International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
Effect of physical parameters of media
particles on the performance of floating
medium filter
Bashir Brika1,*, Steven Bradshaw 2, Ed Jacobs 3
1
Advaced Laboratory of Chemical Analysis, Libyan Authority for Research, Science and Technology,
Tripoli, Libya
2
Department of Process Engineering, Faculty of Engineering, Stellenbosch University, Stellenbosch, South
Africa
3
Department of Chemistry and Polymer Science, Faculty of Science, Stellenbosch University, Stellenbosch,
South Africa
Abstract
Removal of particles by a filter is a complex water treatment process. Several factors are involved in
particles removal mechanisms. Factors include size and shape of filter medium granules, filtration velocity,
coagulant chemical dosage, and media depth. The objective of this study was to investigate the effect of
these factors on the performance of floating medium filter. Four medium grain sizes (2.28, 3.03, 3.30, and
4.07 mm) were used. Two medium shapes (cubic and disc) were evaluated. The filter medium was lighter
than water; thus, the bed floated. The mode of filtration was upward in all the experiments. The raw water
turbidity was kept constant (≈ 60 NTU) and the flow velocity, media depth, and coagulant chemical dosage
were varied. A two level factorial design was chosen as an experimental design to study the effect of the
physical parameters and the interactions between the influencing factors. Results showed that the sharpedged cubic medium gave a better performance in terms of turbidity removal in comparison with smooth
cubic medium and disc-shaped medium. Statistical analysis showed that the best operating conditions to
remove turbidity were found to be: low filtration rate (36.8 L/m2· min), longer media depth (0.6 m) and
optimum coagulant dose (23 mg/L). Media depth seemed to have the greatest influence on the turbidity
reduction.
Keywords: Floating medium filter, Up flow filtration, Particle removal, Experimental design
Polyethylene is available in a variety of grades
I.
Introduction
with different properties to suite processing for
The media used in the floating media filter, is
example by blow moulding, injection moulding
fabricated from polyolefinic materials. These
and
materials are not as dense as water and have
(HDPE) is very tough materials, whereas the low
specific gravities between 0.89 and 0.93 [1,2].
density
ISSN: 2231-5381
extrusion.
High
polyethylene
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density
is
polyethylene
relatively
softer.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
Polyethylene and polypropylene have excellent
II.
Floating medium filter (FMF)
chemical resistance to all chemicals used in
drinking
water
a
Figure 1 shows a diagram of the FMF
are
constructed to evaluate the performance of
available in the form of pellets that come in a
different media in particle removal. The filter
variety of sizes and shapes. These pellets can be
column had an internal diameter of 0.3 m and a
acquired directly from manufactures that produce
height of 2.8 m. In order to ensure that no filter
the polymers from raw materials, or can be
medium particles escaped the filter column, a
obtained from converters. However, the plastic
wire mesh was installed on the upper section of
recycle-industry can supply cheaper granules
the filter column.
concentrated
applications,
form.
Polymer
even
in
materials
(pellets).
Air release valve
For the past fifteen years, there has been interest
in using granular polymer as filtering materials
for particle separation [3-6].
In this kind of
Backwash
water inlet
Filtrate outlet
filtration, the media form a bed in which the
majority of the media floats just beneath the
Media and
retainer grid
surface of the process liquid (Figure 1). The
process liquid is pumped into the bottom of the
filter and flows upward through the bed [7].
Generally, the floating (buoyant) media particles
used in this type of application are of larger
Feed water
inlet
Air inlet
diameter than the media particles used in
conventional sand filters [8].
An acceptable quality of filtrate (< 1.0 NTU) can
Sludge removal
not be provided by all media. This affects the
performance of the floating media filter.
Figure 1: Diagram of the floating medium filter
This paper reviews the outcomes of research into
the use of floating plastic media of different size
III.
Media size and shape
and shape for the treatment of turbid water. A
As can be noticed from Figure 2, apart from the
factorial experimental design was used for
size differences, the main difference that can be
determining the efficiency of the different media
observed between the media is that the medium
used and the physical parameters affecting the
(ii) is sharp-edged whereas medium (i) and (iii)
filtration process.
are round-edged.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
Media (i)(i)
Medium
Media (ii)(ii)
Medium
Media (iii)
Medium
(iii)
Figure 2: Examples of polypropylene beads
The flow profiles within the bed consisted one of
their removal from the fluid being filtered by the
the above mentioned medium would differ. It can
filtration mechanisms [9,10]. The selection of
be proposed that a filter bed contains medium
suitable media for a FMF is critical in the design
particles with sharp edges would provide greater
and operation of the filtration process. In
mixing compared to a filter bed that contains
addition to the importance of the media shape, it
beads with round edges. Figure 3 shows an
is apparent that the effect of media size has an
assumption of a fluid flow around medium
influence on process performance. Therefore it is
particle. Greater mixing would also create more
believed that both media shape and size play a
opportunities for particles to collide and hence
role in the filtration process (particle removal).
