Molecular (osmotic and electro-osmotic) backwash of cellulose acetate hyperfiltration membranes

Journal of Membrane Science, 8 (1981) 173-192 173

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

MOLECULAR (OSMOTIC AND ELECTRO-OSMOTIC) BACKWASH OF CELLULOSE ACETATE HYPERFILTRATION MEMBRANES

K.S. SPIEGLER* and J.H. MACLEISH**

Sea-Water Conversion Laboratory, University of California-Berkeley (U.S.A.)

(Received August 4, 1980; accepted October 2, 1980)

Summary

A technique using osmosis and/or electro-osmosis was developed to clean and possibly decompact contaminated modified (asymmetric) cellulose acetate membranes.

The rejuvenation technique developed is called “molecular backwash”. When contaminated with ferric hydroxide, the membranes (cured at 92°C) exhibited considerable flux decrease. After molecular backwashing, the hyperfiltration flux increased again. In some cases, molecular backwash also partially restored the salt rejection loss which had occurred during contamination. Flux loss recovered by molecular backwashing varied from 30% to over 100% (i.e. the flux of the compacted membrane was greater than before contamination, but not greater than the flux of a clean uncompacted membrane).

-.

Introduction

In desalting plants using membranes, mass-transfer bottlenecks often develop at, or near, the membrane/solution interfaces. These regions fre- quently get contaminated by adsorption, deposition of impurities, or both. Even without this type of contamination, interfacial polarization limits the production rate of the desalting processes utilizing membranes. If, in addi- tion, contamination is present, the interfacial region develops substantial resistance to mass transfer.

Considering that macroscopic filters need periodic macroscopic backwash to restore their original flux (the backwash periods being short compared to the filtration periods) it is reasonable to examine what molecular backwash does for selective membranes which act as molecular filters. It is known that in some cases substantial improvement of membrane performance has resulted from temporary reversal of mass flow through the membrane, but few systematic studies on this subject are available. The “molecular- backwash” technique described here uses osmotic and/or electric forces in

*Present address: Department of Mechanical Engineering, University of California- Berkeley 94720, and Michigan Technological University, Houghton 49931, U.S.A. **Present address: Cordis-Dow Corp., Concord, California 94520, U.S.A.

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an attempt to reduce the mass-transfer resistance at the membrane/feed sur- face. It is quite possible that, simultaneously, the bulk permeabilities of the membranes and/or their backings and supports, are also improved due to re- version of the conformational and geometrical changes brought about by compaction in reverse osmosis.

The work described here developed from previous fundamental work on streaming potentials across membranes [l-4]. In the course of these past experiments it was found that certain modified cellulose acetate membranes, which had often been considered as electroneutral materials, exhibited streaming potentials. The latter are phenomenologically defined as electrical potential differences across these membranes which appear when a pressure is applied to one of two identical homogeneous solutions separated by the membrane. As a result of the pressure difference, liquid flows through the membrane. The following mechanism is believed to cause streaming potentials: When water flows across the membrane, it interacts with mobile ions in the membrane, thus displacing these ions by drag forces in the direction of the water flow. If there are fixed negative ions, and hence, on account of electroneutrality, more mobile positive than negative charge carriers in the membrane, this drag effect causes the low-pressure solution to have a more positive potential than the high-pressure solution. The polarity of the streaming potential is reversed when there are fixed positive ions, but, in principle, streaming potentials can also arise from mobility differences of equal concentrations of mobile cations and anions, i.e. in the absence of fixed charges.

The existence of these streaming potentials implies that there are strong flow interactions between the water and the ions which permeate the membrane. The mutual drag between water and mobile ions causes a streaming potential upon application of a pressure difference across the membrane. Hence, it was reasoned that when reverse osmosis is interrupted and an electrical potential difference applied across the membrane, the resulting electro- migration of counterions across the membrane would cause a flow of water in the same direction. In fact it can be calculated from the measured streaming potentials (by use of Saxen’s law) that each counterion can drag 50-80 water molecules along (as shown later in Theory section 2), the exact amount depending on the solution concentrations and the membrane properties. (Only a small portion of this water is “hydration water” in the sense of equilibrium thermodynamics; the rest is water transported by electro-osmotic drag.) Our preliminary experiments confirmed this large magnitude of the electro-osmotic flow. Therefore it stands to reason that modified cellulose acetate membranes can be effectively backwashed by ap- plying an electrical potential difference across them, and it is possible that this treatment will prove more useful than trying to pass water across them in the direction opposite to hyperfiltration (e.g. in a tubular plant [5,6] from the outside of the tube to the inside), a procedure which requires re- design and strengthening of certain components of existing reverse-osmosis plants, and which can also damage the membrane.

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Because of the necessity to pass electric current - albeit perhaps only for short periods -- a number of additional elements are also required in a reverse-osmosis plant using eEectro-osmotic backwash, but it is quite possible that existing metallic structural components themselves will not have to be appreciably modified and they could at least partially make up the required electric network. Less modification would be required for osmotic backwash. Knowledge on osmotic backwash under realistic hydraulic conditions is scarce. While it is known that periodic release of pressure in reverse- osmosis plants temporarily restores fresh water product fluxes which had gradually declined in the course of the operation of the plant [ 71, no systematic in situ studies under well defined hydrodynamic conditions were found in the literature. Since osmotic backwash is possible even in hollow- fiber units, while electro-osmotic backwash seems primarily suitable for tubular, and perhaps also for spiral-wound units, it is worthwhile to systematically study the use of both molecular backwash procedures. Moreover, knowledge on osmotic backwash is often essential for quantita- tive interpretation of data on electro-osmotic backwash, because the two flow processes occur simultaneously when the feed solution is more con- centrated than the hyperfiltrate. This is usually the case in the normal operation of hyperfiltration and ultrafiltration plants. Electra-osmotic backwash of modified cellulose acetate membranes is, in principle, faster than osmotic backwash, because even a moderate applied voltage (about 2 volts) is equivalent, in terms of water flow, to fairly large osmotic pressures (order of magnitude 100 atmospheres).

Theory

1. Forces and fluxes for molecular backwash It is well known IS-10 ] that the flow through a hyperfiltration mem-

brane can be described by

(J”)i=e = -L,(Ap - uA~) (1)

where J,, is the volume flux (cm3/cm2-set = cm/set), L, the hydraulic permeability of the membrane (cm/set-MPa), Ap the hydraulic pressure difference across the membrane (MPa)*, CJ the reflection coefficient of the membrane, and An the difference between the osmotic pressures of the solutions on the low and high-pressure faces of the membranes, respectively.

It is seen that the driving force for hyperfiltration flow is -(Ap - oAn). The orientation of the membrane with respect to the co-ordinate system is shown in Fig. 1. The dense active layer of the membrane faces to the left.

*l dekabar = 10 bar = 1 Mpascal (MPa) = 9.87 atm. This unit is recommended for dis- cussions of pressure-driven membrane processes, because the applied and osmotic pressures are usually between 0.1 and 10 MPa.

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Negative electrode

Asymetric cellulose acetate hyperfiltration membrane

Positive electrode

Fig. 1. Orientation of asymmetric cellulose acetate membrane with respect to electrodes and coordinate system. A = Quantity on right minus quantity on left. Hyperfiltration pressure difference, Ap, is negative. Hyperfiltration flux is positive. Cation and current flow are negative [12]. By electrical-engineering convention, the direction of the electric current is the direction of the flow of positive charges, i.e. opposite to the flow of electrons.

Transfer of any species from left to right is considered positive; therefore, hyperfiltration fluxes are positive and hyperfiltration pressure differences are negative. The sign of A is defined in the list of Symbols.

The osmotic driving force which increases with the concentration polarization [6] opposes the hydraulic pressure difference across the membrane. The effects of concentration polarization can be lessened somewhat by periodic backwash of the membrane. If this backwash were to be done by directly passing water through the membrane in the direction opposite to the hyperfiltration flow, membrane damage could readily result. (Hydraulic backwash did prove possible for hollow-fiber ultrafiltration membranes which have a smaller diameter than regular tubular hyperfiltration membranes [ll].) While osmotic flow can be used for molecular backwash, electro-osmotic flow can, in principle, often cause relatively rapid molecular backwash of the membrane as shown in the Results and Discussion section.

When an electric current is passed across a membrane separating solutions of the same concentrations, and no pressure applied, the volume flux is

&)Ap=o, AC=0 = pi (2)

where /3 (cm3/A-set) is the electro-osmotic coefficient [8] and i (A/cm*) is the density of the electric current across a membrane separating identical solutions.

When the volume flux is caused by the simultaneous action of pressure and electric current, the two influences are additive, i.e. the total volume flux is obtained by adding eqns. (1) and (2). When the solution concentrations are different, this additivity represents merely an approximation, because the ion and water concentration profiles across the membrane are not the same when (a) pressure only and (b) electric current only are applied. In the interpretation of the results of the present work this approximation has been made, however, i.e. the total volume flux was expressed by

Jv = -L,(Ap - oAn) + pi (3)

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When there is no electric curent, the flux through the membrane is pure- ly osmotic. The osmotic pressure difference across the membrane, Anf, can, in principle, be adjusted at will by changing the concentrations of the flush ing solution (at the low-pressure face) or of the feed solution (at the high- pressure face). During our experiments the osmotic pressure difference was held constant. In this case the volume flux is designated by J,:

J UC = -L,(Ap - oA7rf) for i = 0 (4)

(This flux is not equal to the flux, JvO, occurring under normal plant conditions when there is no flushing on the low-pressure face, i.e. when the concentration of the hyperfiltrate is allowed to reach its own level.) Subscript f stands for “flushing of both membrane faces”.

The electro-osmotic flux through the membrane, Jve (cm/set) is the difference between the observed flux, Jv, and the osmotic flux, J,, which would occur in the absence of electric current. If the same concentration difference between low-pressure and high-pressure solutions is maintained as in the absence of electric current, then:

Jve = Jv - JvC = {[-LP(Ap - oA7if)i=O + pfi] - [-L,(Ap - oA~f)]+o}= pfi

(5)

The electro-osmotic coefficient, pf, depends on the nature and concentrations, c, of the solutions. If the interfacial solutions are different, as in the experiments described here, /3f may be expected to differ somewhat from the electro-osmotic coefficient measured at equal solution concentrations. It should be noted that the concentration at the high-pressure solution/membrane interface is somewhat different from the concentration of the bulk high-pressure solution because of polarization.

2. Relationship between electro-osmosis and streaming potential (Saxkn’s law)

Figure 2a schematically represents hyperfiltration through a membrane with fixed charges. There exists a relationship between electro-osmotic flow and streaming potential. This is “Saxen’s relation”:

Ap=O,Ac=O i=O,Ac=O

where AE is the electric potential difference across the membrane and c the concentration (mol/cm3) [1,10,13].

