Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 204

Biowaste to ethanol
Around half of the total dry matter in plant origin biomass is cellulose, and
since this makes up the majority of the biowaste component in MSW, it rep-
resents a huge potential source of renewable energy. As is widely appreciated,
sugars can be broken down by certain micro-organisms to produce alcohols,
of which ethanol (C 2 H 5 OH) is the most common. This is, of course a well
known application for the production of alcoholic beverages across the world,
typically using fermentative yeasts. These organisms are poisoned by ethanol
accumulation greater than about 10% which means that, in order to derive higher
concentrations, approaches relying on distillation or fractionating are required. In
a wider context, ethanol, either as the typical hydrated form (95% ethanol, 5%
water) or as azeotropically produced anhydrate, makes a good fuel with excellent
general combustion properties.
Historically, the realisation of the huge energy source locked up in the sugars of
the cellulose molecule has always been a practical impossibility. The combination
of the β 1-4 linkage in cellulose itself, coupled with its typically close association
with lignin, making large-scale hydrolysis to sugars a costly and difficult prospect.
Some early attempts employed enzymes from wood rotting fungi working on a
feedstock of old newspapers or pulp, though the energy involved in actually
making the process work often became a limiting factor. In the mid 1990s,
various researchers began to investigate the potential of genetically modified
bacteria, by inserting appropriate sequences from a variety of naturally occurring
wood-rotting organisms. In the following years, a number of technologies have
emerged, based both on whole-organism and isolated-enzyme techniques, and
the commercial processing of cellulose to alcohol now appears to be about to
become a mainstream reality.
A number of countries have begun to show an interest in the potential gains
to be had from developing a biowaste-based ethanol industry. Within the US,
many individual states have undertaken feasibility studies for their own areas.
The California Energy Commission, for example has established that the state-
wide annual generation of biowaste exceeds 51 million dry tonnes, comprising
forestry residue, MSW and agricultural waste and estimates the resultant maxi-
mum potential ethanol yield at more than 3 billion gallons (US).
There are several thriving biomass-to-ethanol production plants elsewhere in
the US and the world, though most of these make their alcohol from primary
crop plants, rather than biowaste. As with biogas, further discussion of the wider
aspects of ethanol and the role of biotechnology in energy production appears in
a later chapter.
Eutrophic fermentation (EF)
Eutrophic fermentation (EF) was the name given by one of the authors to the
experimental, wet, in-vessel, aerobically enhanced biodegradation process he
designed to investigate accelerated decomposition, principally as an alternative to