grow larger-sized floc that will lead to enhance
Figure 3: Assumed fluid flow around media particles
IV.
floating media used during the experimentation
Experimental
is shown in Figure 4. Based on the classification
Floating media used in this study were classified
of Lees, 1964 and Janoo, 1998, media was
according to the shape classification of Lees,
characterized as cubic media (i) and (ii), and
1964 and Janoo, 1998. A photograph of the
disc-shaped, media (iii) [11]. Media (i) has
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
rounded edges (die-face cut), whereas media (ii)
mg/L of bentonite (Protea Chemicals, South
has sharp edges (lace cut). Media (iii) has round
Africa) in a 20 L tank using a stainless steel
smooth edges and was also die-face cut.
high-rate mixer. After 30 min of mixing the
Polypropylene medium particles were used
suspension was transferred to a 1500 L conical
because this polymer is readily available in shape
raw water feed tank. The bentonite in solution
and size. Cubic polypropylene was obtained
was kept in suspension by circulating the feed
from Sasol Polymers and disc polypropylene was
water by means of a centrifugal pump. The raw
obtained from Pelmanco Pty Ltd (South Africa).
water had a typical turbidity of 60 NTU. Ferric
sulphae was used as the coagulant, with 23.0
Artificial
feed
water for
use during the
experiments was made up by dissolving 250
(i)
mg/L being the optimum dosage at an optimum
pH of 5.5.
(ii)
(iii)
(iv)
Figure 4: Media used in the factorial experiments; (i) small cubic, (ii) large cubic,
(iii) disc media, and (iv) small cylindrical.
V.
Two-level full factorial design
Factorial experiments
Two-level full factorial designs are designs that
Factorial design is one tool that can be used in
research to design experiments. An experiment
using factorial design allows the experimenter to
examine, simultaneously, the effects of multiple
independent
factors
and
their
degree
of
test all the two-level combinations of the
factors involved. A two-level full factorial design
with k factors requires 2 k experimental
trials to cover all possible combinations of the
input factors.
interaction [12]. It is stated that when several
The 23 design
factors are of interest in an experiment, a
factorial experimental design should be used.
A
number
of
factorial
Furthermore, factorial experiments are the only
experiments
way to discover interaction between variables
determine the effect of physical parameters
were
designed
conducted
to
[13].
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
on the performance of FMF. The 23 design was
being combined with the preceding terms. This
chosen the conduct the experimental work.
order of writing the treatments is called the
3
The 2 design is a two-level factorial experiment
standard order.
design with three factors (A, B and C). This
Three factors were investigated in 23 full
design tests three (k=3) main effects, A, B and C;
factorial design, studying the effects of filtration
three-two factor interaction effects, AB, BC, AC;
rising velocity, media depth, coagulant chemical
and one-three factor interaction effect, ABC. The
dosage
design requires eight treatment combinations per
performance. The low and high levels for the
replicate. The eight treatment combinations
three factors in the full 23 factorial design are
corresponding to these runs are: (1), a, b, ab, c,
shown in Table 1, where the levels of each factor
ac, bc and abc. The treatment combinations are
for the different treatment options that make up a
written in such an order that factors are
23 factorial design matrix are shown.
and
media
shape
on
the
filter
introduced one by one, with each new factor
Table 1 (a and b): Experimental design of a 23 factorial experiment
(a)
Level
Factor
Details
-
0
+
A
Filtration rising velocity (m/h)
2
3
4
B
Media depth (mm)
200
400
600
C
Chemical dose (mg/L)
11.5
17.25
23
(b)
Trial
Treatment
Factors
A
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B
C
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
VI.
1
(1)
-1
-1
-1
2
a
+1
-1
-1
3
b
-1
+1
-1
4
ab
+1
+1
-1
5
c
-1
-1
+1
6
ac
+1
-1
+1
7
bc
-1
+1
+1
8
abc
+1
+1
+1
(b)
Results and discussion
8
The objective of the experiment was to assess the
7
6
performance of various media [Figure 4] was
5
Turbidity (NTU)
effect of the factors on the turbidity removal. The
evaluated in the experimental FMF. Once the
experiment started, the turbidity (NTU) was
Media (ii)
--+--+++--+
+-+
-++
+++
4
3
measured over filter run time (in hours). Figures
2
5 [a, b and c] show the relationship between
1
turbidity and filter run time for the complete
0
0
1
2
3
4
5
6
7
8
Filter run time (h)
experiment [Table 1] for each of the media (i-iii)
respectively.
