The biomass of small flocs initially increases exponentially at a rate equal to that of cell suspensions. After this first phase, the growth rate gradually decreases and finally the radius becomes a linear function of time. At this time flocs are large and have a kernel of dead biomass. This kernel arises when the substrate concen-tration decreases below the threshold level at which cells are just able to pay their maintenance costs.
We deduce an explicit approximative expression for the interdivision time of flocs, and thereby for the growth of flocculated microbial biomass at constant substrate concentrations. The model reveals that the effect of stirring on degrada-tion rates occurs through a reduction of the floc size at division. The results can be applied in realistic biodegra-dation quantifications in activated sludge tanks as long as substrate concentrations change slowly.