Construction Of A Bioretention System With Improved Fillers And Research On Its Hydraulic And Pollution Removal Performanc

The change of the original urban hydrological regulation system caused by the continuous advancement of urbanization has exacerbated the environmental problems such as urban waterlogging and the deterioration of the water quality of the receiving water body.As a key technical measure of low impact development,bioretention system has a good application prospect in sponge city construction and urban non-point source pollution control in China.However,the traditional bioretention system has some problems,such as poor infiltration and storage effect and unstable decontamination effect,in this study,the broken oyster shells were used to improve the planting soil layer of bioretention system,balance the infiltration and stagnancy of water,enhance the decontamination effect of the system.The best oyster shell addition ratio was selected in this study through the performance of both hydraulic and purification,compared with the unmodified system,the effects of oyster shell addition,inundation area setting,plant growth and oyster shell covering on the hydraulic performance and nitrogen and phosphorus removal efficiency of the system were explored.And meanwhile explored the hydraulic and purification mechanism from three aspects of soil,plants and microorganisms by studying the fluorescence composition and source of dissolved organic matter（DOM）in interstitial water of different systems and its correlation with hydraulic and purification performance,the diversity and composition of microbial community in different systems and different depth fillers in the same system.The main results are as follows:（1）Compared with B1,B3 and B4 systems,B2 system with 20%broken oyster shells has the highest permeability,and the water holding capacity and water retention of the system increased with the addition of oyster shell.The removal rates of NH₄⁺-N、PO₄^3--P and TP by B1 and B2 systems were 100%,84%and 97%respectively,and the removal effect of other nitrogen were also more stable than that of B3 and B4.Therefore,the mixed soil with the ratio of soil,river sand and broken oyster shell of4.5:3.5:2 is selected as the experimental group of the next simulation experiment,and the mixed soil with the ratio of 4.5:5.5:0 is used as the control.（2）The physical and chemical properties of planting soil are changed after adding oyster shell,the bulk density decreases and the density increases;p H value,total porosity,capillary and non capillary porosity increased;and soil total organic carbon content also increased slightly.Both addition of oyster shell and growth of plant increased the permeability,water retention and water holding capacity of the system.In addition,the setting of the covering layer enhances the water holding capacity of the system.（3）The removal efficiency of nitrogen,phosphorus and COD by each system is good,and the average removal rate of each pollution index is more than 80%.The addition of oyster shell had little effect on the removal rate of nitrogen,phosphorus and COD.When there is no submerged area,the removal rate of pollutants in the medium and high load stage is higher than that in the low load stage;when setting the submerged area,the effect of submerged area on the nutrient removal rate of low load is higher than that of medium and high load,especially increasing the removal rate of NO₃^--N and TN under low load.The effect of plant growth on COD removal rate is more obvious than that of other pollutants.The inverted filler,i.e.the setting of oyster shell overburden,has a great impact on the removal rate of low load pollutants,especially on nitrogen and COD.When no plants were planted,the oyster shell covering with a thickness of 5 cm could improve the removal rate of low load nitrogen and COD;when growing plants and setting oyster shell covering layer at the same time,that only improves the removal rate of high load COD and reduces the removal rate of other pollutants.In addition,the average accumulation of available nitrogen in the planting soil of the system was reduced by planting plants and setting oyster shell covering.（4）In this study,there was a significant positive correlation between the hydraulic performance indexes（permeability coefficient,soil moisture content and retention rate）of the system（p<0.05）.The hydraulic indexes were significantly positively correlated with the removal rates of NH₄⁺-N,NO₂^--N and COD,but were significantly or extremely significantly negatively correlated with NO₃^--N,TN and phosphorus.（5）During the research,the DOM fluorescent components in the interstitial water of each system remained unchanged,and there were five fluorescent components:tyrosine like（I）,tryptophan like（II）,fulvic acid like（III）,dissolved microbial metabolites（IV）and humic acid like（V）,and the proportion of standard volume of humic acid like（III and V）was the highest.DOM is mainly composed of endogenous organic matter during the operation of the system.Plant growth increases the exogenous input of organic matter,resulting in a higher degree of humification of dissolved organic matter.The correlation between hydraulic performance and protein like content changed from a significant negative correlation in the early stage to a significant positive correlation in the late stage,while the correlation between hydraulic performance and humus like content changed from a significant positive correlation in the early stage to a significant negative correlation in the late stage.（6）For different bioretention systems,the addition of oyster shell increased the diversity and richness of microbial community in planting soil;plants increased the richness of microbial community in planting soil.The microbial community of C1system without oyster shell is quite different from that of C2 and C5 system,the plant root growth of C5 system weakens its similarity with the soil microbial community of C2 system.The dominant microflora of microorganisms in different systems and different fillers of the same system are the same at the phylum and genus levels.The first four dominant microflora are Proteobacteria,Actinobacteria,Acidobacteria,Chloroflexi respectively;dominant bacteria belong toβThauera,a Proteus like bacterium that can absorb phosphorus in the process of simultaneous nitrification and denitrification.