Impact Of Water-level Change On Nitrogen Transfer And Migration In Horizontal Subsurface Flow Constructed Wetlands

Horizontal subsurface flow constructed wetland has been widely used in various sewage treatment,it has high removal efficiency on nitrogen and phosphorus in water's pollutants.And it mainly uses plants,matrix and microbial to remove pollutants.Though it has high removal efficiency on pollutants,there are also some problems,such as substrate clogging,limited removal efficiency and so on.Water-level change is an important influence factor of hydrologic mechanism.The water level,the period of flooding and the range of water level all affect the distribution of plants in the wetland ecosystem,the growth strategy,the population and quantity of microorganisms,and the physical and chemical properties of wetland.We can sufficient use the ecological factor of being controlled to improve the physical and chemical properties of the wetland environment,so that plants and microorganisms get better growth,to a certain extent,improve system capacity to remove pollutants and slow down the phenomenon of wetlands clogging.By building simulation devices of horizontal subsurface flow constructed wetlands,four wetlands(CW1?CW2?CW3?CW4)which setting up the frequency of water-level fluctuation respectively as 0cm/d?3cm/d?6cm/d?9cm/d were set up to explore the impact of water-level change on nitrogen removal in constructed wetlands.And we made it through the study of four groups of physical and chemical indicators,microbial nitrification and denitrification intensity,as well as the differences between plants and other indicators of variation and the correlation between indicators and nitrogen removal rate.The methods and results used in the study were as follows:(1)The physical and chemical indexes of water quality were determined by national standard method,to investigate the changing regularity of physical and chemical indexes in different water level fluctuation.The average concentration of dissolved oxygen had significant differences,and the average concentration of dissolved oxygen increased with the frequency increasing,but the variation range of pH and Eh were smaller during the whole experiment.COD and TP had no significant differences,but the removal rate of COD was high(> 85%)and the removal rate of TP was low(about 20%).The removal rate of TN varied with the variation of water level fluctuation.The CW3 system had the highest removal rate,CW1 system had the lowest removal rate,CW2 and CW4 system had no significant difference on TN removal.The removal rate of ammonia nitrogen was significantly different,and the removal rate increased with the increase of water level fluctuation.The change of nitrate nitrogen was not obvious.(2)During the experiment,the nitrification and denitrification intensity were measured,the nitrification and denitrification intensity of the four wetland systems had difference.The nitrification intensity increased with the increasing of water level,but the difference between CW3 and CW4 was not significant,which indicated that the nitrification intensity had a certain increasing limiting;the nitrification intensity decreased along the wetland,and the upper layer was significantly higher than that of lower layer.The denitrification intensity was opposite to that of nitrification.CW1(under the condition of static water level)was the highest of all.The denitrification intensity decreased with the increasing of water level,but the difference between CW2 and CW3 was not significant.The denitrification intensity increased first and then decreased along the wetland,while the upper layer of CW1 and CW2 systems were higher than the lower layer in the depth direction,the lower layer of CW3 and CW4 systems were higher than the upper layer.(3)There were also some differences in plant related indexes of four wetland systems,and the variation of different plant indices was different.The biomass dry weight of Typha orientalis were significantly different from CW1?CW2 and CW3?CW4.The biomass dry weight of Oenanthe javanica were significantly different between CW1?CW4 and the other three systems,but CW2 and CW3 had not significant difference.The proportion of dry weight distribution of underground biomass in the plants was also different,and the distribution proportion of above-ground part was basically increased with the increasing of water level fluctuation.The ROL and POR had similar variation of two plants,and they all decreased with the increasing of water level,but increased comparing with before the plant culturing.(4)There were certain correlations between the indexes of experimental study,and there were linear correlation between them and nitrogen removal rate.There was a positive correlation between dissolved oxygen concentration and nitrification intensity,nitrification intensity and ammonia nitrogen removal rate.There was a positive correlation between the total root length and total root surface area and total root volume,and the total root surface area also had a positive correlation with total root volume of Typha orientalis.ROL of Typha orientalis was positively correlated with total root length and total root surface area.There was a significant positive correlation between dry weight of above-ground biomass and dry weight of below-ground biomass of Oenanthe javanica.ROL of Oenanthe javanica was positively correlated with POR.The removal rate of ammonia nitrogen was negatively correlated with the total root volume,the root volume of Typha orientalis and the dry weight of under-ground biomass of Oenanthe javanica.While the total nitrogen removal rate was negatively correlated with dry matter weight of above-ground biomass of Typha orientalis.