Method for simulating migration of pollutants in bioretention pond based on COMSOL

Abstract

The invention discloses a method for simulating migration of pollutants in a bioretention pond based on COMSOL Multiphysics, and the method comprises the following steps: determining design parameters of the bioretention pond by integrating rainfall runoff pollution data of an actual engineering construction area and carrying out related calculation, determining the pollutants, and carrying out the simulation of the migration of the pollutants in the bioretention pond. Pollutant accumulation characteristics and adsorption and desorption characteristics of pollutants in a filler are obtained through a pollutant accumulation experiment, a pollutant adsorption leaching experiment and the like, a bioretention pond pollutant migration model comprises a fluid flow equation and a solute migration equation, and links such as convection diffusion, adsorption and desorption, dissolution and reaction degradation are considered in solute migration. The efficient simulation on the pollutant transportation of the bioretention pond is realized. According to the method, the influence of different filler combinations and design scales on the pollutant removal efficiency of the bioretention pond is explored, the parameter conditions of water outlet target optimization control are determined, and scientific support is provided for the optimization design of the bioretention pond.

Description

Technical Field

[0001] This invention relates to a method for simulating pollutant transport in a COMSOL-based bioretention tank, belonging to the technical field of pollutant transport simulation in porous media. Background Technology

[0002] Bioretention ponds, as a key component of sponge city construction, offer significant advantages in intercepting surface runoff, reducing peak flow, improving runoff quality, and replenishing groundwater resources. However, effectively designing bioretention systems and predicting their treatment efficacy against different pollutants and their rainwater retention capacity during operation remain areas requiring further research and development.

[0003] In recent years, COMSOL Multiphysics, as a powerful finite element simulation software that solves simulation problems in fields such as fluid flow, chemical substance transport, and reaction engineering through multiphysics coupling, has shown great promise in solute transport simulation. However, because bioretention tanks are typically composed of various packing materials, and the pollutant migration mechanisms are complex and diverse, pollutant removal efficiency is affected by many factors, including the properties of the pollutants, the type and properties of the packing materials, design parameters, and manipulated variables. Currently, research on using COMSOL to simulate the migration and transformation of pollutants in bioretention tanks is relatively limited, lacking a theoretical basis that can provide scientific guidance and design suggestions for the optimal design of bioretention tanks under different design objectives and construction backgrounds. Summary of the Invention

[0004] To address the aforementioned problems, this invention discloses a method for simulating pollutant transport in a COMSOL-based bioretention tank, the specific technical solution of which is as follows:

[0005] A COMSOL-based method for simulating pollutant transport in a bioretention tank includes the following steps:

[0006] Step 1: Analyze the pollution characteristics of rainwater runoff in the study area based on meteorological and rainfall-runoff data, determine the rainwater runoff pollution control target, and determine the operating parameters of the pollutant transport model through experimental measurement or empirical methods. The operating parameters of the pollutant transport model include pollutant accumulation characteristics, adsorption isotherms of pollutants in the packing material, packing material porosity, water storage coefficient, saturated hydraulic conductivity, pollutant distribution coefficient in the packing material, molecular diffusion coefficient, pollutant decay rate, initial inflow concentration of pollutants, and lateral and longitudinal dispersion coefficients.

[0007] Step 2: Using the Richards equation to simulate the variable saturation porous media environment in the bioretention tank, based on the convection-dispersion equation, considering adsorption, pollutant reaction and pollutant source-sink terms, the governing equation for the simulation of dissolved phase pollutant transport is established. The simulation of dissolved phase pollutant transport includes two simulation processes: simulation of the fluid flow field and simulation of pollutant transport.

