174 Environmental Biotechnology
as being characterised by a high carbon content, this definition is so wide as to
include the vast majority of the substances for which all environmental biotech-
nologies are viable process options. Hence, in the present discussion, biowaste is
limited to substances which have been derived from recently living matter, with
the approaches available to deal with other carbon-rich materials having already
been examined in the preceding chapters on pollution control, contaminated land
and effluent treatment.
Composition of biowaste
Biowaste of animal origin such as that contained in sewage and soiled animal
bedding contains unabsorbed fats, proteins and carbohydrates, resulting from
incomplete digestion of ingested food of animal and plant origin. In addition,
abattoir waste would include all of the above and a substantial proportion of fats
and protein, derived from the slaughtered animal. In addition, materials excreted
by the animal include metabolic breakdown products such as urea and other small
nitrogen-containing materials, for example partially degraded bile pigments. Live
and dead bacteria, normally resident in animal gut are also present in the biowaste
and so contribute their own fats, proteins, carbohydrates and nucleic acids.
In addition to all the components listed above, biowaste of plant origin will
contain cellulose, hemicelluloses and lignin. Cellulose is worthy of note given
that estimates of over 50% of the total organic carbon in this biosphere is to
be found in the form of cellulose. This is unsurprising, since wood is approxi-
mately 50% cellulose and cotton is almost 100% cellulose. This macromolecule
is an unbranched polysaccharide comprisingD-glucose units linked byβ1–4
linkages (see Figure 2.3). It is thisβlink, rather than theαlink found in its
animal equivalent, glycogen, which prevents cellulose being broken down by the
metabolic pathways in animals. The initial step in the degradation of cellulose
is the removal of a glucose molecule from one end of the long chain which
is a reaction catalysed by the enzymecellulase.Where cellulose is degraded
in animals it is by bacteria resident in the animal rumen or gut, which possess
cellulase. There are also many bacteria living outside the gut, both aerobes and
soil anaerobes (Monserrate, Leschine and Canale-Parola 2001) responsible for
cellulose metabolism. Another major constituent of plant material, the hemicel-
luloses, are also polysaccharides but the subunit in this case is the five-carbon
sugarD-xylose, also joined ‘head to tail’ by aβ1–4 linkage. Otherwise, hemi-
cellulose is not related to cellulose despite the similar name. Unlike cellulose
which comprises onlyD-glucose and in an unbranched structure, the family of
hemicelluloses has side chains and these may comprise any of a variety of sugars
one of which may be the five-carbon sugar, arabinose. The function of hemicel-
luloses in plants is to form part of the matrix which holds the cellulose fibrils
together to improve strength and rigidity of the plant tissue. Lignin is also a very
abundant material in plants and is estimated to comprise almost 25% of the dry
weight of wood. Totally unlike cellulose or hemicelluloses, which are polymers