DESIGN METHOD OF SOIL COVER LAYER OF SOLID WASTE LANDFILL CONSIDERING EFFECT OF PLANT ROOT

Abstract

The present disclosure discloses a design method of a soil cover layer of a solid waste landfill considering the effect of plant root, which relates to the field of designing a soil cover for a solid waste landfill and aims to solve the problem of the prior art that does not consider the non-linear spatial variation of water content and the effect of plant root on the gas migration process. By comprehensively considering the type of plant root architecture, the distribution of water content varying with space, the errors of calculating gas migration caused by assuming that the water content of the cover layer is constant and ignoring the effect of plant root is effectively reduced; and the actual environment of the on-site cover layer is more comprehensively considered, thus improving the calculation accuracy.

Claims

1. A design method of a soil cover layer of a solid waste landfill considering the effect of plant root, including the following steps: Step 1, constructing a mathematical model of the landfill cover layer, wherein the step 1 comprises: based on the structural type of final covering of the solid waste landfill, determining a calculated thickness L of the cover layer; according to the type of vegetation planted in the cover layer, selecting a corresponding root architecture and determining the depth L.sub.2 of the root zone layer and the depth L.sub.1 of the root-free zone layer; based on a geographical location where the cover layer is located and local meteorological conditions, selecting the rainfall intensity q.sub.01, rainfall time t.sub.p, the evaporation intensity q.sub.02, and the transpiration intensity T.sub.p acting at the surface of the cover layer, and constant head conditions acting at the bottom boundary of the cover layer; according to the gas content at the top and bottom boundaries of the cover layer, determining a gas concentration C.sub.t condition acting at the top boundary of the cover layer and a constant concentration C.sub.b or a constant flux F.sub.0 condition at the bottom boundary; and selecting basic parameters of water and gas migration in the cover layer, wherein the basic parameters of water and gas migration in the cover layer further include the soil-water characteristic curve, the saturated water permeability coefficient k.sub.s, the gas intrinsic permeability k.sub.i, the gas diffusion coefficient D.sub.0 and the gas type; Step 2, calculating the distribution of water and gas migration in the cover layer, wherein the step 2 comprises: after constructing the mathematical model of the cover layer and setting the parameters, performing model calculation; and Step 3, evaluating gas sealing performance of the landfill cover layer, wherein the step 3 comprises: comparing and analyzing gas emission rates obtained by calculation under different working conditions; and taking service time t of the cover layer as the x axis and the gas emission rates at the surface of the cover layer as the y axis to draw a change curve of the gas emission rate at the surface of the cover layer with time under the different working conditions; wherein after a period of action of rainfall and drying, if the gas emission rate J at the surface of the cover layer is less than an allowable gas emission standard limit value J.sub.L at the surface of the cover layer, it indicates that the cover layer has a good gas sealing effect and excellent service performance under the working condition, and the gas does not breakthrough the cover layer; on the contrary, when the gas emission rate J at the surface of the cover layer is greater than the allowable gas emission standard limit value J.sub.L at the surface of the cover layer, it indicates that the gas sealing effect of the cover layer is poor under the working condition, and the gas breakthrough the cover layer; the time corresponding to reach the standard limit value J.sub.L is the breakthrough time T.sub.j of the landfill gas.

2. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 1, wherein the model calculation in step 2 comprises the calculation of water content distribution of the cover layer, the calculation of gas distribution of the cover layer and the calculation of the gas emission rate at the surface of the cover layer.

3. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 2, wherein the calculation equation of the water content distribution of the cover layer is:
θ.sub.w=θ.sub.r+(θ.sub.s−θ.sub.r)k* where: θ.sub.s and θ.sub.r are the saturated volumetric water content and the residual volumetric water content of the soil respectively; and k* is the relative permeability coefficient of the soil.

4. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 3, wherein the relative permeability coefficient of the soil considers four different root architectures, namely a uniform root architecture, a triangular root architecture, an exponential root architecture and a parabolic root architecture.

5. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 4, wherein the calculation equations of the relative permeability coefficients corresponding to the four different root architectures are as follows: uniform root architecture: $k^{*} = {\begin{matrix} A + \frac{T_{p}}{k_{s} L_{2}} [\exp (- α z) - 1] (L - L_{1}) \\ A + \frac{T_{p}}{k_{s} L_{2}} {\begin{matrix} [\exp (- α z) - 1] (L - z) + \\ \exp (- α z) [z - L_{1} - α^{- 1} \exp (α z) + α^{- 1} \exp (α L_{1})] \end{matrix}} \end{matrix}$ triangular root architecture: $k^{*} = {\begin{matrix} A + \frac{2 T_{p}}{k_{s} L_{2}^{2}} [\exp (- α z) - 1] [\frac{L^{2} - L_{1}^{2}}{2} - L_{1} L + L_{1}^{2}] \\ \begin{matrix} A + \frac{2 α T_{p}}{k_{s} L_{2}^{2}} {\frac{1}{α} [\exp (- α z) - 1] (\frac{L^{2} - z^{2}}{2} - L_{1} L + L_{1} z) + \\ \begin{matrix} \frac{1}{α} \exp (- α z) [\frac{\exp (α L_{1})}{α^{2}} (α L_{1} - 1) - \frac{\exp (α z)}{α^{2}} (α z - 1) - \\ - L_{1} z + L_{1}^{2} - \frac{L_{1}^{2} - z^{2}}{2} - \frac{L_{1}}{α} \exp (α L_{1}) + \frac{L_{1}}{α} \exp (α z)]} \end{matrix} \end{matrix} \end{matrix}$ exponential root architecture: $k^{*} = {\begin{matrix} \begin{matrix} A + \frac{T_{p}}{k_{s}} \frac{1}{\exp (L_{2}) - L_{2} - 1} [\exp (- α z) - 1] {L_{1} - L + \\ \exp (- L_{1}) [\exp (L) - \exp (L_{1})]} \end{matrix} \\ \begin{matrix} A + \frac{T_{p}}{k_{s}} {\frac{\exp (- α z) - 1}{\exp (L_{2}) - L_{2} - 1} {z - L + \exp (- L_{1}) [\exp (L) - \exp (z)]} + \\ \begin{matrix} \frac{\exp (- α z)}{\exp (L_{2}) - L_{2} - 1} {L_{1} - z - \frac{\exp (α L_{1}) - \exp (α z)}{α} - \\ \exp (- L_{1}) [\exp (L_{1}) - \exp (z)] - \frac{\exp (z - L_{1} + α z)}{α + 1} + \frac{\exp (α L_{1})}{α + 1}}} \end{matrix} \end{matrix} \end{matrix}$ parabolic root architecture: $k^{*} = {\begin{matrix} \begin{matrix} A + \frac{6 T_{p}}{k_{s} L_{2}^{3}} [\exp (- α z) - 1] [\frac{(L - L_{1}) (2 L_{1} + L_{2}) (L + L_{1})}{2} - \\ L_{1} (L - L_{1}) (L_{1} + L_{2}) \frac{(L - L_{1}) (L^{2} + {LL}_{1} + L_{1}^{2})}{3}] \end{matrix} \\ \begin{matrix} A + \frac{6 T_{p}}{k_{s} L_{2}^{3}} {[\exp (- α z) - 1] [\frac{(L + z) (2 L_{1} + L_{2}) (L - z)}{2} - \\ \begin{matrix} \frac{(L - z) (L^{2} + Lz + z^{2})}{3} - L_{1} (L - z) (L_{1} + L_{2})] + \exp (- α z) [z^{2} (L_{1} - \frac{z}{3}) - \\ \begin{matrix} {zL}_{1}^{2} + L_{1}^{3} / 3 - L_{1}^{2} L_{2} / 2 - \frac{L_{2}}{α^{2}} \exp (α z) (α z - 1) + \\ \begin{matrix} L_{1}^{2} L_{2} + \frac{L_{2}}{α^{2}} \exp (α L_{1}) (α L_{1} - 1) + L_{2} z^{2} / 2 - {zL}_{1} L_{2} - \\ \begin{matrix} \frac{L_{1} L_{2}}{α} \exp (α L_{1}) + \frac{L_{1} L_{2}}{α} \exp (α z) + \frac{\exp (α z)}{α^{3}} (α^{2} L_{1}^{2} + 2 α L_{1} - 2 {zL}_{1} α^{2} + \\ z^{2} α^{2} - 2 α z + 2) - \frac{2 \exp (α L_{1})}{α^{3}}]} \end{matrix} \end{matrix} \end{matrix} \end{matrix} \end{matrix} \end{matrix}$ where: A is relative permeability coefficient when ignoring the effect of plant root: q.sub.01 is the rainfall intensity; q.sub.02 is the evaporation intensity; T.sub.p is the transpiration intensity; h.sub.0 is the water head boundary at the bottom of the cover layer; t.sub.p is rainfall time; L is the thickness of the cover layer; L.sub.1 is the thickness of the outside root zone layer; L.sub.2 is the thickness of the root zone layer; α is the desaturation coefficient of the soil; z is the vertical coordinate from the bottom of the cover layer, and k.sub.s is the saturated permeability coefficient of the soil.

6. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 2, wherein the calculation equation of the gas distribution of the cover layer is:
C.sub.g(z,t)=Y.sup.T[exp−(−∫.sub.0.sup.tHdτ)T(0)+exp(−∫.sub.0.sup.tHdτ)∫.sub.0.sup.t exp(∫.sub.0.sup.τHdτ′)Gdτ]+B where: C.sub.g (z,t) is a landfill gas concentration in the cover layer; Y is an n×1-order matrix constituted of $\frac{ψ_{n} (z)}{N_{n}^{1 / 2}};$ ψ.sub.n(z) is a characteristic function and N.sub.n is a constant related to the orthogonality of the characteristic function ψ.sub.n(z); H is an n×r-order matrix constituted of H.sub.nr, H.sub.nr is the time factor related to gas migration; T is an n×1-order matrix constituted of T.sub.n, T.sub.n is the generalized integral transformation formula related to the gas concentration; G is an n×1-order matrix constituted of G.sub.n, G.sub.n is an integral transformation; and B is a function related to boundary conditions.

7. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 6, wherein the variables H.sub.nr, T.sub.n, $\frac{ψ_{n} (z)}{N_{n}^{1 / 2}}$ and G.sub.n can be determined by the following equation: $H_{n r} = - \frac{1}{N_{n}^{1 / 2} N_{r}^{1 / 2}} \int_{0}^{L} \frac{θ_{g} D_{s}}{θ_{g} + H_{g} θ_{w}} \frac{\partial^{2} ψ_{n} (z)}{\partial z^{2}} ψ_{r} (z) dz - \frac{1}{N_{n}^{1 / 2} N_{r}^{1 / 2}} \int_{0}^{L} \frac{\partial (θ_{g} D_{s}) / \partial z - (v_{w} H_{g} + v_{g})}{θ_{g} + H_{g} θ_{w}} \frac{\partial ψ_{n} (z)}{\partial z} ψ_{r} (z) dz + \frac{1}{N_{n}^{1 / 2} N_{r}^{1 / 2}} \int_{0}^{L} \frac{\partial (v_{w} H_{g} + v_{g}) / \partial z + μ θ_{g}}{θ_{g} + H_{g} θ_{w}} ψ_{n} (z) ψ_{r} (z) dz;$ $T_{n} (t) = \frac{1}{N_{n}^{1 / 2}} \int_{0}^{L} ψ_{n} (z) u_{g} (z, t) dz;$ $N_{n} = \int_{0}^{L} ψ_{n}^{2} (z) dz;$ $G_{n} = \frac{1}{N_{n}^{1 / 2}} \int_{0}^{L} G (z, t) ψ_{r} (z) dz;$ where: ψ.sub.r(z) and ψ.sub.n(z) are the characteristic functions: H.sub.g is the Henry's coefficient: θ.sub.6 is the volumetric gas content of the soil: D.sub.s is the effective diffusion coefficient of gas: v.sub.w is the velocity of water flow; and N.sub.r is the constant related to orthogonality of the characteristic function ψ.sub.r(z); z is the vertical coordinate from the bottom of the cover layer, B is the function related to boundary conditions: G(z, t) is the intermediate variable related to the gas concentration and B, ψ.sub.n(z) and G(z,t) can be determined according to different bottom boundary conditions.

8. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 7, wherein the bottom boundary conditions comprise a constant concentration boundary and a constant flux boundary; and for the different bottom boundary conditions, calculation equations of B, ψ.sub.n(z) and G(z,t) are as follows: when the bottom boundary condition of the landfill cover layer is a constant concentration boundary: $B = C_{b} (1 - \frac{z}{L}) + C_{t} \frac{z}{L};$ $G (z, t) = \frac{\partial (θ_{g} D_{s}) / \partial z - (v_{w} H_{g} + v_{g})}{(θ_{g} + θ_{w} H_{g}) L} (C_{t} - C_{b}) - \frac{\partial (v_{w} H_{g} + v_{g}) / \partial z + μ θ_{g}}{θ_{g} + θ_{w} H_{g}} B;$ $ψ_{n} (z) = \sin (\frac{k π z}{L});$ when the bottom boundary condition of the landfill cover layer is a constant flux boundary: ${B = \frac{F_{0}}{θ_{g} D_{s}} .Math.}_{z = 0} (z - L) + C_{t};$ ${G (z, t) = \frac{\partial (θ_{g} D_{s}) / \partial z - (v_{w} H_{g} + v_{g})}{θ_{g} + θ_{w} H_{g}} \frac{F_{0}}{θ_{g} D_{s}} .Math.}_{z = 0} - \frac{\partial (v_{w} H_{g} + v_{g}) / \partial z + μ θ_{g}}{θ_{g} + θ_{w} H_{g}} B;$ $ψ_{n} (z) = \sin [\frac{(2 k - 1) π}{2 l} (L - z)];$ $ψ_{n} (z) = \sin [\frac{(2 k - 1) π}{2 L} (L - z)];$ where: θ.sub.g is the volumetric gas content of the soil; D.sub.s is the effective diffusion coefficient of gas; H.sub.g is the Henry's coefficient; v.sub.w is the velocity of water flow; v.sub.g is the advective velocity of gas; C.sub.b is the gas concentration at the bottom of the cover layer; C.sub.t is the gas concentration at the top of the cover layer; F.sub.0 is the gas flux at the bottom of the cover layer; μ is the degradation rate of the landfill gas; L is thickness of the cover layer; z is the vertical coordinate from the bottom of the cover layer; and k is set to be a positive integer.

9. The design method of a soil cover layer of a solid waste landfill considering the effect of plant root according to claim 8, wherein the calculation equation of the gas emission rate at the surface of the cover layer is as follows: $J = {[C_{g} (z, t) \times (v_{g} + v_{w} H_{g}) - θ_{g} D_{s} \frac{\partial C_{g} (z, t)}{\partial t}]}_{z = L};$ where: C.sub.g (z,t) is the concentration of landfill gas in the cover layer; J is the gas emission rate at the surface of the cover layer.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0026] FIG. 1 is a schematic flow chart of the present disclosure;

[0027] FIG. 2 is a schematic diagram of the calculation model of the present disclosure;

[0028] FIG. 3 is a schematic diagram showing the comparison between the gas emission rate at the surface of a cover layer calculated in the present disclosure and numerical simulation results, and comparison results with and without considering the effect of plant root and water migration; and

[0029] FIG. 4 is a comparative diagram of curves showing the gas emission rate at the surface of a cover layer varying with time, calculated by changing the thickness of the cover layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] In order to make the purpose, technical solutions and advantages of the present disclosure clearer, the present disclosure will be further described in detail with the attached drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present disclosure, and are not used to limit the present disclosure. In addition, technical features involved in various implementations of the present disclosure described below can be combined with each other as long as they do not conflict with each other.

