Composting 89molecular-weight organic acids (Equation 2.3). NH 3 is further oxidized to
become nitrate (NO 3 - ) by the nitrifying bacteria during the maturation or curing
phase. Because it can release more heat energy resulting in a rapid
decomposition rate, aerobic composting has been a preferred technology for
stabilizing large quantities of organic wastes. Anaerobic composting is a slow
process and can produce obnoxious odors originating from the intermediate
metabolites such as mercaptans and sulfides. Depending on the methods of
operation, anaerobic composting can produce temperatures near to or at
thermophilic levels. Because of its simplicity, anaerobic composting has found
some applications in many rural areas of developing countries in the
stabilization of wastes generated from households and farms.
It should be noted that, in contrast to wastewater treatment, the terms
"aerobic" and "anaerobic" for composting have relative meanings. They simply
indicate what conditions are predominant in the process. Since the compost
materials are heterogeneous and bulky in character, in a compost heap, there
always exist "anaerobic" composting, which are few in "aerobic" composting
but abundant in "anaerobic" composting; and vice versa. Some composting
processes, such as composting pits being practiced in rural China, are aerobic at
first and become anaerobic during the later stages of composting period.
Using technology as the key, composting can be classified into 'mechanical'
and 'non-mechanical' processes, or 'on-site' and 'off-site' processes. Composting
can also be divided with respect to the modes of operation, i.e. batch operation
and continuous or semi-continuous operation. These technologies and
operational classifications of composting processes will be described in section
3.6. When temperature is the basis, composting can be divided into 'mesophilic'
composting (whose temperatures in the compost heap are between 25-40°C) and
'thermophilic' composting (whose temperatures are between 50-65°C).
3.1 OBJECTIVES, BENEFITS AND LIMITATIONS OF
COMPOSTING
The main purposes and advantages of composting are classified as follows:
- Waste stabilization. The biological reactions occurring during composting
will convert the putrescible forms of organic wastes into stable, mainly
inorganic forms which would cause little pollution effects if discharged
onto land or into a water course. - Pathogen inactivation. The waste heat biologically produced during
composting can reach a temperature of about 60 °C which is sufficient to
inactivate most pathogenic bacteria, viruses and helminthic ova, provided
that this temperature is maintained for at least 1 day. Therefore, the
composted products can be safely disposed of on land or used as