GAS MOVEMENT
Mostly 90 % of gas produced from decomposition of solid waste includes methane and carbon dioxide. The methane in air in the concentration of 5 to 15 % is explosive. In case of high concentration of methane in landfill, there is no oxygen present so there is no danger of explosion. Most of the methane releases into the atmosphere. Both methane and carbon dioxide are found in concentrations of 40 % at lateral distances of 400 ft from the edges of landfill.
If proper venting is applied than methane does not create any problem. Carbon dioxide is problematic because of its density. Carbon dioxide is 1.5 times denser than air and 2.8 times denser than methane. It tends to move at the bottom of landfill. The concentrations of carbon dioxide in the bottom is thus high.
CONTROL OF GAS MOVEMENT BY PERMEABLE METHODS
The lateral movement of gas could be controlled by installing the vents that are made of materials that are more permeable than surrounding soil. Normally gas vents are made of gravel. The spacing of vents depends upon width of waste cells, mostly it varied between 60 to 200 ft. The thickness of gravel layer should be continuous and recommended between 12 to 18 in. Barrier vents and well vents are also use to control lateral movement of gases.
When well vents are used, waste gas burners are also installed often. It is recommended well penetrates in the upper waste cell. The height of waste burner varied between 10 to 20 ft. The burner can be burned by hand or by pilot flame.
The downward movement of gases can be controlled by perforated pipes in gravel layer at the bottom of landfill.
LECHATE MOVEMENT
Lechate is generally found in the bottom of landfills. It movement is by underlying strata. Lateral movement can also occur depending on properties of surrounding materials.
Darcy's Law:
The rate of seepage of lechate from the bottom of landfill can be estimated by this law. It is written as:
Q= -KA dh/dL
Q= lechate discharge per unit time
K= coefficient of permeability
A= cross sectional area by which lechate flow
dh/dL= hydraulic gradient
The - sign comes from the fact that head loss dh is always negative. Coefficient of permeability is also termed as hydraulic conductivity, effective permeability or seepage coefficient.
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