Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 182

nightmare of impossibility. In any case, the way in which waste is collected and
its resultant condition on arrival at the treatment plant is of considerably greater
influence on its ease of processing and the quality of the derived final product.
There are three general ways in which waste is collected; as mixed MSW,
as part of a separate collection scheme, or via civic amenity sites and
recycling banks. From a purely biowaste standpoint, mixed waste is far from
ideal and requires considerable additional effort to produce a biodegradable
fraction suitable for any kind of bio-processing, not least because the risk
of cross-contamination is so high. By contrast, suitably designed separate
collection schemes can yield a very good biowaste feedstock, as a number of
countries around the world have successfully shown. However, not all separate
collections are the same, and they may vary greatly as a result of the demands
of local waste initiatives and specific targets for recycling. As with all attempts
to maximise the rational use of waste, the delivered benefits of any scheme
inevitably reflect the overall emphasis of the project itself. Where the major
desire is to optimise the recovery of traditional dry recyclables, biowaste may
fare poorly. Systems deliberately put in place to divert biodegradable material
from landfill or incineration routes, however, generally achieve extremely
satisfactory results. In many respects, the same largely holds true for recycling
banks and amenity sites. Dependent on local emphasis, the operation can recover
very specific, narrow waste types, or larger, more loosely-defined, general
groups. Where 'garden' waste is kept separate, and not simply consigned to the
overloaded skip labelled 'other wastes', the biowaste fraction produced can,
again, be of a very high quality and readily acceptable for biological treatment.
Indeed, it is generally accepted that this material is the cleanest source available
for processing and it constitutes something in the region of three-quarters of the
biowaste treated yearly in the UK (DETR, 1999b).
For those approaches to collection which do not involve separation of the
putrescible fraction at source, obviously some form of sorting will be required
before the material can be taken on to any kind of biological processing. It lies
beyond the remit of this work to attempt to describe the methods by which this
can be achieved, or their relative merits. Suffice it to say that whatever on-site
sorting is used must be matched adequately to the demands of the incoming waste
stream, the intended treatment biotechnology and the available local resources.
However, the biowaste-rich fraction is obtained, the major consideration for
processing is its physical form, which is of more fundamental significance to
biowaste than any other refuse-reclaimed material. For traditional dry recyclables,
chipping, crushing or baling are mere matters of convenience; for bio-treatment,
particle size, purity and consistency are indivisible from the process itself, since
they are defined by the requirements of the microbes responsible. In general
terms this means that the biowaste is shredded to break it down into small and
relatively uniform pieces, the exact requirements being dictated by the particular
treatment technology to be used. This not only makes mixing and homogenisa-
tion easier to achieve but also, by increasing the surface area to volume ratio,
makes the material more available to microbial action.