| Facing the problems of "water flooding,water scarcity,water pollution" caused by the accelerated urbanization process,it is urgent to effectively control and utilize urban stormwater runoff,and then the important role of rainwater in building a sustainable water cycle system is being played.As an important part of sponge city construction,the construction of low impact development(LID)system is the focus of future research.The bioretention cell is a typical LID facility for sponge city,but the problems of unstable operation efficiency,lack of design methods,and long-term effects are unclear,which restrict the application of the technology.Media is the key factor for the function of bioretention system,but there are still many shortcomings in the research on the improvement of bioretention system media and its pollutant purification mechanism,which needs further exploration and research.This study uses simulated rainfall tests,actual rainfall monitoring,theoretical analysis and numerical simulation,and typical field applications.It integrates environmental,hydrology,ecology,soil,hydraulics and other disciplines,combining regional characteristics with a view to further solve the three core problems of urban flood control and drainage,non-point source pollution control and rainfall resource utilization.Mainly through the batch and mini-column experiment,taking into account the adsorption capacity,infiltration,water-holding capacity,lifespan and cost of the mixed media,the multi-objective decision-making method is used to establish the hierarchical structure model.The modified media which own best comprehensive performance are developed,and performance parameters are determined;considered the synergistic effect of media-plants-microorganisms,the regulation effect and mechanism of bioretention system on rainfall runoff were analyzed quantitatively through the column simulated runoff test;combined with pilot-scale tests and model simulation,the appropriate method of bioretention system design was determined,the main considerations include media characteristics,hydraulic and pollutants load,submerged zone height,aquifer heights,etc.;combined with rainfall process monitoring and cost-benefit analysis to verify the applicability of typical LID facilities in the loess area.The main research results are as follows(1)According to the principle of strong adsorption capacity,low-desorption rate,strong water-holding capacity,low cost and local accessibility,a design method of bioretention modified media is proposed Through the detection of SEM,CEC,SSA,etc.,the basic characteristics of 10 modifiers such as water treatment residual(WTR)were determined.The mixing ratio of traditional bioretention soil media(BSM)and modifiers were determined by batch experiments;the results of the one-dimensional vertical infiltration experiment show that the permeability of the modified media exceeds 220 mm/h,which is 3.4 times higher than that of the undisturbed soil;the water-holding capacity of the modified media is 0.9 to 1.6 times that of the traditional BSM,and most of them are higher than the traditional BSM.the maximum adsorption capacity and long lifespan media are obtained by batch and mini-column experiments,in which BSM+10%WTR for SRP,BSM+10%green zeolite for NH4+-N.Through the system experiment,a multi-objective decision-making problem consisting of five factors including infiltration,water holding capacity,purification capacity,lifespan and cost was constructed.The hierarchical structure model was established.The comprehensive performance of the improved packing was sorted as follows:BSM+10%fly ash>BSM+10%green zeolite>BSM+10%WTR>BSM+10%medical>BSM(2)According to the results of the column-scale test,the basic characteristics of bioretention media are characterized by media factor(MF)and infiltration rate,taking into account the effects of influent pollutant concentration,discharge area,recurance interval,rainfall duration and antecedent dry time.Based on the Response Surface Methodology(RSM),a multivariate quadratic regression equation was used to establish a quantitative coupling relationship model between the water regulation effect,the concentration removal and the influencing factors(R2?0.715),which can be used to predict operation effect for different bioretention media(3)The temporal and spatial variation characteristics of carbon,nitrogen,phosphorus,heavy metals and enzyme activities in the bioretention facility media were studied by 25 simulated rainfall column-scale tests and four-stage media characteristics.The results show that in the one-year operation period,when the pollutants contents in the media are relatively high,the contents of pollutants in the media decreases first,and the system tends to be stable with the operation of the system,and the system is stable after operation.The content of medium nitrogen and phosphorus showed a trend of upper layer>middle layer>lower layer.There is a difference between the enzyme activity and the pollutant content of different enzymes in the bioretention system through Pearson correlation analysis between the pollutants