Reverse Osmosis and Ultrafiltration - American Chemical Society

3 Nature of Dynamically Formed Ultrafiltration Membranes SHOJI KIMURA1, TOSHIRO OHTANI2, and ATSUO WATANABE2

Downloaded by UNIV OF GUELPH LIBRARY on July 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch003

1

Institute of Industrial Science, University of Tokyo, 7-22-1 Roppongi, Minatoku, Tokyo 106, Japan 2 Ministry of Agriculture, Fishery, and Forestry, National Food Research Institute, 2-1-2 Kan-nondai, Yatabe, Tsukuba Ibaragi 305, Japan Ultrafiltration membranes were made by filtering various high molecular weight solutes and suspended solids on a porous ceramic tube surface, which was originally used for dynamic reverse osmosis membranes. Nature of the dynamically formed ovalbumin membrane depends on concentration and pH of the solution, but generally the rejection of albumin itself is high and the flux is significantly larger than commercial membranes. Membranes formed can easily be removed by washing with NaClO. Since the ovalbumin membrane can also reject other solutes, its molecular weight cut off value was determined. Membranes made of dextran do not show such good results. Modules made of porous ceramic tubes were developed and tested using various solutions, whose components formed self-rejecting membranes. Results of secondary sewage and pulp factory effluents are shown.

A dynamically formed membrane has been applied for various effluent treatments since it was discovered by ORNL (1_), because the formation and the cleaning procedures are rather simple compared to polymeric membranes. Materials used for dynamic membranes were Zr colloid which was later supplemented by polyacrylic acid (_2). The latter membrane is now called ZOPA membrane. The development of the modular form of these membranes and results of various applications have been reported by D.G. Thomas ( 3 ) , by Carre Inc. and Clemson University ( 4 ) , by University of Natal (_5), and by authors (6jJ)» Applications of dynamic membranes to various ultrafiltration processes are very interesting, because they can be used at high temperatures, formed and cleaned very easily. But in the actual ultrafiltration process the gel layer formed on membranes controls the flux and rejection primarily and natures of original membranes become obscure. In this regard the gel layer can be considered as a kind of a dynamically formed membrane, which may be applicable for practical applications. 0097-6156/85/0281-0035$06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


36

REVERSE OSMOSIS AND ULTRAFILTRATION

In this investigation natures of various gel layers formed on a bare porous ceramic tube, which was originally used as a support of ZOPA membrane, are studied as those of a dynamic membrane. The first part of this report deals with various natures of dynamically formed ovalbumin and dextran membranes. It is shown that the former membrane is self-rejecting and can also reject other solutes, but at the same time the concentration polarization effect is significant. The latter membrane is not self-rejecting and the flux decrease is due to the osmotic pressure effect. The second part deals with practical applications of Dynaceram module, which has been scaled up from a single ceramic tube such as the one used above to a module which contains 151 tubes. Results of treating an activated sludge and kraft pulp effluents are shown. Formation of ovalbumin membranes The experimental apparatus used is shown in Figure 1. A ceramic tube, whose O.D. is 10 mm and length 30 cm, is used as a supporting substrate of dynamically formed membranes. This ceramic tube is made of alumina and its pore sizes are from 0.5 to 1.5 microns. The surface layer is made of extremely fine particles with an average size of 0.05 microns produced by a special treatment. When aqueous ovalbumin solutions of various concentrations were fed over the ceramic tube under pressure, flux started to decrease and rejection of ovalbumin itself was increased by the gel layer formation. Both flux and rejection reached steady values after about an hour. These results are shown in Figure 2, where it is found that steady fluxes are dependent on the albumin concentration and rejections are almost over 90%. Experimental conditions in this case are as follows: temperature: 25°C, pressure: 5 atm, and flow rate: 5L/min. The same procedure was applied using dextran as a solute to form membranes. But as it is shown in Figure 3, rejection did not increase, although flux declined considerably. This is considered to show that a gel layer of dextran was not formed and the flux decline was due to the osmotic pressure of dextran (8). A clear distinction between a gel layer formation mechanism and a osmotic pressure mechanism is difficult to define at this stage, but these two figures clearly show the difference of these two mechanisms. Rejections of various solutes Ovalbumin membranes, made from 800 and 5,000 ppm albumin solutions, were used to measure rejections of various molecular weight solutes, such as PEG and dextran, which are listed in Table I. Solute concentrations were kept as low as 200 ppm, so that no gel layer formation occurred. A membrane formed from 800 ppm solution had a large flux value of 30-35 10~ 3 kg/m 2 s, but observed rejections were as low as about 20% for solutes whose molecular weights were less than 100,000. A membrane from 5,000 ppm solution had about a half of the above flux but rejections were about 50-60%. These observed rejections were corrected to give true rejections by eliminating the effect of concentration polarization. The correction procedure is exlained later. Results are plotted in Figure 4, where it is found that true rejections of both membranes are almost the same. So it



3.

SHOJI ET AL.

Dynamically Formed UF Membranes

Figure 1. Experimental apparatus and a test cell.

Figure 2. Dynamically formed ovalbumin membranes; gel layers rejecting ovalbumin.


