Fermentative processes have been described earlier, both in the general wider metabolic context and specifically in regard to their potential use in the treatment of biowaste. Fermentation produces a solution of ethanol in water, which can be further treated to produce fuel-grade ethanol by subsequent simple distillation, to 95% ethanol, or to the anhydrous form by azeotropic codistillation using a solvent.
The relative ease with which liquid fuels can be transported and handled, coupled with their straightforward delivery to, and inherent controllability of combustion in, engines makes them of considerable importance. Ethanol is a prime example in this respect, since it can be used either as a direct replacement for petrol, or as a co-constituent in a mix. Though at 24 G J/m3, it has a lower calorific value than petrol (39 G J/m3), in practice any performance discrepancy is largely offset by its better combustion properties.
There are thriving ethanol industries in many countries of the world, generally using specifically energy-farmed biomass in the form of primary crop plants, like corn in the USA and sugarcane in Brazil. In another example of the importance of local conditions, the production costs of ethanol and the market price realised by the final fuel depend on many factors external to the technology itself. Hence the indigenous economy, employment and transport costs, government policy, taxation instruments and fiscal incentives all contribute to the overall commercial viability of the operation.
Brazil, where ethanol/petrol mixing has been routine since the 1970s is an excellent example. Although the country’s use of ethanol partial substitution has a relatively long history, dating back to the 1930s, the real upsurge of acceptance of ‘gasohol’ lay in an unusual combination of events, partly driven by the energy crisis of the mid-1970s. Rising oil prices, which increased by over 25% in lessthan two years, came at the same time as a fall in sugar revenue following a slump in the world market. The Brazilian sugarcane industry, which had shortly before invested heavily in an extensive national programme of modernisation, faced collapse. Against this background, the production of fuel from the newly available biomass crop became a sound commercial move, simultaneously reducing the country’s outlay on purchased energy and buoying up one of its major industries.
In the preceding discussion of biogas, this involved the marrying together of the goals of biowaste treatment and energy production. In a similar vein, as was described in an earlier, there have been various attempts, over the years, to produce ethanol from various forms of waste biomass, using naturally occurring microbes, isolated enzymes and genetically modified organisms (GMOs). The appeal to obtaining renewable energy from such a cheap and readily available source, is obvious.
In many respects, the situation which exists today with biowaste is very sim-ilar to that which surrounded Brazil’s sugarcane, principally in that there is an abundant supply of suitable material available. The earlier technological barri-ers to the fermentation of cellulose seem to have been successfully overcome. The future of ethanol-from-biowaste as an established widespread bioindustrial process will be decided, inevitably, on the long-term outcome of the first few commercial projects. It remains fairly likely, however, that the fledgling industry will depend, at least initially, on a sympathetic political agenda and a support-ive financial context to succeed. While this application potentially provides a major contribution to addressing two of the largest environmental issues of our time; energy and waste, it is not the only avenue for integrated biotechnology in connection with ethanol production.
As has already been mentioned, specifically grown crops form the feedstock for most industrial fermentation processes. The distillation which the fermen-tate undergoes to derive the final fuel-grade alcohol gives rise to relatively large volumes of potentially polluting byproducts in the form of ‘stillage’. Typically high in BOD and COD, between six and 16 litres are produced for every litre of ethanol distilled out. A variety of end-use options have been examined, with varying degrees of success, but dealing with stillage has generally proved expen-sive. Recently developments in anaerobic treatments have begun to offer a better approach and though the research is still at an early stage, it looks as if this may ultimately result in the double benefit of significantly reduced cost and additional biomass to energy utilisation. The combination of these technologies is itself an interesting prospect, but it opens the door for further possibilities in the future. Of these, perhaps the most appealing would be a treatment train approach with biowaste fermentation for ethanol distillation, biogas production from the stillage and a final aerobic stabilisation phase; an integrated process on a single site. There is, then, clear scope for the use of sequential, complementary approaches in this manner to derive maximum energy value from waste biomassin 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).
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