Studies On Influencing Factors And Reduction Measure Of Ammonia Gas Emissions In Constructed Wetland

With the invention of the ammonia process,the world’s agricultural production capacity has been greatly increased,which in turn contribute to rapid population growth.People’s demand for resources is increasing day by day.In order to ensure high and stable yield of agricultutal and animal husbandry products,the input of nitrogen in production is increasing constanaly.Excessive nitrogen leads to low nitrogen utilization efficiency,and a large number of active nitrogen（NH₃,N₂O,NOx,NO₃^-）enter the environment,resulting in a series of environmental problems.The rapid development of agriculture,livestock and poultry industry not only brings great pressure to the environment,but also brings great challenges to sewage treatment.Constructed wetland,as a new sewage purification technology,has been more and more widely used in the field of water treatment because of its advantages of good treatment effect,simple process,low operation capital and convenient maintenance,and the number and use area of construced wetlands are increasing year by year.However,the comprehensive environmental benefits of constructed wetlands are greatly reduced due to the harmful ammonia gas produced in the process of sewage purification.Therefore,it is urgent to identify the influencing factors of ammonia emissions in constructed wetlands,and to explore ammonia emissions and propose ammonia emission reduction measures.In this study,the influencing factors of water purification and ammonia emission of constructed wetlands were investigated by mesocosm-scale ecological simulation experiment by using livestock and poultry breeding wastewater as treating target.The optimal influent chemical oxygen demand（COD）/nitrogen（N）and the optimal influent pH are obtained.The effectiveness of biochar addition on pollutants removel and ammonia emission reduction were discussed and verified.The main conclusions of this study are as follows:（1）Influent COD/N significantly affects pollutant removal and ammonia emission in constructed wetland by providing electron donor and energy source.The removal rate of NH₄⁺-N increased with the decrease of influent COD/N ratio,and ite highest value（91%）appeared in the treatment group with influent COD/N ratio of 2.5.The change of influent COD/N ratio has no significant difference in COD removal.For ammonia discharge,the cumulative discharge flux of ammonia increased with the increase of influent COD/N ratio.The lowest cumulative ammonia discharge flux（60.48μg/m²）and the lower daily ammonia discharge filx occurred in the treatment group with influent COD/N ratio of 2.5.When the influent COD/N ratio is 2.5,the daily discharge flux of ammonia gas is also at a low level.Microbial community analysis showed that the treatment group with influent COD/N ratio of 2.5 had the largest Shannon index and the highest community diversity.At the same time,the relative abundance of microorganisms related to the degradation of organic matter and the useful role of nitrification and denitrification in sewage are also relatively high.（2）Influent pH affects wetland pollutant removal and ammonia emission through the synergistic effect of physic,chemictry and microorganism.With the increase of influent pH,the removal rate of NH₄⁺-N in constructed wetland gradually decreased,but had no significant effect on COD removal.The removal rate of NH₄⁺-N（68%）was the highest in the treatment group with pH 6.For ammonia emission,the cumulative emission flux of ammonia increased with the increase of influent pH（except for the treatment group with influent pH 9）,and the lower cumulative emission flux of ammonia（100.30-102.31μg/m²）occurred in the treatment groups with influent pH 5 and 6.When the influent pH is 5 and 6,the daily ammonia emission flux is relatively low,and the peak value is significantly lower than that of the other treatment groups.Microbial community analysis showed that Shannon value was relatively larger in the treatment groups with pH 5 and 6,indicating the high diversity of microbial community in constructed wetlands.（3）The environmental temperature noe only affects physical and chemical interactions in constructed wetlands,but also affects the purification effect of pollutants in constructed wetlands by affecting microbial activity,community structure and relative abundance in CWs.With the increase of ambient temperature,the removal rates of NH₄⁺-N in constructed wetlands gradually increased,and the highest NH₄⁺-N removal rate（87%）occurred when the ambient temperature was about 20℃.However,ambient temperature has no significant effect on COD removal in constructed wetland.For ammonia emission,with the increase of ambient temperature,the cumulative ammonia emission flux in the constructed wetland gradually increased,and the lowest ammonia emission flux（86.69μg/m²）occurred when the ambient temperature was low（about 0℃）.Meanwhile,the peak value of ammonia emission flux in the treatment group with the ambient temperature of 0℃was also significantly lower than that in the other treatment groups.Microbial community analysis showed that with the increase of ambient temperature,the microbial species and community diversity in constructed wetland increased significantly,and the relative abundance of microorganisms involved in nitrification and denitrification in constructed wetland also increased significantly.（4）Biochar has strong adsorbability due to its large specific surface area,developed pore structure and abundant surface functional groups,which can effectively remove nitrogen and organic matter in constructed wetlands.Compared with the control group without biochar addition,the removal rates of NH₄⁺-N,TN and COD in the constructed wetland with biochar addition increased significantly,and the removal rates all increased with the increase of biochar addition proportion.The highest NH₄⁺-N removal rates（73%）,TN removal rates（76%）and COD removal rates（99%）occurred in the treatment group with 50%biochar addition.For ammonia emission,the cumulative ammonia emission flux was significantly lower in the 50%（45.4μg/m²）biochar addition group than in the control group（66.5μg/m²）,but significantly higher in the 10%（161.6μg/m²）biochar addition group than in the control group（66.5μg/m²）.Similar to this trend,the peak value of ammonia emission flux in the treatment group with 50%biochar addition was also significantly lower than that in the other treatment groups.Microbial community analysis showed that the addition of biochar could effectively improve the number of microbial species and community diversity in constructed wetlands.