| Water system integration is one of the most efficient technologies for saving freshwater and reducing wastewater. A lot of corresponding strategies have been reported over the past two decades, including the water pinch technology and the internal water main technology. But up to now, most efforts focused on how to minimize the freshwater consumption for the water-using network with traditional structure. The network complexity and the system controllability were overlooked in most research. As to the water-using network with internal water mains, the design methodology is based on algorithmic rules and cannot guarantee the optimum of the final solution. In addition, how to take account of heat integration in the water-using network design is not reported for multi-contaminant water-using systems. In this paper, research covers water-using networks with different structures as following: For the water-using networks with traditional structure, the mathematical model is improved by adding another programming to optimize the network structure. With this procedure, the final design not only can attain the minimum or acceptable freshwater consumption, but also can feature a comparatively simple structure. Due to the less number of connections between water-using processes, the final network is more convenient to construct and need less expense. For the water-using networks with internal water mains, a general design methodology is proposed based on the established superstructure of the network. Firstly, the maximum number of internal water mains is specified to define the scale of the water-using system, and a mathematical model is established to minimize the freshwater consumption of the system. Secondly, with the freshwater consumption limit, another model is established to obtain the number of necessary internal water mains. The last model can be established optionally to obtain the minimum freshwater consumption with specified number of internal water mains. The final design features a less freshwater consumption compared with the design without water reuse. It also has a simpler structure than the optimal water-using network with traditional structure. This contributes to its high controllability. The water-using network with mixed structure is proposed in this paper to balance the freshwater consumption and the network complexity. In such a network, internal water mains as well as the connections between processes can both exist in the final design. Mathematical models are established based on the superstructure to optimize the system. Firstly, the minimum number of potential control for traditional network is calculated as the reference. Then the minimum freshwater consumption under specified number of potential control can be obtained. The last programming is introduced to simplify the network structure. Water-using networks with mixed structure can take account of the freshwater consumption, the network complexity and the system flexibility simultaneously and integrate the advantages of the traditional structure and the internal water main structure. For the heat efficient water-using network, a mathematical model with multiple objectives is built to attain the minimum fresh water consumption and the minimum utilities simultaneously. Three solving strategies are proposed to convert it into sequential models with single objective. The first strategy is utilized in systems dominated by freshwater consumption, and the main goal is to reduce the freshwater consumption to the minimum. The second strategy is applied to systems dominated by utilities, and the main goal is to optimize the utility duty to the minimum. In the third strategy, economical concerns are taken into account to balance the importance of freshwater and utilities. This method can be applied to a water using system with multiple contaminants. In heat integration, not only the process-to-process streams in the water using system, but also the streams out of the water using system can be taken into account. In the end, the design method for the... |
