Biofilm development and bed segregation in a methanogenic fluidized bed reactor

Bacterial biofilm growth on a pilot lab scale anaerobic fluidized bed bioreactor under mesophilic conditions is analyzed in this paper. Biolite was used as substratum and the system was fed with acetic acid as sole carbon source. Bed expansión was maintained around 40%. It was observed that the decrease of the expansion provoked a biomass increase in the reactor and the decrease of the removal rate of organic matter. This can be explained by a mass transfer limitation phenomenon. Volatile fatty acids accumulated within the system when the expansion of the fluidized bed was lower than 10%. An initial bed segregation was observed. Biomass adhesion and growth influenced bioparticle size and density, which contributed to further bed segregation. Biomass concentration in the bottom part of the bed showed higher values than in the upper levels. On the other hand, biofilm density increased towards the reactor bottom, wherein it showed the highest values. Methanogenic tests were conducted using acetic, propionic and butyric acids. These assays indicated that total activity was higher in bioparticle sampled from upper reactor levels. This was explained by the lower biofilm density values in this zone and mass transfer limitation phenomena. Moreover, biomass was unable to degrade acids different from acetic while performing the activity tests. This was probably due to the fact that acetic acid was the sole carbon source fed to the reactor.