SOLIDS RETENTION TIME IN SPHERICAL BIOFILMS IN A BIOFILM AIRLIFT SUSPENSION REACTOR

Fluorescent microparticles were used as tracer beads to measure the dy namics of solids in spherical biofilms in a biofilm airlift suspension reactor. Attachment to, release from, and penetration into the biofil ms of the tracer beads were measured. The coverage of the biofilm surf ace was low and the steady state particle concentration on the surface was dependent on the biofilm surface characteristics. The measured at tachment rate constant was identical in both experiments and appeared to be determined by the hydrodynamic conditions in the turbulent react or. The attachment rate was much faster than the release rate of the t racer beads and, therefore, the solids retention time in the biofilm p article is not due to a simple reversible adsorption-desorption proces s. The heterogeneity of the distribution of tracer beads on different sectors on the biofilm surface decreased during the attachment period. Due to random detachment processes the heterogeneity of the tracer be ad distribution increased during the release period. The tracer beads quickly penetrated into the biofilm and became distributed throughout the active layer of the biofilm. The observed penetration into biofilm s, the nonuniform distribution on the biofilm surface, and the fast up take and slow release of tracer beads cannot be described by a simple model based on a reversible adsorption-desorption mechanism, nor with existing biofilm models. These biofilm models, which balance growth an d advection assuming a uniform biofilm with a homogeneous surface, are inadequate for the description of the observed solids retention time in biofilms. Therefore, a new concept of biofilm dynamics is proposed, in which formation of cracks and fissures, which are rapidly filled w ith growing biomass, combined with nonuniform local detachment, explai ns the observed fast penetration into the biofilm of tracer beads, the long residence time, and the nonuniform distribution of fluorescent m icroparticles. (C) 1994 John Wiley and Sons, Inc.