Agriculture Reference
In-Depth Information
One of the most interesting findings, from the standpoint of conventional com-
posting, is the fact that anaerobic treatment at least sometimes can be effective in
breaking down aromatic structures (Tambone et al.
2013
). These are the very struc-
tures that tend to be most resistant to breakdown during conventional composting. It
follows that anaerobic digestion could be considered as a pretreatment stage, either
before conventional composting or before saccharification and fermentation for the
purpose of making ethanol (Taherzadeh and Karimi
2007
).
3.3.4
The Energy Footprint of Composting
From the standpoint of environmental sustainability, it is important to compare dif-
ferent processing schemes in terms of such factors as their ability to store carbon,
helping to curb increases in gaseous carbon dioxide. Composting can be regarded as
usually having a favorable net impact, since it can minimize the release of methane,
which has a much greater global warming effect than CO
2
. Composting also tends
to stabilize humic carbonaceous substances so that they can persist in the soil for
many years.
To go one step further, recent research has shown that composting can be used as
a pretreatment step in preparation for pyrolysis to produce bio-based fuel products
(Barneto et al.
2010
). This approach can make sense because composting tends to
reduce cellulose and hemicellulose-related materials, while leaving mainly lignin-
related materials behind. Per unit mass, pyrolysis of the lignin component yields
more hydrogen, which then can be used as a fuel in place of fossil fuels. Lemee
et al. (
2012
) likewise used composting as a pretreatment in preparation for catalytic
hydroliquefaction to produce biofuels.
When assessing the value of composting as a preliminary step for various appli-
cations, as just noted, it can be important to gain some knowledge about the energy
content and susceptibility to thermal decomposition. Such information has been
obtained, in the case of compost samples, by thermogravimetric analysis (Bernabé
et al.
2011
,
2013
).
3.4
Chemical Changes During Composting
The humus component of mature compost has chemical structures indicating its
probable origin mainly from the decomposition of lignin (Hubbe et al.
2010
; Hachi-
cha et al.
2012
). Meanwhile, the polysaccharides and various minor chemical con-
stituents of biomass are more fully decomposed and depleted during conventional
composting. This section considers recent research progress in understanding such
chemical changes.