37


38


Figure 3- Dynamically formed dextran membranes; gel layers are not formed and dextran is not rejected.

Figure 4» Rejections of various solutes by dynamically formed ovalbumin membranes.


3.

SHOJI ET AL.

39


becomes clear that low observed rejections are due to the concentration polarization effect. Correction of concentration polarization effect


The experimentally observed rejection, &0bs> R, are defined as

and

tne

true

rejection,

where C-,, C2, and Co are, respectively, the concentration in the bulk, at the membrane surface and in the permeate. The relation between C-, and Co is given by Equation 3 as

where J is a volume flux. To correct the effect of concentration polarization and to obtain the true rejection from data of observed rejections using Equation 3, mass transfer coefficients, k, are needed and they can be obtained by the velocity variation method 0 0 . But results first obtained in this experiment showed a wide scattering of data, which is considered to be both due to the uncertainty of diffusion coefficients reported in literatures, and to the wide molecular weight distribution of solutes. Since the mass transfer coefficient obtained from the reverse osmosis experiment can be applicable for the ultrafiltration case also, the velocity variation method was performed by measuring NaCl rejection using a Zr(IV)-polyacrylic acid dynamic membrane formed on the same ceramic tube. This result is shown in Figure 5, where it is seen that Leveque equation can be used. The latter equation is given as

and is a typical mass transfer correlation for a laminar flow inside a tube. In the case of ultrafiltration Ng, becomes larger than the values obtained from Equation 4 for dextran. This led us to measure diffusion coefficients by a ultracentrifuge method. Results are given in Table I, where it is seen that measured values are larger than literature values. Using measured values it is proved that Leveque equation is also valid for ultrafiltration, as is shown in Figure 6. This correlation and diffusion coefficients were used for the correction of data shown in Figure 4. Effect of pH on natures of ovalbumin membranes As an isoelectric point of ovalbumin is at pH 4.5, the nature of membranes may be changed by changing pH of a solution before and after the dynamic membrane formation. Results are shown in Figure 7, where it is seen that the flux reaches minimum near the isoelectric point in both cases, although there are some



40


Figure 5« Correlation of mass transfer coefficients obtained by NaCl rejection using Zr(IV)-PAA dynamic membranes.

Figure 6. Correlation of mass transfer coefficients obtained by dextran and PEG using ovalbumin membranes.


3.

SHOJI ET AL.

discrepancies of data at large pH. at pH 5-6 is the most dense one. Table I.

This shows the membrane formed

Molecular Weight and Diffusion Coefficients of Solutes

Solute


41


Molecular weight

180 400 504

Glucose PEG #400 Raffinose PEG #1540 PEG #4000 PEG #6000 Dextran PEG #20000 Dextran Dextran Dextran Dextran

1,500 3,000 7,500 10,000 20,000 40,000 70,000 150,000 250,000

D x 10 7 (cm 2 /s)

Reference

69 45 42 23 19 14 13 10 8.0

(10) (10) (10) (10) (11)

5.8* 5.4* 6.2*

(ID (11)

(ID (11) (3.9)(12) (2,.6X12) (2.0)(12)

*) measured in this work by ultracentrifuge. Next, membranes formed at various pH values were used to measure rejections of various solutes listed above. Results are shown in Figure 8, where the general trend is similar to Figure 4. Though the membrane having a larger flux gives lower rejections and vice versa, true rejections of membranes remain the same at a very high level, and the molecular weight cut-off value is about 1000. Development of Dynaceram module Modules, which were scaled up from a single tube to multitubular ones, have been developed by TDK Electronic Co. Ltd., Japan, and named Dynaceram module. These modules were originally developed for the reverse osmosis process. But based on the above experimental data these modules were tested directly, without covering by particular dynamic membranes, for various actual effluents. Some of these results are reported here. The specifications of modules are listed in Table II, and its Table II.

Specifications of Dynaceram Modules DC 0005

Flux (m3/day kg/cm 2 ) No. of tubes Area (m )

Dimensions (mm) Max. temperature (°C) pH range Max. pressure (kg/cm )

DC 0305

DC 0505

DC 0610

0.022

0.46

3.16

6.32

1

22

151

302

0.0077 10x550

0.16 64x540

1.10 120x631

2.20 120xll00

100

100

100

100

1-12

1-12

1-12

1-12

80

80

80

80



42


Figure 7. Effect of pH before and after membrane formation. was changed during (A) and after (D) membrane formation.

Figure 8.

pH

Nature of membranes formed at different pH values.


3.

SHOJIET AL.


43

structure is shown in Figure 9. Dimensions of one ceramic tube is 5 mm O.D. and 550 mm long. DC-0305 has 22 tubes and DC-0605 has 151 tubes. DC-0610 is a combined form of two DC-0605.


