Simulation Of Nitrate Transport And Removal In Wetland

For total nitrogen concentration of lake and reservoir water and nitrate nitrogen(NO3--N) concentration of the centralized drinking water surface source, Surface Water Environment Quality Standard (GB3838-2002) has strict standard limits in china. However, the total nitrogen and NO3--N concentration in general river water is unlimited. Unfortunately, inflow rivers are important transportation channels of non-point source NO3--N pollution into lake and reservoir. Natural or constructed wetland is one of the most promising ways to reduce NO3-N pollutant. Therefore, riparian wetlands play an important role in intercepting non-point source NO3--N pollution. In order to obtain optimum wetland structure and operating parameters, the studies on the influence of wetland physical structure factors and operation parameters on NO3--N removal in simulated riparian wetland were conducted. At the same time, based on first-order kinetic model, the dynamic model of nitrate removal in wetland and its extended form were established, which provided basis for the optimization design and operation management of riparian wetland. The physical structure factors included aspect ratio, plant density and water level, and operation parameters included water temperature, carbon-nitrogen ratio(R), influent NO3--N concentration(Cin), hydraulic loading rate(q) and pH. The following results were obtained.(1)In wetland structure factors, aspect ratio and water level were important influence factors for NO3--N reduction, and plant(calamus) density was not an important factor in NO3--N reduction. The removal rate of wetland with aspect ratio 4:1 was the highest. Compared to the wetlands with water depth 2(0.30m) and water depth 3(0.15m), the wetland with water depth 1 (0.40m) had better NO3--N reduction efficiency. The high water depth was favorable for NO3--N reduction in wetland. The average NO3--N+NO2--N removal rates of wetlands with blank(calamus:0 plant·m-2), density 1(14 plants·m-2) and density 2(28 plants·m-2) had similar values.(2)?The rise in ambient temperature was conducive to NO3--N reduction in wetland. The NO3--N+NO2--N removal rate was increased with the increase of water temperature, and the process was divided into three stages:First stage was the low speed growth stage(the growth rate of 1.3%/?), as water temperature was below 13?; Second stage was the high speed growth stage(2.0%/?), as water temperature was between 13? and 24?; Third stage was the growth rate of slowing down stage(1.5%/?), as water temperature was between 24? and 28?. When the water temperature was lower than 13?, the effluent NO2--N concentration(CNO2--N) decreased with the decrease of water temperature, and when water temperature was higher than 13?, CNO2--N decreased with the increase of water temperature. ?The average NO3--N removal rates of wetlands increased rapidly with the increase of R, and when R was 4, removal rate reached 96%. With the increase of R, CNO2--N increased at first, and then decreased. ?With Cin increased, effluent NO3--N concentration of wetlands increased at first, and then decreased, and CNO2--N increased continuously with growth was being accelerated. Area removal rate of NO3--N+NO2--N increased with the increase of the influent NO3--N concentration.?NO3--N+NO2--N removal rate decreased with the increase of q. With the increase of q, the area removal rate has increased rapidly at first, then growth rate slowed down, and finally tended to be stable.?When influent pH was decreased from 7.5 to 5.0, NO3--N removal rate decreased significantly, and denitrification in wetland was restricted.(3)The extended first-order kinetic model gave a certain accuracy in predicting NO3--N removal results of experimental wetland. The kinetic studies on NO3--N removal in wetland were conducted by using the first-order kinetic model. To reduce the uncertainty of model parameters and optimize the design of wetlands, the relationship between water temperature, R, Cin, q and first-order areal rate constant(K) were discussed, and functional relations between them were established. Meanwhile, the extended kinetic model was constructed. The results showed that, NO3--N reduction in wetland was in accordance with the first-order kinetic model; The relation between K and water temperature could be satisfactorily explained by Arrhenius law(fitting coefficient R2:0.96), and the temperature coefficient(?) value was 1.06; The R had the exponent relation(R2:0.95) with K20; the dependence of K20 on Cin obeyed the linear equation(R2: 0.94); K20 was related q with a quadratic function(R2:0.93). The extended first-order kinetic model, which has been considered the influence of water temperature, R, influent concentration and q on K, gave a certain accuracy(?:0.97; R2:0.99) in predicting NO3--N removal results of experimental wetland.Riparian wetlands were of great value to intercept non-point source NO3--N pollution into the water. The studies on influence of wetland physical structure factors and operation parameters on NO3--N removal in simulated riparian wetland provided important theoretical basis for the design, operation and management of riparian wetlands. The construction of extended first-order kinetic model was significant to improve the reliability and applicability of the NO3--N reduction model.