Issues related to the calibration of a PBM for activated sludge flocculation

1
Issues related to the calibration of a PBM for
activated sludge flocculation
Dresden University of Technology Institute for
Urban Water Management
Author Thilo Koegst Supervisor ir.
Ingmar Nopens1 Promoter Prof. Dr. ir. Peter
Vanrolleghem1 Prof. Dr. sc. tech. Peter
Krebs2 1Gent University - BIOMATH, Department
of Applied Mathematics, Biometrics and Process
Control, Gent, BELGIUM 2Dresden University of
Technology Institute for Urban Water
Management, Dresden, GERMANY

• Modelling part
• For modelling flocculation processes Population
  Balance Equations have proved to be useful. Such
  equations function similar as a mass balance and
  are concerned with keeping track of numbers of
  entities. The observed entities are distinguished
  from one another by one ore more variables.
• In a closed system is the number of particles, ni
  having the same diameter, di described by four
  processes. These processes are schematically
  shown below.
• The main tasks in defining a PBM for flocculation
  of activated sludge are
• define aggregation and breakage processes
• model hydrodynamic influences in a simple way
• specify important floc properties (e.g.
  structure)

Objectives
The effluent of final clarifiers from biological
waste water treatment plants is frequently
deteriorated from small activated sludge flocs.
The overall objective is therefore to gain
knowledge of the flocculation process of
activated sludge flocs. The gained information
can then be used in a Population Balance Model
(PBM) which mathematically describes the
flocculation process according to floc size and
may lead to possible optimizations. The
particular objectives for the thesis were ? to
assemble a setup to conduct flocculation
experiments in a simplified system under various
physico-chemical conditions to produce a database
to calibrate and validate a PBM ? to define and
investigate possible structures of a PBM for
flocculation of activated sludge ? to examine
methods to convert the experimental data for use
in a PBM
Experimental part Aim of the experimental part is
to monitor the total floc size distribution in a
vessel of 5l during flocculation occurs to
produce a database for the calibration and
validation of the PBM. Stable activated sludge
from a Sequencing Batch Reactor (SBR) has been
used for controlled flocculation experiments in
the FLOCUNIT. The FLOCUNIT is a set-up where
physico-chemical conditions of the sludge, such
as temperature, pH, shear rate, dissolved oxygen
and conductivity are measured online and kept
constant. The floc-size-distribution is measured
by a Malvern Mastersizer which applies the method
of light scattering. The Malvern describes the
floc-size-distribution according to volume
fractions of 32 size classes. A typical evolution
of the floc-size-distribution of a flocculation
experiment is shown in Figure 1. Data
conversion The aim of the data conversion is to
move from a volume based fraction distribution to
a concrete number distribution, which can be used
in the PBM. Therefore the Volume Concentration
(VC) has to be determined. The Volume
Concentration describes the ratio between the
total floc volume (VF) to the total measured
volume (VT). In this thesis VC has been
calculated using floc related densities and the
MLSS measurement. Densities that have to be
measured are floc density rF, solid floc density
rS and liquid density rL. VC is calculated as
below To move to a number distribution
spherical shaped flocs have been assumed.
AGGREGATION The aggregation frequency of two
flocs is determined by the product of collision
efficiency and collision frequency. The collision
efficiency takes into account short range forces
that may inhibit a forming of a new floc.
Collision frequency has been found being based on
the following three processes Brownian
motion Shear flow Differential sedimentation
BREAKAGE So far only empirical approaches are
known to describe the breakage of flocs. For
simplicity only binary breakage has been
considered. The breakage frequency of a floc has
been linked to the shear rate in the liquid and
to the size of the observed floc. FLOC
STRUCTURE The structure of activated sludge flocs
is very complex furthermore they are porous and
mostly permeable. In the model they have been
implemented as spheres. To take into account the
porosity the method of the fractal mass dimension
has been applied, there mass, M and size, d are
related as followed Df has a value between 2
and 3 depending on the density of the activated
sludge flocs.
Results In this thesis various PBMs with
different definitions of aggregation and breakage
processes have been applied and flocculation
experiments have been simulated in WEST (Hemmis
NV, Belgium). Model parameters have been
estimated and if possible optimized. The result
of one experiment is shown in Figure 2. Overall
it can be noted that a PBM is generally able to
describe flocculation of activated sludge and if
further developed will be very useful in
clarifier modeling. So far the calibration and
validation is difficult since not all influences
on flocculation have been accurately defined and
implemented in the model. This led to
identifyability problems of model parameters and
insufficient results.
Figure 2 Number distribution at steady state
(at 90 min) model data vs. experimental data
Figure 1 Volume-fraction-distribution of a
flocculation experiment
Thilo.Koegst_at_biomath.rug.ac.be -
http//biomath.rug.ac.be