Biotechnology and Waste 189deal with MSW-derived material and one system which has evolved uses huge
polythene bags, a metre or so high and 60-metres long into which a special filling
machine packs around 75 tonnes of source separated putrescible material. This
particular design also makes use of a fan to force air through the material rather
like the previous technique, with slits in the side wall allowing carbon dioxide
to escape.
The processing time is reduced compared with a similar sized aerated static
pile, since the environmental conditions within the tunnel are easier to control,
though similar cost considerations apply.
Rotary drum
Rotary drums seem to drift in and out of fashion, often being favoured by
those needing to co-compost sewage sludge with more fibrous material, like
crop residues, straw or garden waste. The principle is simple; the waste is loaded
into the drum which then slowly rotates. This gently tumbles the material, mixing
it and helping to aerate it. The drums themselves are usually steel, insulated to
reduce heat loss.
In-vessel
Sometimes also called closed reactor composters, there are a number of designs
of in-vessel systems available, ranging from small steel or plastic tanks, through
larger metal cages to long concrete troughs with high sidewalls. The main char-
acteristic of these systems is that the waste breaks down within an enclosed
container, which allows the internal environmental conditions to be closely con-
trolled. This approach offers a very efficient use of space and close regulation of
the process, since some form of mechanical aeration is also required it is signifi-
cantly more expensive on a tonne for tonne basis than the less resource-intensive
methods. Accordingly, it is less suitable for large capacity requirements, it has
a role in smaller scale operations or where the material to be treated does not
easily fit into other kinds of processing or disposal arrangements.
This is less of a natural group than the preceding approaches to composting,
since it encompasses far greater variety of design. Consequently there is a marked
variance in the capacity, complexity and cost of these systems.
Process parameters
Aside from aeration, which has already been discussed, a number of other param-
eters affect the composting process. Although these are themselves influenced to
some extent by the method being used, in general the most important of these
factors are:
- temperature;
- moisture content;