Question 5 of 5: The Five Phases of a Landfill

Question 5: The Five Phases of a Landfill (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 conventionally divided into five phases (Tchobanoglous), summarized in order below; Phase III is developed in more depth in Question 2(a), and the corresponding gas trend is charted in Question 4(a).

Phase I — Initial adjustment. Immediately after placement and cover, biodegradable components undergo microbial decomposition under aerobic conditions using the oxygen trapped in the refuse's void spaces. Gas in the refuse mass is still close to atmospheric (high N₂, O₂), with CO₂ beginning to rise as respiration proceeds; leachate, if generated at all this early, is dilute and near-neutral, reflecting the moisture initially present rather than significant decomposition products.

Phase II — Transition. As the trapped oxygen is consumed and cannot be replenished (the refuse mass is now covered/buried), conditions shift from aerobic to anaerobic; nitrate and sulphate serve briefly as alternative electron acceptors before they too are depleted. O₂ falls sharply, N₂ begins its decline, and CO₂ continues to increase. The leachate typically develops a lower redox potential and rising organic content as facultative organisms take over decomposition.

Phase III — Acid formation. Obligate and facultative anaerobes hydrolyze and ferment the biodegradable fraction into volatile fatty acids (mainly acetic acid), CO₂ and H₂ (acidogenesis/acetogenesis). This is the phase of most intense biological activity: leachate pH drops to roughly 5–6, BOD₅/COD both peak at their highest values of the landfill's life with a high BOD₅/COD ratio, and metal/nutrient solubility peaks with the low pH. Methane generation is not yet significant. (Full detail in Question 2(a).)

Phase IV — Methane fermentation. A slower-growing population of strict anaerobic methanogens, now established on the hydrogen and acetic acid substrate accumulated in Phase III, converts these intermediates to CH₄ and CO₂. As methanogenesis consumes the volatile acids, leachate pH rises back toward neutral (roughly 6.8–8), and BOD₅/COD both fall substantially from their Phase III peak as the readily degradable organic load is consumed; the gas phase becomes CH₄/CO₂ dominated, settling toward the 50–60% CH₄ / 40–50% CO₂ mixture of Question 4(a).

Phase V — Maturation. As the readily available biodegradable substrate is exhausted, both biological activity and gas generation rate decline gradually (though gas may continue for decades at low rates). Leachate strength drops further — BOD₅/COD both continue to fall and the BOD₅/COD ratio itself decreases, since what remains is progressively less biodegradable, more recalcitrant, humic-like organic matter. Gas composition remains roughly CH₄/CO₂ dominated but at a steadily declining generation rate, which is the basis for design landfill gas collection systems being sized for a peaking-then-decaying, not constant, gas flow over the site's post-closure life.