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In New Zealand the fermentation of fodder beet to ethanol has been identified as the most economically attractive energy farming option (Earl and Brown, 1979). The ethanol produced was to be used for blending with petrol or as a petrol substitute. The hypothetical process design included methane digestion of the wastes from the fermentation to supply in-plant energy by burning the methane produced. The production of hydrogen from the same wastes has been investigated in this thesis. If the wastes from the fodder beet fermentation were fermented to hydrogen, the combustible gas could be used to produce steam or power internal combustion engines. The hydrogen may also be combined with carbon dioxide to give methanol which could be sold as an industrial chemical or used as a fuel or fuel supplement for internal combustion engines. The microorganisms which ferment carbohydrate media to produce the largest volume of hydrogen are members of the genus Clostridium (May et al. 1965). There should be no technical difficulties in operating a pure culture hydrogen fermentation because the Clostridium spp, for example, Cl. acetobutylicum, was used to produce acetone, butanol, ethanol and hydrogen, (the ABE fermentation), on a large scale for many years. In 1952 Beesch reported that the addition of residues from an ethanol fermentation to an ABE fermentation increased the yields of organic chemicals and hydrogen. Other advantages, according to Beesch (1952 and 1953), with the use of ethanol residues in an ABE fermentation were a saving in steam and water and a reduction of foaming in the fermenters. Acetone, butanol and ethanol are valuable industrial chemicals and butanol could be used to reduce the risk of phase separation of alcohol and petrol which occurs at low temperatures or when water was present in the alcohol petrol blend (Graham & Judd, 1978). In this thesis experiments are described in which isolated clostridia and Cl. acetobutylicum fermented ethanol fermentation residues, maize and fodder beet substrates, and maize and fodder beet substrates supplemented with ethanol fermentation residue. Experiments were also carried out to investigate which factors would increase either the hydrogen or the acetone, butanol and ethanol yields.