Direct combination with activated sludge reactor The activated sludge process is a most important water processing technology of sewage effluents. But the key point of this process is to make microbiological floe, which can easily be flocculated. Sometimes bulking phenomena occurs and flocculation becomes difficult. To avoid such a instability of activated sludge process, application of the membrane process, instead of sedimentation, is becoming a great concern of water processing engineers recently in Japan. Following results have been obtained by combining bare Dynaceram modules with a 100 L/day activated sludge unit, which is treating effluent from a research laboratory of a food factory. Figure 10 shows a flux-time behavior of membranes, which depends on the concentration of sludge. Fluxes were very large, considering the low operating pressure of 1 kg/cm , and reached steady values after several hours. Recovery of the flux was possible by a back washing by air, but the most effective washing was performed by NaCIO treatment. An example of flux recovery is shown in Figure 11, where it is seen that about 30 min washing by 500 ppm NaCIO is sufficient. Permeate obtained contained 1300 mg/L TDS, but suspended solids (S.S.) of the feed, 6 mg/L, was completely removed. Amount of bacteria in the feed was 21,000, in which E.coli was 2200, while in the permeate bacteria count was reduced to 300, in which E.coli was 0. Treatment of effluent from bleached kraft pulp process Recently two major pulp factories in Japan installed large ultrafiltration units to process effluents from bleached kraft pulp washing processes, the pH and the temperature of which are 10-11 and over 50°C respectively. The main purpose of this process is to concentrate and recover lignin, which is further concentrated and used as fuel, while the permeate can be discarded without biological treatment. This results in the reduction of the amount of sludge. Membranes presently used are made of polysulfone. Ceramic membranes have been tested for the comparison and results are as follows. Pressure dependence of the flux and the rejection of colors are shown in Figure 12. The particular feature in this case is that flux can be recovered easily by backwashing. Flux did not depend on the concentration ratio up to the value of 5. COD rejection was about 80%, color rejection was about 90%, over 5fold concentration. Conclusion Natures of dynamically formed ultrafiltration membranes were investigated using a ceramic support tube and ovalbumin as a solute. This membrane has intrinsically high rejections, but due to the concentration polarization the observed rejection is low.



44


Figure 9.

Dynaceram module DC-0610.

Figure 10. Direct ultrafiltration of activated sludge effluents of different MLSS concentrations. Pressure, 1 atm; and flow rate, 1 1/min.



3. SHOJIET AL.


45

Figure 11. Recovery of flux by washing with NaClO. Washing conditions: NaClO, 490 ppm; pressure, 1 atm; flow rate, 4 1/min; and temperature, 16-17 °C.

Figure 12. Flux and color removal of effluent from bleached kraft pulp plant. A membrane was formed at 5 atm. Feed velocity, 1 m/s; and temperature, 40 °C.


46


Examples of direct applications of bare Dynaceram modules, developed by TDK based on the above results, were also reported. Nomenclature


o

C D d Jy k 1 N Re NSc NSh u v

= = = = = =

solute concentration (g/cm ) diffusion coefficient (cm/s) equivalent diameter (cm) volume flux (cm /cm s) mass transfer coefficient (cm/s) membrane length (cm) = ud v / = v/D = kd/v = flow velocity (cm/s) = kinetic viscosity ( c m / s )

Acknowledgment Authors are grateful to Mr. Shoichi Wakabayashi, TDK Electronic Co. Ltd. for supplying us their modules and permitting us to publish some of his data. Literature Cited 1. Marcinkowsky, A.E.; Kraus, K.A.; Phillips, O.H.; Johnson, J.S.; Shor, A.J. J. Am. Chem. Soc. 1966, 88, 5744. 2. Johnson, J.S.; Minturn, R.E.; Wadia, P. J. Electroanaly. Chem. 1972, 37, 267. 3. Thomas, D.G. In "Reverse Osmosis and Synthetic Membranes"; Sourirajan, S., Ed.; National Research Council of Canada: Ottawa, 1977; Chap. 14. 4. Brandon, C.A.; Gaddis, J.L.; Spencer, H.G. In "Synthetic Membranes vol. II"; Turbak, A.F., Ed.; ACS SYMPOSIUM SERIES No. 154, American Chemical Society: Washington, D.C., 1981; pp. 435-453. 5. Groves, G.R.; Buckley, C.A.; Cox, J.M.; Kirk, A.; MacMillan, C.D.; Simpson, M.J. Desalination 1983, 47, 305-12. 6. Nomura, T.; Kimura, S. Desalination 1980, 32, 57-63. 7. Kimura, S., Nomura, T. Desalination 1981, 38, 373-382. 8. Jonsson, G.; Christensen, P.M. Paper presented at Europe-Japan Joint Congress on Membranes and Membrane Processes, June 18-22, 1984, Stresa, Italy. 9. Nakao, S., Kimura, S. J. Chem. Eng. Japan 1981, 14, 32-37. 10. Brandrup, J.; Immergut, E.H., Ed. "Polymer Handbook" 1978. 11. Sherwood, T.K., Pigford, R.L.; Wilke, C.R. "Mass Transfer" McGraw-Hill, 1975. 12. Ohta, K.; Yamamoto, H.; Kawahara, K. Poly. Preprints, Japan 1976, 25, 1449. RECEIVED February 22, 1985


Reverse Osmosis and Ultrafiltration - American Chemical Society

Recommend Documents