Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 184

Growth phase: (circa 22-40 ◦ C) growth and reproduction of microbes, result-
ing in a high respiration rate and consequent elevation of temperature to a
mesophilic range.

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Thermophilic phase: (circa 40-60 ◦ C) compost pile achieves peak temperature
and maximum pathogen sterilisation. At the end of this phase the temperature
drops to around 40 ◦ C.

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Maturation phase: (circa 40 ◦ C - ambient) slower, secondary mesophilic phase,
with the temperature gradually dropping to ambient temperature as the micro-
bial activity within the material decreases. Complex organic chemicals are
transformed into humic compounds and residual ammonia undergoes nitrifica-
tion to nitrite and subsequently nitrate.
For a municipal scheme, as shown schematically in Figure 8.1, time and space at
the facility will be at a premium, so the faster the biowaste can be colonised by a
suitable microbial culture, the sooner the treatment space will be ready to accept
a new load for processing. Hence the principal focus of environmental biotech-
nology for the optimisation of conditions for enhanced biological breakdown is
in reducing the time-lag inherent in the latent phase. One of the major means to
achieve this is to ensure that the material to be treated is presented in as suitable
a form as possible. This typically involves some form of grinding or shredding to
produce an ideal physical particle size, but biochemical considerations are every
bit as important in this respect. At the same time as breaking down biowaste
into simpler compounds, the process also brings about a change in the carbon to
nitrogen (C : N) ratio of the material, as substantial quantities of organic carbon
are converted to carbon dioxide. The initial C : N ratio is an important factor in
the success of composting, since a ratio much more than 25 : 1 can inhibit the
mineralisation of nitrogen and adversely affect the product's final maturation.
This latter aspect has clear implications for any intended use of the end product as
a fertiliser or soil enhancer, particularly for a large scale, commercial operation.
To take account of this, facilities accepting mixed-source waste for composting,
often find it necessary to undertake a measure of mixing and blending to ensure
an appropriate balance. It is possible to categorise different kinds of biowaste
according to their carbon/nitrogen content, as illustrated in the Table 8.3.

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Figure8.1 Compostplantschematicflowchart