| Bioretention systems,as a typical Low Impact Development(LID)facility in sponge cities,achieve the purpose of absorbing rainwater at source and reducing runoff pollution through the selection of fill types and combinations.Under long-term operation,the bioretention system’s operational life decreases due to the retention and adsorption of pollutants by the system’s media,which leads to the decay of the bioretention system,mainly in the form of increased infiltration time,increased overflow frequency and the formation of highly polluted layers.In this thesis,we constructed a bioretention system with conventional media(Bioretention Soil Media,BSM),modified media:BSM+10%flyash,BSM+3%biochar,BSM+10%construction waste,BSM+5%air-dried water treatment residual(WTR-1),BSM+5%pyrolytic water treatment residual(WTR-2),six bioretention soil columns were subjected to Total Suspended Solids(TSS)and composite pollution impact conditions to investigate the hydraulic conductivity of the modified bioretention system through simulated water distribution experiments.The study investigated the decay of the hydraulic conductivity of the modified bioretention system through simulated water distribution experiments;identified the causes of fill blockage and its formation mechanism;and conducted a life-cycle environmental impact assessment of the modified bioretention system from construction to operation to scrapped.The main findings of the study are as follows:(1)Total suspended solids were added to 1×10-3 mol/L CaCl2 solution as the experimental influent water,and the effect of the bioretention system on the regulation of total suspended solids and its hydraulic conductivity change process were investigated through 17 field water distribution experiments.The results showed that the conductivity and the concentration of TSS in the effluent of the system gradually decreased,and the TSS was mainly retained and adsorbed on the surface layer of the media,gradually forming a layer of filter cake.At the end of the experiment,the permeability and porosity of the 0～20cm layer of the BSM,BSM+5%WTR-1 and BSM+10%construction waste systems decayed significantly compared to the 20～50cm layer,with the permeability of the 0～20cm layer of the three systems decaying by an average of 51.4%compared to the initial permeability.The permeability coefficient of the 0～20 cm layer of the three systems decayed by 51.4%on average and the 20～50 cm layer by 28.5%on average;the porosity of the 0～20 cm layer of the three systems decayed by 60.5%on average and the 20～50 cm layer by 30.0%on average compared with the initial porosity.(2)Under composite pollution impact conditions,40 simulated rainfall experiments were conducted in three stages to study the pollution load reduction capacity and hydraulic conductivity of bioretention systems,and to clarify the causes of bioretention system blockage and its formation mechanism.The results show that the pollutant removal capacity of each system shows a weak-strong-weak trend in the three stages,with the best and stable effluent effect in stage Ⅱ.The accumulation of pollutants in the media caused the stable infiltration rate of each system in stages Ⅱ and Ⅲ to be significantly lower than that of the initial stage and stage Ⅰ.The accumulation of pollutants in the 0～20cm layer was also greater than that in the 20～50cm layer.the clay and powder content of the 0～20cm layer was greater than that of the 20-50cm layer,while the sand content was the opposite.By comparing the built bioretention facilities rain gardens and their nearby conventional green space site monitoring results:after 10 years of operation,the stable infiltration rate of conventional green space is nearly 10 times that of rain gardens,and the stable infiltration rate of rain gardens at the early stage of construction is more than 5 times that of the current stage;rain gardens buried within 40cm depth of the media in the content of sticky and powdery particles gradually increase with depth,buried below 40cm depth of each component particles The content of each component is basically stable.Based on the experimental results,the clogging mechanism of the modified bioretention system-detection technology-blocking and control measures were summarized,and a clogging prediction framework based on the precipitation module,runoff pollution module,porosity decay function,life prediction function and other modules was established.(3)Through the life cycle environmental impact assessment of each bioretention system,the environmental impact and environmental benefits of each system in the whole life cycle process are analyzed.The results show that the difference in environmental impact between the systems during the raw material acquisition and construction phases is mainly due to the energy consumption generated by each system when acquiring the BSM base material in different proportions.There is no additional energy consumption during the operational phase,while the bioretention systems have some environmental benefits in terms of runoff abatement and purification,with the systems having the most significant impact in terms of reducing greenhouse potential,ecotoxicity and smoke emissions.The scrapped recycling phase has a relatively low environmental impact as 80%of the material in each system is recycled.The BSM+10%flyash system has the highest environmental benefit over the whole life cycle as flyash recycling has a better environmental benefit;the systems have the most significant environmental impact in terms of ecotoxicity,greenhouse potential and smoke emissions,and less significant impact in terms of carcinogen emissions and ozone depletion. |
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