182 Environmental Biotechnology
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 bioprocessing, 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 onsite 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 biotreatment, 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 homogenisation easier
to achieve but also, by increasing the surface area to volume ratio, makes the
material more available to microbial action.