The control of the granulation process seems to be a kind of microbial selection: the enrichment of Methanothrix sp. against the Methanosarcina sp. Because of a lower KS value of Methanothrix, keeping a low (below 200 mg.dm−3) acetate concentration has been advised to be beneficial for granulation. This method results a 70-100 days start-up time. This approach assumes Monod-type kinetics, although a substrate inhibition model may describe better the biomethanation of acetate. We found that the best fit was yielded by a Haldane-type equation modified by an inhibition response coefficient (n). The major difference between the kinetics of raw and granular sludges has been manifested in this dimensionless parameter. The n was 4 times higher for raw sludge (3.6-4.1) than for granular sludge (0.95-1.13) which means that the granular sludge (formed mainly by Methanothrix sp.) is less sensitive to substrate inhibition. Continuous UASB experiments gave a similar result: the n-value continuously decreased (from 2.3 to 0.2) following granule formation. On the basis of the above findings we developed a new strategy for granulation control which ensured fast (35-40 day) granulation on carbohydrate-containing wastewaters.

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