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 (C2H5OH) 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 concentra-tions, 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 combi-nation 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 genet-ically 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 USA, many individual states have started to undertake feasibility studies for their own areas. A recent California Energy Commission Report, for example, has estab-lished that the state-wide annual generation of biowaste exceeds 51 million dry tonnes, comprising forestry residue, MSW and agricultural waste. The same doc-ument estimates the resultant maximum ethanol yield at more than 3 billion gallons (US).
There are several thriving biomass-to-ethanol production plants elsewhere in the USA 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.