Anaerobic digestion: concepts, limits and perspectives

Anaerobic degradation processes are faced with limitations with respect to reaction energetics and reaction kinetics. The small amount of energy available in methanogenic degradation of complex organic compounds allows in most cases only the conservation of minimum amounts of energy in the lowest range of energy exploitable by biochemical reactions for ATP-synthesis. This limit has to be defined in the range of 1/3–1/4 of an ATP unit, or 15–20 kJ per mol reaction. Such small amounts of energy are exploited efficiently by syntrophic microbial communities co-operating e.g. in fatty acid conversion to methane and CO2. Methanogens also set the stage for efficient conversion of sugars or amino acids, and channel electron fluxes to the utmost efficiency. Kinetic limitations are set by the inertness of certain compounds, e.g. hydrocarbons, to react in the absence of a strong oxidant. New reactions have been found recently which activate such compounds, e.g. aromatic hydrocarbons such as toluene, xylenes, naphthalene, methane, or ammonia. Refined techniques for analysis of microbial activities in ill defined natural environments such as digestive tracts of small invertebrates or polluted aquifers have shown an amazing capacity for anaerobic or oxygen-limited degradation processes that are still to be exploited. Thus, anaerobic digestion is still a matter of fast increasing knowledge, both on the side of basic research as well as on the side of application in treatment of soil, waste materials, or in understanding complex living communities.