Agriculture Reference
In-Depth Information
1.2.1
Composting: Microbial Transformation of Raw Materials
Compost and composting process definition have been extensively reported in the
last decades of scientific literature (Feinstein and Morris
1975
; Haug
1993
; Kutzner
2001
). Composting is “sensu stricto, a self-heating, aerobic solid phase biodegrada-
tive process of organic materials under controlled conditions, which distinguishes
it from natural rotting or putrefaction” (Ryckeboer et al.
2003
). Insam and de Ber-
toldi (
2007
) also defined the composting process as “a biodegradation process of a
mixture of substrates carried out by a microbial community composed of various
populations in aerobic condition in the solid state”. Composting transforms raw
organic materials into biologically stable, humic-like substances, easier-to-handle
materials. In this framework, we highlight in our own definition the functional role
of the raw materials as microorganisms' feedstock and the importance of the 'time
factor' in the never-ending process of organic matter maturation. Compost is a re-
sult of microbiological process under human control: the composting. It is a func-
tion of (i) the starting substrates physical, chemical and biological characteristics,
(ii) the percentage of each raw material in the starting mixture, (iii) the process
conditions in terms of water and oxygen range in the core of the mass under com-
posting, and (iv) time (days of process after the starting mixture formulation). The
biological decomposition of organic substrates and organic matter maturation are
the two opposed paths which drive the process all through different phases distin-
guished by time and temperature values. A thermophilic period is representative of
the biodegradation processes as a result of biologically produced heat (Haug
1993
),
while the maturation has been underlined through the organic matter evolution from
the simple and the easy degradable forms in the raw materials to the complex and
humic-like substances of the compost (Lhadi et al.
2006
). The interactions and the
dynamics among these biological forces both of degradation and stabilization result
in a final product sufficiently stable for storage and soil application without adverse
environmental effects.
During the composting process, microbial communities follow one another in a
predictable succession pattern without breaks (Goyal
2005
). Each community growth
changes tremendously the composting mass's parameters (that are the environmental
conditions for the subsequent microorganisms) in terms of temperature, organic sub-
strate compositions, pH and bulk density. This process goes ahead until one micro-
bial community is not any more able to survive in the new environmental conditions
created by its own metabolism and catabolites. At that time one other community
take place carrying on the composting process. As compost temperature increase,
mesophilic zymogenous organism groups leave the place to thermophilic microor-
ganisms, while late in the process, mesophilic microorganisms reemerge but at the
same time they are basically autochthonous groups (Hermann and Shann
1997
).
The composting process causes the loss of the more-degradable organic materi-
als and the concentration of the less-degradable ones resulting in biomass enrich-
ment of the humified materials. Composting is a process where humic substances
are built up in a technological process that produces compost having humic-like
materials. Humification of biomolecules provides environmental benefits through