[0008] Step 3: Based on the governing equations for the simulated transport of pollutants in the dissolved phase, a pollutant transport model in the bioretention tank is constructed using COMSOL Multiphysics software. Through calculation and post-processing, the flow field, pressure, pollutant concentration distribution, retardation coefficient distribution, and effective saturation results within the bioretention tank are obtained. A two-dimensional intercept point is set at the effluent to obtain a graph showing the pollutant concentration change over time at the intercept point. Subsequently, by adjusting the parameters of the pollutant transport model in the bioretention tank, the influence of different packing combinations and design scales on the pollutant removal efficiency of the bioretention tank is investigated. Based on the model simulation results and design requirements, parameters such as the packing combination and design scale of the bioretention tank are adjusted. The optimal parameter combination is compared and determined, providing a scientific basis for optimizing the pollutant treatment effect of the bioretention tank.

[0009] Furthermore, in step 2, considering the unsaturated porous media environment in the bioretention tank, the governing equations for fluid flow in the unsaturated porous media are constructed using the Richards equation interface. The Richards equations are based on Darcy's law.

[0010]

[0011] In the formula: t is time, s; ρ is soil fluid density, kg / m³ 3 ;∈ p Porosity of the medium is dimensionless. For the Laplace operator; Q m For mass source, kg / m 3 / s; u is the Darcy velocity or unit flow vector, m / s; K is the permeability of the porous medium, m 2 Defined as K = K s K r (S e ), where K s m is the saturated permeability coefficient of the medium. 2 K r The relative permeability coefficient is dimensionless and is related to the effective saturation S. e The function of; p is pressure, Pa; g is the gravitational acceleration constant, taken as 9.8 m / s². 2 g is the gravitational acceleration vector, in m / s². 2 D represents elevation, in meters (m). Let m be the unit vector in the direction of gravity;

[0012] Considering the unsaturated conditions and the continuity of fluid flow in porous media, the following governing equations for unsaturated flow, namely the Richards equations, are derived based on Darcy's law:

[0013]

[0014] In the formula: C m For water holding capacity, m -1 S e S represents the relative saturation of the soil. p For elastic water storage capacity, Pa -1 .

[0015] Furthermore, the van Genuchten model is used to describe the soil hydraulic function, representing θ, C m S e and K r Four retention and permeation properties:

[0016] θ=θ r +S e (θ s -θ r (5)

[0017]

[0018] In the formula: θ r m represents the residual moisture content. 3 / m 3 ;θ s m is the saturated water content. 3 / m 3 h p α is the pressure head, m; α is the reciprocal of the air intake value of the moisture characteristic curve, m. -1 β is an indicator parameter for the slope of the soil moisture characteristic curve, obtained by fitting the soil moisture characteristic curve; m = 1 - 1 / β;

[0019] Therefore, the governing equations for the flow field simulation of the bioretention tank established in step 2, i.e., the simulation of the fluid flow field, are as follows:

[0020]

[0021] Furthermore, in step 2, a solute transport model is constructed based on the convection-dispersion equation, considering adsorption, dispersion, and reactive degradation. The equations controlling the adsorption, dispersion, and reactive degradation of pollutants in the porous media packing of the bioretention tank are as follows:

[0022]

[0023] In the formula: ci Let i be the concentration of substance i in the liquid, in mol / m 3 c P,i R is the mass of pollutants adsorbed per unit weight of dry solid, expressed in mg / kg. i S is the expression for the degradation rate of substance i; i For source and sink items; J i The diffusion flux vector is mol / m 2 / s / ;D D,i Let m be the mechanical dispersion tensor of matter i. 2 / s;D e,i Let m be the effective diffusion coefficient of substance i. 2 / s.

[0024] Furthermore, in step 2, in the unsaturated zone, the pores between soil solid particles will be composed of both air and water, i.e., ∈ p =θ l +θ g , where θ l θ represents the volume fraction of water in the soil. g This represents the volume fraction of gases in the soil. Furthermore, it's necessary to consider soil air concentration in addition to the saturated zone, thus changing the concentration variation term in the bioretention tank filler to include the concentration variations of substances dissolved in the soil liquid, adsorbed in the soil solid particles, and volatilized into the porous medium air. The corresponding formula expression is as follows:

[0025]

[0026] In the formula: c G,i Let i be the concentration of substance i in the gas phase, in mol / m 3 c G,i =k G,i c, k G,i The proportion of substance i in the gas phase;

[0027] Therefore, the governing equations for simulating the transport of dissolved pollutants in the bioretention tank described in step 2 are as follows:

[0028]

[0029] θ g =∈ p -θ l (16)

[0030] c G,i =k G,i c (17).