(c)
(a)
8
8
Media (iii)
Media (i)
Turbidity (NTU)
6
5
4
--+--+++--+
+-+
-++
+++
7
--+--+++--+
+-+
-++
+++
6
Turbidity (NTU)
7
3
5
4
3
2
2
1
1
0
0
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
Filter run time (h)
Filter run time (h)
Figure 5: Factorial experimental trials conducted with three media evaluated in the factorial experiment.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
On the analysis of Figure 5 there appears to be a
depth and chemical dose and for a low level of
small difference between the three media.
filtration rising velocity. It could also be
Experimental trial 7 [Table 1] which was coded
observed that media (iii) is outperformed by
as (-++) appears to be the best combination of
media (i) and (ii). It could be argued that the
factors since the maximum turbidity removal was
rapid deterioration of filtrate quality in the case
achieved under these conditions. This maximum
of media (iii) is because of lower mixing
corresponds to a low level for filtration rising
properties of the disc-shaped media.
velocity and high level for media depth and the
Under the operating conditions that was regarded
chemical dose. On the other hand, experimental
as the best combination (Figure 6 b), which is
trial 2, which was coded as (+--) appears to be
lowest filtration rising velocity, deepest media
the worst combination of factors since the
depth and optimal coagulant dosage (condition: -
minimum turbidity removal was achieved under
++), all the media performed well with long
these conditions [Figure 6].
filtration runs, but again media (iii) was
Figure 6 (a and b) also reveals that the filter run
outperformed by the other two evaluated.
time was the longest for high levels of media
(a)
(b)
3.0
9
8
Media (i)
Media (ii)
Media (iii)
7
2.0
Turbidity (NTU)
6
Turbidity (NTU)
Media (i)
Media (ii)
Media (iii)
2.5
5
4
3
1.5
1.0
2
0.5
1
0.0
0
0
1
2
3
4
0
1
2
3
4
5
6
7
8
Filter run time (h)
Filter run time (h)
Figure 6: Performance of three media tested under (a) the worse operating conditions (+--). (b) the best
operating conditions (-++).
VII.
Discussion of the 24 factorial design
shape (factor D) on the removal of turbidity. The
results
first attempt was to investigate the effect of the
media shape by comparing media (i) and media
24 factorial design experiments were conducted
in order to investigate the effect of the media
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(ii) and the second attempt was to compare the
best performing media in the first attempt and
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
media (iii). Figure 7 shows the effect of media
interpreted that factor D (media shape) by itself
shape in which media (i) was compared to media
affect the response (Turbidity). Figure 8 shows
(ii). Media (i) was indicated by the low level (1)
the relationship between the media shape-
of factor D and media (ii) was indicated by the
chemical dose interaction and turbidity removal.
high level (2).
Figure 8 indicates that the maximum turbidity
removal was achieved with media (ii) when the
As can be seen from the Figures 7 media ii
chemical dose was at high level. On the other
(indicated by 2) performed better than media i
hand there was no significant difference between
(indicated by 1) since using media ii leads to
the performances of both media at the low level
greater particle (turbidity) removal. This could
of chemical dose (factor C). That proves the
be attributed to the fact that media ii is a sharp-
point that media shape does not affect the
edged cubic that could lead to a higher mixing.
response by itself, its importance appears to have
a significant effect when it is combined with the
It must be noted that the media shape is involved
effect of chemical dose.
in an interaction. Therefore it cannot be
Design-Expert® Software
Interaction
D: Media shape
Turbidity
2.8
Design Points
D- 1.000
D+ 2.000
Turbidity
X1 = C: Chemical dose
X2 = D: Media shape
1.875
0.95
Actual Factors
A: Flitration rising velocity = 2.00
B: Media depth = 600.00
0.025
-0.9
11.50
14.38
17.25
20.13
23.00
C: Chemical dose
Figure 7: Effect of interaction CD.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
(a)
(b)
Figure 8: Effect of the medium shape on Turbidity at: (a) high level (optimal) chemical dose, (b) low level
of chemical dose.
Based on ANOVA and Pareto chart (Figure 9)
important factor affecting the turbidity removal.
No other factors or interactions appear to be
Design-Expert® Software
Turbidity
Pareto Chart
B
Bonferroni Limit 3.62187
3.67
A: Flitration rising velocity
B: Media depth
C: Chemical dose
D: Media shape
Positive Effects
Negative Effects
significant. The chemical dose-media shape
interaction and the effect of the media shape also
appear to have an influential effect on the
2.75
t-Value of |Effect|
the media depth (factor B) is by far the most
t-Value Limit 2.26216
CD
D
AC
1.83
C
0.92
AB
process.
0.00
1
2
3
4
5
6
7
Rank
Figure 9: Pareto chart of main effects in
the factorial 24 design.
to be the best performing media in terms of
Conclusions
The statistical analysis indicated that media
depth has the most significant effect on turbidity
turbidity removal in comparison with media i
(smooth small cubic) and media iii (disc-shaped).
This is a further support for the argument that the
removal.
higher mixing rate of the lace cut media allows
The statistical analysis also indicated that media
greater removal of turbidity.
shape play a role on the turbidity removal.
However, media ii (sharp-edged cubic) appeared
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International Journal of Engineering Trends and Technology (IJETT) – Volume 32 Number 1- February 2016
Acknowledgments
Thanks are due to the Libyan Government for
[6]
financing this work.
Special thanks to Mrs.
Breakah for her
[7]
contribution in this publication.
[8]
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