| In recent years.due to the lack of water resources,the concept of sponge city has gradually been widely valued.Rainwater recharge groundwater can effectively promote the construction of sponge city.But in the process of rainwater recharge,blockage is the main factor restricting the construction of sponge city.Based on the background of Shenyang Architectural University,this research takes the investigation of rainwater quality,the analysis of blocking factors of recharge,the study of blocking law of suspended solids,the simulation of blocking process of suspended solids,the prevention and treatment of blocking as ideas,and makes use of sampling test analysis,indoor experiment and model simulation to comprehensively study the blocking law of suspended solids in the process of urban rainwater recharge.The main research results are as follows:(1)Investigation of hydrogeological characteristics and rainwater composition in the study area:Through collecting data to understand the hydrogeological conditions and climatic characteristics of Shenyang Architecture University,and laying sampling points,collecting natural rainwater and road rainwater regularly and detecting them.Modeling and analyzing the concentration of various indicators in rainwater,it is concluded that with the change of rainfall time,the concentration of suspended solids gradually decreases and tends to a stable value.At the same time,the test results were evaluated and analyzed by principal component analysis.Natural rainwater and pavement rainwater are seriously affected by suspended solids.Therefore,three concentration gradients of suspended solids were set up according to the different concentration of suspended solids in the rainfall process,which were 50mg/L,100mg/L and 200mg/L.respectively.Three kinds of suspended solids were used in the experiment of recirculation.(2)Study the blockage law of different suspended solids concentration in recirculation:During the blockage process,the water flux and permeability coefficient decreased rapidly at first and then tended to be stable.By measuring the number of microorganisms,microorganisms grow slowly,the difference of water flux and permeability coefficient between sterilization conditions and normal conditions is small,and the blockage caused by microorganisms can be ignored.By measuring the number of microorganisms,microorganisms grow slowly,the difference of water flux and permeability coefficient between sterilization conditions and normal conditions is small,and the blockage caused by microorganisms can be ignored.The plugging degree of porous media increases with the increase of suspended solids concentration.The concentration of suspended solids not only determines the initial water flux and permeability coefficient,but also determines the rate of decline of water flux and permeability coefficient.With the increase of suspended solids concentration,the initial values of water flux and permeability coefficient of porous media are lower,and the reduction speed is faster.(3)Study the blockage law of different suspended solids by recirculation:suspended solids are easy to form surface blockage in the surface and middle layers.Because there are certain pore between porous media,the particle size of suspended solids is smaller than that between porous media.With the action of water flow,suspended solids enter the pore and accumulate continuously in the upper layer of porous media to form "filter cake".The filter cake formed on the surface affects the water flux in the middle layer of sand column and the lower layer of sand column,which results in the decrease of water flux and permeability coefficient and blockage.The suspended particles with small particle size move into the porous media along with the flow,forming a surface-middle layer blockage.Under the same other conditions,when suspended solids are blocked,the smaller the size of suspended solids is,the stronger the migration ability of water flow and the greater the depth of migration.At this time,the migration of particles dominates,and the porous media is prone to surface-internal double blockage.On the contrary,the larger the particle size of suspended solids,the smaller the migration depth in porous media,and the smaller the influence on the permeability of deep porous media.(4)Study the plugging law of different porous media particle sizes by recharge:the change of overall permeability coefficient of sand column is mainly affected by the change law of surface permeability coefficient.The larger the concentration of suspended solids,the larger the particle size of porous media,the wider the blockage range and the more serious the blockage.At the same concentration of suspended solids,the smaller the particle size of porous media,the easier the blockage occurs and the earlier the blockage occurs.During the test,the water flux and permeability coefficient of medium sand and fine sand increase.On the one hand,the air dissolved in porous media leads to the increase of pore size;on the other hand,more and more suspended solids are deposited in porous media,and suspended solids are filled in the pore volume,which leads to the increase of pore volume.Under the combined action of the two factors,the permeability coefficient and water flux increase to the highest point.(5)Establishing and validating suspended solids blockage model:By establishing membrane filtration model and equivalent permeability coefficient model,the changing trend of water flux and permeability coefficient in rainwater recharge process is obtained.According to the simulation results,the time of blockage is related to the concentration of suspended solids.Using this model,not only the time of blockage can be predicted effectively,but also the time of blockage can be taken at the corresponding time point of blockage.Measures to deal with blockage and maintain the operation of recharge can also guide the actual project,the selection of appropriate concentration of suspended solids for rainwater recharge can avoid serious blockage.(6)To study the control and treatment methods of rainwater recharge blockage:precipitation pretreatment of rainwater can effectively alleviate the blockage of recharge by reducing the concentration of suspended solids.The permeability of porous media can be restored by three methods:water injection scouring,gas injection and water injection disturbance.Three methods are compared and analyzed.The gas injection method can effectively restore the permeability of porous media and solve the problem of blockage of rainwater recharge.However,in the process of rainwater recharge blockage,we should follow the principle of controlling the quality of recharge water mainly and supplementing the later treatment.Taking Shenyang Architectural University as an example,this paper grasps the law of suspended solids blockage by means of survey data and indoor seepage sand column blockage experiment,simulates the changing trend of water flux and permeability coefficient in the process of suspended solids blockage by establishing a numerical model of membrane filtration,and puts forward the control and treatment methods of rainwater recharge blockage,which has important theoretical significance and practical significance for the research of rainwater recharge blockage prevention and control.Meaning,provide theoretical basis and practical reference for relevant research. |
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