Comparing nanofiltration and reverse osmosis for

source water This study investigated the use of ultra-low-pressure reverse osmosis (ULPRO) and nanofiltration (NF) membranes for water reuse applications where lower operating pressures and a high permeate quality are desired. A laboratory-scale investigation was performed to compare the rejection and operational performance of RO, ULPRO, and NF membranes and to select two membranes for testing at a California water facility. A ULPRO membrane and an NF membrane were then tested at pilot and full scale at a water recycling plant and monitored for operational performance and rejection of total organic carbon, total nitrogen, and regulated and unregulated organic micropollutants. Pilot- and full-scale testing of the best-performing membranes demonstrated that both ULPRO and NF membranes could be used to meet potable water quality requirements. The presumed advantage of using ULPRO and NF membranes diminished as fouling occurred, resulting in operating pressures only slightly lower than or similar to those found for traditional RO membranes.

Comparing nanofiltration and reverse osmosis for drinking water augmentation BY CHRISTOPHER BELLONA, JÖRG E. DREWES, GREGG OELKER, JOHN LUNA, GERRY FILTEAU, AND GARY AMY

n increasing number of municipalities are using surface water that has been affected by wastewater discharges or reclaimed water for drinking water augmentation. For groundwater injection projects in the United States that use reclaimed water, treatment using an integrated membrane system such as microfiltration (MF) pretreatment followed by reverse osmosis (RO) is the industry standard (NRC, 2004). RO membranes are favored for these applications because of their high removal efficiencies for total dissolved solids, pathogens, and unregulated trace organic chemicals. For direct injection into a potable aquifer in California, the California Department of Public Health (CDPH) draft regulations for groundwater recharge with reclaimed water require RO treatment and effluent water quality of < 0.5 mg/L total organic carbon (TOC) and < 5 mg/L total nitrogen (TN; CDPH, 2007). Currently, there is the potential to lower operating pressures and costs by implementing newer types of membrane processes such as nanofiltration (NF) and ultra low-pressure RO (ULPRO) for these types of applications. However, an understanding of the advantages and disadvantages of using ULPRO and NF compared with conventional RO membranes in terms of operational performance and permeate water quality is lacking. One of the most challenging operational issues associated with municipal wastewater reclamation is membrane fouling and subsequent flux decline (AWWA, 2005; Gwon et al, 2003; Wilf & Alt, 2000; Speth et al, 1998). Consequently, questions exist about whether ULPRO and NF membranes can achieve

A

A full report of this project, Comparing Nanofiltration and Reverse Osmosis for Treating Recycled Water (91212), is available from the AWWA Bookstore (1-800-926-7337) or from awwa.org/bookstore. Reports are free and currently available to Awwa Research Foundation subscribers by calling 303-347-6121 or logging on to www.awwarf.org.

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SEPTEMBER 2008 | JOURNAL AWWA • 100:9 | PEER-REVIEWED | BELLONA ET AL

2008 © American Water Works Association

Samples were taken (left) from individual pressure vessels in the 2.5-mgd full-scale reverse osmosis train at the West Basin Water Recycling Plant in California. A nanofiltration/reverse osmosis pilot-scale membrane skid (right) was used during this study, which was designed to evaluate the viability of a select number of commercially available membranes for water reuse applications.

and maintain operating pressures comparable to those achieved with RO membranes under fouling conditions. The worst-case scenario associated with membrane fouling would be frequent membrane cleanings to restore permeate flux, which would negate the cost savings associated with ULPRO and NF membranes. It has been demonstrated that RO membranes effectively reject most constituents present in municipal wastewater effluents. However, it was found that at a full-scale reclamation plant using RO treatment, trace organics such as 1,4-dioxane and N-nitrosodimethylamine (NDMA) were present in product water at concentrations greater than the California action limit (OCWD, 2002; 2000). In addition, because researchers have reported the incomplete rejection of various endocrine disrupting chemicals (EDCs), pharmaceutically active compounds (PhACs), and disinfection by-products (DBPs) by ULPRO and NF membranes, there is a need for a more tailored investigation into trace organic compound rejection by RO, ULPRO, and NF membranes under conditions representative of full-scale operations (Nghiem et al, 2005; Ng & Elimelech, 2004; Kimura et al, 2003a, 2003b; Schäfer et al, 2003). The purpose of this research was to evaluate the viability of a select number of commercially available ULPRO and NF membranes for water reuse applications in which lower feed pressures, lower operating costs, and high permeate quality with respect to TOC, TN, and trace organic contaminants are desired. A laboratoryscale investigation into the rejection and operational performance of commercially available membranes was performed in order to select two membranes for pilotscale testing at a water reuse facility in California. The competitiveness of NF and ULPRO membranes was then

determined by comparing the performance of the selected membranes at pilot and full scale to commonly used RO membranes. This comparison was based on flux decline resulting from fouling, specific flux (SF) after a period of fouling, and rejection of TOC, nitrogen species, and regulated and unregulated organic solutes.

