Desalination 308 (2013) 41–46

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Desalination

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Evaluation of high productivity brackish desalination membrane

Michelle Chapman ⁎Bureau of Reclamation, M.S. 86‐68221, P.O. Box 25007, Denver, CO 80225-0007, United States

H I G H L I G H T S

► Evaluation of high productivity brackish water desalination membrane► Favorable comparison with two commercial membranes► New membrane water transport up to 20 Lm−2 h−1 bar−1

► Salt passage was 0.02%.

⁎ Tel.: +1 303 445 2264.E-mail address: mchapman@usbr.gov.

0011-9164/$ – see front matter. Published by Elsevier Bhttp://dx.doi.org/10.1016/j.desal.2012.07.047

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 March 2012Received in revised form 23 July 2012Accepted 27 July 2012Available online 30 August 2012

Keywords:DesalinationReverse osmosisHigh productivityHigh flux membrane

High productivity reverse osmosis membrane developed under the Office of Naval Research Expeditionary UnitWater Purification Programwas evaluated at the Bureau of Reclamation Brackish Groundwater National Desali-nation Research Facility (BGNDRF). Performance of the new membrane was evaluated in comparison to twoother commercial high productivity or low pressure reverse osmosis membranes using a high productivity testsystem designed and built to take best advantage of high flux membrane through either lower operating pres-sure or greater productivity. Membranes were evaluated with brackish groundwater over a range of cross flowvelocities and recovery rates. Experimental membrane water transport was approximately twice two to threetimes that of the commercial membrane and salt transport was an order of magnitude less than commercialmembrane at 20% recovery for three modules of four inches by forty inches in series tested at a range of feedflow rates.

Published by Elsevier B.V.

1. Introduction

The Federal Government has invested in technological improve-ments for desalination for over sixty years. Recent programs havebeen offered through the Department of Defense (DoD) Office ofNaval Research Expeditionary Unit Water Purification program(EUWP) andDefense Advanced Research Projects Agency (DARPA)Ma-terials with Novel Transport Properties program (MANTRA), and theDepartment of Interior, Bureau of Reclamation Desalination andWater Purification Research (DWPR) program. Drivers for the DoDprogram were smaller size, lighter weight, smaller logistical burden,and increased productivity, while the drivers for the Reclamation pro-gram were reduction in cost, energy, and environmental impact. In-creased membrane productivity was an objective for several projectsfunded under all three programs.

There have been improvements in formulation and constructionthat have resulted in increased productivity while maintaining highsalt rejection as evidenced by the difference in specified productivityof new commercial membranes compared to those offered for sale20 years ago. In 1992, a commercial 8‐inch spiral wound seawater

.V.

membrane was rated at 20 m3/day or less [1], while today there areproducts on themarket that advertise 50 m3/day at the same operatingconditions. Is there more room for improvement, or have we reachedthe productivity limit for the spiral wound configuration? The challengefor Dr. Winston Ho of Ohio State University under the ONR EUWP pro-gram was to prove that another doubling of productivity was possible.As a collaborator in the EUWP Program, Reclamation offered to evaluatewhether Dr. Ho had succeeded [2].

The productivity of a membrane element is dependent on severalfactors:membrane permeability, thickness, active area, the spacer char-acteristics, and operating conditions such as the feed water composi-tion, cross flow velocity, pressure, and recovery. These concepts arewell documented elsewhere, for instance in J.G. Wijmans' excellentoverview of the solution diffusion theory [3], Geise's discussion of thetradeoff between permeability and salt transport [4] and other studiesof spacermaterial formation and hydraulics [5–7]. The complex interac-tion of these parameters makes it difficult to predict the performance ofa full size membrane module based on small sample flat sheet testresults.

Commercialmembrane specifications reflect the performance of fullsize modules at one set of operating conditions. The module may becapable of higher, but perhaps unsustainable, level of performance ifoperated under different conditions. An innovative new membrane

