Submitted for
Oral Presentation
Sithebe
and Chirwa 2013
A SIMPLIFIED MODEL OF FLOCCULENT SEDIMENTATION
BASED ON FUNDAMENTAL PHYSICS OF SETTLING PARTICLES
N.P
.
Sithebe
*1
and E
.
M.N. Chirwa
1
1
Water Utilisation Division, Department of Chemical Engineering
University of Pretoria, Pretoria 0002
Tel: 27
-
12
-
420 5894;
Fax: 27
-
12
-
362 5089; Email: evans.chirwa@up.ac.za
Topic
16
–
INDUSTRIAL AND EXPERIMENTAL STUDIES
Abstract
In pra
ctice, flocculent settling data
for plotting iso
-
percentage
concentration
removal profiles
is analy
s
ed using a graphical method. The profiles
are utilized to size
a sedimentation tank, yet they are hand drawn (due to their non
-
linear nature) and
therefore inaccurate and tedious. Hence,
an
optimised design of sedimentation tanks
is necessary in order to eliminate the errors incurred during data
analysis.
The main
objective of this project wa
s to derive an automated data analysis
package for plotting iso
-
percentile removal profiles in settling columns. The package
comprise of an interpolation script and a mechanistic flocculation model. The scri
pt
interpolates experimental data and generates suspended solids removal profiles based
on column de
pth and solids residence time.
The model is derived from the physical
principles of a settling particle. It represents the behaviour of particles in quiesce
nt
flow. It incorporates the particle size distribution and the fractal nature of particles
during settling. The model is further modified by substitution into a one dimensional
transport equation to obtain an instantaneous velocity equation.
Model validit
y and competency was tested by a direct comparison with existing
literature models, like Ozer’s model and Sans’ model. The proposed model performed
better than Ozer’s and Sans’. Comparison was based on the goodness of fit, the least
sum of square errors an
d mathematical consistency with known flocculent settling
behaviour
.
R
eaction kine
tics and collision efficiencies of the
flocculation system
,
were deduced from a modified equation (1) developed by
Smoluchowski
.
k
i
i
j
i
k
j
i
k
n
n
j
i
n
n
j
i
dt
dn
1
,
,
2
1
(1)
Key words:
flocculation, mechanistic model, fractal, iso
-
percentile
profiles,
sedimentation, total
suspended solids.
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