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DESALINATION
ELSEVIER Desalination 147 (2002) 37-42 www.elsevier.com/locate/desal
Investigation on the effect of flocculants on the filtration behavior in microfiltration of fine particles
Minh-Tan Nguyen *, Siegfried Ripperger Technische Universitiit Dresden, Lehrstuhl fiir Mechanische Verfahrenstechnik, 01062 Dresden, Germany
Tel. +49 (351) 463 32472; Fax+49 (3.51) 463 37058; email: [email protected]
Received 1 February 2002; accepted 23 March 2002
Abstract
Flocculated particles can form a highly porous filtration cake and prevent the deposition of particles on the membrane in combination with crosflow micro filtration, which would enable a higher permeate flux [8]. Using this concept, a study was carried out to investigate the possibility of using flocculants to enhance the permeate flux in micro filtration of fine particles. The study consists of several experimental series involving the various flocculants, changes in the suspensions pH value and different stirring rates. Promising results, indicating that interactions between membrane-particle-flocculant, which can cause complicated effects on the permeate flux, were obtained. The option of using flocculants to increase permeate flux is also indicated.
Keywords: Microfiltration; Flocculants; Permeate flux; Adsorption
1. Introduction
A common technique for separating particles from a suspending fluid, which are approximately one-tenth of a micrometer up to a few micrometers in size, is micro filtration (MF). This technique is used for concentrating of slurries in the food, beverage and cosmetic industries, for purification and pre-treatment of water, and for microbial separations in the biotechnology industry. The
*Corresponding author.
economic viability of MF operation depends on the permeate flux and the capacity of the module. The higher the permeate flux the faster the feed can be processed. The higher the module capacity, the Iarger the feed volume that can be processed before the module has to be taken off-lined for cleaning. It is well known, that the permeate flux of a MF process is usually limited by the formation of a concentration polarization boundary layer consisting of a high concentration of particles which are retained by the membrane. At the
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.
00 1 l-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOOl l-9164(02)00572-6
38 M.-T Nguyen, S. Ripperger / Desalination 147 (2002) 37-42
beginning of the filtration process, the membrane retrains the particles. But later, the cake layer retains particles. Therefore, the pore size in a MF membrane is generally over an order of magnitude larger than the diameter of the particle. This boundary layer provides an additional hydraulic resistance to the permeate flow. In addition, both the permeate flux and the module’s capacity are compromised by fouling of the membrane as a result of deposition of suspended particles on or in the membrane pores. Hence, the process of MF is primarily determined by the formation of a cake layer on the membrane. Several researchers [ 1][2] have focused on the forces acting on the single particle and obtained similar results: The balance between the lift force resulting of shear flow and the drag force of the permeate flux is responsible for the transport of particles to the layer. At high permeate flux the drag force is higher than the lift force for a wide range of particle sizes. This implies that the particles will be transported to the layer and will be deposited here. Larger particles have a higher lift force and therefore will not touch the layer. With decreasing filtration rate the balance between the lift force and the drag force of the permeate flux shifts to smaller particle sizes. Consequently, only small particles can be deposited on the layer at low filtration rates. Accordingly, a possible approach to enhance the permeate flux in MF would be the creation of a flocculated feed. With the addition of flocculants to the feed, the particle size distribution of the feed can be changed. At the same time, a more porous cake layer, which leads to a higher permeability, would be obtained.
This study focuses on the following points: l The possibility of flocculant usage to enhance
permeate flux in ME l The effect of flocculant addition on filtration
processes.
2. Materials
Experiments were carried out in a stirred cell apparatus. Permeate was collected in a beaker on
a balance connected to a PC. Permeate flux was continuously determined and made available for further processing. As filter-medium, a polyamide membrane (AKZO) with a nominal pore size of 0.2 pm and clean membrane resistance of about 4.4~10’” m-’ was used. A feed suspension was prepared by dispersing Titan Oxide (MERCK), with an average particle size of 0.25 pm, in de- ionised water. Praestol 2500 (polyacrylamide, neutral polyelectrolyte) and Praestol 630 (co- polymer based on Acryladmide, average cationic polyelectrolyte) were used as flocculants. The characterisation of titan oxide and the membrane were described in [I]. The suspension was then introduced into the cell and filtered after the flocculant had been added with the desired concentrations. During filtration the transmembrane pressure (TMP) remained constant. After the filtration was completed, the height of the filtration cake was measured by a laser triangulometer. Following experimental series were carried out at different concentrations of the flocculant, varying pH values and stirring rates: filtration of pure suspension, filtration of pure flocculant solutions, filtration of suspension with different flocculant dosages, and adsorption isotherm measurement of flocculants on titan oxide. The flocculant concentration was measured by UV- adsorption (Photospectometer SPEKOL) based on a calibration.
