A simple model, which includes accmulation of inert organic solids such as decayed biomass and extracellular polymers, has been developed to predict the treatment efficiency, and to evaluate the accumulation of “active biomass” and the biofilm development in a methanogenic fluidized bed. Mass balance equations of substrate and biomass comprised additional rate terms of extracellular polymer production and biomass detachment, respectively.
The results of a steady-state analysis indicated that the biofilm contained a significant amount of inert organic solid when a lower detachment rate was given to the inert solids than the “active biomass”. The accumulation of inert organic solids might explain why reported maximum specific rates of acetate consumption for methanogenic biofilms were much lower than those of enriched and pure cultures of acetate utilizing methanogens. The conventional concept of solid retention time (SRT) seems to overestimate the true SRT of “active biomass”, which is a more useful index for controlling the treatment in fluidized beds. The model experiments also implied that maximum specific activities of total biofilm and washed-out biomass were fundamentally related to the biomass detachment rates at the biofilm surface. Determination of the activities of sloughed biomass would be useful to investigate the bacterial distribution in biofilms.