When streaming potentials across modified cellulose acetate membranes (cured at 94°C) were measured [4,14], some dependence of these potentials on the concentration of the feed solution was found. For a feed concentra-

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Semipermeable &

membrane

::::::: CONCENTRATE

y&later _=+_,--_1 :::. :::.

I I Semipermeable membrane

(a)

(b)

Fig. 2. Comparison between streaming potential and electro-osmosis measurements [ 1,121. (a) Reverse osmosis. Water forced through membrane by pressure difference drags Na+ through membrane causing steaming potential. (b) Electra-osmosis. NaC forced through membrane by applied electric field drags water through membrane.

tion of 0.0098 X lop6 mole/cm 3, the value of the observed streaming potential differential was

(AE/Ap) = -6.8 mV/(lOO psi) = -0.99 X lob2 V/MPa* .

From Saxkn’s Law, eqn. (6), one would predict an electro-osmotic flow of

[Jve/i]ap=~,ac=~ = 0.99 X 10F2 V/MPa = 0.99 X 10U2 V/(watt-set-cm3)

because 1 MPa-cm3 = 1 watt-sec.

= 0.99 X 10m2 cm3/C **

*In this case the hyperfiltrate concentration was 5.3 X lo-’ mole/cm3. **The signs conform with the situation shown in Fig. 1.

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Therefore, at a current density of (-2) X 10m3 A/cm’, the expected electro-osmotic transport is

J ve = 0.99 X lo-’ cm3/C X (-2) X lob3 C/set-cm2 =

= -1.98 X lOA5 cm/set ,

while -1.97 X 10m5 cm/set was measured by the method described later in Experimental Part 3. This agreement is good, but discrepancies between theory and measurement were observed at higher feed-solution concentrations.

It is of interest to interpret these values in terms of the amount of water dragged electro-osmotically by each ion. The measured electro-osmotic flow corresponds to

0.99 X lo-’ cm3/C X (l/18) mole water/cm3 X 0.96 X lo5 C/eq =

= 52.8 mole water/eq

This is quite a large amount of water dragged electro-osmotically per equivalent of electric current (Faraday). Moreover, this modified cellulose acetate membrane is a cation exchanger when in contact with the very dilute NaCl solution (about 10e2 M) used [3,4,15,16]. Most of the electric current is carried through the membrane by the mobile cations; hence it seems that each sodium ion drags several dozen water molecules with it as it electro- migrates through the membrane, provided the sodium chloride solution containing this membrane on the high pressure face is very dilute.

Experimental

1. Basic apparatus The apparatus was a modification of the device used for streaming poten-

tial measurements as described in detail previously [4,17]. Briefly, pressurized feed solution is pumped through an annulus, the outerface of which is a modified cellulose acetate membrane [ 183. The hyperfiltrate flows in a porous support on the low-pressure side of the membrane and is collected. This thin porous membrane support rests on an electrically conductive tube which is used as an electrode. The other electrode is an electrically conductive rod forming the center of the annulus through which the solution flows. Some minor modifications to the basic apparatus were described by MacLeish [12].

The major difference between the measurements described here and previous work is the design and use of a membrane test section which made it possible to flush both faces of the membrane, and to use high current densities in backwashing for extended periods of time. In previous experiments it was not possible to effectively flush the low-pressure side of the membrane, or use high current densities for more than lo-15 minutes. These features were essential, because in the experiments described here,

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electric current was passed through the system for periods long enough to create non-negligible amounts of electrode products (including, often gases) which would have interfered with the measurements unless flushed away. This is not to say that in practical application of molecular backwash, the periods of electric backwash would have to be so long as to necessitate flushing of the low-pressure face of the membrane, but the flushing feature and the concomitant possibilities for use of relatively high current densities were considered desirable for the relatively long backwash periods (several tens of minutes) used in this research.

2. Membrane test section The membrane test section shown in Fig. 3, was 25.4 cm long with outer

diameter 4.44 cm. It was designed to allow control of both electrolyte concentration and fluid flow rates on both sides of the membrane, and passage of an electric current through the membrane. Control of flow rates and electrolyte concentration on the low-pressure side was accomplished through a flushable spacer support and auxiliary hydraulic system. Current was passed by means of two electrodes; one tubular electrode around the outside of the membrane, and a “high-pressure” electrode of diameter 1.27 cm centered in the brine flow channel.

Figure 3 is a cutaway view of the test section, membrane, and high- pressure electrode (spacer support and flare nuts not shown), and Fig. 4 is a cross-sectional view. Details of the construction and the fluid-distribution

Membrane diameter in flushed section Entrance section connector

after pressurizing 2 42 cm I?let-outlet manifolds ( Plastic) Membrane thick”e5s 0.045 err tot?. memDrdne batring tmck”e55 :’

for flushing fluld (PVC) ‘?

0175 cm t before pressurizingI. Rubber gasket-,, \ Flare nut (stn steel)

High-pressure electrode, ,’ ElectrIcal contact diameter 1 27cm for low-pressure Flare sleeve

\ , i , , ,

-1 119

ET

O-Ring sea!-

Membrane

Electrode Spacer ( plaStIcI

x, Membrane backing (porous plastic mesh)

” Membrane backing (polyester cloth)

i

108

7

Fig. 3. Dimensions of molecular-backwash test section [ 121. Note: Dimensions in cm. Left flare nut and entrance section are not shown.