3. Results and discussion
The results obtained in all experimental series were diverse. The mean permeate flux for the filtration of 300 ml suspension at a TMP of 0.25 bar in the case of a Praestol2500 dosage of 50 mg/l and 20 mg/l was 50 l/(m*.h) and 150 l/ (m*.h), respectively, are shown in Fig. 1 and Fig. 2. These values were smaller as compared to the filtration of the pure suspension (190 l/(m*.h)). An increase in the permeate flux from 50 l/(m*.h) to 400 l/(m*.h) as the flocculant dosage was reduced to 10 mg/l can also be seen. In other words, the cake layer resistance was increased as
M.-T. Nguyen, S. Ripperger / Desalination 147 (2002) 3742
M pure suspension
*xx IOmg/l
880 20mgil
---O 50mg/l
Fig. 1. Permeate fluxes for the filtrations of 300 ml titan oxide suspensions with different dosages of Praestol2500, at pH neutral and TMP 0.25 bar.
the flocculant dosage was raised. The addition of 10 mg/l Praestol 2500 caused a positive effect. The mean permeate flux (400 l/(m2.h)) is greater than that in the filtration of the pure suspension.
the cake layer, it was observed that, with flocculant addition, the porosity of the cake layer was higher than for the pure suspension.
The addition of different dosages of Praestol Similar results were obtained at a pH value of 630 had a positive effect on the permeate flux in 3.5 (Fig. 3b,c). This suggests that the effect of most cases (Fig. 3a). An increase in PFM dosage flocculants may be negative or positive and that from 5 mg/l to 15 mg/l led to an increase in per- a critical dosage exists. At the critical dosage the meate flux from 150 l/(m*.h) to 950 l/(m*.h), much amount of flocculant is sufficient to create greater than in the filtration of pure suspension. flocculation with the particles, forming a more At the Praestol 630 dosage of 20 mg/l, however, porous layer and preventing the deposition of the permeate flux decreased to 350 l/(m*.h). Based particles on the membrane in combination with on the triangulometer thickness measurements of crossflow microfiltration. Once the critical dosage
Flocculant dosage, mg/l
Fig. 2. Mean permeate fluxes for the filtration of 300 ml suspension with addition of Praestol 2500, at pH neutral and TMP 0.25 bar.
Praestol 630, pH neutral
Flocculant dosage, mgil
(a>
Praestol630, pH 3.5 300
I!SI
a
z 2:
200 X-
cz s G
100 E EL 2
0 0 10 20 2
Flocculant dosage, mg/l
@)
Praestol2500, pH 3.5
0 10 20 Flocculant dosage, mg/l
Cc)
Fig. 3. Mean permeate flux for the filtration of 300 ml suspension at TMP 0.5 bar: (a) With addition of Praestol 630 at pH neutral; (b) With addition of Praestol630 at pH 3.5 and (c) With addition of Praestol 2500 at pH 3.5.
40 M.-T. Nguyen, S. Ripperger/ Desalination 147 (2002) 37-42
Adsorption isotherm of Praestol630 0.01 1 I I
CX.KJ experiment data - Regression
0 50 100
Free Praestol630 (mg/l)
Fig. 4. Adsorption of Praestol630 on titan oxide at pH 3.5.
is overcome too many flocculant molecules remain in the suspension, as has been shown in the adsorption isotherms of the flocculants on suspended particles, Fig. 4 and Fig. 5. This leads to the adsorption of flocculant on the membrane surface (or in the membrane pores) and the formation of viscous flocculant layer on the membrane surface.
As flocculants were introduced in the filtration system, many processes would have occurred: (a) flocculation of the particles due to the adsorption of flocculant on the particle surfaces; (b) adsorption of flocculant on the membrane surface, and probably within the membrane structure or (c) residue of free flocculant in the suspension. The predominant process among these is decided by the interactions between flocculant-particle- membrane. In order to gain insight into the distri- bution of added flocculant within the filtration system, 3 experimental series were carried out: 1)
2)
3)
Adsorption experiment, in which the particles came in contact with flocculants Similar to (1) but here in addition to the particles also a piece of membrane was brought in contact with the flocculant, and Similar to (2) but after absorption, the sus- pension was filtered.
