Study On The Effect Of Biochar On Nitrogen Removal From Multi-stage Soil Infiltration System

Rural domestic wastewater in southwest China has significant water quality and quantity fluctuations,and contains nitrogen.Current systems for treating rural domestic wastewater have poor denitrification effects,and discharge into rivers and lakes can have a bad impact on the environment,so it is necessary to set up a high-efficiency denitrification treatment system in rural areas.Multi-soil-layering（MSL）system is a new wastewater treatment process with high capacity and long life for soil purification of wastewater.In this study,biochar was used as a filler in the soil layer of MSL system for treating rural domestic wastewater,and straw filler was used as a control group for 45 days of continuous operation under four different influent conditions of high hydraulic continuous influent,medium hydraulic continuous influent,low hydraulic continuous influent,and peak influent.The effects of biochar-based MSL system on rural domestic wastewater treatment were analyzed;the complex nonlinear relationship between nitrogen removal effect and pollutant removal in biochar-based MSL system was analyzed using stepwise cluster regression,and the nitrogen removal mechanism was deeply analyzed from the community structure of microorganisms.The main conclusions are as follows.（1）The biochar-based MSL system was significantly more effective than the straw-based MSL system for pollutant removal and had the best overall performance under the low hydraulic load condition of continuous influent.The removal rates of TN,NH₄⁺-N,NO₃^--N,and COD_Cr by the biochar-based MSL system under this condition were 93.03%,98.30%,69.90%,and 87.21%.The effluent concentrations of TN and NH₄⁺-N from the biochar-based MSL system during the operation period reached the limits of the primary standard in Sichuan Province’s Emission Standards for Water Pollutants from Rural Domestic Wastewater Treatment Facilities（DB51/2626-2019）.（2）In the stepwise cluster regression model,the factors with strong correlation with the effect of TN removal rate were NH₄⁺-N removal rate,COD_Crremoval rate,NO₃^--N removal rate,and p H.Through the correlation analysis,it was found that the changes of the internal factors of the biochar-based MSL system demonstrated that the system had better conditions for nitrogen removal.（3）The removal of nitrogen in MSL systems is the result of the combined action of substrate adsorption and nitrification-denitrification of microorganisms,and the nitrogen conversion pathways of conventional nitrification-denitrification and simultaneous nitrification-denitrification exist in both MSL systems.（4）The microbial community structure demonstrated the differences in the denitrification flora of the two MSL systems,with the dominant phyla of the biochar-based and straw-based MSL systems being Amoebacteria and Thick-walled bacteria,respectively.The nitrification of both systems relied mainly on heterotrophic nitrifying bacteria.The denitrifying bacteria were more abundant in the biochar-based MSL system,reaching a relative abundance of 32.4%in its soil layer,which was significantly higher than that of 6.1%in the soil layer of the straw-based MSL system.And heterotrophic nitrifying-aerobic denitrifying bacteria spp.were present in the soil layer of the biochar MSL system with a relative abundance of 8.58%.