Study On The Reduction Of Surface Runoff Pollution In The Biological Retention Pond Conditioned By Biochar/composite Microbial Agents

With the acceleration of urbanization,surface runoff carries a large amount of pollutants into receiving water bodies,leading to eutrophication of water bodies such as rivers and lakes.In this thesis,the two tributaries in the Xianghe area of the North Canal have been facing the problem of surface runoff pollution.The pollution status of the tributaries and the water quality of surface runoff were analyzed.On this basis,the reactors of biological retention ponds conditioned with biochar and composite microbial agents were constructed.The thesis studied the efficiency of pollutant removal under different operating conditions,analyzed the changes in the microbial community structure of the device,and proposed a process plan for building a functional biological retention pond on the river bank to reduce surface runoff pollution.The main findings were as follows:The main pollutants in the two tributaries in the Xianghe area of the North Canal were ammonia nitrogen and total phosphorus,and the water quality gradually improved from upstream to downstream.The initial surface runoff pollutant concentration was significantly affected by the dry period before rain and rainfall intensity.The initial pollution load of surface runoff from the road was much higher than that of the roof.The annual pollution inputs from the initial surface runoff to the two tributaries were estimated as:COD 469.4 t/a,ammonia nitrogen 25.3 t/a,TN 38.3 t/a,and TP 2.1 t/a.The biological retention tank conditioned by the biochar/composite microbial inoculants possessed the higher efficiencies on pollutant removal than those of the unconditioned biological retention tank under different working conditions.The research results showed that the biochar particularly promoted the reduction of nitrogen.The removal of phosphorus using biochar was limited,while the compound microbial inoculants had the excellent effects.Under the optimal hydraulic retention time,the COD,TN,ammonia nitrogen,and TP concentrations of influent were 200 mg/L,12 mg/L,10 mg/L and 1.5 mg/L,respectively.The COD and TP concentrations of the effluent from the conditioned biological retention pond reached level IV of surface water quality,and the ammonia nitrogen effluent concentration reached level V.The average removal rates of COD,TN,ammonia nitrogen and TP were increased by 4.18%,8.05%,14.67%and 9.52%,respectively,compared with unconditioned ones.The biological retention pond conditioned by biochar/composite microbial agents could resist the impact of high pollution load.When operating under high pollution load conditions,the COD,TN,ammonia nitrogen,TP concentrations of influent were 300 mg/L,15 mg/L,12 mg/L and 2 mg/L,respectively.The effluent COD,ammonia nitrogen and TP concentrations of the conditioned biological retention tank all attained the surface water standard of level V,and the pollutant removal rate was higher under the condition of lower pollution load.When operating in the alternate dry and wet state,the drought interval had a greater impact on the removal of nitrogen.The removal of phosphorus was not as high as that of nitrogen,and there was no significant difference in the removal of ammonia.However,the conditioned biological retention pond had a faster recovery capacity,and the diversity and abundance of microorganisms in the conditioned biological retention pond device were increased compared with the unconditioned one.Based on on-site investigations and combined with experimental results,a demonstration project of biological retention ponds was designed on the bank of Xiaoyoufa Canal.The technique flow was:the runoff flows through the curb gap into the buffer zone to pre-treat the surface runoff in advance;then the runoff is discharged into the biological retention tank for treatment;and finally runoff is directly entered into the municipal rainwater pipe network after treatment.The area of the biological retention pond designed for the demonstration project is 480 m~2;the depth of the pond body is 0.75 m;and the effective storage volume is 790 m~3.The expected effluent of the system meets the GB3838-2002 level V standard.