Agriculture Reference
In-Depth Information
6.4
Role of Thermophilic
Bacilli
in Composting
On the basis of temperature, composting can be divided into two types (a) hot com-
posting—this is the most preferred type, as it is rapid with three different phases and
is carried out under a more or less controlled environment, and (b) cold compost-
ing—this method requires longer time periods and may last for many years.
Thermophilic composting is the most widely used procedure for the conversion
of waste/undesired material into important by-products and involves three differ-
ent phases, (a) mesophilic, or the moderate temperature phase (~ 40 °C), that lasts
up to 2-3 days, this phase is characterized by an exponential increase in microbial
biomass with the rapid breakdown of soluble and easily biodegradable substrates.
The microbial activity results in heat generation causing an increase in temperature.
A succession of microbial community from mesophiles to thermophiles is observed
during this stage, (b) the thermophilic phase, usually lasts for several days, which is
determined by the physico-chemical and the nutritional parameters of the compost
under study. The temperature in large piles and/or man-made compost systems can
reach up to 55-80 °C during this stage, however in small bioreactors the tempera-
ture only reaches up to 45-50 °C. The temperature during the thermophilic phase
has a major influence on the microbial community structure, although the nature of
the available nutrients and the corresponding physico-chemical factors also influ-
ence the community structure. The microbial population of thermophiles are domi-
nated by the bacterial population; however the role of thermophilic fungi cannot be
overlooked. Irrespective of the nature and the type of compost, thermophilic
Bacilli
account for the most prevalent group of thermophiles and are phenotypically rep-
resented by the diverse members of the genera
Bacillus
, and play a vital role in the
breakdown of complex substrates including, fats, proteins, cellulose, hemicellulose,
pectins and lignins. This also assists in the eradication of many pathogenic microbes
and weeds. The exhaustion of nutrients results in a decrease of the microbial load
and the thermophilic phase moves onto (c) 'curing' or maturation phase, character-
ized by the presence of mesophiles and slow breakdown of the organic matter over
long periods with humus as the final product. An important aspect of thermophilic
Bacilli
during the entire process of composting is their remarkable ability to utilize
diverse compounds as carbon source and also the spectrum of their thermostability,
as many representatives of the class
Bacilli
are able to grow both at 30 and 50 °C,
which allows them to survive and remain metabolically active over all three phases.
6.5
The Spectrum of Thermophilic
Bacilli
Thermophilic bacterial strains have been isolated from almost every type and nature
of compost, including heterotrophs, autotrophs and mixotrophs. Beffa et al. (
1996
)
reported the presence of a large number of heteroteophic spore forming bacteria
notably
Bacillus
spp. during the thermophilic phase of garden manure using the
enrichment culture technique. Blanc et al. (
1997
) used amplified ribosomal DNA
