LANDFILL GAS

COLLECTION DESIGN LAYER

DESIGN METHODS & CONCEPTS

Landfill gases are generated from the biodegradation of solid waste in a landfill. The actual rate of gas generation depends on waste composition, moisture content, age, etc. The purpose of a gas collection layer is to facilitate the collection of the generated gases so that they do not cause uplift of the cap. The typical configuration of a landfill gas collection layer is presented in Figure 7.1. The primary design criterion for geocomposites is to provide enough flow capacity to reduce the landfill gas pressure to an acceptable level in terms of factor of safety for slope stability, as illustrated in the following equation:

max             cover               cover

tanδ

[

u    = γ      × t      × cosβ –

[

(FS   × γ      × t      × sinβ)

s                cover                cover

EQUATION 7.1

WHERE

u        = allowable gas pressure (kPa)
γ         = cover soil density (kg/ m  )

t          = soil cover thickness (m)
FS   = factor of safety against sliding
δ = interface friction angle (degrees) for geocomposite-geomembrane interface
β = slope angle

max

cover

s

cover

3

LandfillGas_Figure7.1.png

FIGURE 7.1 – SCHEMATIC OF A LANDFILL GAS COLLECTION LAYER

The incoming flow rate for landfill gas will be gauged in terms of flux. The equation used to calculate the landfill gas flux is presented as follows [Thiel, 1998]:

q  = r  × t      × γ     

g             g             waste              waste

EQUATION 7.2

WHERE

g

3

q    = landfill gas supply rate (m/sec)
r    = landfill gas generation rate (m  /sec/kg of waste)
t          = thickness of waste (m); 
γ           = unit weight of waste (kg/m  ).

g

waste

waste

3

The required transmissivity of the gas drainage layer can be calculated as follows:

θ     =

greq

q  × γ

g

g

u

max

(

D

2

8

)

EQUATION 7.3

WHERE

g

3

q    = landfill gas supply rate (m/sec)
γ   = unit weight of gas (kg/m  )

θ         = required gas transmissivity for geonet or geocomposite (m  /sec per m width)

D = slope distance between drains (m)

g

greq

3

Ultimate gas transmissivity can be calculated using Equation 7.4

θ        = θ   × FS × RF  × RF  × RF  × RF  

ultimate              req                                         in                   cr                    cc                    bc 

EQUATION 7.4

WHERE

ultimate

θ              = ultimate gas transmissivity
θ      = the transmissivity of the geocomposite (m  /sec-m) 

FS = overall factor of safety

RF     = reduction factor for intrusion 

RF     = reduction factor for creep to account for long-term behavior 

RF     = reduction factor for chemical clogging 

RF     = reduction factor for biological clogging

req

cc

in

bc

cr

2

Notice that the above equation provides the required transmissivity for the flow of gas, not water. Therefore, transmissivity value from actual in-plan airflow should be used for evaluating geocomposite performance.

Table 1 provides density and viscosity values for various fluids for use in Equation 7.5. Again we note that a very significant side benefit of providing a gas collection layer under the final cover is that it will also serve to collect side slope seeps. The seeps would be collected at the toe of the geocomposite gas collection layer, as illustrated in Figure 7.2.

TABLE 7.1 – DENSITY AND VISCOSITY OF VARIOUS FLUIDS [THIEL, 1998]

9.99E+02

1.20E+00

1.83E+00

6.66E-01

1.31E+00

kg/m

3

N/m

3

9.80E+03

1.18E+01

1.79E+01

6.54E+00

1.28E+01

Notes:
( 1 ) Values for LFG (landfill gas) were assumed to be prorated as 55% properties of carbon dioxide, and 45% properties of methane. This ratio was used to match the LFG characteristics for the Coffin Butte case history, which may be different than other landfills.

( 2 ) Values are at standard temperature and pressure.

N-s/m   or kg/(s-m)

2

1.01E-03

1.79E-05

1.50E-05

1.10E-05

1.32E-05

m  /s

2

1.01E-06

1.48E-05

8.21E-06

1.65E-05

1.01E-05

Water

Air

CO2

Methane (CH   )

4

LFG: 55% CO2, 45% CH

4

FLUID

DENSITY (ρ)

UNIT WEIGHT (γ)

DYNAMIC VISCOSITY (μ)

KINEMATIC VISCOSITY (ν=μ/ρ)

LandfillGas_Figure7.2.png

FIGURE 7.2 SEEP COLLECTION AT TOE OF GAS COLLECTION LAYER UNDER FINAL COVER SYSTEM

LFG pressure gradient varies linearly with its maximum at the strip drain location, and zero in the center of the geocomposite gas venting blanket. 

Maximum pressure gradient is shown below in equation 7.5:

Maximum pressure gradient is shown in equation 7.5:

i     =

max

q

g

θ

greq

(

D

2

)

EQUATION 7.5

WHERE

q    = landfill gas supply rate (m/sec)
i         = maximum pressure gradient

θ         = required gas transmissivity for geonet or geocomposite (m  /sec per m width)

D = slope distance between drains (m)

g

max

greq

3

LANDFILL GAS

EQUATION SHEET

Maximum Gas Pressure

Landfill Gas Supply Rate

Required Air Transmissivity and Gradient

Maximum Gas Pressure

Landfill Gas Supply Rate

Required Air Transmissivity and Gradient

Maximum Gas Pressure

Landfill Gas Supply Rate

Required Air Transmissivity and Gradient

Solve For Maximum Gas Pressure

Input parameters

β =

degree

Slope Angle

γ          =

cover

Unit weight of cover protective soil

kN/m 

3

δ =

degree

Interface friction angle for geocomposite-geomembrane interface

FS   =

s

dimensionless

Factor of safety against sliding

t          =

cover

meter

Thickness of cover protective soil

SOLUTION

kPa

Allowable maximum gas pressure underneath the cover geomembrane

µ        =

max

γ         x

cover

t         x

cover

cosβ –

(FS   x

s

γ         x

cover

t         x

cover

sinβ)

tanδ

Solve For Landfill Gas Supply Rate

Input parameters

r   =

m  /sec/kg-waste

Landfill gas generation rate

3

g

t           =

meter

Thickness of waste

waste

γ           =

kg/m 

Unit weight of waste

waste

3

SOLUTION

m  /sec-m 

3

2

q   =

g

r   x

g

t           x

waste

γ            

waste

Landfill gass supply rate

Solve For Required Air Transmissivity and Gradient

Input Parameter

3

2

m  /sec/m  

D =

meter

Slope distance between drains

μ        =

kPa

Maximum allowable gas pressure

max

RF    =

dimensionless

Intrusion Reduction Factor

in

FS =

dimensionless

Overall Factor of Safety

dimensionless

Creep Reduction Factor

RF    =

cr

RF     =

cc

dimensionless

Chemical Clogging Reduction Factor

dimensionless

Biological Clogging Reduction Factor

RF     =

bc

N/m  

Unit weight of gas

γ       =

gas

3

Gas generation Rate

q    =

g

SOLUTION

θ         =

greg

m  /sec

2

Required gas transmissivity of the geocomposite layer

θ        =

greg

q    x

g

γ     

g

U         

max

[

8

D   

2

[

Ultimate gas transmissivity

m  /sec

2

θ        = FS x RF    x RF    x RF    x RF    

greq

in

cr

cc

bc

θ              =

ultimate

dimensionless

i =

Maximum pressure gradient

D  

[

8

[

q  

g

i        =

max

θ         

greq