Comparison Of Efficiency And Mechanisms In Eutrophic Water Treatment Using Different Types Of Aquatic Plants

Excess nitrogen input is one of the major factors causing eutrophication in aquatic ecosystems. In order to monitor the eutrophication status of water bodies accurately, rapidly and precisely, a rapid detection method should be established for analyzing various forms of nitrogen (N). As nitrate is the major form of N in eutrophic lakes, rivers, reservoirs, a rapid method for analyzing nitrate in eutrophic water using continuous flowing analyzer was developed in the first place. The developed method can provide the technical support for fast and accurate detection of nitrate in the collected water samples, and provide a good analyzing method for monitoring the effect of aquatic plant on wastewater treatment at the same time. Then, experiments were carried out to study the treatment efficiency of eutrophic water using different types of aquatic plant. Four different types of aquatic plants (Eichhornia crassipes* Pistia stratiotes、Trapa、Hydrilla verticillata) were used to construct microcosms for treating eutrophic water at different plant growth stages, e.g. initial growth period (June to July), fast growth period (July to August) and slow growth period (September), respectively. During water treatment, four different types of plant were compared of:1) the capability to accumulate N、P; 2) the removal efficiency of N、P in underlying water; 3) the ability to mediate the gas production from biological processes (including N2O、N2 emission driven by microorganisms); 4) the ability to mediate the abundance of denitrifying microorganisms and physicochemical environment; 5) the ability to mediate release and adsorption of nitrogen and phosphorus from the sediment. Based on these, we use N-15 stable isotope tracing technology to study the nitrogen transportation and transformation process under the influence of Eichhornia crassipes. The major results include:1) A rapid method for analyzing nitrate in eutrophic water using continuous flowing analyzer has been established:The regress equations were y=8E-06x+0.012, correlation coefficient(R2) is 0.9999. The relative standard deviation (RSD) is less than 1%and the recovery was above 97.24%-103.60%.2) According to the results, there were obvious differences in NH4-N, NO3-N and TN removal efficiency between different types of plant. Two species of free-floating plants, E. crassipes and P. stratiotes, showed the highest N removal efficiency and removal rates, with TN removal rates of 99.67±0.27%at initial growth period,96.26±0.88% at fast growth period and 63.86±3.77% at slow growth period, TP removal rates of 92.67±3.77% at initial growth period,92.82±2.11% at fast growth period and 98.04±0.45% at slow growth period. P. stratiotes showed the fastest P removal efficiency, with the highest TP removal efficiency at slow growth period. The submerged plant H. verticillata had the lowest removal efficiency for N in underlying water. Trapa had the lowest removal efficiency for P in underlying water. Among three growth stages, except H. verticillata, the fast growth stage had the highest N、P removal efficiency for all plant types, followed by the initial growth stage. The capability of four types of aquatic plant to accumulate N was in order of E. crassipes>P. stratiotes> Trapa>Hydrilla verticillata, the capability of four types of aquatic plant to accumulate P was floating plant stronger than Trapa and Hydrilla verticillata.3) The total amount of gas production as well as the individual gas (N2、N2O、O2) flux in water with submerged plant were significantly higher than other plant types, N2O flux in water with Trapa was highest. The ability to mediate the abundance of denitrifying gene (nosZ, nirS, nirK) and physicochemical environmental factors in water varied with different plant types, which can lead to the differences in the capability to mediate microbial N transformation processes of water with different types of plant.4) The E. crassipes can assimilate nitrate and ammonia from the water simultaneously, but has a preference for ammonia assimilation. The presence of water hyacinth can stimulate nitrification and denitrification reaction in eutrophic water. Through the N-15 tracing experiment using a self-designed chamber that can separate the apartment with plant shoots from the apartment with roots, it has been found that 15N2O and 15N2 in the apartment with growth of plant shoots under the condition that there were no exchange of nutrients and gas between the two compartments. The results indicate that the plant aerenchyma within water hyacinth can be an important pathway through which the gaseous nitrogen products, produced from biological processes in water, are transported into the air.