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 in to methane is investigated. As bed stratification occurs, the behaviou r of biofilms is different along the height of the column. The experim ental 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 rests are achieved with glucose, a cetate and propionate as carbon sources in order to estimate the speci fic activity of each trophic group. The experimental sections shows th at, for thick biofilms, the acidogenic activity is lower and the metha nogenic activity is higher than for thin films. The modelling part des cribes the relationship between mass transfer limitations (when severa l trophic groups are involved) and biofilm size. The dynamic simulatio ns gave a good fit to the experimental batch tests. Both simulations a nd experiments indicates that the biomass composition of the biofilm d epends on biofilm size. When biofilm thickness increases, the amount o f 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 importa nt parameter. This shows that bed stratification has an influence not only on the hydrodynamics of the fluidized layer but also on the kinet ics of organic carbon elimination by methanogenic biofilms. This pheno menon has to be taken into account for further modelling and design of fluidized bed fermenters. (C) 1998 Elsevier Science Ltd. All rights r eserved.