Aerobic decomposition of organic matter depletes the waste deposit of oxygen (O2) as buried
waste in the landfill or refuse dump becomes compacted and circulation of air is inhibited. As
oxygen becomes depleted, it is replaced as the oxidising agent by in succession, nitrate
(NO3
-
), manganese (as MnO2), iron (as Fe(OH)3) and sulphate (SO4
2-). In general, the aerobic
stage is short, no substantial volumes of leachate are produced, and aerobic conditions are
rapidly replaced by anaerobic conditions. The main stages of anaerobic digestion are (i)
acetogenic (acid) fermentation, (ii) intermediate anaerobiosis, and (iii) methanogenic
fermentation, all three of which can be operating simultaneously in different parts of the
landfill.