系统思考和决策试验.pdf

Exercise in application of System Thinking in the water purification system High School Affiliated to Nanjing Normal University (No.37 Chahaer Road, Nanjing, P.R.China 210003) Heng Wenyu*, Wang Majie, Dai Xingjie (senior 3) Advisors: Kong Jun, Ding Zhijun, Yao Yuqin 1. The background There is a lake named “WEIMING” in our school, however, this lake, an important interest place in school, has been polluted seriously, especially in summer, when students often suffer from the effluvium of the lake. As members of System Thinking Club, we hope to improve the terrible situation of the lake by using System Thinking and STELLA. We need to explore the causes affecting the water quality and model the real situation in the lab, and then find the efficient way to purify the water in the lake. 2. The theme and the goal Fig.1 In the lab, we built the sewage treatment procedure to simulate the artificial wetland, and we utilize the method of System Thinking to research the operating mechanism of wetland and modify the water quality. With the help of System Thinking, we would build the eco-machine model. This method can be applied widely to different fields in the industry. Our research goal is to improve the water quality through a series of analysis and processing. We found two elements that affect water quality: one is the amount of dissolved oxygen; the other is the amount of phosphorus (P) and nitrogen (N). Therefore, we set the goal of water modification as follows: to increase the amount of dissolved oxygen and to reduce the amount of P and N. 3. The explanation of model Because our research object is a complicated ecosystem, we didn’t build the model in our first step, but listed all of elements affecting the amount of DO and P the right is the data from the experiment) Therefore, we can draw a conclusion: the model we built accords with the real situation. Then through tests on the simulation results, we can get the following detailed conclusions: 1. The amount of DO is an important factor which can help us to quantify the water quality. The DO does increase due to the integrated effect of emergent plant, submerged plant, aquatic species and pumping. 2. Pumping helps increase the amount of DO, although the higher pumping rate can cause the better effect on DO. The relationship between these two factors is not linear. Besides that, cost should be taken into consideration. So we should choose an economical method instead of operating the pump to the higher power rate. 3. Enhancing the photosynthesis by planting some black algae is an effective way to increase the amount of DO and reduce the effluvium in water. The larger the amount of black algae is, the better the effect is. In addition, the increase rate is not a constant. So that too much black algae is not a good option. 4. Simulator shows that pumping in the day time has a better effect on increasing the amount of DO compared with in the night time. 5. Judging the eco-machine from the aspect of energy consumption, we can say that eco-machine is green, economical and effective because electrical energy is the only energy source besides the aquatic species we required. 6. Our model is not a mature one so far. For the P due to the limitation of the suitable apparatus, we are not able to measure the amount of P&N continuously. So we can not find the formula involved N/P. Our job is to design a hypothetic value into the model. This inevitably brings a great uncertainty in the system. 7. In addition, according to our research, we know the influential factors include outside ones like temperature and pressure, especially the temperature is capable to affect both solubility of oxygen and the rate of metabolism. Unfortunately, we did not find a good way to involve the factor pressure, for the temperature, we chose a constant value corresponding to the situation—20℃. Because molecules move in a faster speed in higher temperature, the water pollution problem is worse in summer than in winter. Noname 5oxygen in emergment aquatic plant tankNoname 6solubiliutyamount of dissolved P&N in aquarium convert rateexcermentfrom fishSediment in aquariumconverted to sedimentincreaseP&N from inputinputP&N per unitconvert to sedimentconvert to dissolveddelivering P&N from aquarium to physical treatment tankvolume of water in aquariumfish weightexcrement per fishsediment in physical treatment tank 2cleaned 2oxygen added by bulrushamount of bulrushAmount of dissolved P&N in emergment aquatic plant tankdecreaseincreagrowth rateoxygen produced per bulrushP&N consumed per bulrushdecreaamount of dissolved P&N insubmerged plant tankNoname 4~ N P indexFig.4 The corresponding charts in the conclusion part: Chart 1: Horizontal axes: Time/s Vertical axes: dissolved oxygen/ mg/litre Chart 2: (a) (b) The blue ones show the situation with the slow pumping rate, while the red ones show the situation with the fast pumping rate. Chart 2(a): both in the comp。