Research On Nitrogen Removal Efficiency And Transformation Process Of High Ammonia Nitrogen River Bypass Pond-wetland System

The excess of nitrogen nutrients has seriously affected the river water environment in China and has gradually became the main water pollution problem in China's rivers.The Ziya river system of the Haihe River Basin in northern China exhibits typical characteristics of unconventional water supply,it accpeted domestic sewage and industrial wastewater which reach or not reach the standard,nitrogen nutrient pollution,especially ammonia nitrogen pollution,is the most serious.The pond-wetland treatment system is widely used in river ecological restoration projects due to its good treatment effect,low infrastructure investment and daily operation cost,and easy management and maintenance.This study selected Niuwei River,a typical high ammonia-nitrogen river,in the Hai River Basin as research object,using the pond-wetland combined treatment system to explore the effect of purifying nitrogen and phosphorus nutrients in unconventional supplied water,analyzing nitrogen occurrence form,spatial distribution characteristics and transformation process in wetland water.By monitoring the changes in water quality in different periods,analyzing the effects of nitrogen and phosphorus removal and the influencing factors on various forms of nitrogen in the wetland system,calculating the nitrogen conversion rate and adsorption-desorption characteristics of ammonia nitrogen in surface sediments,analyzing the distribution of inorganic nitrogen in the vertical plane and estimating the diffusion flux and annual pollution load of inorganic nitrogen.The major results are as follows:?1?The removal effect of nitrogen and phosphorus indexes in the Niuwei river water during the startup period was poor,the average removal rates of TN,NO₃^--N and NO₂^--N were 20.4%,80.4%and 8.8%,respectively,and NH₃-N increased by 51.0%.Removal rates of TP and SRP were 8.7%and 3.2%,COD and TOC removal rates were19.3%and 4.9%,respectively.The operation period had a good removal effect on the nitrogen and phosphorus indexes of the Niuwei river water,the average removal rates of TN,NH₃-N,NO₃^--N and NO₂^--N were 35.0%,41.9%,93.5%and 52.9%,respectively.The average removal rates of TP and SRP were 24.4%and 30.4%.COD and TOC average removal rates were 55.6%and 31.7%.The correlation analysis of different forms of nitrogen in surface sediments and water shows that the occurrence forms of nitrogen in sediments are affected by both water and sediments.?2?The overall ammoniation rate?AR?of the surface sediments was greater than the potential nitrification rate?PNR?,and the AR was about 16 times that of the PNR,the 4-3 point of the secondary wetland was the typical ammonia nitrogen accumulation point.AR and PNR were affected by physical and chemical indicators of sediments and have obvious spatial differences.The adsorption process of NH₃-N in surface sediments was divided into rapid adsorption phase?0-60 min?and adsorption equilibrium phase?60-480 min?,the adsorption kinetics of NH₃-N can be well fitted by using the amanded Elovich model.The isothermal adsorption process can be well fitted by the Langmuir model,the adsorption-desorption equilibrium concentration varied from 5.21 to 12.98 mg·L^-1,and the background adsorption amount varied from 67.27 to242.84 mg·kg^-1.The surface sediment desorption experiment reached its equilibrium state after desorpted until 12 hours,the desorption equilibrium varied from 32.49 to57.44 mg·kg^-1,the desorption amount was affected by NH₃-N and C/N in the sediment.?3?The concentration of inorganic nitrogen in overlying water varies little with depth,but it fluctuates greatly in pore water,and each sampling point peaks near the water-sediment interface in the vertical scale.On spatial scale,NH₃-N of overlying water and pore water in upstream is higher than downstream,the concentration of NO₃^--N in overlying water and pore water is opposite to NH₃-N,and the overall content of NO₂^--N is low.There were obvious spatial differences in inorganic nitrogen diffusion flux and annual pollution load at the water-sediment interface in the pond-wetland system,NH₃-N,NO₃^--N and NO₂^--N present a tendency to release from sediments to overlying water throughout the process,sediments appear as the"source"of each inorganic nitrogen.The diffusion flux and annual pollution load of each wetlands were at different levels.There are varying degrees of nitrogen pollutant release risk,and endogenous pollution loads cannot be ignored.