8. 5 Stages of Biodegredation in Landfills
Each cell contains the waste deposited in one day
Landfills are constructed one cell at a time over a period of 20 to 30 years
A the end of the day, each cell is capped with a layer of sand or soil
At any given time, different parts of the same landfill are in different stages of decomposition
The time frame for decomposition is variable, depending on the nature of the waste, the landfill conditions, and the local climate

9. Stage 1: Initial Adjustment
Waste absorbs moisture, compacts, and settles slowly
Oxygen is present as microbes and animals begin to consume and digest organic matter
When the oxygen supply is exhausted, aerobic organisms die off and anaerobic microbes take over as principal decomposers
This aerobic to anaerobic transition occurs 6 to 18 months after waste is placed in cell

10. Stage 2: Transition
Carbon dioxide completely replaces oxygen as the primary landfill gas
Anaerobic bacteria convert the substances created by aerobic bacteria to acetic, lactic, and formic acids and to alcohols, including methanol and ethanol
The acids produced during this stage percolate through the waste as highly acidic leachate
“Transition” stage lasts for several months

11. Stage 3: Acid Formation
Anaerobic decomposition continues to produce leachate with its high concentrations of organic acids
As pH drops, waste metals such as chromium, iron, and magnesium to dissolve in the leachate
Nitrogen & phosphorous slowly become available to anaerobic bacteria, which facilitates population growth
Over time fermenting bacteria convert organic acids, alcohols, hydrogen and carbon dioxide to acetate
Landfill settles unevenly

12. Stage 4: Methane Fermentation
Methane-producing bacteria transform the acetate, methanol, and other chemical to methane, carbon dioxide and water
The methanogenic bacteria produce significant amounts of methane and carbon dioxide (landfills account for about 20% of human-generated methane emissions)
Landfill continues to settle
This stage lasts from 15 to 60 years

13. Stage 5: Final Maturation
There is little scientific data on the details of this phase & its long-term characteristics are uncertain
Bacterial populations decline
Gas and leachate production eventually ends
Landfill topography stabilizes

14. Problems and Questions
Does the threat to human health end when biological activity ends?
The regulations requiring capping and lining effectively prevent air and water from entering the landfill, slowing down decomposition processes.
In the absence of decomposition, waste remains “mummified”
Groundwater contamination is a continual threat