| With the gradual control of point source pollution in recent years,the environmental quality of urban water is greatly affected by non-point source pollution caused by storm water runoff and other factors.In terms of the treatment of rainfall runoff,the land percolation treatment system has become one of the main treatment methods at home and abroad,and the artificial rapid percolation system has been applied for many times due to its advantages of low investment cost,good effluent effect and simple operation and management.However,different media combination schemes and operating conditions directly affect the processing capacity of the artificial rapid filtration system.Therefore,through the optimization of media combination and operation process,this paper optimizes the best construction scheme suitable for Beijing area,and further improves the purification performance of the system through the modification of vermiculite.Through the screening of five natural medium such as vermiculite,vermiculite has the best adsorption effect on nitrogen and phosphorus,with the maximum theoretical saturated adsorption capacities of 686.27 mg/kg and 667.47 mg/kg,respectively.By combining vermiculite with soil and sand in different proportions,the optimal medium was selected:the ratio of soil to sand was 1:1,vermiculite was added with 15%and limestone was mixed with 8%.The average removal rates of ammonia nitrogen,TP and COD are 90.5%,73.1%and 57.6%respectively.Through the optimization of operating conditions,it was determined that the hydraulic load was 2m/d(water for 3 h,dry for 45 h),the thickness of soil layer was 70?100 cm,and the operation mode of segway water inlet with 2:1 ratio from the surface of the medium and 30 cm from the surface had the best effect,The removal rates of COD,TP,ammonia nitrogen and TN were 87.7%,90.1%,89.5%and40.5%,respectively.During the submerged-water stage,the system adsorbed and trapped organic matters such as nitrogen and phosphorus in the medium,degraded nitrogen and organic matters under the action of microorganisms when the system dried,and provided a certain carbon source for the anaerobic section of the system to support the denitrification reaction of this section,further promoted the complete removal of nitrogen.According to the detection of microorganisms in different soil depths,the microbial communities in each layer of the artificial rapid percolation system were abundant,with Proteobacteria and Actinobacteria as the dominant bacteria groups.Through pilot operation,it had been verified that the artificial rapid filtration system can maintain certain stability under the condition of long-term operation,and its removal rates of COD,TP,ammonia nitrogenand TN were 69.3%,59.3%,85.1%and 75.7%,respectively.Under the condition of high concentration water inlet,better treatment effect can be achieved,and Constructed Rapid Infiltration system also has certain removal effect on low concentration water inlet.In order to further improve the purification ability of the system,chitosan as modified vermiculite,modification of vermiculite adsorption effect of ammonia nitrogen increased by 18.4%,The adsorption effect of phosphorus was also improved,chitosan by hydrogen bonding load in vermiculite surface,did not insert into the vermiculite,modified vermiculite Na~+content increased,specific surface area increased significantly.Under the above optimal operating conditions,the removal rates of TP,COD,ammonia nitrogen and TN were 90.2%,91.8%,94.4%and 46.5%,respectively.The removal effect of TP,COD,ammonia nitrogen and TN was better than that of unmodified vermiculite column,which can achieve the purpose of treating organic pollutants such as nitrogen and phosphorus in rainwater runoff. |
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