[0031] Furthermore, in step 2, the adsorption process of pollutants is considered during the migration and transformation of pollutants, using the Langmuir model or the Freundlich model, or by experimentally determining a custom adsorption curve.

[0032] The Langmuir model uses the following governing equations:

[0033]

[0034] In the formula: c P,i,L It is the Langmuir equilibrium adsorption capacity of substance i in the adsorbate, in mg / g; K L,i This is the Langmuir adsorption constant, L / mg; c Pmax,i It is the maximum adsorption capacity of the adsorbent, mg / g; c i It is the concentration of substance i in the solution, in mg / L;

[0035] The Freundlich model uses the following governing equations:

[0036]

[0037] In the formula: c P,i,F It is the Freundlich equilibrium adsorption capacity of substance i on the adsorbent, mg / g; K F,i N F,i c is the Freundlich constant of substance i, mg / g; i c is the concentration of substance i in the solution, in mg / L; ref,i It is the standard sample concentration of substance i in the solution, in mg / L.

[0038] Furthermore, the modeling process based on COMSOL Multiphysics in step 3 includes the following steps:

[0039] 3.1: Based on the characteristics of rainwater runoff pollution in step 1 and the rainwater runoff control targets of the construction area, determine the structural design of the required bioretention pond, identify the pollutants to be simulated, and establish a geometric model of the bioretention pond;

[0040] 3.2: Adding a multiphysics interface: The "Richards equation (dl)" interface is used to simulate the flow field in the porous media packing, and the "dilute mass transfer in porous media (tds)" interface is used to simulate the migration and transformation process of dissolved phase pollutants in the packing. Parameter properties are set and the equations are constrained by initial conditions and boundary conditions. The left and right boundaries of the bioretention tank are set as non-flowing boundaries, and the bottom boundary is set as a permeable layer boundary.

[0041] 3.3: Custom mesh generation for the geometric model of the bioretention tank;

[0042] 3.4: Add transient studies, adjust the absolute tolerance, time step, termination method and criterion configuration in the solver according to the requirements, perform post-processing on the calculation results, and display the Darcy velocity field, pressure head, dissolved phase pollutant concentration, effective saturation and retardation coefficient views respectively;

[0043] 3.5: Using the two-dimensional intercept points in the dataset, create a one-dimensional plotting group to extract the concentration change data of a certain point at the outflow location at different times, and show the change of pollutant outflow concentration;

[0044] 3.6: Based on the model simulation results and design requirements, adjust the design parameters such as the packing material combination and design scale of the bioretention tank model. Based on the simulation results after debugging, compare and determine the optimal parameter combination for different design objectives (such as pollutant control, runoff control, etc.) to provide a scientific basis for optimizing the pollutant treatment effect of the bioretention tank.

[0045] The beneficial effects of this invention are:

[0046] This invention uses the collection of rainwater runoff pollutant data in the bioretention pond construction environment to determine the pollutant input links and emission fluxes in the study area, laying the foundation for model construction.

[0047] This invention considers adsorption, dispersion and degradation in the process of simulating solute transport in a bioretention tank. Under the corresponding physical field nodes, the control equations for unsaturated flow field and dissolved phase pollutant transport are respectively placed to realize the simulation of the migration and transformation of dissolved phase pollutants under unsaturated conditions in a bioretention tank.

[0048] This invention utilizes two-dimensional rotation to simulate a three-dimensional columnar biological retention tank model based on a constructed two-dimensional model. It combines traditional columnar experiments with model simulation, making it easier to fit the model to the columnar biological retention tank under experimental conditions, and also facilitating model debugging.

