Environmental Engineering Reference
In-Depth Information
Stage I. Hydrolysis/aerobic degradation
The hydrolysis/aerobic degradation stage occurs under aerobic (in the presence of
oxygen) conditions. This occurs during the emplacement of the waste and for a period
thereafter which depends on the availability of oxygen in the trapped air within the waste. The
micro-organisms are of the aerobic type, that is, they require oxygen and they metabolise the
available oxygen and a proportion of the organic fraction of the waste to produce simpler
hydrocarbons, carbon dioxide, water and heat. The heat generated from the exothermic
degradation reaction can raise the temperature of the waste to up to 70-90 °C (McBean et al.
1995; Waste Management Paper 26B 1995). However, compacted waste achieves lower
temperatures due to the lower availability of oxygen. Water and carbon dioxide are the main
products, with carbon dioxide released as gas or absorbed into water to form carbonic acid,
which gives acidity to the leachate.
The aerobic stage lasts for only a matter of days or weeks depending on the availability of
oxygen for the process, which in turn depends on the amount of air trapped in the waste, the
degree of waste compaction and how quickly the waste is covered.
Stage II. Hydrolysis and fermentation
Stage I processes result in a depletion of oxygen in the mass of waste and a change to
anaerobic conditions. Different micro-organisms, and the facultative anaerobes, which can
tolerate reduced oxygen conditions, become dominant. Carbohydrates, proteins and lipids are
hydrolysed to sugars which are then further decomposed to carbon dioxide, hydrogen,
ammonia and organic acids. Proteins decompose via deaminisation to form ammonia and also
carboxylic acids and carbon dioxide. The ammonia is derived largely from the deaminisation
of proteins, which also form carboxylic acids and carbon dioxide. The derived leachate
contains ammoniacal nitrogen in high concentration. The organic acids are mainly acetic acid,
but also propionic, butyric, lactic and formic acids and acid derivative products, and their
formation depends on the composition of the initial waste material. The temperatures in the
landfill drop to between 30 and 50 °C during this stage. Gas concentrations in the waste
undergoing stage II decomposition may rise to levels of up to 80% carbon dioxide and 20%
hydrogen (Waste Management Paper 26, 1986; Waste Management Paper 26B, 1995).
Stage III. Acetogenesis
The organic acids formed in Stage II are converted by acetogen micro-organisms to
acetic acid, acetic acid derivatives, carbon dioxide and hydrogen under anaerobic conditions.
Other organisms convert carbohydrates directly to acetic acid in the presence of carbon
dioxide and hydrogen. Hydrogen and carbon dioxide levels begin to decrease throughout
Stage III. Low hydrogen levels promote the methane-generating micro-organisms, the
methanogens, which generate methane and carbon dioxide from the organic acids and their
derivatives generated in the earlier stages. The acidic conditions of the acetogenic stage
increase the solubility of metal ions and thus increase their concentration in the leachate. In
addition, organic acids, chloride ions, ammonium ions and phosphate ions, all in high
concentration in the leachate, readily form complexes with metal ions, causing further
increases in solubiliation of metal ions. Hydrogen sulphide may also be produced throughout
the anaerobic stages as the sulphate compounds in the waste are reduced to hydrogen sulphide
by sulphate-reducing micro-organisms (Christensen et al 1996). Metal sulphides may be a