Table 1Composition of brackish water from wells at BGNDRF.a

Parameter Units Well 1 Well 2 Well 3 Well 4

pH 7.27 6.63 6.56 6.79TDS mg/L 1640 550 3860 4150Conductivity μS/cm 1887 6009 4650 4790Total hardness mg/L 220 3310 79 1710Total alkalinity mg/L 24 58 34 50Bicarbonate mg/L 96 230 136 200Nitrate mg/L BDL 29 23 18Chlorides mg/L 32 430 518 520Sulfates mg/L 745 2640 1606 1770Phosphate mg/L BDL N/A N/A N/APhosphorous mg/L 0.07 N/A N/A N/ACalcium mg/L 60 440 290 395Iron mg/L 0.017 0.048 0.13 0.029Magnesium mg/L 16 425 155 179Potassium mg/L 5 2.4 3 3Silicon mg/L 4.75 6.25 1.75 3.25Sodium mg/L 320 680 535 484Strontium mg/L 0.025 0.017 0.011 0.014Manganese mg/L 0.009 0.003 0.002 0.003Barium mg/L 0.058 0.09 0.092 0.089Zinc mg/L 0.003 b0.002 b0.002 b0.002

a μS/cm=microsiemens per centimeter; pCi/L=picocuries per liter. For all wells tur-bidity is b0.1 NTU; lithium is below detection limit of 1 mg/L; boron and aluminum areless than 0.002 mg/L.

42 M. Chapman / Desalination 308 (2013) 41–46

formulation may have a different optimum performance window. Toallow a true comparison of new membrane modules with the state ofthe art in commercialmodules it is necessary to test themodules in par-allel over a range of operating conditions using the same source water.

2. Methods and resources

2.1. Test facility and source water

Testingwas performed at BGNDRF at 500 Lavelle Road, Alamogordo,NewMexico, 90miles north of El Paso, Texas. BGNDRF is a Reclamationfield office under the management of the Reclamation Upper ColoradoRegion in Salt Lake City, UT with funding from the Reclamation Re-search and Development Office in Denver, CO. The facility is availableto the public for pilot testing advanced water treatment and alternativeenergy technology. Three outdoor test pads are capable of 230 L/minfeed flow. Six indoor test bays have access to 110 L/min. The rest ofthe groundsmay be used for alternative energy and agricultural studies.There are four wells at the BGNDRF ranging in total dissolved solids(TDS) from 550 to 4150 mg/L TDS. Table 1 lists the water compositionfor each of the wells. Well 3 was used for the membrane evaluation.

Fig. 1. Raw water from well three was stored in center tank (tank

2.2. Equipment

Water from well three was pumped to the BGNDRF storage tank 2(Fig. 1). Rawwater pre-treated by the Expeditionary UnitWater Purifier(EUWP) ultrafiltration (UF) system to remove any oxidized iron and de-bris (Fig. 2) and then stored in tank 1. Underground plumbing allowstransfer of water between wells, tanks, outdoor and indoor test sites.Pretreated water was pumped from tank 1 to the main building.

Reverse osmosis performance testing was carried out inside the fa-cility using the Reclamation 4″ element evaluation system shown inFig. 3. This system, designed for evaluating a wide range of membraneperformance potential, consists of two parallel trains capable of operat-ing three 4-inch-diameter by 40-inch-long modules in series. The sys-tem has separate pumps, effluent sensors, and flow controls. Feedwater conductivity, pH, and temperature are measured on one train.Permeate and concentrate conductivity, flow, and pH are monitoredon both trains. The pumps are controlled manually or automaticallyvia variable frequency drives (VFDs). The system is designed to operateat up to 110 L/min feed flow to each train at pressures up to 34 bar. Na-tional Instruments Compact DAQ is used for monitoring sensor outputand control of VFD. Table 2 lists instrumentation and controls includedin the system.

System “I” pump is in the front, system “II” pump is in the back. Fourmultiparameter controllers display operating parameters on the skidand transmit data to Compact DAQ.

Vessels are Codeline UB4 with ½-inch permeate ports and ¾-inchfeed/concentrate ports. To accommodate high productivity, these di-mensions should be increased. The feed piping on the system is 1½inches with ¾-inch permeate and concentrate lines, reduced at the ves-sel to fit the end cap and module permeate tube dimensions.

2.3. Membrane material

The experimental membrane [2] was formed into 4″ spiral woundmodules. Eleven of these modules were shipped to BGNDRF for evalua-tion in comparison with two commercial membrane products promotedas being “high productivity” or “low pressure.” Table 3 lists membranecharacteristics from theirmanufacturer specification sheets and the resultof factory tests with the experimental membrane.

2.4. Analysis

Parameters used to evaluate the performance ofmodules are the netdriving pressure, cross flow velocity, differential pressure across thethree-element vessel, recovery, water transport coefficient, and saltpassage. These were calculated from measured parameters in the fol-lowing manner.