MATERIALS Candidate membranes. Eleven candidate membranes were selected to represent a variety of commercially available RO, ULPRO, and NF membranes (Table 1). Two of the membranes—a ULPRO, ESPA2,1 and a lowpressure brackish water RO, TFC-HR2—are used at fullscale water reuse facilities and therefore were chosen as the benchmark ULPRO and RO membranes to which all other candidate membranes were compared.

ANALYTICAL METHODS The following water quality analyses were performed to quantify membrane performance in removing constituents of concern during laboratory-, pilot-, and fullscale investigations. Conductivity, pH, TOC, nitrate, ammonia, and ultraviolet absorbance. Conductivity and pH were measured following methods 2510 and 4500-H+, respectively (Standard Methods, 2005). TOC was measured using a TOC analyzer3 according to method 5310 C (Standard Methods, 2005). Ultraviolet absorbance (UVA) was measured with a scanning spectrophotometer4 according to method 5910 B (Standard Methods, 2005). Ammonia was measured using a spectrophotometer5 according to Nessler Method 8038 (Hach, 1997) and an ion-selective probe5 according to method 4500-NH3 (Standard Methods, 2005). Nitrate

BELLONA ET AL | 100:9 • JOURNAL AWWA | PEER-REVIEWED | SEPTEMBER 2008


103

TABLE 1

Summary of candidate membrane specifications

Manufacturer Specifications

Project Specifications

pH Range

Pure Water Flow gpd

Element Area m2 (sq ft)

Material

4040/ 8040 Element Used at:

NaCl*

4–11

39,750

37 (400)

Polyamide

Full scale

General Specifications

Membrane

Type

Manufacturer

Salt Rejection %

TFC-HR-400

Brackish water RO-8040

Koch Membrane Systems, Wilmington, Mass.

99.5

TFC-HR-4040

Brackish water RO-4040


99.5 NaCl*

4–11

2,100

7.2 (78)

Polyamide

Lab and full scale

TFC-ULP

Low-pressure RO-4040


98.5 NaCl*

4–11

1,750

7.2 (78)

Polyamide

Lab scale

XLE


Dow/Filmtec, The Dow Chemical Co., Midland, Mich.

99 NaCl†

2–11

2,400

7.6 (82)

Polyamide

Lab scale

TMG20-430


Toray Industries (America) Inc., New York

99.5 NaCl†

2–11

41,640

40 (430)

Polyamide

Full scale

TMG10


Toray Industries (America) Inc., New York

99.5 NaCl†

2–11

2,400

8.1 (87)

Polyamide

Lab, pilot, and full scale

NF-90

NF- 4040


85-95 NaCl‡, 97.7 MgSO4§

2–11

1,850

7.6 (82)

Polyamide

Lab and pilot scale

NF-200

N-4040


45-65 CaCl2,** 97 MgSO4§

3–10

1,350

7.6 (82)

Polyamide

Lab scale

NF-4040

NF-4040


99 MgSO4§

2–11

1,950– 3,000

7.6 (82)

Polypiperazine

Lab and pilot scale

NF-270

NF-4040


40-60 CaCl2,** 97 MgSO4§

2–11

2,925

7.6 (82)

Polyamide

Lab scale

TFC-S

NF-4040


85 NaCl,* 98.5 hardness total*

2–11

2,000

7.2 (78)

Polyamide

Lab scale

ESPA2-8040


Hydranautics, Oceanside, Calif.

