110 Organic waste recycling: technology and management
- Presence of white or grey colour due to the growth of
actinomycetes.
In cases where the composted products are to be applied to crops and public
health aspects are of concern, the time required for pathogen die-offs during
composting is another important criterion to be considered (see Figure 3.1).
More information about pathogen die-off during composting and the public
health risks are discussed in section 3.7.
Because compost materials usually contain some biologically resistant
compounds, a complete stabilization during composting may not be achieved. The
time required for a satisfactory degree of composting would depend on the
environmental factors in and around the compost heap as were described in section
3.4. Under suitable conditions, aerobic composting normally takes about 10-30
days, while anaerobic composting can last from 45 to 100 days. The maturing or
curing time of compost may be approximately the same as those required for
organic stabilization. Some manufacturers have produced mechanical composting
reactors which claim to yield satisfactory compost within a short period, e.g. 24
hours. However, these reactors are both expensive and difficult to operate, and the
composted materials will usually need an additional time for curing or nitrification.
It should be noted that in batch composting, the temperature pattern and
biological succession developed in the compost piles are similar to those shown
in Figures 3.2 and 3.3, respectively. Because both the stages of waste
stabilization and consequently curing occur during batch composting, the
composted product is suitable to be used in agriculture or horticulture.
The continuous composting process is normally aerobic and has a semi-
continuous plug-flow passage of composting materials through a reactor
structure, and temperatures of the reactor contents are continuously in the
thermophilic range. Depending on the specific design, the material is
translocated through the structure as part of the tumbling action in a large
revolving drum, by the action of augers force through the mass, or by gravity.
These approaches may provide the gas exchange by forcing air through the
mass. Residence time in the reactor is 1-10 days with 5 days being typical. The
material displaced from the continuous stage is biologically stabilized (i.e. the
reaction of Equation 2.1 is achieved, but mostly not nitrified (i.e. reaction of
Equation 3.5 is not achieved). It should be processed further in the curing stage
to allow for nitrification to occur and to make the composted products suitable
for agricultural reuse. Composting may be employed for the purpose of sludge
dewatering or drying. In this case, there is no need for the nitrification reactions
to take place and the composted products from continuous reactors can be
disposed off in sanitary landfills or reused in non- agricultural activities.