Organic Waste Recycling

Composting 95

pathogen inactivation. Haug (1980) conceptually described the various rate-
controlling phenomena occurring during aerobic composting which include:

• Release of extracellular hydrolytic enzymes by the cell and transport of
  the enzymes to the surface of the substrate;


• Hydrolysis of substrate molecules into lower molecular weight, soluble
  fractions;


• Diffusion transport of solubilized substrate molecules to the cell;


• Diffusion transport of substrate into the microbial cell, floc or mycelia;


• Bulk transport of oxygen (usually in air) through the voids between
  particles;


• Transport of oxygen across the gas-liquid interface and the unmixed
  regions which lie on either side of such an interface;


• Diffusion transport of oxygen through the liquid region;


• Diffusion transport of oxygen into the microbial cell, floc or mycelia;
  and


• Aerobic oxidation of the substrate by biochemical reaction within the
  organism.

In practice, the design of a composting plant is based on such criteria as: type
and quantity of materials to be composted, time required for waste stabilization
and pathogen inactivation, degree of compost maturity, type of composting
process to be employed, and area and location of the composting plant. Data
from laboratory and pilot-scale investigation together with knowledge of past
experiences greatly help in designing an efficient composting plant.

3.3 Biological succession

Composting is a biological process in which organic wastes are converted into
stabilized humus by the activity of complex organisms which are naturally
present in the wastes. These include microorganisms such as bacteria, fungi and
protozoa and may also involve invertebrates such as nematodes, earthworms,
mites, and various other organisms (Figure 3.3).
The organic wastes are firstly decomposed by the first level consumers such
as bacteria, fungi (molds), and actinomycetes. Waste stabilization is
accomplished mainly through the bacterial reactions. Mesophilic bacteria are the
first to appear. Thereafter, as the temperature rises, thermophilic bacteria appear
which inhabit all parts of the compost heap. Thermophilic fungi usually grow
after 5-10 days of composting. If the temperature becomes too high, i.e., greater
than 65-70°C, the fungi, actinomycetes and most bacteria are inactive, and only
spore-forming bacteria can develop. In the final stages, as the temperature