Reverse-Osmosis and Ultrafiltration Membrane Compaction and

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Downloaded by OHIO STATE UNIV LIBRARIES on July 1, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch029

Reverse-Osmosis and Ultrafiltration Membrane Compaction and Fouling Studies Using Ultrafiltration Pretreatment BRIAN J. RUDIE, TIM A. TORGRIMSON, and D. D E A N SPATZ Osmonics, Inc., Minnetonka, M N 55343 Is it compaction or fouling that causes membrane flux to deteriorate over time? For years, compaction of the membrane itself has been considered a primary reason for flux decline. Recent findings indicate that compaction does not play the role suggested by many and is much less important than fouling, even on closed loop tests using purified water. The data on numerous membranes run for a minimum of 1000 hours at temperatures varying from 25°C (77°F) to 50°C (122°F) and pressures from 25 psig (172 kPa) to 800 psig (5512 kPa) was collected. Comparison of membrane compaction rates indicates that fouling is a much greater cause of flux degradation than has previously been reported. The use of ultrafiltration prior to the membranes on test helped to substantiate the effect of fouling. The system used for these tests is a much improved method of obtaining compaction results and it is suggested that all compaction studies use UF pretreatment immediately prior to the membrane being tested. The ability to predict membrane performance over time is a necessity to properly design any RO or UF system. Whether the RO or UF system membrane configuration employed is of the hollow fiber, tubular, plate and frame, or spiral wound configuration; system designers must consider changes in solute passage and permeate rate that occur with operating time to design a system that will meet the requirements of the end user. Predicting system performance requires a thorough understanding of the physical and chemical properties of the solution being processed as well as a thorough understanding of the basic physical and chemical properties of the membrane employed. Essential to the understanding of how a membrane performs on any given solution is the understanding of how the membrane performs under controlled conditions. This paper discusses a better approach to 0097-6156/85/0281-0403S06.00/0 © 1985 American Chemical Society

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

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REVERSE OSMOSIS AND ULTRAFILTRATION

understand and predict membrane flux based on data obtained through closely controlled experiments.


Discussion Initial attempts were made to collect meaningful data on membrane flux over time in a closed-loop test cell system. The objective was to get the non-fouling induced decline in flux, i.e. the compaction rate, of the membrane. This initial system contained a combination of brass, stainless steel and plastic components. Due to contamination of the feed solution from corrosion of the metal components, airborne particulates and microorganisms, meaningful data could not be obtained due to significant fouling. Two new test systems were designed and built at Osmonics in an attempt to reduce contamination of the feed solution and to collect meaningful data. One test system was designed such that the feed solution to the test cells was continuously prefiltered through ultrafiltration spiral-wound membrane elements immediately prior to the test cells. The other test system did not employ the use of ultrafiltration of the feed solution. Both systems were constructed of all 316SS and plastic components. The high pressure plumbing of both systems was constructed of 316 stainless steel components. The low pressure plumbing was constructed of either 316 SS or inert plastic components. Flow and pressure was generated by multi-stage centrifugal TONKAFLO pumps using all SS and plastic wetted parts. Flow schematics of the two test systems are shown in Figures 1 and 2. The flow through the UF test system is as follows: the feed solution is contained in a 55 gallon polyethylene tank. The feed solution is fed by gravity to the inlet of the low pressure pump. A throttling valve at the discharge of the pump allows for flow and pressure adjustment. Five micron cartridge filters are installed to remove any large particulate matter that may be present in the feed solution. The 316 SS filter housing is located at the discharge end of the first pump rather than ahead of the inlet to the pump to prevent cavitation of the pump. A 316 stainless steel heat exchanger with a tempering valve is used to maintain the desired temperature of the solution. In the system containing the ultrafiltration prefiltration (Figure 2 ) , the pressurized feed solution passes through the ultrafiltration housing containing two OSMO-411-PT2(PS) spiralwound UF elements. Note that this system contains a second multistage centrifugal pump. If low pressure testing (£400 psi) is desired, the second pump is by-passed. If high pressure testing (600-1000 psi) is desired, the flow is directed through the second pump to increase pressure to the UF housing. The concentrate flow through the UF elements is controlled by a valve at the concentrate discharge side of the element housing. In this system, the recovery is established at 100% (i.e. no concentrate flow)• Periodically, the concentrate valve is opened to increase concentrate flow over the membrane to flush away foulant materials. The permeate from the UF housing provides the direct feed to the test cell manifold. It is important to recognize that the UF runs with a transmembrane pressure of 40-50 psi and that



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RUDIE ET AL.

Figure l a .

RO and UF Membrane Compaction and Fouling Studies

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Flow schematic of t e s t c e l l loop without UF p r e f i l t r a t i o n .

