190 Environmental Biotechnology
- particle size;
- nature of the feedstock;
- accelerants;
- processing time.
Temperature
The temperature changes over the stages of composting have important implica-
tions for the efficiency of the process. It is widely agreed that for satisfactory
sanitisation, the material should reach at least 55◦C, though there is less of a
consensus over the required duration of this exposure.
On the other hand, the temperature should not be permitted to exceed 70◦C,
since above this most of the compost microbes either die off or become inac-
tivated, causing the biological breakdown to slow or stop. In a commercial
operation, lost processing time has inevitable financial consequences.
Moisture content
A moisture content of around 60% is the ideal target for optimum composting,
though anything within a range of between 40–70% will suffice. While some
biowastes meet this requirement naturally; others forms can be surprisingly dry,
sometimes with a moisture content as low as 25–30%, which approaches the
levels at which severe biological inhibition can occur. Equally, too wet a material
may be a problem as this may restrict aeration and even encourage leaching.
Even when the initial mix is right, composting matter gradually loses moisture
over time and evaporative losses from the surface of the composting biowaste
can cause problems, especially in frequently turned windrow regimes. Careful
monitoring and appropriate management is necessary to ensure that the optimum
range is maintained.
Particle size
The optimum particle size for composting is, of necessity, something of a com-
promise. The smaller the individual pieces, the larger the surface area to volume
ratio, which makes more of the material available to microbial attack, thus speed-
ing up the process of decomposition. However, particles which are shredded too
finely will tend to become compacted and so reduce aeration within the material.
Consequently, a balance must be struck, providing the smallest possible particle
size which does not interfere with air flow. Individual design features may need
to be considered; dependent on the system used, bed depth, aeration method and
the nature of the biowaste itself can all have an influence.
Nature of the feedstock
The importance of the carbon to nitrogen (C:N) ratio and the need for careful
management to ensure a proper balance has already been discussed. In addition,