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Flushing-fluld

inlet

StaInless-steel outer tube

Annular flow channel for saline feed

H!gh-pressure

I I _ = _ _, ,._I,

-Low-pressure elect rode (graphite)

Electrvxl connection /

to graphite electrode I Cellulose acetate membrane

Fig. 4. Cross-section of membrane test section [12].

system which serves to flush the low-pressure face of the membrane are pre- sented in the original thesis [12], which also specifies the preparation of a permeable membrane backing and a method for the insertion of the membrane and its backing into the test section. This backing consists of several layers of porous cloth (100% polyester lining fabric, “Zephyr”, Skinner Fabric Co., New York, N.Y.) and several layers of “Spacer-S” electrodialysis spacer mesh (As&i-Dow Co., Tokyo)*.

3. Measurement of electro-osmotic and/or osmotic flow The equipment for studying the osmotic and electro-osmotic flux con-

sisted of a system to flush the low-pressure face of the membrane and the graphite electrode with a suitable electrolyte, and measuring devices to determine the volume of fluid passing through the membrane. The flushing system included the porous backing of the membrane (Fig. 4). The flushing fluid was pumped from a 4-liter capacity polyethylene bottle through the support to a similar bottle. Both bottles were simultaneously weighed (Fig. 5). One bottle was placed on the balance pan (6-kg capacity, Mettler P-5N, Mettler Instruments, Princeton, New Jersey), and the second was hung from a weighing hook which is accessible from the bottom of the balance. The suspension system for the hanging reservoir was a combination

*The authors thank Dr. Y. Tsunoda, President, Asahi-Dow Co., Ltd., Tokyo (Japan) for the gift of the spacer material.

F Three-way valve

- Direction of flow

Membrane test sectvan

Fig. 5. Schematic of system for measurement of electro-osmotic flow [12].

of a three-point and one-point suspension so that the bottle always hung straight.

The pump was a Model 955 syringe metering pump (Harvard. Apparatus Co., Inc., Millis, Massachusetts). To meet the requirements of our system, two modifications had to be made to the pump. Firstly, the flexible tubing, clamp valves, and glass pumping syringes supplied as original equipment were replaced by stainless-steel tubing and “Swage-Lok” connectors (Crawford Fitting Co., Solon, Ohio), “Skinner” electric valves, and 100 ml syringes with stainless steel barrels and “Teflon” plungers, respectively. This is a standard modification done by the Harvard Apparatus Company which converts the unit to “high pressure use” (200 psi). The second modification was to replace the pinion assembly supplied as original equipment by a different one (Cat. No. 600-078, also Harvard Apparatus Co., Inc.). This had the effect of increasing the maximum flow rate from 120 cm3/min to 225 cm3/min when 100 ml syringes were used.

Precision-bore “Pyrex” glass tubing (3.2 mm i.d. X 6.4 mm o.d.) was used to withdraw and return the flushing fluid from and to the bottles. The glass tubing extended 3.8-5.1 cm out of the tops of the bottles and was bent at a 90” angle. The short arm was connected to “Tygon” tubing which carried the fluid to the pump, through the membrane backing, and back through the return precision-bore tube to the other reservoir (Fig. 5). In ad- dition to the two bottles which were weighed, there is a reservoir which is not part of the weighing system and was used when the system was brought to a steady state in preparation for measurements. Figure 5 also shows the

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valving system, which provides for a recirculation mode in which the flushing fluid is returned to the reservoir. It was also possible to reverse the direction of flushing flow in the spacer-support, and to this end the flushing-flow system was connected to the test section by “Swage-Lok” connectors.

One of two power supplies was used to provide electric current for electro-osmotic backwashing, depending on the current and voltage ranges required. For currents less than 1 ampere, a regulated D.C. power supply (Mod. No. CIC40-0.8m, KEPCO, Flushing, New York, O-O.8 A) was used in the current regulation mode. For currents from 1.0 to 4.0 A, the power supply was KEPCO Model No. PR-155-4m, O-155 V, with manual current regulation. A variable resistor (Rex Rheostat, Roosevelt, New York), 2.6 ohm, 14.5 A, was wired in series between the negative output connector of the power supply and the central electrode of the test section. This resistor maintained the current output of the voltage-regulated power supply fairly stable. The total current was read from the ammeters built into these power supplies.

The flushing fluid for osmotic backwash was deionized water. In order to prevent oxygen-bubble formation and excessive pH changes at the anode (usually the low-pressure electrode), sodium sulfite and sodium acetate were added to the flushing fluid for the electro-osmotic backwash.

The “active” area of the membrane was calculated from the length (16.68 cm) and the internal radius of the membrane backing after expansion under pressure (1.21 cm). In the flux calculations the “active” area of the membrane was accordingly taken as 127 cmz.

