| In recent years, with the development of industry and agriculture, water environment problem is getting worse, how to treat and reuse wastewater has become a top priority. There are a lot of problems of traditional physical and chemical methods in practical application, such as large investment and operating difficulties, secondary pollution, so that people turning to the use of higher plants to purify sewage.Biological floating island as one of the biological treatment technology has been increasingly used in treatment of eutrophic water. Biological floating island is in accordance with the law of nature, with a floating bed as a carrier, making aquatic plants or improved terrestrial plants growing on the surface of eutrophic water. Through the plant root absorption, sorption and species competition to reduce water nutrients such as nitrogen, phosphorus and organic matter, so as to achieve the effect of water purification. Besides, biological floating island can also create a water landscape as well as bring certain economic benefits.In this study, through surveys, experiments and other methods, the status quo of landscape water in Shanghai was studied. After analysis and comparison, we selected some native aquatic species suitable for the floating island and with good water purification effect and good landscape effect. Lythrum salicaria,Arundo donax and Zizania latifolia were selected as the experimental plants.By setting the indoor and outdoor experiments, we analyzed the average fresh weight of the three plants, removal of nitrogen and phosphorus from water and nitrogen and phosphorus content of plants, to draw their purifying effect. The absorption and release of nutrient elements from water of coir as the floating island plant support matrix was also studied. The results showed that:1 All three plants growing well in low nitrogen and phosphorus, biomass increased with the nutrient concentration; while at high concentrations the biomass decreased.2 The curve of biomass accumulation, nitrogen and phosphorus in the body and removal of nitrogen and phosphorus in water of three plants were generally similar, they all increased and then decline with the passage of time, and the content of nitrogen and phosphorus in the stems and leaves was higher than that of roots. Therefore, in order to achieve the best effect of water purifying, plant stems and leaves should be harvested when reach largest biomass, to remove nitrogen and phosphorus from water, to avoid secondary pollution caused by the litter of plants.3 Results of indoor experiments showed that, the removal of nitrogen and phosphorus from water were different between three plants, and the removal effect of TP was better than TN. Arundo donax performed best in removing nitrogen and phosphorus from water, as high as 86.12%and 72.60%, respectively; Lythrum salicaria performed the second good, the rate of TN and TP removal were 78.61% and 59.00%, respectively; nutrient absorption ability of Zizania latifolia was worst, the rate of TN and TP were 67.60% and51.30%, respectively.4 Coir could provide attachment sites for microorganisms, thus to reduce the suspended substance and improve transparency in water. Coir would also release a small amount of nitrogen and phosphorus into the water, leading to increased content of nitrogen and phosphorus.This happened in just the early stages of experiments. In this study, the stage takes about two weeks'time. Overall, the adsorption capacity of coir on removing nitrogen and phosphorus from water was stronger than that of release nutrient into water. The coir used as floating island supporting substrate, with the plant root systems could provide more living sites for microorganisms, thereby bringing the system to better water purifying effect.5 In practice, selection of plants applying on the floating island should be based on the degree of eutrophication, native plants with great vitality, rapid reproduction, nutrient absorption ability and good visual effects were recommended.
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