[0031] Mainly according to weather conditions and the landfill environment of a landfill cover layer, the present disclosure obtains water and gas migration parameters of the landfill cover layer, selects a type of plant root architecture of the cover layer and sets a depth of the root zone in the cover layer, determines water and gas migration boundary conditions acting on and under the cover layer, and calculates the gas emission rate at the surface of the cover layer under the effect of plant root in combination with the service process of the landfill cover layer, so that the service performance of the cover layer under this condition can be comprehensively evaluated. Further, parameters can be changed to calculate different working conditions; and the type of cover structure that meets the service requirements can be selected, or the existing cover structure can be maintained to meet the service requirements.

Embodiment 1

[0032] This embodiment takes migration of methane as a landfill gas in the cover layer as an embodiment to further explain the present disclosure.

[0033] As shown in FIG. 1, the present disclosure discloses a design method for a soil cover layer of a solid waste landfill considering the effect of plant root. According to an adopted technical solution, the method includes 3 steps: step 1 is the construction of a mathematical model of the cover layer, which mainly determines the thickness of the soil cover layer, the type of root architecture, the depth of the root zone, and the top and bottom boundary conditions of water and gas migration; step 2 is the calculation of the distribution of water and gas migration in the cover layer, which mainly calculates the gas concentration in the cover layer and the gas emission rate at the surface of the cover layer considering the effect of plant root and water migration; step 3 is evaluation the service performance of the cover layer, according to the gas emission rate limit value given by relevant specifications, if the calculated gas emission rate at the surface of the cover layer is less than the gas emission rate limit value, it indicates that the cover layer has excellent service performance under the working condition; and on the contrary, when the calculated gas emission rate at the surface of the cover layer is greater than the gas emission rate limit value, it indicates that the gas sealing effect of the cover layer is poor, and the gas breakthrough the cover layer under the working condition. Further, model parameters can be reset as needed to assist designers to optimize the cover structure at the early stage of design or to maintain and control the existing filed cover layer to meet the emission requirements.

[0034] Step 1): Constructing a Mathematical Model of the Cover Layer

[0035] As shown in FIG. 2, the cover layer calculation model of the present disclosure mainly includes the following parts: vegetation layer soil, soil outside the root zone and the industrial solid waste layer. Specific soil parameters: soil layer thickness L=0.9 m, the depth of the root zone L.sub.2=0.3 m, the depth of the outside root zone L.sub.1=0.6 m; the uniform root architecture is adopted for calculation; the cover layer soil is compacted loess; the basic parameters of the vegetated soil are: SWCC: θ.sub.s=0.47 and θ.sub.r=0.1; the desaturation coefficient α=0.2 m.sup.−1; the saturated permeability coefficient of the soil is 1.0×10.sup.−7 m/s; the intrinsic permeability of soil is 4.8×10.sup.−16 m.sup.2; and the diffusion coefficient of gas in free gas phase is D.sub.0=2.12×10.sup.−5 m.sup.2/s.

[0036] According to regional weather conditions of typical landfills in China and according to the design of once-in-50-year heavy rain, it is assumed that the continuous rainfall time is t.sub.p=24 h. Hence, the rainfall intensity of the cover layer is q.sub.01=40 mm/day. Then, there is continuous drought with an evaporation rate of q.sub.02=4 mm/day and a transpiration rate of T.sub.p=6 mm/day; and the constant water head boundary at the bottom of the cover layer is taken as h.sub.0=−3.5 m.