99.5 NaCl††

3–10

34,100

37 (400)

Polyamide

Full scale

ESPA2-4040



99.5 NaCl††

3–10

2,600

7.9 (85)

Polyamide

Lab scale

ESNA1-LF

NF-4040


80 NaCl†

3–10

1,700

7.9 (85)

Polyamide

Lab scale

CaCl2—calcium chloride, MgSO4—magnesium sulfate, NaCl—sodium chloride, NF—nanofiltration, RO—reverse osmosis TMG10 and TMG20-430 are equivalent products in 4- or 8-in. configurations, respectively. Test conditions:*700 mg/L (> 45% monovalent), †500 mg/L NaCl, ‡2,000 mg/L NaCl, §2,000 MgSO4, **500 mg/L CaCl2, ††1,500 mg/L NaCl

was measured with an ion chromatograph (IC)6 using method 4110 C (Standard Methods, 2005) using an anion exchange column7 and a sodium bicarbonate eluent. DBP analysis. Chloroform and bromoform analysis followed US Environmental Protection Agency (USEPA) method 551.1 (1995a). Analysis of trichloroacetic acid (TCAA) and dichloroacetic acid (DCAA) followed method 552.2 (USEPA, 1995b). NDMA samples were analyzed following modified method 1625 C (USEPA, 1989) with a reporting limit of 2 ng/L.8 Unregulated organic chemicals. Selected trace organic compound analysis (Table 2) followed the method by Reddersen and Heberer (2003). One-litre samples collected during membrane testing were extracted using solid-phase extraction,9 eluted, derivatized, and analyzed 104

by gas chromatography/mass spectrometry (GC/MS). Detection and quantification limits (3:1 and 11:1 signalto-noise ratio, respectively) were in the low nanogramper-litre range and similar to those reported by Reddersen and Heberer (2003).

LABORATORY-SCALE TESTING OF CANDIDATE MEMBRANES Following is a summary of the materials and methods used to quantify membrane performance at the laboratory scale. Membrane performance quantified during laboratory-scale testing was used to select membranes for subsequent pilot- and full-scale investigations. Fouling apparatus. A bench-scale fouling test was performed with precompacted (12 h, with deionized water



at 760 kPa) membrane specimens using two cross-flow flat-sheet membrane units10 in parallel, with a total membrane surface area of 139 cm2. Membranes were fouled in parallel for ~ 220 h (or until flux stabilized) with 50 L of microfiltered (0.2-µm nominal pore size), pH-adjusted (6.3–6.5), nonnitrified secondary effluent from the Metro Wastewater Reclamation Facility in Denver, Colo. (Metro effluent). Experiments were conducted at 22.5 ± 1.5oC at fluxes between 15 and 20 L/m2-h and a feed flow rate of 1 L/min (superficial cross-flow velocity of 0.19 m/s) with both concentrate and permeate streams recirculated to the feedwater tank. Laboratory-scale membrane testing unit. Operation and rejection performance of candidate membranes was evaluated using a 35-L/min (feed flow) unit with two 4040 spiral-wound membrane elements in series and two feedwater matrixes, both at 23oC and an adjusted pH of 6.1–6.3 (using hydrochloric acid). The feedwater matrixes were ion strength–adjusted deionized water (sodium chloride and calcium sulfate added to achieve 200 mg/L of hardness as calcium carbonate and a conductance of

TABLE 2

1,400 µS/cm) and microfiltered (0.2 µm) Metro effluent. During experiments, permeate and concentrate flows were recycled to the 200-L feed tank. Operational data including flow rate, pressure, conductance, and temperature were logged by a supervisory control and data acquisition (SCADA) system. Membrane performance was evaluated under two operational regimes with varying recoveries (20 and 80%, respectively), which resulted in two permeate flux rates (30.5–37 L/m2-h and 20–24 L/m2-h, respectively). High recoveries were simulated by returning ~ 80% of the concentrate flow to the feed pump. Individual experiments were conducted for 2 h before samples were collected. Membrane testing protocol. Candidate membrane performance was investigated as follows: rejection of TOC (feed concentration of ~ 11 mg/L) and nitrogen (ammonia and nitrate at 40 mg/L N each), conductance, and SF were evaluated with the 4040 laboratory testing unit using nonnitrified feedwater; trace organic contaminant (spiked at 400–600 ng/L) rejection was evaluated using the 4040 laboratory testing unit with ion strength–

Results from unregulated trace organic compound analysis

Compound (CAS number)

Compound Classification

Molecular Weight g/mol

pKa*

Limit of Detection ng/L

Method Reporting Limit ng/L

Method of Detection

17ß-Estradiol (50-28-2)

Steroid hormone (endocrinedisrupting chemical)

