The objectives in this study are to develop a layered model with attrition for the growth process of biofilm in a methanogenic fluidized bed reactor, to discuss the possibility of existence of “dormant” biomass in the biofilm, and to estimate distributions of active biomass in the biofilm by the model. The model expresses characteristics and distributions of microorganisms inside the biofilm. Each substrate consumption activity of attached, sloughed, and remaining biomasses is measured by batch experiments. Comparing their specific activities, the possibility of existence of “dormant” biomass in the biofilm is indicated. Dormant biomass is inactive in the biofilm, but its activity recovers by the supply of substrates. Based on results of the experiments, a layered biofilm model is developed, into which the concept of “dormant” biomass is incorporated. The accumulation process of active and dormant biomasses in the biofilm is expressed by simulation with this newly developed model.

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