244 Environmental Biotechnology
in a way which also permits nutrient and humus recovery. Thus, the simultaneous
sustainable management of biologically active waste and the production of a sig-
nificant energy contribution becomes a realistic possibility, without the need for
mass-burn incineration. In many respects this represents the ultimate triumph
of integration, not least because it works exactly as natures does, by unifying
disparate loops into linked, cohesive cycles.
Clearly, both AD and ethanol fermentation represent engineered manipulations
of natural processes, with the activities of the relevant microbes optimised and
harnessed to achieve the desired end result. In that context, the role of biotechnol-
ogy is obvious. What part it can play in the direct utilisation of biomass, which
generates energy by a quite different route, is less immediately apparent. One
of the best examples, however, once again relates to biological waste treatment
technologies, in this instance integrated with short rotation coppicing (SRC).
Short rotation coppicing
Short rotation coppicing differs from simple tree husbandry, being more akin to
an alternative crop grown under intensive arable production. Typically using spe-
cially bred, fast-growing varieties or hybrids, often of variousSalixorPopulus
species, SRC involves establishing plantations which are then harvested on a sus-
tainable basis, to provide a long-term source of biomass material for combustion.
There is often a substantial land requirement associated with SRC and routinely
a 2–4 year lead-in period. Once established, however, a yield of between 8–20
dry tonnes per hectare per year can reasonably be expected, with a calorific
value of around 15 000 MJ/tonne. Harvesting the crop forms a rotational cycle,
as different sections of the plantation reach harvestable size, year on year. In this
form of energy cropping, the trees themselves are effectively pruned, rather than
felled, regrowth ensuring a continuing supply. Utilisation is by burning, usually
in the form of chips or short lengths, most commonly for heating purposes in one
form or another. In addition, the potential for producing electricity is becoming
increasingly important.
The practicalities and limitations of generation from such a fuel source largely
lie beyond the scope of the present work to examine. In general, though, ensur-
ing continuity of supply and adequate production can be problematic. In addition,
while much interest has been shown in the idea of using the biomass produced by
a number of individual growers in a single generator, the logistics and transport
costs are major obstacles to overcome. It is possible to characterise any given
fuel in terms of its calorific value per unit mass, which is referred to as itsenergy
density(ED). Clearly, high ED confers obvious advantages in terms of storage
and delivery. Wood, however, is a relatively low energy density fuel and hauling
it to a centralised facility, thus, becomes costly, both in economic and environ-
mental terms, especially over long distances. There is a clear advantage, then, in
maximising the final yield of energy cropped trees and integrated biotechnology
can assist in this regard.