4. Effect of molecular backwash on artificially contaminated membranes In the contamination phase of these experiments the feed was subjected

to a hyperfiltration pressure of 600 psi. First the flux of the solution through the membrane was measured until the initial rapid flux decline ceased and a quasi plateau was reached. This flux was taken as the precon- tamination base line. During this time a high-pressure (five micron) filter and an ion-exchange column were on line (normal mode) [17]. Just before the contaminant was introduced to the brine tank, a new bypass line, which had been added to the basic circuit [4], was opened, isolating and re- moving the filter and ion-exchange column from the flow system. The contaminant was prepared by mixing one flask of FeCl, solution and one flask of NaOH soWion. In most experiments one liter each of solutions of 30 g FeCl,-6H,O per liter and 13.3 g NaOH per liter were used [19]. The con- tents of the two flasks were emptied simultaneously into the brine feed tank. (For membrane No. 8, the FeCl, and NaOH solutions were mixed in a flask and added to the brine tank). The pH was adjusted to between 4.5 and 5.5. Tests with ammonium thiocyanate and potassium ferricyanide solutions showed that, at this stage, no appreciable amounts of free ferric and ferrous ions respectively were in the solution. The hyperfiltration flux was

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measured at regular intervals to determine the effect of contamination. Samples of the hyperfiltration flux were taken in order to determine the salt rejection. The flux was observed to decline to a constant value, at which time backwashing experiments were begun. Four types of backwashing experiments were made:

(a) Osmotic backwash with no flushing. The hyperfiltration pressure was lowered to 5-30 psi. (Since there was no flushing, the flux due to osmosis was not a steady-state flux). After this treatment, the hyperfiltration pressure was raised to 600 psi and the hyperfiltration rate measured again.

(b) Osmotic backwash with flushing. The hyperfiltration pressure was lowered to 5-30 psi, and the low-pressure face of the membrane flushed with deionized water. After this treatment, the hyperfiltration pressure was raised to 600 psi and the hyperfiltration rate measured again.

(c, d) Electra-osmotic backwash with and without flushing. The procedure was similar to (a) and (b) respectively, except that in these cases, electric current was passed through the membrane for 20-30 minutes, the positive current flowing from the external to the internal electrode. After this treatment (at 5-30 psi), the hyperfiltration pressure was measured again. A typical electric current density was -32 mA/cm* .

In these experiments with contaminated membranes, the flow rate through the membrane was not measured while molecular backwash was being performed, but the hyperfiltration rates and salt rejection before and after the backwash were closely monitored. The rejection was determined by argentometric titration.

The membranes used in the molecular backwash experiments had been slightly expanded (at 30 psi) during pressurization. This caused the salt rejection to drop from a nominal 99% to about 80%. This effect had also been observed by Loeb [18] who attributed it to a modification of the microstructure of the membrane. After this expansion, the membranes used for the experiments described in Figs, 8 and 9 were recured with slightly pressurized hot water, so as to restore their high salt rejection. The salt rejection of a membrane recured at about 92°C was found to be 0.94 for an 0.024 M NaCl feed.

It should be noted that the test section for this work was designed primarily, but not exclusively, to produce molecular backwash, and to determine if this backwash would subsequently improve either the hyperfiltration flux or the salt rejection of contaminated cellulose acetate membranes, or both. The fluxes associated with an osmotic or electric driving force were not measured with the usual restrictions inherent in the definition of osmotic and electro-osmotic flux. In particular, the osmotic flux is usually measured with zero pressure difference across the membrane and no electric current; the electro-osmotic flux is usually measured with zero pressure difference and zero concentration difference across the membrane. In all osmosis and electro-osmosis measurements done in this work, however, there were both a pressure and concentration difference across the mem-

185

I I I I I I I

Osmotic backwash Electra-osmotic backwash

\

18

‘. _-__ I

--Temporary pressure- loss to 100 p51

I

\

\ I

56- ----.--- 54- 5.2 -

x 5.0-

;,

0 I I I I I 1 I I I

0 IO 20 30 40 50 60 70 00 90

Time from start of contamtnatlon of membrane (hour)

Fig, 6. Effects of iron hydroxide contamination and molecular backwash on hyperfiltration flux (0.1 M NaCI). Feed brine: 0.1 M NaCl. Ap = -600 psi. Temperature 25°C. Contaminant: Ferric hydroxide, pH 5.5. Flushing fluid on low-pressure side of membrane (for electro-osmotic backwash): (0.1 M Na,SO, + 0.15 M Na acetate)/liter. Flushing fluid for osmotic backwash: deionized water. Hyperfiltration pressure during molecular backwash: Ap = 30 psi. (a) osmotic backwash duration: 35 min, (b) electro- osmotic backwash duration: 15 min. Current density: 40 mA/cmZ. Membrane No. 6, modified cellulose acetate. Membrane curing temperature: 92°C [ 121.

HOUr5 from stat? Of COntamlnatlOn

7. Effect of iron hydroxide contamination and molecular-backwash on hyperfiltration flux (0.03 M NaCl). Feed brine: 0.03 M NaCl (0.01 M NaCl for first 25 hours). Ap = -600 psi, Temperature 25°C. Contaminant: ferric hydroxide, pH 5.5. Flushing fluid on low-pressure side of membrane (for electro-osmotic backwash): (0.03 M Na,SO, + 0.045 M Na acetate)/liter. Hyperfiltration pressure during molecular backwash: Ap = -7 p.s.i. (except experiment (e)). ( a molecular backwash duration: 40 min, (b-e) molec- ) ular backwash duration: 30 min, (f) molecular backwash duration: 25 min. Membrane No. 7, modified cellulose acetate. Membrane curing temperature: 92°C [ 121. AU electro- osmotic backwashes, except (e), with flushing.