[0049] This invention sets a two-dimensional intercept point at the outlet to obtain a curve showing the change of pollutant concentration at the outlet over time. This allows for an intuitive understanding of the treatment efficiency of the bioretention tank. By changing the design parameters and operating variables of the bioretention tank, the changes in the treatment efficiency of the bioretention tank are analyzed. Furthermore, through model debugging, the optimal design parameters of the bioretention tank are summarized, achieving the optimal design of bioretention tanks under different construction backgrounds and treatment objectives, and providing a scientific basis for their design in practical engineering. Attached Figure Description

[0050] Figure 1 This is a flowchart of the present invention.

[0051] Figure 2This is an example diagram showing the geometric modeling settings and related parameters for the flow field of the biological retention tank in this invention.

[0052] Figure 3 This is a schematic diagram of the grid division of the bioretention pool model of the present invention.

[0053] Figure 4 The images show the simulation results of the flow field distribution over 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days.

[0054] Figure 5 The images show the flow field pressure distribution results for 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days, respectively.

[0055] Figure 6 The images show the results of the three-dimensional columnar model of the flow field pressure distribution for 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days, respectively.

[0056] Figure 7 This is a set of graphs showing the simulated pollutant concentration results for 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days.

[0057] Figure 8 This is a set of graphs showing the results of the three-dimensional columnar model of pollutant concentrations for 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days.

[0058] Figure 9 The images show the changes in the resistance coefficient of the packing material in the biological retention tank over 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days, according to the present invention.

[0059] Figure 10 This is a series of graphs showing the changes in the effective saturation of the packing material in the biological retention tank over 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, and 15 days.

[0060] Figure 11 This is a series of graphs showing the simulated pollutant concentration results at the two-dimensional cutoff points set in this invention. Detailed Implementation

[0061] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0062] This invention relates to a method for simulating pollutant transport in bioretention tanks based on COMSOL, with reference to... Figure 1 As shown, the specific process includes the following:

[0063] Step 1: Analyze the pollution characteristics of regional stormwater runoff using meteorological and rainfall-runoff data, determine the stormwater runoff pollution control targets, and determine the model parameters for simulating pollutant transport in the bioretention tank through experimental measurements or empirical methods. The model parameters include pollutant accumulation characteristics, adsorption isotherms of pollutants in the packing material, packing material porosity, water storage coefficient, saturated hydraulic conductivity, initial inflow concentration of pollutants, distribution coefficient of pollutants in the packing material, molecular diffusion coefficient, pollutant attenuation rate, and lateral and longitudinal dispersion coefficients.

[0064] Step 2: Using the Richards equation to simulate the variable saturation porous media environment in the bioretention tank, and considering adsorption, pollutant reaction and pollutant source and sink terms based on the convection-dispersion equation, establish the governing equation for simulating pollutant transport in the dissolved phase.

[0065] As a further aspect of this invention, this solution mainly relates to saturated and unsaturated flow, with the calculation of the saturated flow model primarily based on Darcy's law:

[0066]

[0067]

[0068] In the formula: t is time, s; ρ is soil fluid density, kg / m³ 3 ;∈ p Porosity of the medium is dimensionless. For the Laplace operator; Q m For mass source, kg / m 3 / s; u is the Darcy velocity or unit flow vector, m / s; K is the permeability of the porous medium, m 2 Defined as K = K s K r (S e ), where K s m is the saturated permeability coefficient of the medium. 2 K r The relative permeability coefficient is dimensionless and is related to the effective saturation S. e The function of; p is pressure, Pa; g is the gravitational acceleration constant, taken as 9.8 m / s². 2g is the gravitational acceleration vector, in m / s². 2 D represents elevation, in meters (m). Let m be the unit vector in the direction of gravity;

[0069] Considering the unsaturated conditions and the continuity of fluid flow in porous media, the following governing equations for unsaturated flow, namely the Richards equations, are derived based on Darcy's law:

[0070]

[0071] In the formula: C m For water holding capacity, m -1 S e S represents the relative saturation of the soil. p For elastic water storage capacity, Pa -1 .

