In open anaerobic ecosystems where biomass is not sterilized, the degra­dation carried out by ubiquitous microorganisms normally follows a rather well-defined pathway as shown in Fig. 1. If no inorganic electron acceptors such as sulfate or nitrate are present, methane is the inevitable terminal biofuel product since all intermediates from the fermentative bacteria can be degraded to methane, carbon dioxide, and water. The natural end prod­ucts of the fermentative bacteria in such open systems are short-chained volatile fatty acids, hydrogen, and carbon dioxide. Alcohols are only formed in small amounts. Approximately 90% of the energy of the converted biomass is conserved in the end products, and only 10% is used by the fermentative bacteria [1]. In the terminal formation of methane from the fermentation

products, the biomass carbon is sequestered completely to the most oxi­dized (CO2) and the most reduced (CH4) states. Only 4% of the original biomass energy is utilized by the terminal link, leaving 86% of the ori­ginal energy content in the formed methane (Fig. 1), which constitutes the sound energetic rationale for the extensive exploitation of the biogas pro­cess. The obligate biology leading to methane formation has an intrinsic stability governed by thermodynamics, which ensures that methanogenesis proceeds within a wide spectrum of physical and chemical conditions. In most methanogenic fermentations the methane yield lies close to the theor­etical maximum of 3 mol of methane per mol of glucose, calculated from the Buswell equation [12,13]:

CaHbOc + (a — b/4 — c/2)H2O ^

(a/2 — b/8 + c/4)CO2 + (a/2 + b/8 — c/4)CH

4