2) while UF filtrate was stored in the tank on the left (tank 1).

Fig. 2. EUWP UF system filtering well water.

43M. Chapman / Desalination 308 (2013) 41–46

Net driving pressure (bar)

NDP ¼ Pf−Pp−πf þ πc

2−πp: ð1Þ

Cross flow velocity (m/s)

Vx ¼ Qf þ Qc

2 � Ax: ð2Þ

Differential pressure (bar)

ΔP ¼ Pf−Pc: ð3Þ

Recovery (%)

Rec ¼ Qp

Qf: ð4Þ

Water transport coefficient (L m−2 h−1 bar−1)

A ¼ QpTcorArea �NDP : ð5Þ

Fig. 3. Reclamation 4-inch element evaluation system.

Salt passage (dimensionless)

B ¼ CpCfRec � ln 1

1−Rec

� � : ð6Þ

In the above equations, P is pressure, Q is flow, and C is concentra-tion. Subscripts indicate where it is measured: “f” for feed water, “c”for concentrate, and “p” for permeate. Pi (π) is the osmotic potential.“Ax” is the cross section area of the space betweenmembrane envelopesin the modules calculated as the length of a module leaf (estimated at0.9 m) times the number of leaves times the spacer thickness. “Area”is the active area of membrane in the module. Tcor is the temperaturecorrection factor from Dow Filmtec Technical Manual [8]. “Rec” is thewater recovery. Concentrations are estimated from conductivity basedon the TDS 442 Standard Solution developed by MyronL [9]. This solu-tion is prepared from 40% sodium sulfate, 40% sodium bicarbonate,and 20% sodium chloride. Conductivity values are available for a widerange of TDS levels. Fitting a power function equation to the conductiv-ity versus TDS data results in the following conversion factor:

TDS ¼ 0:4138 � C1:0781: ð7Þ

C is the conductivity in μS/cm. TDS is converted to concentration inmol/L by dividing by the average ionic mass of the standard solution.

3. Experimental

Elements from all three sources were evaluated over a range ofcross flow velocities and recovery rates over a period of seven days,as described in Table 4, to determine the performance response ofeach of the three membrane products. Table 4 lists the actual averageoperating parameter values. Controlling exactly to the target value

Table 2Instrumentation provided for systems I and II.

Purpose Manufacturer Model number Feed Concentrate Permeate

Pump T&T NH76C2F021E15A I, IIVFD Baldor VS1SP415-1B I, IIFlow Signet 2551 I, II I, IIPressure Cole Parmer EW-86001-27 I, II I, IIConductivity Signet 2850 Cell const. 1 II I, II

2850 Cell const. 10 I, IIpH Signet 2750/2754 IITemperature Signet 2350 IIControllers Signet 8900 II I, II I, II

Table 3Factory specifications for 4″×40″ membrane modules.

Supplier Membrane A Membrane B OSU factory test

Productivity (m3/day) 9.8 9.5 16.3Area (m2) 8.1 7.2 7.6Chloride rejection (%) 99.0 99.5 98.6Recovery (%) 15 15 12Test solution (mg/L NaCla) 500 2000 2000Applied pressure (bar) 6.9 10.3 15.5Pressure normalized flux(L/m2 hbar)

7.31 5.34 5.77

Maximum feed flow (L/m2) 53 60 n.a.b

Maximum pressure (bar) 41.4 41.40 n.a.b

Maximum dP (bar) 0.9 1 n.a.b

Flux (L/m2/h) 50 55 89Spacer thickness (mil) 28 34 28

a NaCl = sodium chloride.b Information not available.

Fig. 4. Variation of differential pressure across a three-module vessel with increasingfeed flow rate and recovery. Shaded line indicates recommended maximum pressuredifferential. Data is given for two sets of experimental membrane from both sides ofthe parallel test system. “Sys I” and “Sys II”.

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was not possible due to harmonic fluctuations in water delivery fromthe outdoor storage tank. The two systems were controlled manuallyfor this test to maintain recovery and feed flow for an hour while let-ting the pressure respond accordingly. Conductivities and permeateflow rates were verified manually and recorded automatically duringeach operating condition. The system was flushed with feed water atthe end of each day and shut down for the night.

