Water network integration is one of the most efficient technologies for energy saving and pollution reduction.During the past two decades,both the water pinch technology and the mathematical programming method have been frequently discussed and widely applied in the industry.With the rapid development of water network integration technology,some practical problems are becoming more and more popular.These problems include the optimization of regeneration reuse water networks with membrane units,the flexible design of multiple plant water networks and the optimal design of energy efficient water networks.This thesis focuses on the optimal solution of the single contaminant water allocation networks with the above mentioned characteristics,the main contents are as follows:1.A new approach for targeting minimum freshwater and minimum effluent treatment flow rate of a water allocation network is presented.By defining pinch point,the relationship between freshwater consumption and pinch concentration is established first.Then,a grand composite curve based on the concept of Wang and Smith's limiting composite curve is constructed firstly to determine the wastewater line with minimum flow rate.Subsequently,the wastewater line is modified for the case of regeneration-reuse.The minimum treatment target is obtained through a wastewater composite curve,which is generated from the grand composite curve.The proposed approach can be applied to the fixed flow rate as well as the fixed contaminant load problems with a single contaminant.Finally,the reuse,regeneration-reuse and wastewater treatment problems can be solved in a single concentration versus contaminant load diagram.2.A NLP model for the optimization of regeneration reuse water networks with membrane units is established.The necessary and sufficient conditions for the optimal solution of this problem are proved:(1) the regeneration concentration should be larger than the concentration of maximum mixing;(2) the regeneration inlet flow rate should be set at its maximum value corresponding to the concentration;(3) the pinch points of the system are set at particular regions.According to these necessary and sufficient conditions,a step-by-step algorithm to achieve the minimum freshwater usage is also proposed.3.A design methodology for flexible multiple plant water networks is introduced.By combining pinch insight with mathematical programming,target freshwater usage and cross plant interconnections(CPIs) conditions are obtained firstly without considering the detailed network design.Subsequently,an MILP model is established for the design of flexible water networks of individual plants.The proposed systematic approach can be applied for both fixed contaminant(FC) operations and fixed flowrate (FF) operations,limited to a single contaminant.Example from literature and a practical example of chlor-alkali complex are used to illustrate the applicability of the approach.4.An optimal design approach for energy efficient water networks is developed.A new kind of NLP model for utility targeting under△T_(min)=0℃is presented which can reduce the number of variables greatly.Moreover,an improved design approach considering the specified△T_(min) is generated.This approach,which treats the direct and indirect heat transfer separately,provides the information to construct the water utilization system as well as the corresponding heat exchanger network without any△T_(min) violation.5.The design of energy efficient water utilization systems allowing operation split is explored.Practical features such as operating flexibility and capital cost have made the number of sub operations an important parameter of the problem.By treating the direct and indirect heat transfers separately,target freshwater and energy consumption as well as the operation split conditions are first obtained.Subsequently,a mixed integer non-linear programming(MINLP) model is established for the design of water network and the heat exchanger network(HEN).The proposed systematic approach is limited to a single contaminant.Example from literature is used to illustrate the applicability of the approach.