Experimental Study On The Rainwater Retention And Infiltration System Based On Sponge City

In recent years,in order to promote the quality and safety of urban water environment,the construction of sponge city is vigorously promoted in our country.The rainwater retention and filtration system is one of the typical measures for the sponge city construction which has the functions of retaining rainwater,purifying pollutants and recharging groungwater.However,each part of a country has its own rainfall characteristics and soil types,and the application of rainwater retention percolation system is generally lack of the study of regional adaptability.How to adopt and re-innovate this technology would be the difficulties faced in the sponge city construction.Based on the characteristics of soil type,rainfall and surface runoff water quality in Nanning City,this study aims at improving the packing layer of the existing rainwater retention and filtration system and and classifying and optimizing its plant species so as to construct a new system.This study explored the influence of external factors on the purification effects and the contaminants purification rules of the internal vertical layers.The research outcomes are as follows:(1)In the experiment of the composition and optimization of the filling layer of the retention and filtration system,packing layer was mainly composed of 0?10%of the local red loam and grit mixtures,and its average permeability coefficient is 28.47 cm/h?30.90 cm/h.After screening five kinds of adsorption and purification media and four kinds of carbon source additives and according to the removal effect of water quality and PCA analysis results,the experiment chose zeolite and vermiculite as filler layer adsorption purification medium,and sugar cane bagasse as a filler layer carbon source additive.The optimal ratios of filler material layers are;60%coarse sand and soil mixture,15%zeolite,15%vermiculite,10%bagasse.The homogeneous composition effect was the optimal when the filler layer thickness was 120cm.According to the results of PLS regression analysis,the addition of zeolite and vermiculite was the primary factor for the permeability coefficient of NH3-N,TP and soil column,the filler thickness is the key factor to the purification effect of COD,NO3-N and NO2-N,and the homogeneous and layered combination has less effect on the above indexes.(2)After screening of plants for rainwater retention and filtration system,the results showed that:the removal rate of the pollutants in the experimental group of 7 kinds of plants was increased by 0-20%than that of the blank group,which meant that the presence of plants increased the purification effect of the rainwater retention and filtration system.PCA analysis and cluster analysis were used to analyze the average rate of the damage of plant cell injury rate,soluble sugar content,malondialdehyde content,leaf relative water content,relative soil water content and chlorophyll fluorescence index(Fv/Fm,Y(II),qP,qN).The submergence tolerance of seven species was divided into three groups:Schefflera arboricola,Ixora chinensis,Acorus calams for the strong group,Ophiopogon japonicus for the intermediate group,Hymenocallis littoralis,Setcreasea purpurea Boom,Nephrolepis auriculata for the weak group.The drought resistance of 7 species was divided into 4 groups:Schefflera arboricola were the strong group,Hymenocallis littoralis,Ophiopogon japonicus,Ixora chinensis were intermediate group,Nephrolepis auriculata were the weak group,and Acorus calams and Setereasea purpurea Boom were non-drought-resistant group.Therefore,the woody plants and herbaceous plant combinations can be selected according to the functional requirements of the rainwater retention and infiltration system.(3)In the experimental study of the rainwater retention and infiltration system,the correlation analysis between external factors and pollutant removal rate showed that NH3-N was significantly correlated with influent loading and influent flow,TN and COD were significantly correlated with sunny days,and TP was significantly correlated with sunny days and influent flow.This study used multivariate linear regression analysis to fit the external factors and the removal rate of water quality indicators,and then compared the predicted value with the measured value.The fitting degree of NH3-N and TP was high(R2:0.750?0.800),and the fitting degree of TN and COD was low(R2:0.578?0.533).When the device depth was 0?40 cm,the removal effect of pollutants was about 75%,which mainly depended on the interaction of planting layer and packing layer.The depth of the device and the removal rate of the 4 indicators all showed a good distribution of quadratic equation,and the fitting coefficients were above 0.9.