Agriculture Reference
In-Depth Information
composting process and their association with maturity must be carefully studied.
The present chapter reviews the physical [color, odor, temperature, organic matter
(OM) loss], chemical [pH, electrical conductivity (EC), water soluble C (WSC), C:N
ratio, water soluble organic carbon (C
w
):organic N (N
org
) ratio, NH
4
+
-N and NO
3
−
-N,
humic acid (HA): fulvic acid (FA) ratio, humification index (HI) and cation exchange
capacity (CEC):total organic carbon (TOC) ratio)] along with biological [seed ger-
mination index (GI) and microbiological parameters (bacterial, fungal and actino-
mycetes counts, O
2
and CO
2
respiratory, enzyme activities and microbial diversities
(coliform, fecal coliforms and fecal enterococci population)] parameters affecting
stability and maturity of high quality compost prepared from organic waste materials.
5.2
Concepts of Composting Process
While composting occurs, efficient composting requires the control of several fac-
tors to avoid nuisance problems of such as odour and dust and also for obtaining a
quality agricultural product. The controlled conditions are basic for a composting
procedure, distinguishing it from aerobic fermentation. Over the last decades, re-
search has been focused on the study of the complex interaction amongst physical,
chemical and biological factors that occurs during composting.
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 are (Fig.
5.1
):
a. Latent phase, which corresponds to the time necessary for the microorganisms to
acclimatize and colonize in the new environment in the compost heap.
b. Growth phase, which is characterized by the rise of biologically produced tem-
perature to mesophilic level.
c. Thermophilic phase, in which the temperature rises to the highest level. This is
the phase where waste stabilization and pathogen destruction are most effective.
d. Maturation phase, where the temperature decreases to mesophilic and, subse-
quently, ambient level. A secondary fermentation takes place which is slow and
favours humification; that is, the transformation of some complex organics to
humic colloids closely associated with minerals and finally to humus. Nitrifi-
cation reactions, in which ammonia is biologically oxidized to become nitrite
(NO
2
−
) and finally nitrate (NO
3
−
), also take place (Metcalf and Eddy
1991
).
The organic wastes are initially decomposed by the first level consumers such as
bacteria, fungi (molds), and actinomycetes. Mesophilic bacteria are the first to ap-
pear. Thereafter, as the temperature rises, thermophilic bacteria, which inhabit all
parts of the compost heap, appear. Thermophilic fungi usually grow after 5-10 days
of composting. If the temperature becomes too high, i.e., greater than 65-70 °C,
fungi, actinomycetes, and most bacteria become inactive and only spore forming
bacteria can develop. In the final stage, as the temperature declines, members of the
actinomycetes become the dominant group which may give the heap surface a white
or gray appearance. After these stages the first level consumers become the food of
