Agriculture Reference
In-Depth Information
Many factors affect the composting process, some of these factors have an im-
portant role in the process and others can influence its direction. The major factors
affecting the composting process are oxygen, moisture and bulk density. Tempera-
ture must be monitored carefully because it is a function of the process wellness; its
values drive the control procedures of the composting process. Once the compost-
ing process starts, the aerobic transformation takes place causing heat production
with consequent oxygen consumption and water evaporation.
Daily temperature values of the process are the most important index of the
composting status and it is the result of the microbial activity. It can be regulated
within certain ranges only by indirect methods, decreasing temperature can be done
through: (i) heat removing by turning, (ii) heat decreasing by wetting, while in-
creasing temperature can be done through (a) oxygen supply by turning, (b) water
supply by wetting. Other important factors that could limit the composting process
are nutrients and pH. Nutrients especially C and N play an important role in the pro-
cess as they are essential for microorganisms growth (Epstein
1997
). These factors
cannot be regulated during the process and can be managed only by an appropriate
starting mixture.
Decomposition occurs mainly in the thin biofilm on the organic residues sur-
face, which requires proper moisture content. Moisture is a key element affecting
the composting process in a dynamic composting system where biological drying,
metabolic water production and changes in compaction and porosity are all occur-
ring over time (Richard et al.
2002
). Compost mass humidity is measureable and
can be regulated through watering.
The bulk density increases during the composting process due to decreasing par-
ticle size of the composting biomass as a result of decomposition. Bulk density af-
fects the natural airflow from and to the composting pile, which in turn affects the
heat dissipation form the pile's core to surrounding environment, which is impor-
tant to determine the frequency of turning process or of the aeration flux. Bulking
agents should be included in the starting mixture, e.g., different types of barks or
wood chips. Larsen and McCartney (
2000
) reported 20-35 % of free air space in the
composting mass as optimum range during the whole process. However, the effect
of composting itself on the bulking agent could decrease its size, therefore, increas-
ing mass compaction and limiting oxygen transfer.
It is worth noting that at the bottom of the pile, especially in the case of turning
windrows, mostly used at farm scale, some niches (microenvironments) of anaero-
biosis may occur even under optimal process management (Ryckeboer et al.
2003
).
Yamamoto et al. (
2009
) reported the evidence of Clostridia not only in the non-well
aerated compost samples, as expected, but also in the well-managed process.
Clos-
tridium
sp. converts organic compounds to sugar, acids and alcohol (Wiegel et al.
2006
) playing an important role in compost maturation, i.e., anaerobic communi-
ties carry on the decomposition after that aerobic bacteria had consumed oxygen in
the areas, or in the periods, when gas exchange is very slow. It has been estimated
that 1 % of all the bacteria found in municipal solid waste compost are anaerobic
(Atkinson et al.
1996
). Anaerobic activities have been revealed by CH
4
release from
composting pile (Fig.
1.1
). The anaerobic microbial communities take parts in the
decomposition process increasing its complexity and microbial biodiversity.