[0072] The van Genuchten model is used to describe soil hydraulic functions, representing four retention and infiltration properties—θ, C, and φ. m S e and K r :

[0073] θ=θ r +S e (θ s -θ r (5)

[0074]

[0075] In the formula: θ r m represents the residual moisture content. 3 / m 3 ;θ s m is the saturated water content. 3 / m 3 h p α is the pressure head, m; α is the reciprocal of the intake value of the moisture characteristic curve, m. -1 β is an indicator parameter of the slope of the soil moisture characteristic curve, which is obtained by fitting the soil moisture characteristic curve; m = 1 - 1 / β.

[0076] Therefore, the governing equations for the unsaturated flow field simulation in the biological retention tank established in step 2 are as follows:

[0077]

[0078] Furthermore, a solute transport model is constructed based on the convection-dispersion equation, considering the effects of adsorption, dispersion, and reactive degradation. The governing equation is as follows:

[0079]

[0080] In the formula: c i Let i be the concentration of substance i in the liquid, in mol / m 3 c p,i R is the mass of pollutants adsorbed per unit weight of dry solid, expressed in mg / kg. i S is the expression for the degradation rate of substance i; i For source and sink items; J i The diffusion flux vector is mol / m 2 / s / ;D D,i Let m be the mechanical dispersion tensor of matter i. 2 / s;D e,i Let m be the effective diffusion coefficient of substance i. 2 / s;

[0081] In the unsaturated zone, the pores between soil solid particles will be composed of both air and water, i.e., ∈ p =θ l +θ g , where θ l θ represents the volume fraction of water in the soil. g This represents the volume fraction of gases in the soil. Furthermore, it's necessary to consider soil air in addition to the saturated zone, thus changing the concentration variation term in the bioretention tank filler to include three terms: dissolved in soil liquid, adsorbed in soil solid particles, and volatilized in the porous medium air. The corresponding formula expression is as follows:

[0082]

[0083] In the formula: c G,i Let i be the concentration of substance i in the gas phase, in mol / m 3 c G,i =k G,i c, k G,i denoted as i, representing the proportion of substance i in the gas phase.

[0084] Therefore, the governing equations for simulating pollutant transport in the bioretention tank described in step 2 are as follows:

[0085]

[0086] θ g =∈ p -θ l (16)

[0087] c G,i =k G,i c (17)

[0088] Furthermore, the adsorption process of pollutants is considered during their migration and transformation, using either the Langmuir model or the Freundlich model, or by experimentally determining a custom adsorption curve. This scheme uses the Langmuir model, employing the following governing equations:

[0089]

[0090] In the formula: c P,i,L It is the Langmuir equilibrium adsorption capacity of substance i on the adsorbate, in mg / g; K L,i This is the Langmuir adsorption constant, L / mg; c Pmax,i It is the maximum adsorption capacity of the adsorbent, mg / g; c i It is the concentration of substance i in the solution, in mg / L.

[0091] Step 3: Based on the rainfall runoff data and stormwater runoff control targets in Step 1, determine the required structural design of the bioretention pond and establish its geometric model. Combining the flow field control equations and solute transport control equations from Step 2, construct a pollutant migration and transformation model for the bioretention pond in COMSOL. The constructed geometric model of the bioretention pond must be determined based on the actual engineering dimensions, taking into account rainfall runoff characteristics and stormwater runoff pollution control targets.

[0092] Step 3.1: In COMSOL's "Model Wizard", select the "2D Axisymmetric" spatial dimension. By rotating the 2D model, the 3D biological retention tank column model can be simulated.

[0093] In terms of physical field interface selection, the "Richards equation (dl)" interface is used to simulate the flow field in the porous media packing, and the "dilute mass transfer in porous media (tds)" interface is used to simulate the migration and transformation process of dissolved phase pollutants in the packing.

[0094] Step 3.2: The study area of ​​this scheme is a 1.5m × 1.5m two-dimensional XZ section. Above the 0.75m composite filler layer 1 is another 0.75m composite filler layer 2. Points are inserted geometrically to simulate the inlet location of the rainwater pipe, as follows. Figure 2 As shown.