272.4

10.4

0.4

1

HPLC-ELISA

Bisphenol-A (80-05-7)

Plasticizer (endocrine-disrupting chemical)

228.3

9.7

5

10

GC/MS

Caffeine (58-08-2)

Stimulant

194.2

1.4

20

40

GC/MS

Carbamazepine (298-46-4)

Anticonvulsant, antimanic agent

236.3

14.0

20

3

GC/MS

Clofibric acid (882-09-7)

Blood lipid regulator

214.7

3.2

2

10

GC/MS

Dichlorprop (120-36-5)

Pesticide

235.1

3.0

1

10

GC/MS

Diclofenac (15307-86-5)

Analgesic

296.2

4.2

1

2

GC/MS

Gemfibrozil (25812-30-0)

Blood lipid regulator

250.3

4.8

2

10

GC/MS

Ibuprofen (15687-27-1)

Analgesic

206.3

4.4

5

10

GC/MS

Ketoprofen (22071-15-4)

Analgesic

254.3

4.2

2

5

GC/MS

Mecoprop (93-65-2)

Pesticide

214.7

3.2

2

10

GC/MS

Naproxen (22204-53-1)

Analgesic

230.3

4.4

1

5

GC/MS

Phenacetine (62-44-2)

Analgesic

179.2

N/A

20

4

GC/MS

Primidone (125-33-7)

Antiepileptic

218.3

12.3

50

4

GC/MS

Salicylic acid (69-72-7)

Analgesic

138.1

3.0

5

10

GC/MS

Testosterone (58-22-0)

Steroid hormone (endocrinedisrupting chemical)

288.4

N/A

0.5

1.0

HPLC-ELISA

Tris(2-chloroethyl) phosphate (115-96-8)

Chlorinated flame retardant

285.5

N/A

20

40

GC/MS

Tris(1-chloro-2-propyl) phosphate (13674-84-5)


327.6

N/A

20

40

GC/MS

Tris(1,3-dichloro-2-propyl) phosphate (13674-87-8)


430.9

N/A

20

40

GC/MS

*Source: Syracuse Research Corp.’s PhysProp Database CAS—Chemical Abstracts Service, a division of the American Chemical Society, HPLC-ELISA—high-performance liquid chromatography-enzyme-linked immunosorbent assay, GC/MS—gas chromatography/mass spectrometry, N/A—not applicable



105

adjusted deionized feedwater. Flux decline as a result of fouling was investigated using the flat-sheet fouling unit with Metro effluent.

MF11 followed by RO using the ESPA2 and TFC-HR membranes. These full-scale RO membranes generally operate at 80–85% recovery, resulting in ~ 17.0–20.4 L/m2-h (10–12 gfd) permeate flux.

PILOT- AND FULL-SCALE TESTING On the basis of laboratory-scale performance testing, membranes were selected for pilot- and full-scale evaluations at a water reuse facility. The following materials and methods were used to perform pilot- and full-scale evaluations. Pilot-scale testing of candidate membranes. A mobile four-stage pilot-scale membrane skid using 4040 spiralwound NF/ULPRO membrane elements was used in a 2:2:1:1 pressure-vessel array with each pressure vessel holding 4, 3, 4, and 3 elements, respectively. The pilot-scale unit simulates the hydrodynamic conditions of a two-stage full-scale train with seven 8040 elements per vessel. The pilot was equipped with a customized SCADA system to monitor and log flux, pressure, and selected water quality parameters online (e.g., pH, temperature, conductance). Full-scale membrane testing and monitoring. Full-scale testing and monitoring was performed at a water recycling plant that desalinates secondary effluent for drinking water augmentation. Prior to the RO treatment trains, the non-nitrified secondary effluent feedwater is microfiltered, pH adjusted (sulfuric acid), dosed with antiscalant, and chlorinated to form chloramines with excess ammonia. The integrated membrane system consists of

