Effect Of Aquatic Plants On Purification Of Eutrophic Water And Nutrients Release From Sediment

Water pollution is one of the most important factors that cause global water shortage, while water eutrophication is the most common phenomenon that has became a problem in water pollution control. Correspondingly, understanding the mechanisms involved in eutrophication process as well as the related remediation technologies that have developed for controlling water eutrophication has became the research frontier and hot topics worldwide in the fields of environmental ecology and plant nutrition. Phytoremediation is a energy-effective and cost-effective technology that have developed for effectively controlling water eutrophication in recent years, which has increasingly been attracting academic attention woldwide.The concentrations of N and P are generally in a state of dynamic equilibrium between the overlying water and the sediment in the eutrophic water. When aquatic plants were applied to treating the eutrophication water, the nutrient concentrations in the overlying water were reduced and the nutrient balance between the overlying water and the sediment would be broken. In order to provide the theoretical basis of using aquatic plants to effectively treat eutrophic water and reduce the endogenous pollution load in eutrophic lakes, simulation experiments were carried out to study the effect of three types of aquatic plants (water hyacinth, water lettuce and cattail) on purification of eutrophic water with different eutrophic levels as well as the dynamic changes of the nutrient concentrations in porewater of sediment. The major results and conclusions obtained are summarized as follows:(1) When daily average temperature ranged5℃～15℃, water hyacinth and water lettuce normally grew in eutrophic water with initial concentrations of3.0mg/L～15mg/L TN and0.2mg/L～1.0mg/L TP. Water lettuce were rapidly adapted to a lower temperature, with a fast increase in root growth and biomass production, while water hyacinth has a longer adaptation period and a more slowly increase of biomass production when compared with lettuce.(2) In eutrophic water with three different concentrations of TN and TP, the removal rates of TN and TP in water lettuce purification system were63.1%～88.3%and93.3%～97.4%respectively. The removal rates of TN and TP in water hyacinth purification system were86.5%～65.9%and90.0%～95.0%respectively. The removal rates of TN and TP in the water lettuce purification system were obviously higher than those in the water hyacinth. With the increase of initial nutrient concentrations, the removal efficiency of TN in water lettuce system and water hyacinth system decreased, while the removal efficiency of TP increased. Release of N and P nutrients from porewater of sediment toward the overlying water were observed. Cultivation of these two aquatic plants promoted the release of N and P from porewater of sediment toward the overlying water. Cultivation of water lettuce has a better promotion in releasing of N and P in the porewater when compared with water hyacinth.(3) When daily average temperature ranged20℃～35℃, water hyacinth grown in eutrophic water with different concentrations of TN and TP had a more rapid increase in biomass when compared with cattail. This may indicate that water hyacinth had a better adaptability to different TN and TP concentrations while cattail had a longer adapting period. Cultivation of both water hyacinth and cattail effectively reduced TN and TP concentrations of the overlying water with initial concentrations of3.2mg/L-14.2mg/L TN and0.2mg/L-1.0mg/L TP. The removal rates of TN and TP in water hyacinth purification system were72.0%～94.0%and82.5%～98.1%respectively. The removal rates of TN and TP in cattail purification system were66.0%～92.8%and77.0%～93.8%respectively. The quality of effluent water in purification systems cultivated with these two aquatic plants both reached the surface water quality standard III class.(4) Under the conditions of the experiments, the contribution of water hyacinth and cattail uptake to nitrogen removal in eutrophic water with different concentrations of TN and TP were111.0%～492.9%and52.3%-271.5%respectively. The contribution of water hyacinth and cattail uptake to phosphorus removal in eutrophic water with different concentrations of TN and TP were164.6%～717.4%and139.3%～573.7%respectively. The lower the nutrient concentrations in overlying water were, the higher the contribution of uptake by aquatic plants to nutrient removal was.(5) Cultivation of two aquatic plants (water hyacinth and cattail) promoted the release of N and P from porewater of sediment toward the overlying water. The lower the nutrient concentrations in the overlying water were, the higher the release rates of nutrients from sediment were. Cultivation of water hyacinth has a better promotion in releasing of N and P also effectively reduced TN and TP concentrations in the overlying water. At the late stage of the experiments, the concentrations of ammonia and nitrate in the porewater in the cattail system decreased significantly, which were much lower than those in the water hyacinth system and mud-water system.This might be due to the fact that cattail has a very well-developed root system that can directly absorb nutrients from the sediment. Therefore, the release rate of nitrogen from the porewater of the sediment to the overlying water was reduced and the nutrients release of the porewater in the sediment was inhibited.