[0095] Step 3.3: Set the parameters in "Global Definition". The fluid flow parameters set in this scheme are shown in Table 1 below:

[0096] Table 1. Fluid Flow Parameter Settings for Bioretention Tank

[0097]

[0098] The solute migration parameters set in this scheme are shown in Table 2 below:

[0099] Table 2. Setting of Solute Migration Parameters for Biological Retention Tanks

[0100]

[0101] Step 3.4: In the Richards equation (dl) physics field, set the fluid flow parameters for the two composite filler layers respectively, and constrain the equation using initial and boundary conditions. This includes selecting "hydraulic conductivity" as the permeability model and "van Genuchten" as the water-holding model in "Porous Matrix 1" under "Unsaturated Porous Media," and inputting the required parameters. For the initial value, select "pressure head." This scheme assumes an initial groundwater level of -1.4m. Considering gravity, specify "height" as z. Set the "pressure head" of the inflow boundary to Hp0. Set the bottom boundary as the permeable layer boundary, the left boundary as an axisymmetric boundary, and the right and upper boundaries as five-flow boundaries.

[0102] Step 3.5: In the physics field of "Dilute Mass Transfer (TDS) in Porous Media", the solute migration parameters of the unsaturated porous media are set for the two composite packing layers. The left boundary is set as an axisymmetric boundary, the right and upper part of the boundary is a flux-free boundary, the bottom boundary is an outflow boundary, and the boundary of the upper geometric point is set as concentration c0. In the "Unsaturated Porous Media" section, the "saturation" setting for the "liquid" part is set to "liquid volume fraction", and the time domain variation is the pressure head time variation. The specific volumetric water content is input as "dl.Cm", the convection is selected as "Darcy velocity field", and adsorption and dispersion are added. Based on the previous experiments, the adsorption and dispersion parameters of pollutants in the packing are determined. The Langmuir isothermal adsorption model is used in this scheme.

[0103] Step 3.6: As Figure 3 As shown, the constructed biological retention tank model is meshed. The meshing function built into COMSOL can be used. Select "User Controlled Mesh" and calibrate with "Fluid Dynamics" and predefine it as "Refined". In addition, the boundary layer is refined to simulate the boundary layer more accurately. In "Boundary Layer Properties", the number of layers and stretch factor are set. In this scheme, the number of layers is set to 8 and the stretch factor is 1.2.

[0104] Step 3.7: Add transient studies. Adjust the absolute tolerance, time step, termination method, and criterion configuration in the solver according to the requirements. This scheme uses days (d) as the time unit, and the output time step is "range(0,0.1,0.9)range(1,1,5)range(7,2,15)", that is, outputting 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 7, 9, 11, 13, 15 days. The tolerance is controlled by the physics field.

[0105] Step 3.8: Post-process the calculation results to obtain the flow net by default (see appendix). Figure 4 ), pressure head (see appendix) Figure 5 ), Dissolved phase contaminant concentration diagram (see appendix) Figure 7 Three-dimensional pressure is obtained through two-dimensional rotation (see appendix). Figure 6 ), 3D concentration map (see appendix) Figure 8 By adding plotting groups in post-processing, the hysteresis coefficient can be obtained (see appendix). Figure 9 ), effective saturation view (see appendix) Figure 10 ).

[0106] Furthermore, by setting streamlines and contour lines in the result graph, the changes in the flow field and concentration during pollutant transport can be visually displayed.

[0107] Furthermore, by utilizing the two-dimensional intercept points in the dataset, such as Figure 2 At the bottom left corner, create a one-dimensional plotting group to extract concentration change data at a specific point at the outflow location over different time periods, displaying the changes in pollutant outflow concentration. See [link / reference]. Figure 11 .

[0108] The above description is merely an embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Based on the above-described ideal embodiments of the present invention, and through the above description, those skilled in the art can make various changes and modifications without departing from the technical concept of the present invention.