After completing the sequence of flow and recovery for both com-mercial membranes, the first set of experimental membrane wereloaded into system II and the sequence was repeated to directly com-pare them with membrane A. To determine the variability among theexperimental modules a second set of was loaded into system I andthe sequence was repeated once again.

4. Results and discussion

With a new membrane formulation and module design, it is firstnecessary to determine the range of feed flow rates that the module

Table 4Average operating conditions for target settings in chronological order.

System I System II

Membrane X-flow Vel(cm/s)

Recovery(%)

Membrane X-flow Vel(m/s)

Recovery(%)

A 26 19.9 B 27 19.6A 25 25.6 B 25 25.0A 24 31.8 B 24 31.7A 23 38.9 B 23 39.0A 35 23.3 B 34 24.0A 34 18.7 B 34 19.5A 33 29.3 B 33 29.5A 31 34.5 B 31 34.1A 33 22.3 B 34 24.5A 28 21.5 B 27 22.5A 26 18.9 OSU — set 1 26 19.1A 25 24.1 OSU — set 1 26 23.9A 24 28.9 OSU — set 1 24 28.6A 24 35.1 OSU — set 1 24 34.8A 35 19.4 OSU — set 1 36 19.3A 33 24.0 OSU — set 1 34 24.2OSU — set 2 34 19.0 OSU — set 1 33 19.3OSU — set 2 34 24.6 OSU — set 1 34 22.3OSU — set 2 27 17.3 OSU — set 1 27 17.5OSU — set 2 32 20.4 OSU — set 1 32 19.3OSU — set 2 30 24.3 OSU — set 1 31 23.7OSU — set 2 29 29.0 OSU — set 1 29 29.1OSU — set 2 29 34.1 OSU — set 1 28 34.2OSU — set 2 32 19.2 OSU — set 1 30 19.9OSU — set 2 26 20.0 OSU — set 1 26 19.2OSU — set 2 25 23.3 OSU — set 1 25 23.4OSU — set 2 24 29.5 OSU — set 1 24 28.7OSU — set 2 24 34.7 OSU — set 1 24 34.4

can tolerate within traditionally acceptable pressure differentialsacross the vessel. Feed flowwas varied from 50 to 90 liters per minute(L/min) and recovery from 15 to 35%. In this range of operation, pres-sure drop across three elements rose from 0.6 to 3.2 bar. Maximumrecommended pressure drop is 2 bar (200 kPa) for a vessel or0.45 bar for one module. Fig. 4 shows the change in pressure dropwith feed flow rate over all testing for this study. The variation inpressure differential within one feed flow rate is due to change in re-covery, which is discussed below. Membrane B has a thicker feed/concentrate channel spacer than membrane A and the OSU experi-mental membrane. As a result, it can handle higher feed flow rateswithout exceeding the recommended pressure difference across thevessel.

Fig. 5 shows the net driving pressure required to achieve the tar-get feed flow and recovery operation points. Figs. 6 and 7 depict theresulting water transport coefficients and salt passage, respectivelyat the two target feed flow rates.

The experimental modules required significantly lower drivingpressure than commercial membranes A or B. The initial runs withthe experimental membrane resulted in water transport two tothree times higher over the entire range of conditions. Salt passagewas an order of magnitude lower than the commercial membranesat the lower recovery point but increased to equal commercial mem-branes at 30% recovery and to an unacceptable level at 35%. Mem-brane B behaved in a similar manner at the lower feed flow but athigher recovery it's salt passage rose to match that of membrane A

Fig. 5. Net driving pressure required to achieve target feed flow and recovery levels.Error bars represent two standard deviations of operation over a 1-hour period.

Fig. 6. Change in water transport with increasing feed flow and recovery rate. Errorbars represent two standard deviations over 1 h of operation. Error bars for somemembrane A and B points are smaller than the symbols.

Fig. 8.Water transport, recovery, and cross flow velocity for both systems during mem-brane variability testing. Data points for system I before the large symbol are for mem-brane A. Each data point represents the average of an hour of operation.

45M. Chapman / Desalination 308 (2013) 41–46

and its own performance at the higher flow rate. Both commercialmodules maintained water transport coefficient and acceptable saltpassage level over the range conditions.

