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22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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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.