Modeling and experiments on the influence of biofilm size and mass transfer in a fluidized bed reactor for anaerobic digestion

The kinetics of fluidized bed bioparticles for glucose fermentation into methane is investigated. As bed stratification occurs, the behaviour of biofilms is different along the height of the column. The experimental part of this study concerns the characterization of bioparticles coming from different heights of a pilot-scale reactor. Biofilm sizes and activities are measured. Batch tests are achieved with glucose, acetate and propionate as carbon sources in order to estimate the specific activity of each trophic group. The experimental sections shows that, for thick biofilms, the acidogenic activity is lower and the methanogenic activity is higher than for thin films. The modelling part describes the relationship between mass transfer limitations (when several trophic groups are involved) and biofilm size. The dynamic simulations gave a good fit to the experimental batch tests. Both simulations and experiments indicates that the biomass composition of the biofilm depends on biofilm size. When biofilm thickness increases, the amount of acidogens decreases and the amount of methanogens increases, mainly because acidogen growth is diffusion-limited. The biomass distribution in the biofilm among each trophic group appeared to be a very important parameter. This shows that bed stratification has an influence not only on the hydrodynamics of the fluidized layer but also on the kinetics of organic carbon elimination by methanogenic biofilms. This phenomenon has to be taken into account for further modelling and design of fluidized bed fermenters.