various phases of gas generation in a landfill.
The generation of principal landfill gases is thought to occur in five more or less sequential phases. Each of these phases is described briefly here.
Phase I: Initial adjustment. Phase I is the initial adjustment phase, in which the organic biodegradable components in municipal solid waste begin to undergo bacterial decomposition soon after they are placed in a landfill. In phase I, biological decomposition occurs under aerobic conditions because a certain amount of air is trapped within the landfill.
Phase II: Transition phase. In phase II, identified as the transition phase, oxygen is depleted and anaerobic conditions begin to develop.
Phase III: Acid phase. In phase III, the bacterial activity initiated in phase II is accelerated with the production of significant amounts of organic acids and lesser amounts of hydrogen gas. The first step in the three-step process involves the enzyme-mediated transformation (hydrolysis) of higher molecular mass compounds (e.g., lipids, organic polymers, and proteins) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in
the process (acidogenesis) involves the bacterial conversion of the compounds resulting from the first step into lower molecular weight intermediate compounds, as typified by acetic acid (CH,COOH) and small concentrations of fulvic and other more complex organic acids. Carbon dioxide (CO2) is the principal gas generated during phase III.
Phase IV: Methane fermentation phase. In phase IV, a second group of microorganisms that convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to methane (CH) and CO2 becomes more predominant.
Because the acids and the hydrogen gas produced by the acid formers have been converted to CH, and CO₂ in phase IV, the pH within the landfill will rise to more neutral values in the range of 6.8 to 8.