[0037] According to the emission situations of polluted gas in the domestic landfills, the present disclosure adopts methane gas as the migration gas for design. The initial methane concentration in the cover layer is selected as C.sub.0=0 mol/m.sup.3, the top methane concentration of the cover layer is C.sub.t=0 mol/m.sup.3, and the methane concentration at the bottom of the cover layer is taken as C.sub.b=8 mol/m.sup.3 according to the gas production at the bottom of the solid waste landfill. Reference specification: Australian Design Standard CFI, 2013 stipulates that an emission limit of landfill gas methane is 7.2 g/m.sup.2/day, so the allowable methane emission rate at the surface of the cover layer here is specified as J.sub.L=7.2 g/m.sup.2/day.

[0038] Step 2): Calculating the Distribution Situation of Water and Gas Migration in the Cover Layer

[0039] After establishing the mathematical model and setting parameters of the cover layer considering the effect of plant root, the distribution of the volumetric water content of the cover layer with space is obtained by calculation at first; and then the variation of the gas concentration of the cover layer with space and time is obtained.

[0040] According to the calculation results of gas concentration of the vegetation cover layer obtained from the above steps, and then according to the calculation equation of the gas emission rate, the gas emission rate curves of the cover layer with time under different working conditions can be obtained.

[0041] Step 3): Evaluating Service Performance of the Landfill Cover Layer

[0042] The curve of the gas emission rate on the surface of the cover layer with time calculated by the present disclosure is shown in FIGS. 3 and 4. As can be seen from FIG. 3, the gas calculation method provided by the present disclosure is basically consistent with the change trend of numerical results obtained by numerical software COMSOL based on a theoretical equation, which fully shows that the design method of the soil cover layer considering the effect of plant root designed by the present disclosure is high in accuracy. Meanwhile, as shown in FIG. 3, the calculation result considering the effect of plant root and the influence of water migration is more accurate, while ignoring the effect of plant root and water migration will underestimate the gas emission rate and overestimate the gas breakthrough time. It is thus clear that the traditional design calculation method ignoring the influences of plants and water migration has certain risks, which also proves that the present disclosure is more accurate.

[0043] In addition, according to the calculation results of the present disclosure under the working condition shown in FIG. 3, it is found that the gas emission rate has exceeded the gas emission limit value J.sub.L, so it is necessary to continue to perform the first step. Here, the cover layer design is completed only by changing the cover layer thickness as an example. FIG. 4 shows a comparison diagram of the gas emission rate at the surface of the cover layer under two optimized designs (L=1.2 m and L=1.5 m) and an original working condition (L=0.9 m). It is thus clear that increasing the thickness can greatly reduce the gas emission rate. In this way, the cover layer structure meeting the requirements is designed. Meanwhile, the parameters of the cover layer can be set flexibly according to an actual situation and economic effects, so as to obtain a more economical cover layer structure that meets the requirements of gas sealing. It can be seen that the present disclosure is convenient and can be set flexibly according to the needs of users.

[0044] The present disclosure provides a design method for the soil cover layer of a solid waste landfill considering the effect of plant root, which comprehensively considers factors such as the type of plant root architecture, boundary conditions, water content distribution changing with space and the like, effectively reduces the calculation error caused by assuming constant water content distribution of the cover layer and ignoring the effect of plants, and further fits with an actual service environment where the cover layer is located at the engineering site. The calculation method is flexible and simple, and solves the problems of complex numerical simulation calculation and unclear variable relationships.

[0045] Although the specific embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments, and various changes can be made within the knowledge of those ordinarily skilled in the art without departing from the purpose of the present disclosure, while modifications or deformations without creative labor are still within the protection scope of the present disclosure.

DESIGN METHOD OF SOIL COVER LAYER OF SOLID WASTE LANDFILL CONSIDERING EFFECT OF PLANT ROOT

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G06F2111/10

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Y02W30/30

GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS

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G06F30/10

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G06F30/10

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Abstract

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Description