92 Organic waste recycling: technology and management
3.2 Biochemical reactions
Organic wastes suitable for composting vary from the highly heterogeneous
materials present in municipal refuse and sludge to virtually homogeneous
wastes from food processing plants. The courses of biochemically breaking
down these wastes are very complex, encountering several intermediates and
pathways. For examples, the breaking down of proteins includes the following
pathways:
Proteins ĺ peptides ĺ amino acids ĺ ammonium compounds ĺ bacterial
protoplasm and atmospheric nitrogen or ammonia.
For carbohydrates:Carbohydrates ĺ simple sugars ĺ organic acids ĺ CO 2 and bacterial
protoplasm.
The precise details of the biochemical changes taking place during the
complex processes of composting are still lacking. The phases which can be
distinguished in the composting processes according to temperature patterns are
(Figure 3.2):
- Latent phase, which corresponds to the time necessary for the
microorganisms to acclimatize and colonize in the new environment in
the compost heap. - Growth phase, which is characterized by the rise of biologically produced
temperature to mesophilic level. - Thermophilic phase, in which the temperature rises to the highest level.
This is the phase where waste stabilization and pathogen destruction are
most effective. This biochemical reaction can be represented by Equations
2.1 and 2.3 for the cases of aerobic and anaerobic composting,
respectively. - Maturation phase, where the temperature decreases to mesophilic and,
consequently, ambient levels. A secondary fermentation takes place
which is slow and favors humification, that is, the transformation of some
complex organics to humic colloids closely associated with minerals
(iron, calcium, nitrogen, etc.) and finally, to humus. Nitrification reactions
in which ammonia, a by-product from waste stabilization as shown in
Equations 2.1 and 2.3 in the form of ammonium ion, is biologically
oxidized to become nitrite (NO 2 - ) and finally nitrate (NO 3 - ) also occur, as
follows: