Model analysis and its evidence for particle removabilityof pressed non-woven fabric membrane*T. Oike, *S. Manabe, **M. Kim, ***K. Yoshimura, ***K. Shuzenji
Sepa-Sigma Inc., **MK-Sigma Inc., ***Fukuoka Industrial Technology Center, Japan*Abstract— Low cost membranes for water treatment using a pressed non-woven fabric membrane are proposed. The
pressed non-woven fabric membranes are applied in the novel separation technologies of the pore diffusion separation
under the flow fractionation. The particle removal was evaluated by both computer simulation and experimental method.
The simulation demonstrated that when the particle size increased the probability of the particle ejection through themembrane pores decreased and when the particle located far from the membrane surface the above probability decreased
also. The experimental results showed the similar tendency to the model computer analysis.

I. INTRODUCTION
Due to the increase in the world population and theeconomic development, the water shortage has becomea serious problem. To solve the water shortage, manytechnologies of water treatment have been developed.
The water treatment required for developing countries
must be small size, easy to handle and low cost, since it
is not easy to build large-scale infrastructure. A problem
of a centralized city is also needed to concern. Due toold facilities of domestic infrastructure, it will be goodchance to convert them to such small size equipments.
In the case of installing small equipments, theoperation system must be simple. A membrane
separation may be the most effective for these demands.
The current membrane separation is known to be veryexpensive due to the high production cost of themembrane.
In the previous paper, the pressed non-woven fabricmembrane for water treatment has been proposed as thelow cost membranes[1], and the novel low cost
separation technology such as the pore diffusion and theflow fractionation has been evaluated.
The pore diffusion, defined as the diffusion of a
substance in a pore of a membrane, is the separation
technique under no plugging [2]. Water molecule passes
through pores in the membrane by diffusion and also isfiltered through a bulk flow under the low transmembrane pressure such as less than 0.1 atm. The
particles shift their position to the place where shows
the higher flow rate. This shift has been named as theflow fractionation[3],[4] originated from the σ effect[3].
The particles cannot pass through pores of theLarge particle of toxic

Water

membrane, and then, is separated from water. One of
the features of the pore diffusion is high level of particle
removability. On the other hand, slow filtration rate is adisadvantage.
The flow fractionation separation has been proposed
for the porous membrane[4]. The separation mechanism
is based on the flow fractionation effect that occurs byquickened flow rate[5]. The flow fractionation effect isobserved in blood flow[6]. By the use of this effect we
may maintain a stable long-term filtration performance.
For example, red blood cells move to the center of theblood vessel by the effect resulting the stablefiltration[7][8]. When we use this effect in the filtration
process of aqueous solution, it is possible to remove
particles by low pressure filtration. The flow
fractionation effect is observed only in the case of theshear stress to the particles exceed over the criticalvalue that originates the rotational motion of theparticles in a flow stream. This indicates that the flow
stream of a given aqueous solution must be the laminarflow and the flow should give the shear rate to theparticles. The shear rate, τ, is given by the following
equation (1).
τ=V/t

(1)

Where V is the flow rate (mm/s), t is the width of theflow path (mm). When the particles size are a
sub-micron, more than 20 sec-1 shear rate works
effectively on the rotational motion of the particles.
Even in the case of the pressed non-woven fabricmembrane, the flow fractionation and also the porediffusion may be applicable since the surface of theFlow rate

MembraneFig.1: Schematic representation of the flow fractionation mechanism:
The lengths of arrows stand for the amount of flow rate and arerepresented by a parabolic curve.

membrane is smooth[1] although its mean pore size
exceeds over 1 μm. We can expect the development of
the novel low cost and small scale equipment for watertreatment.
In this paper, we will evaluate the particle
removability using both methods of the computer
simulation and experimental filtration with themembrane. The wastewater containing particles may be
employed and the separation performances including
particle removability may be evaluated. We intend toshow (1)the separation mechanism for the novel
separation technology including the pore diffusion andthe flow fractionation, and (2)confirmation of theeffectiveness of the no-woven fabric membrane to thesolution treated with the nucleating reagent.
II. METHODS
A. Pressed non-woven fabric membrane
Regenerated cellulose filament non-woven fabric(The non-woven fabric membrane) was prepared bycopper ammonium process (100g / m² basis weight, 390
μm thickness, Ra=21.1 μm surface roughness), androller pressed to more than 50 % of the compression
rate in a wet condition by a mechanical roller pressmachine. The average pore size of the original
non-woven fabric was about 100 μm including support
mesh pore size.
Average pore size of the membrane 2rf wascalculated by putting the filtration rate of distilled water
J into the following formula (2).
(2)
Where J is the observed value in (mL / min), d is themembrane thickness (μm), ⊿P is pressure difference(mmHg), A is the membrane area (m2). Prρ is porosityof the membrane and in calculated though the eq. of "1 (cellulose density / membrane density)". η is theviscosity of water.
The compression ratio was calculated with thethickness of the membrane before and after pressing.
Compression ratio is given by the following equation(2).

Particle Flow
vin=92mm/s

Compression ratio = (T1-T2)/T1×100

Where T1 is the thickness of non-woven fabricmembrane before pressing, T2 is the thickness afterpressing.
B. Nucleation of particles in raw water
A sewage waste water was employed as a raw waterwhere the particles were generated. The sewage was
sampled from a sewage farm of the city of Kitakyushu
in Japan. Original COD of the solution was 185 ppm.
After, 40 ppm of sodium hypochlorite and 50 ppm offerric chloride added to the sewage waste water foroxidation and mixed by aeration in 30 minutes, 500
ppm of Ca(OH)2 was added for nucleation and mixed
by aeration in 30 minutes. The size and quantity ofparticles were measured by Laser particle size analyzer
LS 230 (Beckman Coulter inc.).
C. Computer simulation of particle ejection
A computerized simulating software, STREAM ver.8(CRADLE Inc.), was used for the estimation of thelocation of the particle in the laminar flow as thefunction of time and of the position of the particle ingeneration. The ejection means that the particle goes
through out from primary side(original waste waterside) to secondary side(filtrate side) via membrane.
The flow pass had the orthogonal cross section of 2mm x 2 mm x 200 mm (Fig.2). The flow rate in the
primary side, Vin, was 92 mm/s. The particle size wasselected 3 sizes of 1, 10, 100 μm. The distance fromparticles to membrane surface, h, was 250-750 μm. The
filtrate speed condition, Vyout, was 0.022-0.33 mm/s.
D. Experimental evaluation of filtration performance
The filtration performance was evaluated by filtering
waste water through the pressed non-woven fabricmembrane using the filtration equipment designed forthe pore diffusion and the flow fractionation. The waste
water includes the nucleated particle of toxic materials.
The particles were filtered through the pressed
non-wovenfabricmembrane.
TheFiltration
performance was evaluated by measuring the particle
size and the quantity of the particles in the raw waterand the filtrate.

2mm

200mm

Filtration
vyout=0.022, 0.091, 0.33mm/s
yVinh

Vyout

Membrane surface(2)

xFig.2: Model analysis condition

2mm

III. RESULTS AND DISCUSSION
A. Pressed non-woven fabric membrane
The pressed non-woven fabric membrane wasprepared under the same condition in the previouspaper[1]. The compression rate was 52.6%. The averagepore size was about 8 μm after the compression.
B. Nucleation of particles in raw water
The particle size in the raw water was 17 μm as apeak in the distribution curve vs log (particle size) and31 μm as average size (Fig.3). There were two peaks
located at 17 μm and 1 gentle curve of 0.1-1μm. It
might be occurred by stirring of a circulation pump of
the filtration equipment.
3.53Raw water

2
1.5
10.5
00.01

0.1

110100

1000

10000

Particle size / um

Fig.3: Particle size distribution of the raw water
C. Model analysis of particle ejection
The simulation results are summarized in Fig.4. The
figure shows the area, expressed by the co-ordinates ofVyout (y-axis) and particles size (x-axis), where theparticles at the heights of 250 μm, 500 μm and 750
μmfrom the membrane surface retain in the flow
without ejection through pores. In the case of further
distance between a particle and membrane surface, theparticle ejection was decreased. In the case of bigger
particle, the ejection probability of the particle at thesame position is decreased.
0I
Retention

0.1

hEjection

0.15

250 um

II0.2

0.25

Retention

500 umEjection

750 um

III0.30.35

Fig.6 is the particle size distributions of raw waterand two filtrates. Filtrate 1 indicates the filtrate obtained
under 0.33mm/s of Vyout, and the Filtrate 2 indicates thefiltrate obtained under 0.091mm/s. The Filtrate of0.022mm/s could not be evaluated through a lightscattering method because of too clear. A comparison
between the solution before treatment and the filtrate 1indicates that particles over 40 μm were removed by thefiltration, and the filtrate 2 indicates that particles over 3μm were removed.
181614Raw water

12Filtrate1

10Filtrate2

864200.01

0.1110100

1000

10000

Particle size / um

0.05

Vyout / mm･s-1

Fig.5: Observation of solutions before and after treatment;
From left side, the solutions are treated under
Vyout=0.022mm/s, 0.091mm/s and 0.33mm/s,
respectively, and the solution at right end is thesolution before treatment.

Volume / %Volume / %2.5color. With slower Vyout speed, its color becomes clear.
It means slower Vyout gives higher removability of theparticle with wide range of size. The experimental
filtrates showed mostly the same results of the model
simulation.

Retention

01

10100

Particle size / um

Fig.4: Particle retentive zone exhibited by two-dementional
map of Vyout and particle size; Zone I stands for theregion where the particle located at h=250um remains
in the flow solution under V in of 92mm/s, Zone I+II
stands for the remaining region for the particle ath=500um and Zone I+II+III stands for the remaining
region for the particle at h=750um.
D. Experimental evaluation of particle removal
Fig.5 shows experimental filtrate of each of Vyout.
The rightmost is the raw water and it has the most vivid

Fig.6: Particle size distribution of the raw water andthe filtrate;
Filtrate1: Vyout=0.33mm/s, Filtrate2:Vyout=0.091mm/s
In the filtrate2, it has no peak around 10μm. It means
the nucleated particles around 10 μm stay in the primaryside in 0.091mm/s of Vyout. As for two big peaks around
1 μm of the filtrate 2, bigger particles than 1 μm in the
primary side might be comminuted by the shear stressof the circulation pump of the filtration equipment.
IV. CONCLUSION
The novel membrane separation technology
employed both of the pore diffusion and the flow
fractionation mechanisms worked effectively to thesolution treated with the nucleating reagent for theremoval of particles with the non-woven fabricmembrane. The removability was originated from theflow fractionation mechanism and the activity of theBrownian motion in addition to the sieving of pores.The contribution of the flow fractionation wasreproducible by the computer simulation of themovement of a particle in fluid flow.
V. ACKNOWLEDGEMENTS
We are deeply grateful to Mr. Takeuchi, Asahi
Kasei Fibers Corp. for his enormous supports suchas providing membrane samples and helpfulcomments. Also, we would like to thank FAIS,
Kitakyushu Foundation for the Advancement ofIndustry, Science and Technology, for its financial
supports.

VI. REFERENCES
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[2]
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[4]

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[6]

[7]
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