Biomedical Engineering Reference
In-Depth Information
of contamination and the need for post-user segregation typically foisted on the
operators of centralised facilities. Thus, although domestic initiatives of this kind
are unlikely ever to make the sort of difference to biowaste treatment demanded
by legislation on their own, it seems likely that they will always have a role to
play, perhaps most especially in remoter areas where collection for processing
elsewhere might prove uneconomic.
Centralised composting
The biochemistry and microbiology of all composting remains essentially the
same, irrespective of the details of the operation. However, the scale of schemes
set up to deal with a municipal biowaste stream in terms of the physical volume
involved imposes certain additional considerations, not least amongst them being
the need to ensure adequate aeration. In the back-garden compost heap, oxygen
diffuses directly into the material; large-scale composting cannot rely on this
method, as the large quantities involved lead to a lower surface area to volume
ratio, limiting natural oxygen ingress. To overcome this, various techniques make
use of mixing, turning or pumping, but, clearly, the additional energy required
has its own implications for a commercial operation.
Approaches suitable for municipal scale use fall into five main categories:
•
windrow;
•
static pile;
•
tunnel;
•
rotary drum;
•
in-vessel.
A sixth form, termed 'tower composting', may occasionally be encountered,
but it is generally much less common than the other five.
No one system is the universal ideal. The decision as to which approach is
likely to be the most suitable for specific conditions is dependent on a number
of factors, including the nature and quantity of biowaste available, the required
quality of the end-product, the time available for processing, local workforce
and land availability and financial considerations. In the brief descriptions of
each system which follow, the main features are set out, but it is important to
realise that there are many other issues which have a bearing on the selection of
appropriate technologies than the necessary brevity of this chapter permits us to
explore fully.
Windrow
The biowaste is laid in parallel long rows, around two or three metres high and
three or four metres across at the base, forming a characteristically trapezoid
shape. Windrowing is usually done on a large scale and, though they can be situ-
ated under cover, generally they tend to be outdoor facilities, which exposes them
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