0 1 2 3 4 5 6 7 8 9 10

Time from first measurement (days)

Fig. 8. Effect of pressure, contamination, and molecular backwash on hyperfiltration flux (0.03 M NaCl). Feed brine: 0.03 M NaCI. Ap = -600 psi. Molecular-backwash duration: 25 min. Contaminant: ferric hydroxide, pH 5.5. Flushing fluid for electro-osmotic backwash: (0.03 M Na,SO, + 0.045 M Na acetate)/liter. Flushing fluid for osmotic backwash: deionized water. Hyperfiltration pressure during molecular backwash: Ap = -7 psi. Temperature: 25°C. Membrane No. 9, modified cellulose acetate. Membrane curing temperature: 92°C [12].

Lowered pressure to 4 PSI Brine concentration raised to 0.03 M NaCl

Feed brine 0.024 M NaCl

” 5

All Value5 of hyperfiltratlon flux measured at Ap: -6OOps1

0 n I

V- 4 0.6

04 1 i 02 0 11 ’ 11 11 1 11 1 I I 1 11 I I / 1

0123456789 10 11 12 13 14 15 16 17 18 19 20

Time from first measurement (days)

9. Effect of pressure variation, contamination and osmotic backwash on hyperfiltra- tron. All values of hypertiltration flux, J,,, measured at Ap = -600 psi. Feed brine: 0.024 and 0.03 M NaCl. Contaminant: Fe(OH),. pH 5.5. Flushing fluid for osmotic backwash: deionized water. Temperature: 25°C. Membrane No. 8 (with flush); modified cellulose acetate; during temperature: 94°C; recuring temperature after being seated: 94°C WI.

187

brane. The pressure difference was required to assure that the membrane remained properly seated against the membrane backing when the backing was being flushed, and the concentration difference was determined by the requirements of the flushing solution for the outer electrode.

In all experiments, the temperature was maintained at 25 + 0.4”C. The circulation rate of the feed solution was 25 cm3/sec which corresponds to a (superficial) flow velocity of 7.56 cm/set in the test section*. The alkalinity of the recirculating brine feed (due to the anode reaction) was periodically neutralized with a solution of hydrochloric acid, so as to minimize membrane hydrolysis which occurs at high pH.

Results and discussion

Figures 6, 7 and 8 show the hyperfiltration flux of membranes which were contaminated with iron hydroxide and then decontaminated by molecular backwash procedures. The flows shown in these figures were measured at a hyperfiltration pressure difference of Ap = -600 psi (i.e. the pressure of the brine inside the cylindrical tube was 600 psi higher than the atmo- spheric pressure). Certain observations can be made:

(1) Molecular backwashing of contaminated cellulose acetate membranes improved both the hyperfiltration flux and in some cases the salt rejection,

(2) The electro-osmotic backwash had a more beneficial effect on flux and salt rejection than did osmotic backwash.

Figure 6 shows both osmotic and electro-osmotic backwash experiments for a feed brine of 0.1 M NaCl containing as contaminant 11.97 g/l of Fe(OH)3 in suspension. After the contaminant was added, the flux declined 14.5% in 20 hours. The osmotic backwash recovered one half of the flux loss caused by contamination. There was a time span during which no data were taken (65 hours), and a period in which there was a pressure loss (pump shut down). After 88 and 90 hours the hyperfiltration flux was measured and found to have dropped 9% in the two hours after the pressure was raised from 100 psi back to 600 psi. The electro-osmotic backwash restored 73% of the flux lost due to contamination. In these two experiments the osmotic and electro-osmotic backwashes both brought the flux to the same absolute value, but the electric backwash caused a larger percentage of recovery.

Figure 7 shows the results of several backwashing experiments, The contaminant was 24 g Fe(OH), per liter in suspension.in the feed brine. The brine concentration was 0.01 M NaCl for the first 25 hours of the experiment. Since the flux decline was faster in the earlier experiment with a brine coni3entration of 0.1 M NaCl (Fig. 6), it was decided to increase the brine concentration to determine if this would increase the rate of flux decline. Increasing the brine concentration from 0.01 M to 0.03 M NaCl indeed in-

*lo% stroke setting of the feed pump.

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creased the rate at which the hyperfiltration flux decreased. For backwash (a) and (c) in Fig. ‘7, there was no flushing on the low-pressure side of the membrane. In step (a) the pressure was dropped to 30 psi for 40 minutes and in (c) to 5 psi for 30 minutes. Since the salt rejection is very low at low pressures, salt was expected to pass through the membrane and equal- ize the salt concentration on the two sides of the membrane. Thus with no flushing, a transient, diminishing flow through the membrane was expected since the osmotic pressure difference decreased. Step (b) shows osmotic backwash with continuous flushing. The flow rate of the flushing fluid (deionized water) on the low-pressure side of the membrane was 225 cm3/min. At this flow rate, the salt concentration of the salt leaking into the flushing fluid on the low-pressure side of the membrane remained low. Step (b) (osmotic backwash with flushing) shows a greater improvement in hyperfiltration flux than either (a) or (c), for which there was no flushing. In steps (d), (e) and (f) the membrane was electro-osmotically backwashed. (d) was performed at 19.7 mA/cm’ and (f) at -32 mA/cm2. (In the electric backwash indicated by (e) in which the brine pressure was maintained at 600 psi, flushing of the low-pressure electrode was not possible; hence gas evolution at this electrode took place and the electric current density through the membrane was probably not uniform.). The hyperfiltration flux increased to about the same value for both experiments but started from a lower value in step (f).

