Studies On Enhancement Of Nitrogen Removal Characteristics In Biochar Tidal Flow Construted Wetland

The strong re-oxygenation capacity of tidal flow constructed wetland can efficiently oxidize NH₄⁺-N to NO₃^--N,so that the nitrification reaction is no longer the rate-limiting step of TN removal,but the rapid degradation of COD can easily lead to a lack of organic carbon for denitrification,so that the denitrification reaction is inhibited,affecting the removal of TN.Therefore,supplementing carbon sources to increase denitrification efficiency is the key to enhancing the removal efficiency of TN in constructed wetlands.Biochar is a kind of carbon-rich material with a well-developed pore structure.It can be used as a new type of organic carbon source to strengthen the denitrification process of low C/N ratio sewage,and can also be used to adjust the oxygen environment of wetland.At present,there are few researches on using biochar as the matrix of tidal flow constructed wetland to strengthen the operation effect of wetlands.The project establishes biochar tidal flow constructed wetland and gravel tidal flow constructed wetland,and analyzes the impact of biochar on the removal efficiency of contaminants in tidal flow constructed wetland.Study the enhanced nitrogen removal characteristics of a biochar tidal flow constructed wetland.In this paper,the physical and chemical properties of biochar and gravel were measured,and the re-aerobic capacity of the different constructed wetlands under long-term operating conditions and the overall removal effect of various pollutants were analyzed,and the different forms of nitrogen were analyzed during the tidal operations cycle of constructed wetlands.Transformation and degradation characteristics and reoxygenation rules,linear coupling of multiple water quality parameters and the denitrification of wetlands,and analysis of the effects of pollutant composition and load on the denitrification effect of tidal flow constructed wetlands,reached the following conclusions:（1）The specific surface areas of biochar and gravel were 780 m²/g and 3.08 m²/g.The isothermal saturated adsorption capacity of NH₄⁺-N was 1.411 mg/g and 0.702mg/g,,and isothermal saturation adsorption of TP were 0.62 mg/g and 0.142 mg/g.（2）The hydraulic loading is 0.27m³/（m²·d）,the influent TN,NH₄⁺-N,NO₃^--N,and NO₂^--N concentrations are 25.16±1.10mg/L,22.12±1.43mg/L,2.21±0.26mg/L and0.15±0.18mg/L,the removal rate of TN in biochar tidal flow constructed wetland and gravel tidal flow constructed wetland were 86.92%and 32.61%.Due to the stronger reoxygenation capacity,the biochar tidal flow constructed wetland has achieved 98.81%efficient removal of NH₄⁺-N,which is much higher than the removal rate of 74.11%of gravel tidal flow constructed wetland.The effluent NO₃^--N concentration of the biochar constructed wetland was 2.55±1.84mg/L and 10.08±2.15mg/L.The biochar could be used as a slow-release carbon source to promote the denitrification reaction,and the effluent NO₃^--N of the biochar constructed wetland was significantly lower than that of gravel constructed wetlands（p<0.05）.The removal rate of NO₂^--N in biochar constructed wetlands was 88.09%,while the concentration of NO₂^--N in the effluent of the gravel constructed wetland was significantly higher than that in the influent（p<0.05）,indicating that the biochar constructed wetland has a strong denitrification stability.（3）During the first stage of tidal operation cycle,the removal rate of NH₄⁺-N from biochar tidal flow constructed wetland was 93.35%.Because of the release of carbon source from biochar,it has a certain role in promoting denitrification,and NO₃^--N is not significant accumulation occurred.The nitrification and denitrification process was completed synchronously.The removal rate of TN was 83.33%.Due to poor reoxygenation capacity and lack of endogenous carbon source recharge,the removal rate of NH₄⁺-N in the gravel constructed wetland was only 49.61%.And there was a significant accumulation of NO₃^--N,and the TN removal rate was 31.26%.During the second phase of tidal operations,the removal rate of NH₄⁺-N from biochar constructed wetlands increased to 98.81%,the removal rate of TN increased to 86.92%,and the removal rate of NH₄⁺-N from gravel constructed wetlands reaching to 74.11%,the removal rate of TN increased to 36.21%.（4）The oxygen environment created by tidal operation can effectively reduce the inhibitory effect of increasing organic matter concentration on the removal of NH₄⁺-N.When the influent COD/NH₄⁺-N value is 10.22¹7.96,The removal rate of NH₄⁺-N per unit area in the biochar constructed wetland and gravel constructed wetland are 5.15⁶.91 g/（m²·d）and 3.15⁵.73 g/（m²·d）.Because the organics were degraded efficiently in the early stage of the reaction,when influent COD/NO₃^--N was 94.62¹79.24,and COD/TN was 9.31¹4.92,influent organic matter did not significantly promote denitrification.As a slow-release carbon source,biochar can increase the effluent COD/TN value from 0.77³.11 to 1.89¹03.92,making up for the lack of carbon source to suppress the denitrification reaction defects,and further enhancing the removal efficiency of TN by the constructed wetland to make outflow TN concentration decreased significantly.（5）When the influent TN load is 6.22⁷.53g/（m²·d）,the removal rate per unit area of TN by biochar tidal flow constructed wetland and gravel tidal flow constructed wetland is 4.43⁶.95g/（m²·d）and 1.3³.74g/（m²·d）,the linear fitting relationship with the influent load is poor.There was a significant difference in the removal of NH₄⁺-N between the two constructed wetlands（p<0.05）.When the influent load was5.18⁷.07 g/（m²·d）,the removal of NH₄⁺-N per unit area of the biochar constructed wetland was 5.15⁶.91g/（m²·d）can effectively resist the fluctuation of the influent load.The removal of NH₄⁺-N per unit area is 3.15⁵.73g/（m²·d）in the gravel constructed wetland,with a poor ability resisting the impact of influent water loading.