  We present a model for a biological reactor in which bacteria tend to
aggregate in flocs, as encountered in wastewater treatment plants. The
influence of this flocculation on the growth dynamics of the bacteria is
studied. We argue that a description in terms of a specific growth rate is
possible when the flocculation dynamics is much faster than the other processes
in the system. An analytical computation shows that in this case, the growth
rate is density-dependent, i.e., depends both on the substrate and the biomass
density. When the flocculation time scale overlaps with the other time scales
present in the system, the notion of specific growth rate becomes problematic.
However, we show numerically that a density-dependent growth rate can still
accurately describe the system response to certain perturbations.
