plants is carried out for further water conservation. Two central mains for cooling water and condensed water storage are placed among the six plants when inter-plant integration is considered. Compared with the in-plant water network optimization design, the result for inter-plant integration corresponds to a further 9% reduction in overall fresh water requirement and 70% in combined wastewater discharge. Lastly, region-wide water resources allocation network is integrated and optimized using the multi-objective optimization model proposed in the thesis. Water resources, water demand systems (including drinking use, industrial use, agricultural use, etc.) and wastewater treatment plant are integrated into a system, taking into account recycling of treated wastewater, to improve the sustainability of regional water system. The results show that optimization of the water allocation network of the region where the company located has the further potential to decrease freshwater consumption and wastewater discharge, reducing the total volume of freshwater used by 2.5% and wastewater discharged by 41.3%.