Agriculture Reference
In-Depth Information
ever, this factor is very relevant for controlling N-losses by ammonia volatilization,
which can be particularly high at pH > 7.5.
5.2.1.3
Aeration
Aeration has an indirect effect on temperature by speeding the rate of decomposi-
tion and therefore, the rate of heat production. The air requirement depends upon
the type of waste (type of material, particle size), the temperature of the compost
and the stage of the process. Air supply can be controlled to some extent by the
use of a system of aeration. Under natural conditions warm air diffuses from the
top of the windrow drawing fresh air into the base and sides (Hellmann et al.
1997
). Aeration is further encouraged by periodic turning of the windrow. Alterna-
tively, air may be actively forced into the pile, usually within a closed or in-vessel
system with the aim of maximizing the rate of microbial decomposition. This is
relatively costly, but is useful for materials that pose health risks. Forced aeration
has also been used successfully on static piles giving a high degree of process con-
trol (Sasaay et al.
1997
).
5.2.1.4
Microbial Activity
Microbial activity is influenced strongly by moisture content: activity decreases
under dry conditions, and aerobic activity decreases under water-logged conditions
due to the resulting decrease in air supply. The recommended optimum water con-
tent is 40-60 % on a mass basis (Epstein
1997
). The microorganisms involved in
composting develop according to the temperature of the mass, which defines the
different steps of the process (Keener et al.
2000
). Bacteria predominate early in
composting, fungi are present during all the process but predominate at water levels
below 35 % and are not active at temperatures > 60 °C. Actinomycetes predominate
during stabilization and curing, and together with fungi are able to degrade resistant
polymers.
5.2.1.5
Moisture Content
Changes in moisture content are related to aeration and temperature; in an aerated
static pile system approximately 90 % of the heat loss is due to evaporation of water
(Sasaay et al.
1997
). Systems which actively encourage aeration can lead to desic-
cation and result in a decrease in the rate of decomposition in windrow composting
(Itavaara et al.
1997
). The compost must be kept aerobic to avoid the production
of odours. Moisture is essential but if the compost is too wet then anaerobic condi-
tions develop. Anaerobic conditions are also undesirable because of the loss of N
by denitrification. There may also be build-up of organic acids, such as acetic acid,
which can be toxic to plants.