TABLE 3

Membrane

RESULTS AND DISCUSSION The following sections summarize testing results from laboratory-, pilot-, and full-scale evaluations performed during this study. Laboratory-scale. Pressure, SF, and fouling. During 4040 membrane unit experiments with secondary effluent feedwater, the TFC-HR membrane required the highest feed pressure (~ 1,345 kPa) to achieve a flux of 30.5 L/m2-h (18 gfd) and subsequently displayed the lowest SF value (0.03 L/m2-h-kPa or 0.116 gfd/psi; Table 3). The ULPRO membranes TFC-ULP,12 TMG10,13 and ESPA2 required feed pressures similar to those for the NF membranes NF-200,14 TFC-S,15 and ESNA1-LF16 (760–965 kPa), resulting in SF values between 0.037 and 0.057 L/m 2 -h-kPa (0.15–0.23 gfd/psi). Four membranes— XLE,17 NF-90,18 NF-4040,19 and NF-27020—had SF values that were significantly higher than those for the other membranes tested, with membrane NF-270 being highly permeable (SF value of 0.11 L/m2-h-kPa [0.45 gfd/psi]). Although membrane manufacturers report that permeability among membrane elements of the same type can vary significantly (± 15–40%), the permeability results generated during testing generally followed the expected

Summary of laboratory-scale operational and rejection performance for candidate membranes

Manufacturer

Flux Decline* %

Initial Specific Flux† L/m2-h-kPa (gfd/psi) 0.03 (0.12)

Adjusted Specific Flux‡ L/m2-h-kPa (gfd/psi)

TOC Rejection† % (±)

Nitrate Rejection† % (±)

Ammonia Rejection† % (±)

0.025 (0.10)

98.6 (0.5)

95.8 (0.1)

98.2 (1)

TFC-HR


18

ESPA2


15

0.049 (0.2)

0.042 (0.17)

98.6 (0.5)

95.7 (1.7)

98.8 (0.5)

TFC-ULP


22

0.042 (0.17)

0.032 (0.14)

97.7 (0.8)

92.6 (0.4)

96.3 (0.5)

TMG10

Toray Industries (America) Inc. New York

16

0.057 (0.23)

0.048 (0.20)

96.7 (0.8)

95.1 (0.4)

97.2 (0.1)

XLE


36

0.071 (0.29)

0.047 (0.19)

98.9 (0.1)

85.1 (3.6)

92.4 (3.6)

ESNA1-LF


80

0.037 (0.15)

0.007 (0.03)

92.5 (3.3)

91.4 (2.5)

96.5 (0.2)

TFC-S


48

0.057 (0.23)

0.03 (0.12)

94.3 (1.2)

77.6 (2.2)

91.5 (2.1)

NF-90


30

0.074 (0.30)

0.052 (0.21)

97.1 (0.5)

79.3 (0.8)

88.2 (0.3)

NF-200


8

0.054 (0.22)

0.048 (0.20)

92.4 (0.1)

6.7 (4)

49.2 (2.3)

NF-4040

Dow/Filmtec, The Dow Chemical Co., Midland, Mich

24

0.091 (0.37)

0.069 (0.28)

93.5 (2.9)

0.5 (5)

47 (1.3)

NF-270

Dow/Filmtec, The Dow Chemical Co., Midland, Mich

39

0.11 (0.45)

0.067 (0.27)

93.5 (1.9)

–10.1 (5)

35.9 (1.8)

*Stabilized flux decline determined during bench-scale fouling experiments †Measured during 4040 lab-scale experiments with secondary effluent feedwater ‡Calculated by multiplying remaining flux during fouling tests with initial specific flux

106



Samples were taken from a full-scale, 2.5-mgd reverse osmosis train at West Basin Water Recycling Plant in California—a recycling plant that desalinates secondary effluent for drinking water augmentation.

trend: NF > ULPRO > RO. By using membranes with greater SF values, there is the potential for lowering both energy consumption and operating costs. The SF values determined for each membrane reflect the flux of a clean membrane and will decrease significantly with membrane fouling. Membrane fouling was investigated with the bench-scale fouling equipment and secondary treated effluent feedwater. Stabilized flux decline values and relevant characteristics for all candidate membranes are shown in Table 3. Because the candidate membranes displayed a wide range of fouling propensities and initial permeabilities, the fouling tests were useful in determining the operating characteristics of a membrane after a period of flux decline resulting from fouling. For example, one of the NF membranes, NF-90, exhibited twice the flux decline (30%) of two ULPRO membranes, TMG10 and ESPA2 (15–16%), but maintained a greater SF overall because of its high initial permeability. Of the candidate ULPRO and NF membranes tested, the NF-200, TMG10, TFC-ULP, and NF-4040 exhibited the least amount of flux decline while treating microfiltered secondary effluent, maintaining ~ 92, 87, 78, and 76% of initial flux by the time the experiment was terminated (~ 220 h), respectively. By combining the flux decline values with the SF values determined through laboratory 4040 testing, a certain degree of flux decline caused by fouling was deemed acceptable as long as an individual membrane’s adjusted SF value was significantly greater than the value achieved with the TFC-HR membrane, indicating the potential for cost savings. The ULPRO membrane with the highest

