Question 5 of 5: The Five Phases of Landfill Decomposition

Question 5: The Five Phases of Landfill Decomposition (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A landfill's biological and chemical evolution is classically divided into five phases, each defined by a distinct microbial regime and a corresponding, recognizable signature in leachate quality and gas composition.

Phase I — Initial adjustment (aerobic). Immediately after placement, aerobic microorganisms consume the oxygen trapped within the refuse's void spaces and in the cover soil; this phase is short (days to a few months) because the trapped oxygen supply is finite and is not replenished once the cell is capped/isolated. Gas at this stage still resembles ambient air, dominated by nitrogen and oxygen with CO₂ beginning to rise as a first decomposition product; if leachate is generated this early it resembles fresh infiltrating water carrying elevated BOD from readily soluble surface organics, and mild exothermic heating from aerobic respiration can be measured.

Phase II — Transition (anaerobic onset). Once the trapped oxygen is exhausted, the environment shifts to anaerobic, and facultative organisms take over, initially using alternative electron acceptors (nitrate, then sulphate) before true methanogenesis becomes possible. Nitrogen begins declining sharply as it is no longer being replenished and gas volume grows; leachate pH begins to fall as the first organic acids form, and BOD/COD in the leachate start rising as fermentation intermediates accumulate.

Phase III — Acid phase (anaerobic, non-methanogenic). Hydrolysis and fermentation break down complex organics (cellulose, hemicellulose, proteins) into simpler soluble compounds — sugars and amino acids — which acid-forming bacteria further convert to volatile fatty acids (acetic, propionic, butyric), hydrogen, and CO₂, with some H₂S and NH₃ also produced. This is when leachate is at its most aggressive: pH drops as low as 5–6, and BOD, COD and conductivity all reach their peak, with the low pH additionally mobilizing metals from the waste into solution. Gas composition is dominated by CO₂ (which can approach 90% of the dry gas at its peak); CH₄ is minimal or absent because methanogenic archaea are inhibited by the low pH this phase itself produces.

Phase IV — Methane fermentation (anaerobic, methanogenic, steady state). Methanogens now convert the volatile fatty acids and H₂/CO₂ accumulated in Phase III into methane and carbon dioxide in roughly steady proportions (CH₄ ≈45–60%, CO₂ ≈40–50%, with N₂/O₂ near zero). As the acids are consumed, leachate pH recovers toward neutral-to-slightly-alkaline (≈6.8–8), and BOD/COD drop substantially from their Phase III peak; the BOD:COD ratio also falls, reflecting declining leachate biodegradability as the readily degradable fraction is used up. This is typically the longest phase — it can persist for years to decades — and is when landfill gas-to-energy recovery is most productive, since gas generation rate and CH₄ content are both at their most stable and favourable.

Phase V — Maturation (final phase). With the readily biodegradable substrate largely depleted, both gas generation rate and leachate strength decline steadily. Remaining leachate becomes dominated by more recalcitrant, humic-like organic material, and ammonia-nitrogen — which is not consumed by methanogenesis the way BOD is — declines far more slowly than BOD/COD, often persisting as the leachate's most difficult remaining constituent long after organic strength has fallen. In the gas, trace nitrogen and oxygen slowly reappear as air infiltrates the now much less biologically active refuse mass through the cover and gas collection system. This phase can continue for decades after closure, and is the direct biological reason the post-closure monitoring period discussed in Question 1(iii) must be so long.