Figure l b .

Test c e l l .



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Figure 2. Flow schematic of test cell loop with UF prefiltration. Test cell is shown in Figure lb.



29. RUDIE ET AL.

RO and UF Membrane Compaction and Fouling Studies

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the permeate is at the desired test cell operating pressure. Thus, there is no repressurization of the permeate as it feeds directly to the test cells with minimal contact of metals prior to contacting the membrane in the test cells. The flow through each test cell is controlled by the 316 SS concentrate valve on the discharge side of each cell. The permeate and concentrate are returned to the feed tank, closing the loop. Both systems were equipped with numerous control devices to protect the system components and to maintain the desired flows, temperatures, and pressures. In the event of reduced pressure due to blinding of the cartridge filters or a leak in the system, the low pressure switch automatically cuts off the power to the pumps. A high pressure switch automatically shuts off power to the pump in the event of over-pressurization due to a restriction in the plumbing. A temperature probe senses the temperature of the feed solution and controls the cooling water flow through the heat exchanger via a solenoid valve to maintain the desired temperature. Data was collected on numerous CA and PS membranes run on a feed solution consisting of 2000 ppm NaCl and 0.5% formaldehyde in RO H20« Formaldehyde was used to eliminate potential bacteria mold or algae growth which can be a major problem on closed-loops without bactericides. Each membrane was tested for a minimum of 1000 hours continuous operation at various operating temperatures and applied pressures to determine the effects of temperature and pressure on membrane flux decline. New membrane samples were used for each test conducted. Table I reflects the battery of tests performed on both test loops. Table I.

Membranes Tested and Associated Operating Pressures and Temperatures Transmembrane Pressure (psig)

Membrane Type Polysulfone (UF) Polysulfone (UF) Cellulose Acetate Cellulose Acetate Cellulose Acetate Cellulose Acetate

(UF) (RO) (RO) (RO)

25 50 100 400 600 800

Operating Temperatures °C (°F) 25(77) 25(77) 25(77) 25(77), 25(77), 25(77)

40(105), 50(122) 40(105) 50(122)

Visual examination of the membranes after long terra testing indicated that both new all 316SS/plastic systems significantly reduced membrane fouling when compared to the first test loop which contained brass components. The membranes tested on the new test system without UF prefiltration still showed significantly more membrane fouling than membranes tested on the system employing UF prefiltration. Nevertheless, some membrane fouling was still observed on the membranes even with UF prefiltration, presumably, from inorganic materials that passed through the UF membrane.


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Procedure Multiple 2 inch diameter samples of each membrane were tested simultaneously to reduce experimental error. The temperature of the test solution was maintained at ^2°C of the desired test temperature throughout each test. Sodium chloride passage was monitored throughout the test as a measure of membrane integrity. Membrane flux was monitored as a measure of membrane compaction and/or fouling. Feed flow rate was maintained at 0.3 to 0.5 gpm (1.3 to 1.9 1pm) for each cell. This flow rate range corresponded to a A P across the cell of 3-8 psid, respectively. Flow conditions across the cell were determined empirically by a plot of A. P vs. flow rate. Non-laminar flow was found when the A P was greater than 1.25 psid. Operation with a A P of 3-8 psid across the cell, in conjunction with cell design, insured transitional to turbulent flow conditions across the membrane surface. The test cells used for these tests are shown in Figure 3. The flow through the test cell is shown in Figure 4. Feed enters the cell at the base and is directed to the center of the membrane sample. The concentrate then flows radially across the membrane surface to the concentrate outlet port located at the periphery of the cell. The concentrate then passes through a concentrate valve and flow meter. The concentrate flow is controlled by adjusting the concentrate valve. The membrane is backed by a SS porous disc. The pure water (permeate) that passes through the membrane passes through the porous disc, and is collected through a port in the top of the cell. The permeate then exits the test cell and returns to the feed tank. For each battery of tests conducted, initial membrane flux values were taken at 0.5 hours after the initiation of the test. Additional flux values were measured and recorded at two hour intervals during the first 24 hours of the test and then every other day throughout the duration of the test. Short intervals were used in the initiation of the test because most literature shows a sharp drop in flux at the beginning of a test. (J_) Our data confirms that the most significant flux reduction occurs within the first 10 hours of operation. The flux data obtained for each membrane was statistically analyzed via a linear regression program. The time and percent of original flux value was converted to log form and entered into the computer. A "least-squares"(.2) linear regression was used to determine the best fit line and the resulting lines were displayed on log-log graphs. By taking the values from the graphs, the slopes(m) were then calculated using the following equation: (3^ M = Log F? - Log Fj Log T2 - Log T

Reverse-Osmosis and Ultrafiltration Membrane Compaction and

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