Study On Eutrophication Forecasting In Linjiang River Backwater Region And Ecological Floating Bed Technology

With formation of The Three Gorges Reservoir, the hydrological condition variations of branch backwater regions reduce the diffusion effect of pollutants and weaken self-purification capacity of backwater region, then lead to water quality deterioration, especially that accumulation of nutrient salts will aggravate eutrophication. Because the researches on branch backwater regions was relatively less, taking Linjiang river which is a branch of Yangtze River as research object to study the eutrophication problem of Linjiang river backwater regions. Main influencing factors of Linjiang river backwater region eutrophication were studied, then, eutrophication forecasting model of Linjiang backwater region was built, and analyze the forecasting results. For the sake of controlling eutrophication, the pollution processes and self-purification capacity of Linjiang river backwater region was simulated. Based on the simulating results, ecological floating bed technology was adopted to control Linjiang river backwater region eutrophication.①In order to investigate the main influencing factors of Linjiang river backwater region, nine water quality indexes, include water temperature, diaphaneity, velocity of flow, TN, TP, COD, Chl-a, pH, DO, were monitored, then factor analysis method was used to analyze these indexes. Studying result showed that water temperature, diaphaneity, TN, TP, COD, chlorophyll-a were main influencing factors of Linjiang river backwater region eutrophication. Multiple regression analysis results of six main influencing factors showed that chlorophyll-a concentration had had significant correlation with water temperature, TN and TP, decreasing content of TN and TP can achieve the goal of reducing chlorophyll-a concentration.②Based on the studying results of main influencing factors, BP neural network model was utilized to forecasting water nutritional status of Linjiang river backwater region on different nitrogen and phosphorus levels, the forecasting results indicated, when removing rate of nitrogen and phosphorus was 40%, chlorophyll-a concentration reduce from 55.591 mg·m-3 to 23.314 mg·m-3, water nutritional status convert from moderate eutrophication to mild eutrophication. When removing rate of nitrogen and phosphorus was 70%, chlorophyll-a concentration reduce to 9.127 mg·m-3, water nutritional status convert from moderate eutrophication to mesotrophy, so eliminating danger of eutrophication. ③Water quality model was used to analyze pollution processes and self-purification capacity of Linjiang river backwater region, results showed that, after pollutants discharged into water, pollutants concentration reached steady state eight hours later. Slow diffusing velocity result in accumulation of nitrogen and phosphorus. Self-purification of Linjiang river backwater region can remove 11.98 percent of COD, 12.25 percent of TN, 13.98 percent of NH3-N and 11.59 percent of TP. The self-purification capacity is far from enough for decontaminating heavy Pollution water of Linjiang river backwater region, so heavy polluted water of Linjiang river backwater region must be treated.④For studying purification effect of ecological floating bed for Linjiang river heavily polluted water, ecological floating bed tester was built at the side of Linjiang river backwater region, study comparatively the purification effect of Canna indica, Acorus calamus Linn, Cyperous alternifoliu and Vetiver zizanioides floating bed system. Purification experimental results showed that four floating bed systems have good COD, TN and TP removal effect, and Canna indica floating bed system had the best removal effect among four floating-bed systems, so it can be applied to treat polluted water of Linjiang river backwater region. Forecasting model of eutrophication was used to predict practical application effect of ecological floating bed in Linjiang river backwater region. Forecasting results showed, when the area of floating bed was 0.6 km2 in Linjiang river backwater region, the forecasting value of chlorophyll-a concentration reduce to 9.68 mg·m-3, water nutritional status convert from moderate eutrophication to mesotrophy.⑤Growth characteristics and absorbency to nitrogen and phosphorus of four plant species cultivated on floating-bed were studied comparatively based on purification experiments. During experimental process, plant growth rate present conic characters, the rapid growth period of plants was from sixty days to ninety days after planting. Nitrogen uptakes of four plants accounted from 18.1% to 35.6% of floating-bed system removal, and phosphorus uptakes occupied from 36.3% to 52.1%. Results showed that plant uptake is the important part of floating bed system nitrogen and phosphorus removal.⑥According to the actual instance that pollutants removal rate of ecological floating bed system was not steady, based on reactor kinetics theory, the relation between COD, TN, TP removal rate of floating bed system and effluent concentration was studied, the results showed that degradation reaction of COD was first order reaction, degradation reaction of TN, TP was complicated.