Study On Removal Efficiency Of Pollutants From Water And Sediment Of Shahe Reservoir By Daphnia Magna-Submerged Macrophytes System

Aiming at the problems such as weak self purification capacity,poor water quality and lack of aquatic macrophytes in Shahe reservoir,this paper investigated a combined system of daphnia magna and submerged macrophytes integrating water purification and landscape functions to effectively remove water and sediment pollutants in the reservoir area,providing technical support for water environment treatment of Shahe reservoir.After confirming the optimal density of daphnia magna and the optimal planting coverage of submerged macrophytes,the practical application of the combined system in Shahe Reservoir pilot project was carried out to investigate the influence of water temperature and water depth on the growth of daphnia magna and three submerged macrophytes in the combined system,as well as the influence of the combined system on the removal efficiency of water and sediment pollutants.The density of daphnia magna affected its reproduction and removal efficiency of water pollutants.Under the real water quality condition of Shahe reservoir,the suitable density range of daphnia magna was 5～15 ind/L.Daphnia magna could increase the transparency from 40 cm to 100 cm,reduce the turbidity from 19.6 NTU to 2 NTU and keep it for a long time,which provides a good precondition for the cultivation of submerged macrophytes.After adding daphnia,the COD concentration of water body did not change obviously.The removal rates of TN and TP were 70.2%and 54.9%.The addition of daphnia magna increased the concentration of NH₄⁺-N by 32.2%,and the removal rate of NO₃^--N was71.9%.The organic matter,total nitrogen and total phosphorus of sediment increased after adding daphnia magna.The initial planting coverage of submerged macrophytes affected the removal efficiency of pollutants in water and sediment.When the planting coverage of submerged macrophytes was 50%,the removal efficiency of pollutants was the best.After the system was stable,the removal efficiency of COD,TN,NH₄⁺-N,NO₃^--N and TP could reach 45.4%,42.8%,66%,46.4%and 85.9%respectively.The removal rates of organic matter,total nitrogen and total phosphorus were 15.8%,33.3%and 19.6%respectively.At the initial planting coverage of 30%and 50%,the content of chlorophyll,protein and plant height of three submerged macrophytes,Ceratophyllum,Myriophyllum and Hydrilla Spicatum,increased significantly,but at 80%coverage,the growth of Ceratophyllum and Myriophyllum was under stress.It was found that the temperature affected the operation of the combined system in the pilot test.Under the water quality condition of Shahe reservoir,when the water temperature was higher than 10℃,the combined system had a certain removal effect on water and sediment pollutants.The removal rates of COD,NH₄⁺-N,NO₃^--N,TN and TP in water gradually stabilized at 30%,15%,66%,17%and 13%,and the removal rates of organic matter,total nitrogen and total phosphorus in sediment reached 8.7%,6.2%and 19.3%.However,when the temperature was lower than 10℃,the removal efficiency of pollutants was very poor.The concentrations of NH₄⁺-N,TN,TP in the water body increased,and the pollutants in the sediment also increased significantly.When the water temperature droped below 10℃,daphnia magna produced dormant eggs and even died.The lowest tolerance temperature of Ceratophyllum was 5℃,that of Myriophyllum was 7℃,and that of Hydrilla Spicatum was 3℃.The tolerance depth of Ceratophyllum and Hydrilla Spicatum was more than 1.5 m,and that of Myriophyllum was 1 m.In conclusion,when constructing the combination system to deal with water and sediment pollutants in Shahe reservoir,the operation parameters are as follows:Daphnia magna is suitable for5～15 ind/L;the initial planting coverage of submerged plants is suitable for 50%;the suitable operation temperature of the combination system is above 10℃;the suitable water depth is 1 m.