In-situ Purification Of River Water In The Enclosure By Submerged Plants

In recent years,eutrophication in the river and lake is becoming more and more serious.Once the aquatic ecosystem is destroyed,water quality and ecological environment will deteriorate,river and lake will lose ecological stability and biodiversity,self-purification capacity will also decrease.As an important part of the river and lake ecosystem,the submerged plant plays a regulatory role on the nitrogen and phosphorus migration or transformation in the "water-sediments" interface.Submerged plant species is a key factor affecting these processes.This paper relies on the major national water project “The key technologies and comprehensive demonstration to improve the quality of Haihe water environment”.In this paper Ceratophyllum demersum Linn.,Myriophyllum verticillatum and Potamogeton labyrinth,which are typical local submerged aquatic plants in Tianjin,were chosen to study their growth situation as well as their effects on the forms of nitrogen and phosphorus in sediments in self-designed submerged beds.The experiments lasted for three months and were carried out in the Haitai North-South River near Tianjin Urban Construction University.A multi-medium model of nitrogen transfer(including atmosphere,water,plants and sediments)was established and fitted with the data of M.verticillatum.The mechanism of nitrogen purification in water phase by M.verticillatum was analyzed.The main conclusions are as follows:1.The enclosure condition was suited to growth demands of M.verticillatum and P.labyrinth whose biomass increased by 170% and 253% during the experiment,while C.demersum was dead quickly after the initial 15-day of experiment due to limited nutrient concentrations.During the experiment,M.verticillatum and P.labyrinth's daily average cumulative amount of nitrogen fixation ranged from 23.3 to 42.1 mg/m2.d and 17.2 to 33.2 mg/m2.d,respectively,and daily average cumulative amount of phosphorus fixation ranged from 4.3 to 5.0 mg/m2.d and 3.2 to 9.9 mg/m2.d,respectively.Submerged plant's nitrogen and phosphorus fixation can be used as an important way to remove nitrogen and phosphorus from water by harvesting submerged plants.2.At the end of the experiment,total nitrogen and total phosphorus concentrations of the sediments increased.Total nitrogen concentration in sediments in the enclosures of C.demersum,M.verticillatum and P.labyrinth increased by 10.8%,15.1%,31.3% and 18.0%,respectively;total phosphorus concentration increased by 5.3%,6.0%,15.4% and 11.5%,respectively.The concentrations of total nitrogen and phosphorus in sediments with plants were higher than that without plants,and the increment was the highest in the enclosure of M.verticillatum,indicating that submerged plant can facilitate the transfer of nitrogen and phosphorus from water to sediments.3.Ion exchange forms of nitrogen and phosphorus in sediments with plants were used first.Plant root growth during the initial period could promote the microbial growth,and then organic matter could be turned into inorganic salt form by mineralization which led to the increase of WAEF-N.During exuberant growth period,WAEF-N was also used.Submerged plant could also promote the co-precipitation of calcium carbonate and phosphate in the overlying water.4.This paper builds a multi-medium nitrogen transfer model and fitted with the data of M.verticillatum.The results showed the model could satisfactorily predict the trends of organic nitrogen and nitrate nitrogen in water phase,R2 numbers were 0.9025 and 0.9403.The organic nitrogen concentration was very sensitive to initial concentration of organic nitrogen N10 and organic nitrogen ammonification rate k1.The concentration of nitrate nitrogen was very sensitive to N10,initial concentration of nitrate nitrogen N30 and the net mass transfer rate of nitrate nitrogen k6.