Composting 109It is generally known that compost temperatures greater than 60-65 °C, above
thermophilic range, will significantly reduce the rate of biodegradation in
compost piles. A recent work using compost samples from a full scale
composting plant showed the optimal temperature for composting, as measured
by microbial activity (incorporation of [^14 C] acetate) was consistently below
55 °C (McKinley et al. 1985). On the other hand, most pathogenic
microorganisms are inactivated effectively at temperatures above 50°C (Figure
3.1). So the key concern is to control temperatures in the compost piles in such a
way as to optimize both the breakdown of organic material and pathogen
inactivation (approximately 55°C). Temperature can be controlled by the
adjustment of aeration and moisture content and the utilization of screened
compost as insulation cover of the compost piles.
Temperature patterns in compost piles influence the types and species of
microorganisms' growth. Mesophilic temperature (25-45°C) is developed first in
composting, followed by thermophilic temperature (50-65°C). After this phase,
most organic substrates will have been stabilized, resulting in the decline of
temperature to mesophilic and eventually to ambient level (Figure 3.2). In many
cases, the thermophilic temperature can even reach 55-65°C and last for a few
days, causing an effective inactivation of the pathogens.
Aerobic composting normally proceeds at a neutral pH and rarely encounters
extreme pH drop or rise. A slight pH drop may occur during the first few days
of anaerobic composting due to the production of volatile fatty acids. After this
period, the pH becomes neutral again when these acids have been converted to
methane and carbon dioxide by the reactions of methane-forming bacteria.
3.5 Composting maturity
There are many criteria to judge the maturity or completion of a composting
process. In general, a composted product should contain a low organic content
that will not undergo further fermentation when discharged on land, and the
pathogens inactivated. Some of the approaches to measure the degree of
compost stabilization are (Haug 1980):
- Temperature decline at the end of batch composting
- Decrease in organic content of the compost as measured by the
volatile solid (VS) content, chemical oxygen demand (COD),
percent carbon content, and C/N ratio - Presence of particular constituents such as nitrate, and the absence
of others such as ammonia - Lack of attraction of insects or development of insect larvae in the
final product - Absence of obnoxious odour