Environmental Engineering Reference
In-Depth Information
will tend to reduce gas production because they obstruct the passage of moisture. Leachate
recirculation back into the waste tends to increase the rate of gas production.
7. Atmospheric conditions
Atmospheric conditions affect the temperature, pressure, and moisture content within a
landfill. Landfill covers and liners help to isolate waste from atmospheric conditions by
limiting oxygen intrusion, limiting infiltration of precipitation, and buffering the effects of
temperature changes.
8. Oxygen concentration
The activity of anaerobic microorganisms is affected by the presence of oxygen. Thus,
the absence of free oxygen concentration in the landfill is required in order to grow and
degrade the MSW into methane and CO
2
.
In reality, the oxygen that diffuses from the
atmosphere into the landfill is consumed by aerobic bacteria in the top layers of the landfill
(aerobic zone) (Warith 2003).
If oxygen is present, aerobic biodegradation will proceed rapidly and the resulting gas
will comprise mostly carbon dioxide. A few modern engineered landfills are designed to
function aerobically. However, in a typical engineered landfill where waste is quickly
compacted and covered, aerobic degradation only occurs until the entrained oxygen is used up
in newly deposited waste. Where oxygen is not available, the waste is broken down by
anaerobic bacteria that produce a “classic” landfill gas, containing roughly equal amounts of
methane and carbon dioxide. In uncontrolled “dumps”, where waste is left loose and
uncovered, breakdown of the waste may be almost entirely aerobic, and in such cases no
methane will be produced.
9. Hydrogen concentration
The fermentative and acidogenic bacteria produce hydrogen during the biodegradation of
MSW, while the methanogenic bacteria use the hydrogen as a substrate to produce methane.
Low partial pressure of hydrogen is required for the acidogenic processes (hydrogen
producing bacteria) and methanogenesis processes.
An increase in the partial pressure of hydrogen causes the generation of propionic and
butyric acids with no further conversion, resulting in an accumulation of volatile organic acids
which reduce the pH and inhibit the methanogenic bacteria. The conversion of propionic acid
requires a hydrogen pressure lower than 9x10
-5
atmospheres (Christensen et al. 1989).
10. Precipitation
Precipitation dramatically affects the gas generation process by supplying water to the
process and by carrying dissolved O
2
into the waste with the water. High rates of precipitation
may also flood sections of the landfill, which will obstruct gas flow. The amount of
precipitation that reaches the waste is highly dependent on the type of landfill cover system.
11. Density of the waste
The density of waste fills is highly variable. An estimate of waste density is often
required for estimating landfill gas generation rates. Several researchers reported density
values; Stecker, (1989) reported that MSW density values range from 474 to 711 Kg/m
3
,
