Enhancement Of Nitrogen Removal Of Low C/N Wastewater Using A Biochar-amended Aerated Vertical Flow Constructed Wetland

With the development of economy and improvement of the living standard of the residentsin in China,the higher nitrogen concentrations of municipal domestic sewage have been observed,resulting in the rise of low ratio of carbon to nitrogen（C/N）sewage discharge.Although there are many technologies and methods for low C/N wastewater treatment,such as activated sludge process.However,it has a series of disadvantages,such as the higher operating cost,existing the secondary pollution.In recent years,constructed wetlands（CWs）have been widely used for wastewater treatment owing to low operating cost,easy maintenance and great landscaping.However,traditional CWs could not achieve high removal efficiencies for treating low C/N wastewater due to insufficient dissolved oxygen（DO）and organic carbon sources,resulting in low total nitrogen（TN）removal.In this study,intermittent aeration and biochar were used to enhance the TN removal rates of low C/N wastewater in vertical flow constructed wetlands（VFCWs）.Firstly,the optimal aeration time and aeration rate were determined,and studied the long-term wastewater purification efficiency by comparison with non-biochar and non-aeration VFCW,biochar and non-aeration VFCW as well as non-biochar and aeration VFCW.Then,the nitrogen removal efficiencies in biovhar-amended intermittently aerated VFCWs under different influent strengths were also studied.Finally,the content of dissolved organic matter（DOM）released by fresh biochar and aged biochar was analyzed by Three-dimensional Fluorescence Spectroscopy（EEM）and UV-visible absorption spectrum,and analysed the mechanisms of denitrification enhancement by biochar.The main conclusions of this study are as follows:（1）Fisrtly,the optimum operating condition of biochar amended intermittently aerated VFCWs has been identified.The results indicated that higher pollutants removal efficiencies could be achieved in biochar amended intermittently aerated VFCWs under the aeration time of 2 h and aeration rate of 0.6 L min^-1.Then,under this optimal condition,the higher removal efficiencies of COD,NH₄⁺-N and TN were observed in the biochar amended intermittently aerated VFCW compared with non-biochar and non-aeration VFCW,biochar and non-aeration VFCW as well as non-biochar and aeration VFCW,which were 95.9%,97.6%and 74.7%,respectively.Furthermore,the N₂O emissions showed obviously in non-biochar and non-aeration VFCW,biochar and non-aeration VFCW,non-biochar and aeration VFCW and biochar and aeration VFCW,and the average N₂O emissions fluxes were 646.9μg m^-2 h^-1、505.33μg m^-2 h^-1、398.92μg m^-2 h^-1 and 296.9μg m^-2 h^-1,respectively,indicating that biochar and intermittent aeration could be conducive to reducing N₂O emissions in VFCWs.（2）The variations of the removal efficiencies of COD,NH₄⁺-N and TN in biochar-amended intermittently aerated VFCWs under different influent strengths were also mastered.In the non-aeration VFCWs,much higher removal of COD,NH₄⁺-N and TN were achieved under different influent strengths in the biochar added VFCWs compared with the non-biochar added VFCWs,and the lower N₂O emissions also investigated in the biochar added VFCWs.In the aerated VFCWs,the removal efficiencies of COD,NH₄⁺-N and TN under different influent strengths were significantly higher than those in the non-aeration VFCWs.There is no influence on COD removal efficiency in the aerated VFCWs with increasing influent strength,and the removal efficiencies were all achieved 95%.Nevertheless,with the increase of influent strength,the removal efficiencies of NH₄⁺-N and TN decreased gradually.By contrast,the TN removal efficiencies under different influent strengths in the biochar added VFCWs were higher than those in the non-biochar added VFCWs.Furthermore,biochar addition could reduce N₂O emissions,which was in favor of slowing down the greenhouse effect.（3）The releasing characteristic of dissolved organic matter（DOM）by biochar was determined,and the mechanism of reinforcement of denitrification by biochar was illuminated preliminary.The results showed that the concentration of DOM released by aged biochar was much higher than that by fresh biochar.Based on the characteristic parameters analysis,the spectral slope ratio（S_R）,the biological index（BIX）and the freshness index（β:α）of DOM released by aged biochar were significantly higher than those by fresh biochar.Besides,the humification index（HIX）and SUVA₂₅₄ of DOM released by aged biochar were lower than those by fresh biochar,indicating that a portion of DOM released by biochar itself could not degraded by microorganisms.Biochar could improve denitrification mainly by the process of adsorption-desorption of influent organic matters.