Study On The Bioremediation Of Eutrophic River By Flouting Net Bed With Submerged Plants

Water environment pollution is still one of the most serious global problems. The phenomenon of eutrophication is still common in many rivers and lakes in China, which seriously affected the people’s life and production. Therefore, how to reduce the level of eutrophication and recovery of freshwater ecosystems has become an important iusse.We developed adjustable submerged plant net bed technology in 2010, and which made a good effect in the ecological restoration engineering. While there are many problems need to be solved in the the imperfect technology. Such as how to ensure the high survival rate of the submerged plant, what is the optimum planting density and how to regulate and sink of the net bed. Hence, we, firstly, found the specific problems of the technology though the field test. And then we designed experiment to improve the adjustable submerged plant net bed technology. Eventually, Using the improved technology to restore the eutrophic waters of river-network of Litang. At the same time we evaluated the effect of the ecological restoration.The research contents are as follows:(1) The eutrophication assessment and grade division of Luchaogang riverDuring the wet season in 2012 and the dry season in 2013, we investigated water quality of the river-network of the luchao port.According to the results of the water ecosystem health evaluation, we can see that the health status of aquatic system was at a low level.The value, among the sites 1 to 9, of the health comprehensive index is between 0.2 ~ 0.4,whose health level for the poor but site 6 in the river-network,while the sites 10~24 was moderate.(2)The key planting technique improvement of submerged plants bed netsBased on the the problem of the low survival rate of submerged vegetation in restoring eutrophic water with adjustable submerged plant cultivation nets technology, we planted three kinds of submerged vegetation through three different ways of planting:Binding cultivation nets,Thread gluing cultivation nets,Net cages. The result show that Vallisneria spiralis,Cabomba.sp and Hydrilla verticillata in binding cultivation nets had negative △ W growth in 15 days,and the △ W value is-0.0117g·100g-1、-6.615g·100g-1、-13.095g·100g-1, respectively, and the △ W negative value is increasing as time goes on.While submerged vegetation‘s △ W value of the thread gluing cultivation nets and net cages had positive growth, but H-2 in 60 days.Compared with the 15 th days,the △ W value of three kinds of submerged plants had increased on the 30 th days. However, the 45 th days‘ △ W value went an apparent decline compared with the 30 th day, and the △ W value of Vallisneria spiralis,Cabomba.sp and Hydrilla verticillata in binding cultivation nets decreased by 95.1%, 98.3%, 99.9%, respectively.In addition,the concentration of TN, TP, CODMn was absrobed very well by submerged plants in the three planting ways after 60 days, but significant difference(p<0.05), the result happened to coincide with the result of submerged vegetation?s growth situation in this experiment, and their advantages and disadvantages for the sequence as follows: Binding cultivation nets<Thread gluing cultivation nets<Net cages. Eventually, we recommend Thread gluing cultivation nets in ecological restoration project due to the cost sper square meter of net cages is 2 times more expensive than net bed. But Thread gluing cultivation nets hanging time should not be too long in ecological restoration project, which should be settled to the bottom before 30 days. In this way,the submerged plants of thread gluing cultivation nets can grow well, and the water quality purification efficiency is also improved as well.(3)The key sinking technology improvement of submerged plant bed netsIn order to study the optimum planting density, the regulation and sinking of the net bed, Vallisneria spiralis, Cabomba.sp were planted by the thread gluing cultivation nets by 300g·m-2, 700g·m-2, 1000g·m-2, 1500g·m-2 of planting density and control group, respectively. The result show that the higher of planting density, the faster of removabilities of N and P. However, the rate of turbidity water purification in the planting density of 1000g·m-2 much the same as 1500g·m-2, but far faster than the density of 300g·m-2, 700g·m-2. Field experiment, we found that there are too large buoyancy to conveniently regulate the net bed when the planting density is 1500g·m-2. Ten days later, the net bed of the planting density of 1000g·m-2 and 1500g·m-2,which were the first to sink to the bottom, while the planting density of 700g·m-2 need 13 days, and the longest time is 300g·m-2 group, for 21 days. And we found that the more time needed for sink, the greater the loss rate of submerged plant. After the net bed were sinked to bottom, the rooting ability of Vallisneria spiralis is stronger than Cabomba.sp.(4)Effect and evaluation of ecological restoration engineering using submerged plant bed nets technology in JinshugangBased on adjustable submerged plant net bed technology, we conducted a water quality purification project of ecological restoration experiment on a brook of Gong Hu Jin Shu port in Suzhou, Jiangsu province. During the ecological restoration experiment, we carefully monitored the common water quality indicators, including NO3--N, NO2--N, PO43--P, DO, NH4+-N, TN, TP, CODMn and SD and evaluated the ecological restoration effect of submerged plant net bed through comprehensive water quality identification index method as well. The results show that the loss rate of submerged plant was only 20% after the implementation of the project. And after 4-5days, submerged plants began to produce new roots in restoration area, which would be fully covered after another 15-20 days. Two months later the coverage ratio of the submerged plant was up to 95%, and the rate of the dominant species of Cabomba.sp was as high as 70%. In addition, SD had an obvious increase from 0.45 meters to 1.27 meters, which made the bottom of the water see. After six months’ restoring, the concentration of NO3--N, NO2--N, PO43--P, NH4+-N, TN, TP, CODMn decreased by 74.1%, 82.8%, 90.1%, 92.6%, 71.8%, 95.6%, 48.6% respectively, while the concentration of DO increased by 65.3%. Besides, X1.X2 of the comprehensive water quality identification index decreased from 5.1 to 2.7(saying a fall of 47%),showing stark contrast to the control area(X1.X2 = 4.7,P < 0.05). Apparently, adjustable bed nets play a very good role as guider in submerged plants recovery, which in turn help to quickly improve the water quality. But the submerged plant in the net bed has a low survival rate.(5) The construction of submerged plant communities and the effect of river water quality purification through the Improved submerged plant bed nets technology in LuchaogangFour restoration areas in Luchaogang were named after number1, 2, 3, 4, whi ch were restored through submerged plant communities recovery by Vallisneria spiralis, mixed submerged plant(Vallisneria spiralis+Cabomba.sp+Ceratophyllum demersum L.), Cabomba.sp and Hydrilla verticillata, respectively. After months of ecological engineering repair, the four areas formed the different characteristic meadow, of which the four areas‘s dominant species of Vallisneria spiralis, mixed submerged plant, Cabomba.sp and Hydrilla verticillata coverage ratio increased by 87.5%, 82.7%, 88.3% and 82.7% respectively. the concentration of TP, TN and CODMn were reduced obviously, compared with the control group significant difference(p < 0.05). In addition, the transparency increased significantly of the four meadow, water clear bottoming out, and formed "underwater forest" landscape effect.(6) Changes of eutrophic state using submerged plant bed nets technology before and after the river ecological restoration in LuchaogangIn June 2014, using the improved adjustable submerged plant net bed technology to restore the eutrophic Litang river-network of the Luchao port. Before the restoration, the concentration of TP, TN and CODMn were Ⅴ class or substandard 5 classes of the four restoration areas and the control group,and the transparency was between 45 to 60 cm, which are severe eutrophication. After 2 months of the ecological restoration, the concentration of TP, TN and CODMn of the four restoration areas had been reduced significantly, compared with the control group significant difference(p<0.05). And in the four restoration areas‘s water was clear and formed the "underwater forest" landscape effect by different pasture. To March 2015, the concentration of TN, TP and CODMn the 4 restoration areas were maintained the class between Ⅲ to Ⅳ, and the nutritional status of 4 restoration areas were maintained in the moderate and mild eutrophic.