Adsorption isotherm of Praestol250( 0.01
.10-3
.10-4
.10-5
.lo-6
.lo-’
.1o-8
OQO experiment data
0 - Regression
0 50 100
Free Praestol2500 (mg/l)
Fig. 5. Adsorption of Praestol 2500 on titan oxide at pH 3.5.
At the end of each experiment, the flocculant concentration was measured with the SPEKOL. In terms of the mass balance of the flocculant in the system with respect to the adsorption isotherm, the distribution of the flocculant was calculated. The results are demonstrated in Figs. 6 and 7.
As can be seen, the fraction of flocculants, which was filtered on the membrane surface, is the largest portion, followed by the portion of free flocculant in the suspension, and finally the adsorbed flocculant on the particle surface, which created floes with titan oxide particles. The amount of flocculant adsorbed on the membrane was negligible. Although the increase in concen- tration of the added flocculant leads to another equilibrium, the distribution ratio did not change significantly. The adsorbed fraction of flocculant on particles tended to stay stable because of the adsorption plateau reached, which, according to Fuch and Killmann [4], caused by the zeta- potential plateau value. The amount of negative zeta potential of the particles decreases with increasing polyelectrolyte concentration because of a screening of the surfactant charges [4]. The free flocculant molecules remaining in the suspen- sion formed a gel layer, which contributed to the increase in viscosity of the solute, on the
M.-T. Nguyen, S. Ripperger / Desalination 147 (2002) 37-42 41
c 100% .g 3 80% PL 2 60% ‘0 E 40% _m a 20% :: G 0%
Praestol2500
0 be filtered on membrane
[_I adsorpbed on men-brane
q on particles
n free
s 100% E a 80% g Z 60% 5 z 40% _m 3 20% 8 G 0%
10.93 20.57 Floculant concentration (mgll)
Fig. 6. Relative fractions of Praestol 2500 in the filtration systems, pH 3.5.
membrane surface during filtration. This resulted in an increase in hydraulic resistance i.e., the decrease in the permeate flux due to the formation of a viscous layer of flocculant on the membrane surface. This phenomenon was also reported in the other investigation [ 111. Among the four frac- tions, the free surfactant portion had most signifi- cant effect on the permeate flux. This is manifested in the result of the filtration of pure flocculant solutions.
It is clearly shown that a small amount of flocclulant (10, 20, 50 or 100 mg/l) can already have a significant effect on the permeate flux. The same phenomenon was observed for filtration of
200 400 Times
Fig. 8. Permeate flux for the filtration of 300 ml pure Praestol 2500 solutions at pH neutral and TMP 0.5 bar.
Praestol630
0 be fiitered on mmbrane
0 adsorpbed on matirane
on particles
n free
10.73 20.76 Flocculant concentration (mgll)
Fig. 7. Relative fractions of Praestol 630 in the filtration systems, pH 3.5.
Praestol630 solutions. In terms of effective floe formation, the optimum dosage of the Paraestol 2500 and 630 was determined to be 5 mg/l, [3][6][7][10]. With a dosage of 50 mg/l, 100 mg/ 1 a suspension, in which the flocculant amount is equal to 33 mg/l and 79 mg/l pure flocculant solution, respectively, can be obtained. In this case, in addition to the filtration of a flocculated suspension, the system has to filtrate the flocculant solution. Due to that the hydraulic resistance was dramatically raised, which lead to the decrease in permeate flux in comparison with filtration of pure suspension.
a Pure Prado1 2500 solution 3 ” 500 r
G
Flocculant dosage mg/l
Fig. 9. Mean permeate fluxes for the filtration of 300 ml Praestol2500 solution at pH neutral and TMP 0.5 bar.
42 M.-T. Nguyen, S. Ripperger / Desalination 147 (2002) 37-42
4. Conclusions
The following conclusions can be drawn: 1. The possibility to enhance the permeate flux
in micro filtration of suspensions of fine particles is confirmed.
2. The effect of flocculants on permeate flux is complex. It can be positive or negative. There is a critical dosage at which an enhancement of permeate flux can be obtained.
3. Small amount of flocculant can already have a significant effect on the permeate flux.
4. Once the flocculant is introduced into the filtration systems, it can not only create floes with particles but also adsorb on the walls of the membrane pores and form a gel layer on the membrane surface. These duplicated activities can lead to an increase in hydraulic resistance.
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