The second set of experimental membranes were similar to mem-brane A. Figs. 8 and 9 show the chronological results of testing withthe experimentalmembranes showing cross flow velocity and recoverywith water transport and salt passage respectively. The first six datapoints for system I in these figures are from membrane A. After thelarge symbol, the data represents results for OSU-Set 2. Fig. 8 shows agradual decline in water transport over the test that extended overtwo days. Water transport is higher when the recovery is below 20%.Salt passage increased dramatically for the OSU-Set 1 whenever recov-ery was over 20%. At the end of this series, the modules were removedfrom the system to check the o-rings. Two of the OSU-Set 1 permeatetube o-rings had become deformed over the testing. Neither of thetwo sets of commercial membranes had that problem.

The most probable reason for the high salt passage at high recov-ery is that the o-rings were not suitable for the pressure differentialunder those operating conditions, but the spikes in salt passage didnot correspond to high differential pressure. Fig. 10 shows differentialpressure over time and salt passage. Actually, the return to low saltpassage occurred at the highest differential pressure. High salt pas-sage periods correspond to high recovery periods. This would indicatethat if surface charge is the primary mode of salt rejection, then thehigh concentration polarization occurring at high recovery is neutral-izing the charge and allowing salt to pass through the membrane. Theo-rings were deformed though by the end of the test.

Fig. 7. Salt passage. Error bars represent two standard deviations.

This probably happened sometime after 11 h of operation in thesecond phase of testing. Samples of the membrane were sent to OSUfor analysis. They found traces of calcium sulfate residue on the surface,which would explain the decline in water transport over time.

5. Conclusions

The experimental membrane did indeed have a 2–3 times higherwater transport than the commercial membranes, and at lower recov-ery rates, it also had an order of magnitude lower salt passage. Due tothe charge repulsion mode of salt rejection, these membranes wouldbe better for water sources with lower concentrations than used forthis test (3800 mg/L). Higher end operating conditions for this studydid exceed the recommended operating limits of the commercial mem-branes. The lower end feed flow rate of 55 L/min is actually highaccording to membrane A specifications. However, the commercialmembranes did perform well over the entire test range, which showsthat they have built in durability that was not evident with the experi-mental membrane modules. A thorough module design for robustnessand perhaps a thicker spacer to enhance mixing would allow fuller ex-pression of the performance advantage of the experimental membrane.

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Fig. 9. Salt passage, recovery, and cross flow velocity for both systems during mem-brane variability testing. Data points for system I before the large symbol are for mem-brane A. Each data point represents the average of an hour of operation.

Fig. 10. Differential pressure and salt passage for system II during the membrane vari-ability testing. Includes all data points recorded over the operation time.

46 M. Chapman / Desalination 308 (2013) 41–46

Acknowledgments

The Office of Naval Research provided funding for the membraneevaluation. Reclamation Research and Development Office providedfunding to design and build the membrane evaluation system for

BGNDRF. The staff at BGNDRF was extremely helpful in getting the sys-tem installed and running. Andrew Tiffenbach and Yuliana Porras of theReclamation Desalination Research teamwere very helpful in operatingthe system for the tests. Special thanks to JohnWalp, of the ReclamationMechanical Engineering team, for keeping the EUWP operating.

References

[1] M. ChapmanWilbert, Guide tomembranes for municipal water treatment, in: Bureauof Reclamation, DWPR Report No. 1, 1993.

[2] L. Zhao, P.C.-Y. Chang, W.S.W. Ho, High-flux reverse osmosis membranes incorporatedwith hydrophilic additives for brackish water desalination. Desalination (this issue).

[3] J.G. Wijmans, R.W. Baker, The solution-diffusion model: a review, J. Membr. Sci.107 (1995) 1–21.

[4] G.M. Geise, et al., Water permeability and water/salt selectivity tradeoff in poly-mers for desalination, J. Membr. Sci. 369 (2011) 130–138.

[5] A.R. Da Costa, et al., Optimal channel spacer design for ultrafiltration, J. Membr. Sci.62 (1991) 275–291.

[6] J. Schwinge, D.E. Wiley, A.G. Fane, Novel spacer design improves observed flux, J.Membr. Sci. 229 (2004) 53–61.

[7] P. Erikson, New design feed channel spacer in spiral wound elements forpretreatment cost reduction, in: Bureau of Reclamation, DWPR Report No. 45, 1998.

[8] Dow FilmTec technical manual excerpt — section 9.6. available at: http://www.dowwaterandprocess.com/support_training/literature_manuals/filmtec_manual.htm(7/18/12).

[9] Myron L Application Bulletin: Standard Solutions and Buffers. SSBAB01-08 MyronL Company, 2008.