Claims

1. A method for simulating pollutant migration in a bioretention pond based on COMSOL, characterized in that: The steps include: Step 1: Analyze the pollution characteristics of rainwater runoff in the study area based on meteorological data and rainfall runoff data, determine the rainwater runoff pollution control target, and determine the operating parameters of the pollutant transport model through experimental measurement or empirical value method. The operating parameters of the pollutant transport model include pollutant accumulation characteristics, adsorption isotherms of pollutants in fillers, filler porosity, water storage coefficient, saturated hydraulic conductivity, distribution coefficient of pollutants in fillers, molecular diffusion coefficient, pollutant attenuation rate, initial concentration of pollutant inflow, and lateral and longitudinal diffusion coefficients; Step 2: Use the Richards equation to simulate the variable saturated porous media environment in the bioretention pond. On the basis of the convection-diffusion equation, consider the adsorption, pollutant reaction and source and sink of pollutants, and establish the control equation for dissolved phase pollutant transport simulation. The dissolved phase pollutant transport simulation includes two simulation processes, namely the simulation of fluid flow field and the simulation of pollutant transport. Step 3: Based on the control equations of dissolved phase pollutant transport simulation, COMSOL Multiphysics software is used to construct a pollutant transport model in the biological retention pond. The flow field, pressure, pollutant concentration distribution, resistance coefficient distribution, and effective saturation result diagrams in the biological retention pond are obtained through calculation and post-processing. A two-dimensional intercept point is set at the outlet to obtain the result diagram of the pollutant concentration changing with time at the intercept point. Subsequently, by adjusting the parameters of the pollutant transport model in the biological retention pond, the effects of different filler combinations and design scales on the pollutant removal efficiency of the biological retention pond are investigated. According to the model simulation results and design requirements, the filler combination and design scale parameters of the biological retention pond are adjusted, and the optimal design parameter combination is compared and determined to provide a scientific basis for optimizing the pollutant treatment effect of the biological retention pond.

2. The method for simulating pollutant migration in a bioretention pond based on COMSOL according to claim 1, characterized in that: In step 2, the unsaturated porous media filler environment in the bioretention tank is considered, and the Richards equation interface is used to construct the governing equation for fluid flow in the unsaturated porous media. The Richards equation is based on Darcy's law: Where: t is time, s; ρ is soil fluid density, kg / m 3 ∈ p is the porosity of the medium, dimensionless; is the Laplace operator; Q m is the mass source, kg / m 3 / s; u is the Darcy velocity or unit flow vector, m / s; K is the permeability of the porous medium, m 2 , defined as K = K s K r (S e ), where K s is the saturated permeability coefficient of the medium, m 2 , K r is the relative permeability, dimensionless, which is related to the effective saturation S e function; p is pressure, Pa; g is the gravitational acceleration constant, which is 9.8m / s 2 ; g is the gravity acceleration vector, m / s 2 ; D is the elevation, m, is the unit vector in the direction of gravity, m; Considering the unsaturated conditions in porous media and the continuity of fluid flow, the following unsaturated flow control equation, namely Richards equation, is obtained based on Darcy's law: Where: C m is the water content, m -1 ; S e is the relative saturation of soil; S p is the elastic water storage rate, Pa -1 .

3. The method for simulating pollutant migration in a bioretention pond based on COMSOL according to claim 2, characterized in that: The van Genuchten model is used to describe the soil hydraulic function, which represents θ, C m , S e and K r Four retention and permeation properties: θ=θ r +S e (i s -θ r ) (5) Where: θ r is the residual moisture content, m 3 / m 3 θ s is the saturated moisture content, m 3 / m 3 ;h p is the pressure head, m; α is the inverse of the air intake value of the moisture characteristic curve, m -1 ; β is the indicator parameter of the slope of the moisture characteristic curve, which is obtained by fitting the soil moisture characteristic curve; m = 1-1 / β; Therefore, the control equation for the flow field simulation of the biological retention pond established in step 2, that is, the simulation of the fluid flow field, is:

4. The method for simulating pollutant migration in a biological retention pond based on COMSOL according to claim 1, characterized in that: In step 2, a solute transport model is constructed based on the convection-diffusion equation, and the adsorption, dispersion and reaction degradation effects are considered. The equations for controlling the adsorption, dispersion and reaction degradation of pollutants in the porous medium filler of the biological retention tank are as follows: Where: c i is the concentration of substance i in the liquid, mol / m 3 , c P,i is the mass of pollutants adsorbed per unit weight of dry solid, mg / kg; R i is the degradation rate expression of substance i; S i is the source-sink term; J i is the diffusion flux vector, mol / m 2 / s / ;D D,i is the mechanical diffusion tensor of substance i, m 2 / s;D e,i is the effective diffusion coefficient of substance i, m 2 / s.

