System Analysis And Optimization For Wastewater Reclamation And Reuse In Northwest Typical Cities With Water Shortage

Wastewater reclamation and reuse (WARR) has been widely accepted as an effective approach to solve various problems relating to water shortage and pollution. However, there are many obstacles in the large-scale usage of reclaimed water in China, especially in less developed Northwest Area. In addition to technical problems, there exist some important society factors such as poor planning, misplacement of funds, people's misunderstanding and unreasonable pricing in WARR system. Based on a key project funded by the National Science Foundation of China, this thesis developed a method integrating system dynamics model(SD) with nonlinear programming model (NLP) for WARR planning in cities with water-shortage problems in Northwest Area.Main results obtained were as following:1) A SD-NLP model for WARR systems was proposed based on characteristics of northwest water-deficient cities, and the Eco-Environment was considered as a main factor in this model.2) A cost analysis model was established based on customer designation and Project Types;a minimal economical scale parameter was derived for on-site WARR projects based on the function of willingness to pay.3) A critical distance concept was introduced and used as economic criterion for setting up either central or decentralized on-site wastewater treatment facilities. A critical distance model was also developed.4) A NLP model was developed and applied in the planning of WARR plants and optimal distribution of reclaimed wastewater;and it could be used to derive input parameters for the SD model.5) In light of the characteristics of wastewater reuse in northwest area, and based on the analyses of the internal detailed structure of the WARR systems, a SD model was set up by analyzing the system functional structure and relationship among system variables, and situation analysis method was applied in SD model as a tool of policy analysis.6) The SD-NLP model was applied to evaluate the planning and trend of WARR system in the city of Xi'an. Based on the requirement of community population and land, as well as reasonable distribution of users were determined for decentralized WARR system, the NLPmodel was used to determine the increase rates of WARR facilities and the ratio between central and decentralized treatment scales;the situation analysis method was applied to analyze the strategies of wastewater reuse. The results indicated:? Compared with the current benchmark situation (2003), untreated wastewater would increase by 3.10 times, BOD5 3.08 times in Xi'an, and the water-shortage rate would reach to 22% by 2030. Therefore, WARR should be increased to relax the future water shortage and pollution problems.(2) Wastewater treatment scales in Xi'an city must reach or exceed that planned by Construct Ministry, and the effect of pollution reduction of WARR must be taken into consideration in order that the future pollution discharge would not increase.(3) To maintain the balance between supply and demand, ratio of wastewater reuse must be higher than that planned by Construct Ministry. The problem of the water shortage would be solved without long distance water transfer when wastewater reuse rate reach about 45% in Xi'an.(4) Wastewater reuse had no significant effect on pollution reduction, and as each 10% increase of wastewater reuse only contributed to 2% of BOD5 reduction. Therefore, to ensure the improvement and sustainable development of water environmental, a reasonable pollutant reduction rate should be established based on local economic development in the future.(5) The price of water used for ecological purposes was too low, which affected the growth of wastewater reuse scales in Xi'an. Wastewater ecological reclamation would be profitable only when the annual increases of water price reach 8%.(6) Under the condition of water resources being used adequately in Xi'an city, the fresh water supply per capital only can reach 150m3/cap in the future, which will be far lower than precaution water shortage of 200m3/cap. Water resources exploitation would be lower than precaution exploiture rate of 40% in the future by WARR.It would be the period with the greatest investment increase on WARR facilities in the near future. The investment requirement to reach the goal of BOD5 reduction would be the biggest, which accounts for 0.16% of local GDP. But compared with foreign countries in investment peak times, it was still in the middle level, and it was an acceptable number considering by local economic development.