This paper presents a synthetic review of studies conducted recently and consolidated by other literature insights that substantiate a multilayer structured model of the population organization in the anaerobic granule. Syntrophic bacteria associations would be located between an external, predominantly acidogenic, layer including also H2-consuming bacteria, and an aceticlastic core. This was evidenced conjointly by electron microscopic observations of cross-cleaved granules, by the specific activity analysis of stepwise abraded granules and by the changes in the specific metabolic activities with granule size distribution and enlargement. Such a structure was deduced from observations of propionate degradation in spite of endergonic conditions in the bulk liquid, of the two-step transfer of CH4 and H2 from liquid to the gas bubbles and from a substrate and product diffusion model which differentiates active zones for trophic groups along the granule radius. Such a structured aggregate is a stable metabolic arrangement that creates optimal nutritional conditions for all its members.

Upflow anaerobic sludge bed reactor, granulation, multilayer ultrastructure, electron microscopy, specific activity, size distribution, substrate diffusion, dissolved hydrogen, liquid-to-gas mass transfer
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© IWA Publishing 1992
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