The lower part of Fig. 7 shows the beneficial effects of the molecular backwash process on the salt rejection. The Figure does not show the results of a follow-up experiment in which the membrane-test section was first removed from the hyperfiltration plant and cleaned with cotton swabs. The hyperfiltration system was drained and cleaned with citric acid, and new brine was introduced. The membrane was stored for 20 hours in deionized water while the system was cleaned. The membrane was put back into the hyperfiltration plant, and hyperfiltration with contaminated brine restarted. After 170 hours the flux was 8.57 X 10m4 cm/set; after 171 hours it had dropped to 8.08 X 10m4 cm/set. It is not known how much of the increase was due to decompaction and how much to cleaning the membrane. This rate of flux decrease was higher than usually found during the contamination experiments.

Figure 8 describes a series of contamination-decontamination experiments performed ‘with a membrane for which the salt rejection was 86% at 600 psi (0.03 M NaCl feed solution). The effects of both osmotic and electro-osmotic backwash are evident.

Figure 9 shows additional experiments on the effect of periodic pressure release on a membrane which was artificially contaminated only during the last hours of the experiment. This membrane was recured for fifteen minutes at 94°C after its expansion in the seating procedure. In hyperfiltration experiments with uncontaminated 0.024 M NaCl circulating brine on the high-pressure face and no flushing on the low-pressure face, the salt re-

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jection was 94% at 600 psi. The membrane was subject to three different pressure-decrease episodes as shown in the Figure. On day one, the hyperfiltration pressure was lowered from 600 to 4 psi for 20 hours. After this depressurization the hyperfiltration flux (measured at 600 psi) rose by 11%. On day fifteen, the hyperfiltration pressure was lowered from 600 psi to 10 psi for 30 minutes. After depressurization, the hyperfiltration flux (measured at 100 psi) increased by 12%. On day sixteen, the pressure was temporarily released for the third time, conditions were the same as for the experiment on day fifteen, and the flux increased by 7%. The backwash on day eighteen included an osmotic backwash of 30 minutes with flushing as part of depressurization of 45 minutes. During the osmotic backwash, the flushing fluid on the low-pressure face was deionized water. The flux increase was 34%, or about three times the maximum percentage increase due to decompaction alone. Because the membrane was inadvertently damaged, it was not possible to perform an electro-osmotic backwash afterwards.

In the course of the backwash experiments, it was found that high pressure operation. decreased the permeability of the flushable spacer backing. Before high-pressure operation (600 psi) at a hyperfiltration pressure of 30 psi and a flushing flow of 225 ml/min, the flushing pressure was 3-4 psi. After operating for 17 days at high pressure (600 psi) and then lowering the hyperfiltration pressure to 10 psi it was found that the flushing pressure necessary to maintain the same flushing flow had increased to 8-9 psi. In electric backwashing experiments after high-pressure operation, some gas bubbles could be seen when the flushing fluid emerged from the low- pressure electrode area. Compaction of the spacer support possibly caused some channeling, resulting in a portion of the electrode being unevenly flushed.

Comparison of the driving forces for osmotic and electro-osmotic backwash The generalized forces for osmosis and electro-osmosis are -A/+ and

-FAE respectively, where Ap, is the difference of chemical potential of water across the membrane (watt-see/mole), AE the electrical potential difference (V) and F is Faraday’s constant (0.965 X lo5 C/es). (The sign con- ventions for driving forces and fluxes are consistent with Fig. 1; positive potential gradients (which are negative driving forces) cause negative fluxes) [201.

The total chemical potential difference, ApcLw, is composed of a concentration-dependent term and a pressure-dependent term respectively

-A/J, = -A&; - iwAp (7)

where & is the average partial molar volume of water (cm3 /mole) [8,21]. During the osmotic backwash, Ap = 0. We calculate A$, from the osmot-

ic pressure difference, An, of the aqueous media separated by the mem-

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brane. The approximate osmotic pressure difference, An, between pure water on the hyperfiltrate side of the membrane and a 0.03 M NaCl solution on the feed side is calculated from van ‘t Hoff’s equation for ideal solutions (see, for instance, Reference 9, p. 324)

An = uRTAc (8)

where R is the universal gas constant [watt-see/K-mole], T the absolute temperature and v the total number of cations and anions produced by complete dissociation of one molecule of electrolyte. For the dilute NaCl solution v is taken as 2; AC (mole/cm3) is the concentration difference across the membrane.

watt-set mol 10 -3 liter An = 2 X 0.32 x 298 K X 0.03 - X

mol-K liter cm3

watt-set = -0.149 =

cm3 -0.149 MPa = -1.47 atm

The signs refer to a membrane with concentrate on the left and diluate (in this case, pure water) on the right, as in Fig. 1. From this value of the osmotic pressure difference we can calculate the osmotic driving force, -Ac(L [see, for instance, Reference 9, eqn. (A 3)]

-A&, = iWAn = 18 cm3 watt-set ----X -0.149

mol cm3

Hence (-A#) = -2.62 watt-sec/mol. Compare this to the electro-osmotic driving force in stage (f)

experiment described in Fig. 7. In this case, the current density -0.031 A/cm’.

of the WaS

Assuming that the resistance of unit area was about 80 ohm-cm2 [22], we obtain for this driving force

F[-AE] = F X [( current density) X (resistance of unit area of membrane)]

= 0.96 X lo5 C/eq X (-0.032 A/cm2) X (80 ohm-cm2)

= -2.47 X lo5 watt-sec/mol

(The number of mols and ionic equivalents are equal here, because the current is carried primarily by the monovalent ions, Na+.)

