Simulation Study Of Enhanced Bioretention Cell To Reduce Campus Runoff Pollution

In view of the increasingly serious urban surface pollution problem and the challenge of flooding in extreme weather,it is urgent to promote the development of sponge cities.Bioretention cell is a popular technology because of its ability to reduce stormwater runoff,delay peak flood times and remove pollutants.As the nitrogen removal effect of traditional bioretention cell is not satisfactory in practice,this study was conducted to find the combined media ratio with better overall performance through the filter column experiment.A lab-scale simulation of double-layer media bioretention cell was set up to investigate the combined effect of plants,media and microorganisms,and evaluate the operation effect of bioretention cell,resistance to hydraulic load and pollution load.The ability of lab-scale bioretention cells to treat nitrate pollution using either inorganic or organic electron donors was clarified to understand the relationship between the electron donors and the effectiveness of denitrification.By constructing a SWMM model for the study area,the stormwater storage and pollutant removal capacity of bioretention cells were illustrated to provide a reference for the application of bioretention cells.The main research results are as follows.（1）The optimization and proportioning of the medium materials were carried out using the filter column experiment.In the screening process of individual media,dolomite and zeolite showed a satisfactory infiltration ability.Meanwhile,the first four media with fine removal efficiency on NH₄⁺-N and TP were iron-modified biochar,dolomite,limestone and zeolite.The combination of dolomite,limestone,zeolite and iron-modified biochar was finally selected considering the infiltration,adsorption performance and lifetime of the media.When the ratio of the combined media was 50%dolomite+25%zeolite+25%iron modified biochar,the removal efficiency of nitrogen and phosphorus reached more than95%,and the disturbance to the p H of natural water environment was small.（2）Lab-scale bioretention cells were carried out to study the treatment effect and ability to resist different load.The constructed double-layer media bioretention cell maintained the removal efficiencies of TN and TP in simulated runoff at 84.8%-89.2%and78.3%-93.9%,and had a better performance in removing NO₃^--N with a removal efficiency of 91.28%.Microbial abundance and diversity were lowest in the upper medium,and the dominant genera were Acidovorax,Saccharimonadaceae and Curvibacter,which mainly played the role of degrading organic matter and heterotrophic nitrification-aerobic denitrification,and the setting of the flooded zone in the lower medium promoted the relative abundance of denitrifying bacteria of Burkholderiaceae in the soil.The perform of bioretention cell showed certain resistance to hydraulic load and pollution load,and the control efficiency of total runoff was 88%and above,and the removal efficiencies of TN and TP were maintained above 80%and 93%respectively.（3）The role of different electron donors was discussed via the operation of lab-scale bioretention cells.The addition of pyrite significantly improved the removal capacity of bioretention cell for NO₃^--N（P<0.05）,and the removal efficiency of NO₃^--N was maintained between 95.2%and 100.0%.By contrast,the addition of paulownia leaves reduced the removal efficiency of NO₃^--N and the maximum efficiency was only 38.2%,accompanying with the high concentration of organic matter in effluent which was not environmental-friendly.Pyrite increased the relative abundance of Thiobacillus in the soil and promoted the process of autotrophic denitrification,while phoenix tree fallout promoted the relative abundance of Enterobacter,Ruminiclostridium＿1,Chlorobaculum and other genera associated with plant decay and inhibited the growth of nitrifying and denitrifying bacteria.（4）A model was constructed to further clarify the effectiveness of bioretention cell applications.The SWMM model was constructed for the campus of Huazhong Agricultural University,and the relevant hydrological,hydraulic,water quality and double-layer media bioretention cell related parameters were set up as well.Compared to the study area without bioretention cell,the study area with bioretention cell could control 98.0%of surface stormwater runoff,82.2%reduction of peak flow,8 overloaded stormwater pipes.Furthermore,an average increase of 77.9%and 64.0%in TN and TP removal under the special rainfall event simulated by the long rainfall period.This suggested that the use of bioretention basins has the potential to both effective runoff control and improved interception of nitrogen and phosphorus.