  A non-negligible part of the biological reactions in the activated sludge
process for treatment of wastewater takes place in secondary settling tanks
that follow biological reactors. It is therefore of interest to develop models
of so-called reactive settling that describe the spatial variability of
reaction rates caused by the variation of local concentration of biomass due to
hindered settling and compression. A reactive-settling model described by a
system of nonlinear partial differential equations and a numerical scheme are
introduced for the simulation of hindered settling of flocculated particles,
compression at high concentrations, dispersion of the flocculated particles in
the suspension, dispersion of the dissolved substrates in the fluid, and the
mixing that occurs near the feed inlet. The model is fitted to experiments from
a pilot plant where the sedimentation tank has a varying cross-sectional area.
For the reactions, a modified version of the activated sludge model no. 1
(ASM1) is used with standard coefficients. The constitutive functions for
hindered settling and compression are adjusted to a series of conventional
batch settling experiments after the initial induction period of turbulence and
reflocculation has been transformed away. Further (but not substantial)
improvements of prediction of experimental steady-state scenarios can be
achieved by also fitting additional terms modelling hydrodynamic dispersion.
