Pollution Characteristics Of Initial Rainfall Runoff In Typical Urban Residential Areas And Removal Efficiency Of Nitrogen And Phosphorus By Biochar

The initial rainfall runoff in residential areas is one of the important sources of urban non-point source pollution.The main pollutants include nitrogen,phosphorus,heavy metals,organic matter,suspended particulate etc.,which can lead to eutrophication of receiving water body or the generation of black and odorous water body,thus causing harm to the water environment safety and urban landscape in urban residential areas.At present,the research focuses on the pollutant concentration level of urban traffic runoff,commercial district,industrial district,but the research on the pollution characteristics and pollutant reduction methods of initial rainfall runoff in residential areas is relatively insufficient.This study takes the residential areas of typical cities（Wuxi and Nanjing）as the research object,in which the methods of field monitoring and simulation experiment were adopted,and focused on the research of initial rainfall runoff coefficient,main pollutant characteristics and pollution load of runoff on different underlying surfaces.Through multi-channel characterization,adsorption kinetics,thermodynamics and real water experiments,the characteristics and adsorption mechanism of different biochar materials were studied,and the adsorption performance and reduction effect of biochar materials on the main pollutants in the initial rainfall runoff were explored,so as to provide reference for the ecological interception and theoretical support to control engineering practice of urban rainfall runoff pollution.The main conclusions of this study are as follows:（1）Under the same conditions,the surface runoff coefficient of different underlying surface types showed decreasing order:roof>traffic avenue>road in residential area>parking lot>park green space,the main reason may be caused by the difference of water permeability of underlying surface;the runoff coefficient increases significantly with the increase of rainfall intensity,and the surface runoff coefficient of roof,traffic avenue,road in residential area,parking lot and park green space increases respectively.The surface runoff coefficients were 0.96-0.97,0.95-0.97,0.84-0.87,0.52-0.55 and 0.09-0.23,respectively.The main reason may be that the underlying surface type in residential areas was mostly impermeable.（2）The main pollutants in the initial rainfall runoff of typical residential areas were nitrogen and phosphorus.The concentrations of total nitrogen（TN）,total phosphorus（TP）,nitrate nitrogen（NO₃^--N）and ammonia nitrogen（NH₄^--N）vary from 2.18mg/L to 10.09mg/L,0.39mg/L to 2.32mg/L,0.12mg/L to 3.19mg/L and 0.12mg/L to 2.56mg/L,respectively.The contents vary greatly with rainfall duration,season and underlying surface.From the analysis of rainfall duration,the concentration of pollutants in the initial rainfall runoff is higher,and with the extension of rainfall duration,the concentration of pollutants mainly showed a decreasing trend firstly and then becoming stable.In the analysis from different seasons,the pollution degree of rainfall in the four seasons showed decreasing order as:summer>spring>autumn>winter,which is mainly due to the intensive rainfall and rainfall frequency in summer;comparative analysis of the average concentration of pollutants in each time shows that the pollution process of rainfall in the four seasons showed decreasing order as:spring>autumn>summer>winter.It may be due to the long dry period before spring rainfall.From the analysis of different underlying surface types,the pollutant concentrations in the runoff of traffic avenue,path flow in residential area and park green space are higher than those of other underlying surfaces.It is found that the high correlation coefficient between suspended particulate matter（SS）and heavy metal pollutants,which may be related to the homology of heavy metals.（3）The annual pollution loads of TN and TP per unit area of traffic avenue,roads in residential area,roof,park green space and parking lot were 4.17,2.82,0.88,0.69,0.62kg/hm² and 1.78,0.56,0.22,0.06,0.18kg/hm² respectively.Analysis from the estimated pollution load per unit area,the pollution load per unit area generated by the runoff of traffic avenue is the highest,but the roads area in residential area was 4.76 times of traffic avenue,so the total pollution load generated by the roads flow in residential area was the highest.According to the estimated pollution load of 14 initial rainfall runoff,the large rainfall lead to the higher pollution load may be.（4）Batch adsorption experiments showed that the modified iron biochar was the best adsorbent,and magnetic modified iron biochar could realize the secondary recovery of adsorbent.The adsorption capacity of NO₃^--N and PO₄^3- was the highest under acid condition,and the fitting effect of second order kinetic equation and Freundlich thermodynamic equation were better.The highest adsorption capacity of TN and TP in the actual polluted water can reach 24.24mg/g and 3.58mg/g.For reducing the load of TN and TP in the actual rainfall runoff,modified iron biochar（BC-Fe）has the least addition and the lowest cost,but magnetically modified iron biochar（BC-Fe₃O₄）has the advantages of better recyclable and no secondary pollution.