adjusted SF was the TMG10, followed closely by the XLE (Table 3). The NF membrane with the highest adjusted SF was the NF-270, followed by the NF-4040, NF-200, and NF-90, which exhibited adjusted SF values > 0.048 L/m2-h-kPa (0.20 gfd/psi; Table 3). TOC, nitrate, and ammonia rejection. TOC, nitrate, and ammonia rejection values (determined as the average of 20 and 80% rejection experiments; permeate flux rates of 30.5–37 L/m2-h and 20–24 L/m2-h, respectively) for candidate membranes during rejection experiments with secondary treated effluent feedwater are presented in Table 3. The RO and ULPRO membranes achieved TOC rejections > 95% with permeate concentrations of < 0.5 mg/L. The NF-90 membrane achieved a TOC rejection similar to that achieved with the RO and ULPRO membranes; NF200, NF-270, and NF-4040 membranes produced permeate TOC concentrations two times greater (~ 0.8 mg/L), resulting in ~ 90% rejection. The TFC-HR, TMG10, and TFC-ULP membranes exhibited > 95% rejection of ammonia (at permeate fluxes of 20–24 and 30.5–37 L/m2-h), resulting in permeate concentrations below 5 mg/L N (Table 3). The XLE and NF-90 membranes were only slightly less efficient in rejecting ammonia, with rejection values of 93 and 89%, respectively (Table 3). Nitrate and, to a lesser extent, ammonia rejection experiments with the candidate membranes highlighted the differences among the membranes that were used in this study. The NF-200, NF-4040, and NF-270 membranes were found to moderately reject ammonia (between 30 and 50%), and the NF-270 and NF-4040 membranes



107

Spiral-wound elements were loaded into a test vessel and integrated into a full-scale reverse osmosis train in order to compare the performance of two test membranes and to determine potential cost savings.

exhibited negative rejection of nitrate (Table 3). In contrast, the NF-90, TFC-S and ESNA1-LF membranes displayed ammonia and nitrate rejections similar to those achieved with the ULPRO and RO membranes (in the range of 80–90%). Permeate nitrate concentrations, however, exceeded the drinking water threshold of 10 mg/L N during most NF experiments, with the exception of the previously mentioned three membranes, NF-90, TFC-S, and ESNA1-FL, at permeate flux rates of 20–24 L/m2-h. One limitation of certain NF membranes for potable reuse applications arises when nitrogen species rejection (especially nitrate) is desired and feedwater nitrate concentrations exceed 10 mg/L N. Nitrate ions have been shown to be much more difficult to reject than ammonia ions, and rejection is highly membrane-dependent, with membrane electrical properties being the most important factor (Lee & Lueptow, 2001; Van der Bruggen et al, 2001). The observed negative rejection of nitrate by certain NF membranes is likely the result of the more mobile nitrate anion permeating preferentially to balance the transport of cations (i.e., sodium and hydrogen ions), which has been reported by various researchers investigating NF membranes (Bowen et al, 2002; Tsuru et al, 1991). It is also noteworthy that nitrate-rejection experiments were conducted with feed concentrations of 40 mg/L N, which is approximately four times higher than nitrate concentrations commonly observed in nitrified/denitrified secondary treated effluents. As a consequence, feedwater nitrate concentrations typical of nitrified effluents will likely result in combined permeate concentrations that comply with the drinking water maximum contaminant limit for nitrate. Trace organic compound rejection. Candidate membranes were evaluated for removal of the trace organic contaminants listed in Table 2. For select nonionic trace 108

