Agriculture Reference
In-Depth Information
Fig. 3.3
Changes in pH with
the passage of time during the
composting of wheat straw,
cattle manure, and chicken
manure. (Data replotted from
Lü et al.
2012
)
esters within the easily-degraded hemicellulose component of biomass (Hubbe
et al.
2010
). However, the carboxylic acids created by saponification are subject to
breakdown, resulting in the release of CO
2
, which is an acidic gas, during the further
progress of composting. Such loss of an acidic component renders the remaining
material less acidic. An additional contribution to pH rise is provided by accumula-
tion of amines or of the ammonium ion in the compost (see section 5.2).
3.3
Thermal Effects: Cellulose as a Fuel for Heating
during Composting
The overall chemical reaction taking place during composting is essentially the same
as that of combustion. As in combustion, two of the main products are carbon dioxide
and heat. Water vapor is released as well. All of these facts are consistent with a view
that the polysaccharide components in the mixture somehow serve as a fuel to drive
the process forward (Hubbe et al.
2010
). Unlike burning, however, the objective of
composting is to only partially consume the organic material. Substantial organic
content, i.e., the humus component of matured compost, is needed in order to retain
moisture and minerals in soil. Overheating, which sometimes leads to self-ignition
of compost piles, is usually taken as an indication that the starting conditions in the
pile were somehow inappropriate, e.g., too large a pile (Zambra et al.
2011
,
2012
).
3.3.1
Distribution of Temperature within a Compost Pile
One of the most challenging aspects of composting is the strong contrast between
the temperature at the outside of a compost pile and the simultaneously much higher
temperatures that can be measured in the interior (Zambra et al.
2011
). The cited
