The solids retention time in biofilms was determined by using fluorescent microbeads as solid tracers. Attachment to and detachment from the biofilms, as well as surface concentration of the microbeads were measured. The surface concentration depended on the surface characteristics of the biofilms. The attachment rate was equal in three experiments and dependent on hydrodynamic conditions in the reactor. In all cases the detachment rate was much smaller than the attachment rate and larger than the average solids retention time of the active biomass. Release of the beads was determined by local, nonuniform detachment phenomena. Microbeads were observed to penetrate into the biofilms. This is not in accordance with conventional biofilm models. Therefore a new biofilm model concept is proposed. The biofilm is a dynamic structure, in which formation of cracks and fissures is taking place continuously. The crevices are filled with fresh biomass rapidly and particulates which are present are entrapped inside the biofilm.

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