Design And Application Of Ecological Floating Bed System Based On Numerical Simulation Of Three-dimensional Flow Field

With the rapid development of economic and urbanization since reforming and opening stage, the industrial wastewater and domestic sewage increasing constantly producted and has been directly drained into water bodies of the rivers and lakes with lack of water treatment facilities, a large number of nitrogen and phosphorus accumulated in water bodies and leads to water degradagion and eutrophication beyond water self-purification capacity, which would destroy the water ecological balance, reduce the biological diversity, endanger human health, aggravate water resources crisis in watershortage area, and affect the social and economic development.The ecological floating bed system(EFBS) was designed to restore the ecosystem of contaminated water, by reducing the nutrientsinside it, such as nitrogen and phosphorus. The nutrients may be reduced through the absorption of matrix, sucking up by the floating bed plants and the way of changing them into other substances, with of help Root Ecosystem. On the technical principle of soilless culture,EFBS can be achieved by planting higher aquatic plants or modified terrestrial plants on the surface of contaminated water, with carriers such as buoyant material or light material.In this thesis, three-layer structure of EFBS was designed through the material selection and structure design. According to the selection principle and characteristic analysis, Iris pseudacorus L., Lythrum salicaria L. and Canna were chosen as floating bed plants. The captive test of the removal of TN and TP in water using the three kinds of aquatic plants during different hydraulic retention time were made. Visual MODFLOW software was applied to simulate the current speed and residence time of water particles through the EFBS under different hydraulic conditions.Then according to the results of the captive test, the optimal numbers of floating bed were selected under different hydrodynamic conditions. The results were verified by testing, thus can be treated as a scientific basis for field project design.The selected float material was a kind of rigid foam floating balls with the diameter of 10cm and the aperture of 1.5cm aperture. The testing results showed that the structure of floating balls were probably unchanged after the water were freezed and melted repeatedly. Two-way stretch plastic geogrid were used as support frame. The analysis on some basic structural test showed that the buoyancy of the double-layer floating beds, using the geogrid as their framework, are much better than the single-layer ones. And the more float balls the EFBS contains, the more substances it can afford. Thus a new flexible three-layer EFBS with complex functions is proposed, considering the weak points of rigid floating body such as the structure of the floating body is too simple, lack of energy dissipation performance, and too expensive.The floating bed is mainly composed of geogrid, floating balls and geotextile. Lythrum salicaria L.,Iris pseudacorus L.and Canna indica L. perform good on reducing TN and TP from water, and another advantage is that these plants have a very long growing season, thus the purifying time is extended. Moreover, they have strong tolerance ability and good landscape effect. According to the small scale static test of the three kinds of plants, the results showed that all of them perform well on getting rid of TN and TP during the test period (5 to 6 days).The result of the simulation by visual MODFLOW software showed that when the average speed in EFBS was 0.21m/d, the optimal number of the floating beds was one and 250 plants were needed. When the average speed was 0.43m/d, the optimal number of the floating beds was also one and 250 plants were needed.When the average speed was 4.35m/d, the optimal number of the floating beds was 4 and 1000 plants were needed. When the average speed was 20.80m/d, the optimal number of the floating beds was 21 and 5250 plants were needed. To verify the prediction,4 ecological floating beds were placed in a reservoir with the maximum velocity of 1m/s. The result indicates selection over time, removal TN rate gradually increased, the maximum removal rate in 5 days was 7.42%, which was consistent with a small pilot test in TN removal trends, demonstrating the feasibility and reasonableness of the design model to simulate the flow of water in the internal floating bed system flows.