5. The method for simulating pollutant migration in a bioretention pond based on COMSOL according to claim 1, characterized in that: In step 2, in the unsaturated zone, the pores between the soil solid particles will be composed of air and water, that is, ∈ p =θ l +θ g , where θ l is the volume fraction of water in the soil, θ g is the volume fraction of gas in the soil. At the same time, it is necessary to add consideration of soil air on the basis of the saturated zone, so that the change term of substance concentration in the bioretention pond filler is changed to the change term of substance concentration dissolved in the soil liquid, adsorbed in the soil solid particles, and volatilized in the porous medium air. The corresponding change of the formula expression is: Where: c G,i is the concentration of substance i in the gas phase, mol / m 3 , c G,i =k G,i c, k G,i is the proportion of substance i in the gas phase; Therefore, the governing equation for the simulation of dissolved phase pollutant transport in the bioretention pond described in step 2 is as follows: i g =∈ p -θ l (16) c G,i =k G,i c (17)。 6. The method for simulating pollutant migration in a bioretention pond based on COMSOL according to claim 1, characterized in that: In the step 2, the adsorption process of pollutants is considered during the migration and transformation of pollutants, using the Langmuir model or the Freundlich model, or determining a custom adsorption curve through experiments; The Langmuir model uses the following governing equations: Where: c P,i,L is the Langmuir solute equilibrium adsorption capacity of substance i in the adsorbent, mg / g; K L,i is the Langmuir adsorption constant, L / mg; c Pmax,i is the maximum adsorption capacity of the adsorbed substance, mg / g; c i is the concentration of substance i in the solution, mg / L; The Freundlich model uses the following governing equations: Where: c P,i,F is the Freundlich equilibrium adsorption capacity of substance i on the adsorbent, mg / g; K F,i 、N F,i is the Freundlich constant of substance i, mg / g; c i is the concentration of substance i in the solution, mg / L; c ref,i is the standard sample concentration of substance i in solution, mg / L.

7. The method for simulating pollutant migration in a bioretention pond based on COMSOL according to claim 1, characterized in that: The modeling process based on COMSOL Multiphysics in step 3 is as follows: The following steps are involved: 3.1: Based on the rainwater runoff pollution characteristics in step 1 and the rainwater runoff control objectives in the construction area, determine the structural design of the required bioretention pond, determine the pollutants to be simulated, and establish the geometric model of the bioretention pond; 3.2: Add multi-physics interface: Use the "Richards Equation (DL)" interface to simulate the flow field in porous media fillers, and use the "Transport of Diluted Species in Porous Media (TDS)" to simulate the migration and transformation process of dissolved phase pollutants in fillers. Set parameter properties and use initial conditions and boundary conditions to constrain the equations. The left and right boundaries of the bioretention tank are set as no-flow boundaries; the bottom boundary is set as the permeable layer boundary; 3.3: Customize meshing of the bioretention pond geometry model; 3.4: Add a transient study, adjust the absolute tolerance, time step, termination method and criterion configuration in the solver according to needs, post-process the calculation results, and display the Darcy velocity field, pressure head, dissolved phase pollutant concentration, effective saturation and resistance coefficient views respectively; 3.5: Using the two-dimensional cutoff points in the data set, create a one-dimensional plot group to extract the concentration change data of a certain point at the outflow location at different times to show the change in pollutant outflow concentration; 3.6: According to the model simulation results and design requirements, adjust the design parameters of the biological retention pond model, such as the filler combination and design scale. According to the simulation results after debugging, compare and determine the optimal design parameter combination for different design objectives.

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