content in bioretention media and urease,protease,dehydrogenase,invertase,acid phosphatase and catalase enzyme activity.The correlation was significantly inconsistent,and catalase was significantly correlated with all pollutants contents(P<0.01),while acid phosphatase was not significantly correlated with all pollutants contents(4)Forming four modified bioretention media by mixing high-efficiency modifiers with traditional bioretention media,and constructing 10 pilot-scale bioretention systems through different media combinations(layered/mixed,submerged zone heights)to clarify the correlation between the pollutant load reduction and the hydrological/hydraulic factors at the pilot-scale facility.The results show that when fly ash(layered/mixed)and WTR(layered/mixed)are used as modifiers,the modified media removal effect is higher than other media(>68%).For the BSM+10%WTR bioretention cell,the nitrate removal rates at the three submerged zone heights were 54.0%,59.9%and 62.1%,respectively.Considering the water reduction and the previous research results,the recommended submerged zone height was 150 mm.According to the results of the pilot-scale tests,the scale of the unit simple and complex bioretention facilities was designed by the aquifer height method and the complete water balance volume reduction method(5)Based on the pilot-scale tests and HYDRUS-1D model,the sensitivity analysis of key parameters of bioretention facilities was carried out.The results showed that the sensitivity parameters which affect water migration are inflow volume(V),media thickness(Hm),empirical parameters(n),saturated water content(?s)and saturated hydraulic conductivity(Ks).the sensitivity parameters which affect pollutant transport are influent volume(V),influent concentration(C),media thickness(Hm)and isothermal adsorption constant(Kd).In this study,the water hydraulic and quality parameters of the bioretention media layer were obtained through the calibration and verification of the model.In the case of 1h duration in two-year recurrence interval,and the discharge ratio is 20:1.It is recommended that the design thickness of BSM+10%WTR mixed media should be more than 50 cm,and the design thickness of BSM+10%fly ash mixed filler should be more than 70 cm(6)Two rain gardens were constructed to treat roof runoff which using traditional BSM and BSM+10%WTR as media.The water regulation effects are:38.1?68.6%(median=53.4%)and 56.5?88.1%(median=72.8%),respectively,and the water reduction rate of BSM+10%WTR rain garden is about 20%higher than the traditional BSM rain garden.After the operation of the facility is stable,compared with the BSM rainwater garden,the removal rate of pollutant concentration in BSM+10%WTR rain garden increased by 6.2%?36.4%,and the load reduction rate is above 84%,which increased by 10.2%?23.3%than BSM rain garden.(7)Performing dozens of actual rainfall process monitoring on four typical bioretention facilities.Combined with the actual rainfall process monitoring of the four bioretention facilities,mixed-media rain garden and layered media bioswale reduce the discharge area,setting internal water storage area,increase the media thickness to improve the runoff regulation effect compared with pure planting soil rain garden.Under the medium and light rainfall conditions(<25 mm),the water reduction rate is more than 68.2%.The statistical results of planting soil rain garden,mixed-media rain garden and layered media bioswale showed that,the probability of effluent TN concentration exceeding the ? limit of surface water environmental quality standard(GB 3838-2002)is 32.7%,42.3%and 19.8%,respectively;The TP effluent concentration exceeded probability was 18.5%,6.5%and 8.9%,respectively;the COD effluent concentration exceeded probability was 50.3%,47.4%and 39.0%,respectively.The annual pollution load reduction rate of the facility is basically over 60%,however,COD and nitrate showed obvious leaching in the initial stage of operation(8)According to the hydrological characteristics of the northwestern China,if the study area has no special requirements for the purification effect of pollutants,and the soil infiltration performance is greater than 10-6 m/s,the bioretention facilities filled with local loess can effectively regulate runoff from the source.For limited construction areas or severe runoff pollution areas,improvement measures should be made for the media or structural parameters of LID facilities.Based on the proportion of different rainfall levels in the annual rainfall events,land use type,annual runoff total control rate and pollutant load reduction rate,the LID facility design scale prediction method was established.Taking the built-up area of Xi'an as an example(the construction land area was 625.3 km2 in 2017),when the pure planting soil rain garden and mixed-media rain garden are used in the construction area,and the road area adopts layered bioswale,by 2030,in order to reach the total runoff control rate 85%and pollutant load reduction rate 60%,the total cost is about 36 billion CNY. |
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