8 5 Stages of Biodegredation in Landfills Each cell contains the waste deposited in one dayLandfills are constructed one cell at a time over a period of 20 to 30 yearsA the end of the day, each cell is capped with a layer of sand or soilAt any given time, different parts of the same landfill are in different stages of decompositionThe 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 slowlyOxygen is present as microbes and animals begin to consume and digest organic matterWhen the oxygen supply is exhausted, aerobic organisms die off and anaerobic microbes take over as principal decomposersThis aerobic to anaerobic transition occurs 6 to 18 months after waste is placed in cell
10 Stage 2: TransitionCarbon dioxide completely replaces oxygen as the primary landfill gasAnaerobic bacteria convert the substances created by aerobic bacteria to acetic, lactic, and formic acids and to alcohols, including methanol and ethanolThe acids produced during this stage percolate through the waste as highly acidic leachate“Transition” stage lasts for several months
11 Stage 3: Acid FormationAnaerobic decomposition continues to produce leachate with its high concentrations of organic acidsAs pH drops, waste metals such as chromium, iron, and magnesium to dissolve in the leachateNitrogen & phosphorous slowly become available to anaerobic bacteria, which facilitates population growthOver time fermenting bacteria convert organic acids, alcohols, hydrogen and carbon dioxide to acetateLandfill settles unevenly
12 Stage 4: Methane Fermentation Methane-producing bacteria transform the acetate, methanol, and other chemical to methane, carbon dioxide and waterThe methanogenic bacteria produce significant amounts of methane and carbon dioxide (landfills account for about 20% of human-generated methane emissions)Landfill continues to settleThis 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 uncertainBacterial populations declineGas and leachate production eventually endsLandfill 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
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