“Biogas,” that is, a mixture of CH4 and CO2 prepared usually from the anaerobic digestion of waste materials by methanogenic bacterial species (Methanosarcina, Methanosaeta, Methanobrevibacter, etc.), is a technology applied globally and is ideally suited for local use in rural communities in developing economies as a cheap source of nonbiomass direct fuel.20,21 As a low-technology but established approach to wastewater treatment, it is applicable on an industrial scale, its only disadvantages being the need to remove malodorous volatile sulfur compounds. Anaerobic diges­tion is also a relatively efficient means of capturing the energy present in biological materials (figure 2.2). As links to biofuels production, biogas production not only has a historical claim for practical implementation but is also an ideal means of purifying wastewater from bioethanol facilities for detoxification and recirculation, thus reducing production costs by generating locally an input for combined heat and power or steam generation.22

Much less widely known are the bacteria that can form H2 as an end product of carbohydrate metabolism. Included in the vast number of species capable of some kind of biological fermentation (figure 2.3) are a wide array of microbes from anaer­obic environments (including Escherichia coli) that were known as active research topics as far back as the 1920s (and some of which were even discovered by Louis Pasteur in the nineteenth century).23,24 The ability of microorganisms isolated from the digestive tract to produce H2 from cellulosic substrates is another scientific research subject with a surprisingly long history.25