It is seen that in typical experiments, the electro-osmotic driving force was much larger than the osmotic driving force. It should be noted that electro-osmotic backwash proved more effective than osmotic backwash, but as the solution concentration was increased, the permselectivity of the membrane decreased; co-ions carried water in the direction opposite to the counterion flow, and thus lowered the electro-osmotic transport rate (per unit electric current).

191

These molecular-backwash experiments demonstrated that under a wide variety of concentrations and current densities, molecular backwash can be used to decontaminate membranes. It might find future application in water reuse, salt-water purification and, in general, in processes utilizing membranes. Under conditions in which the membranes become anion- selective (e.g. at NaCl concentrations larger than for the systems studied in this research), the polarity of the electrodes for electro- osmotic backwash has to be reversed (i.e. high-pressure electrode positive, low-pressure electrode negative). In more dilute solutions, the modified cellulose acetate membranes studied here (“sea-water cellulose acetate membranes”) are cation-selective [2,3,4,15,16,23]. It is important to note that the electro-osmotic backwash technique is different from conventional membrane-cleaning techniques, which were recently summarized [ 241.

Acknowledgment

The authors thank the Office of Water Research, United States Depart- ment of the Interior, for support of this work.

List of symbols

; concentration, mole /cm3 electric potential, V

F Faraday’s constant, 96,500 C/eq

;, electric current density, A/cm2 volume flux, cm/set

L, hydraulic permeability, cm4 /watt-set P pressure, MPa (1 MPa = 1 dekabar = 9.87 atm.) R universal gas constant, 8,314 watt-see/mole-K T absolute temperature, K v molal volume, cm3/mole (subscript w stands for water) P electro-osmotic coefficient, cm3 / C Acl chemical-potential difference, watt-set/mole A# chemical-potential difference in a system at uniform pressure and

temperature A osmotic pressure, MPa 0 reflection coefficient Positive flow direction is from left to right. Operator A stands for difference between a parameter on the right and left, respectively: Ax = x (on right) - x (on left); see Fig. 1.

References

1 J.G. McKelvey, K.S. Spiegler and M.R.J. Wyllie, in: Adsorption, Dialysis, and Ion Exchange, Chem. Eng. Prog. Sympos., Series 55, 24 (1959) 199.

192

2 C.P. Minning, Electrochemical studies on cellulose-acetate hyperfiltration membranes, M.S. Report, Mechanical Engineering Department, University of California, Berkeley, 1966.

3 KS. Spiegler, in: Saline Water Conversion Report for 1967, Office of Saline Water, U.S. Department of the Interior, U.S. Government Printing Office, Washington, D.C. 20402.

4 C.P. Minning and KS. Spiegler, in: E. Selegny (Ed.), Charged Gels and Membranes, D. Reidel Publishing Co., Dordrecht, Holland, Vol. 3, 1976.

5 S. Loeb, Desalination, 1 (1966) 35. 6 F. Harris, G. Humphrey and K.S. Spiegler, in: P. Meares (Ed.), Membrane Separation

Processes, Elsevier, Amsterdam, 1976. 7 I. Nusbaum, ROGA Div., United Oil Products Co., San Diego, California, personal

communication. 8 A. Katchalsky and P.F. Curran, Nonequilibrium Thermodynamics in Biophysics,

Harvard University Press, Cambridge, M.A., 1965. 9 K.S. Spiegler and 0. Kedem, Desalination, 1 (1966) 311.

10 T.S. Brun and D. Vaula, Ber. Bunsenges., 71 (1967) 824. 11 D.C. Weber, Flux maintenance with hollow-fiber ultrafiltration, Seminar on

membrane separation technology, College of Engineering, Clemson University, Clemson, S.C. (August 1976).

12 J.H. MacLeish, Molecular (osmotic and electro-osmotic) backwash of cellulose- acetate reverse-osmosis membranes, Ph.D. thesis, College of Engineering, University of California, Berkeley, 1977.

13 P.B. Lorenz, J. Phys. Chem., 56 (1952) 775. 14 C.P. Minning, Streaming potentials in hyperfiltration (reverse osmosis) of saline

waters, Ph.D. thesis, Mechanical Engineering Department, University of California, Berkeley, 1973.

15 R. Gropl and W. Pusch, Desalination, 8 (1970) 277. 16 H.U. Demisch and W. Pusch, J. Electrochem. Sot., 123 (1976) 370. 17 Office of Saline Water, United States Department of the Interior, Research and

Development Report No. 613, 1971; Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

18 S. Loeb, A composite tubular assembly for reverse-osmosis desalination. Research report No. 66-40, Department of Engineering, University of California, Los Angeles, 1966.

19 J. Jackson and D. Landolt, Desalination, 12 (1973) 361. 29 P. Chartier, M. Gross and K.S. Spiegler, Applications de la Thermodynamique du

Non-equilibre, Hermann, Paris, 1975. 21 I. Prigogine, Introduction, to Thermodynamics of Irreversible Processes, Interscience,

New York, 3rd edn., 1967. 22 D.N. Bennion and B.W. Rhee, Ind. Eng. Chem. Fundam., 8 (1969) 36. 23 D.E. Fayes, Relative transport of sodium halides and sulfate through modified

cellulose-acetate membranes, MS. thesis, School of Engineering, University of California, Los Angeles, 1970.

24 G. Belfort, in: HI. Shuval (Ed.), Water Renovation and Reuse, Academic Press, New York, 1977, Chap, 6.