organic contaminants (Figure 1), where rejection depends on size exclusion and solute-membrane interactions, increasing permeate concentrations were observed for membranes with higher permeate productivity and lower salt rejection, traits generally considered to separate “tight” membranes from “loose” membranes (Table 1; Nghiem et al, 2005; Bellona et al, 2004). The nonionic pharmaceutical residue phenacetine was quantified in all candidate membrane permeate samples collected during the 80% recovery experiments (20–24 L/m 2 -h permeate flux). Chlorinated flame-retardants tris(2-chloroethyl) phosphate (TCEP) and tris(1-chloro-2-propyl) phosphate (TCPP) were more frequently detected in permeate samples from NF membranes than from ULPRO and RO membranes. The largest nonionic trace organic compound used in the study, tris(1,3-dichloro-2-propyl) phosphate (TDCPP; molecular weight = 430.9 g/mol), was not detected in any of the permeate samples. The rejection of nonionic solutes increased with increasing size (e.g., molecular weight), which is expected based on solution diffusion theory and size exclusion (Bellona et al, 2004). Ionic (anionic) trace organic contaminants (Table 2) covering a size range of 138–296 g/mol, however, were well rejected (> 90%; Figure 2) by all membranes tested, because rejection of these solutes is achieved by both size and electrostatic exclusion (Bellona & Drewes, 2005; Nghiem et al, 2005). Electrostatic (or Donnan) exclusion is caused by an electric potential across a membrane or within a membrane polymer that restricts the passage of a charged solute (Bowen et al, 2002). Good overall rejection was observed for all membranes tested because of their overall negative surface charge (as quantified by streaming potential measurements, data not presented)



TF C -H R ES † PA TF 2 ‡ C -U TM LP† G 10 § X ES LE N ** A 1LF ‡ TF C -S † N F90 ** N F20 0* N * F40 40 ** N F27 0* *

Rejection—%

also be considered viable for this type of application. and the size of the ionic organic contaminants relative In contrast, the candidate NF membranes displayed a to inorganic ions such as sodium and chloride. range of rejection and operational characteristics, with Although no candidate membrane was found to reduce the NF membranes NF-90, TFC-S, and ESNA1-LF disall trace organic compounds below detection levels of playing characteristics of ULPRO membranes. Although the analytical methods used in this study, the RO and selection of the highly permeable NF-4040 and NF-270 ULPRO membranes TFC-HR, TFC-ULP, TMG10, and membranes would have been advantageous from an XLE efficiently rejected the trace organic compounds operational standpoint, these membranes did not meet targeted in this study. In addition, permeate concentrathe required permeate water quality. tions were generally below 10 ng/L or nonquantifiable, In another study, the NF-4040 membrane was chowith the exception of the low-molecular-weight pharsen for pilot-scale testing on a nitrified tertiary treated maceutical residue phenacetine. Ionic trace organic confeedwater and operated at a significantly lower feed prestaminants were efficiently removed by the RO and sure in comparison with the TFC-HR and ESPA2 memULPRO membranes because of the contributions of steric branes (Bellona & Drewes, 2007). Although NF-4040 hindrance and electrostatic exclusion. Although ionic permeate water quality during pilot-scale testing was simtrace organic solutes were more frequently detected in NF ilar to that for RO and ULPRO in terms of TOC and permeate samples, concentrations were < 40 ng/L, resulttrace organic compound removal, nitrate rejection was ing in rejection values > 90%. As expected, the limitation close to zero, which limits the application of a low-presof NF membranes was observed to be the removal of sure NF membrane to feedwaters that have nitrate conlow-molecular-weight nonionic organic solutes such as centrations of < 5 mg/L N. phenacetine and TCEP, representing sizes close to the membrane molecular-size cutoff. Selection of candidate membranes for pilot-scale testing. PILOT- AND FULL-SCALE CANDIDATE For this study, laboratory-scale membrane test results MEMBRANE TESTING were used to select membranes for treating municipal Pilot scale. Membrane operational performance. Durwastewater effluents at pilot scale. Candidate membranes ing pilot-scale testing, the TMG10 and NF-90 membranes were considered viable when the following conditions were met: FIGURE 1 Summarized laboratory-scale candidate membrane rejection adjusted SF values were greater than of nonionic (neutral) trace organic compounds* the adjusted SF value of the TFC2 HR RO membrane (0.03 L/m -hkPa or 0.116 gfd/psi), permeate con100 centrations of trace organic contaminants were similar to those 90 observed for the TFC-HR and ESPA2 membranes, nitrate rejection 80 resulted in permeate concentrations 70 below the drinking water standard (10 mg/L N), combined ammonia 60 and nitrate rejection resulted in per50 meate concentrations below 10 mg/L N, and TOC rejection resulted in 40 permeate concentrations of < 0.5 mg/L in accordance with CDPH’s TDCPP (MW 430.9) Groundwater Recharge Draft RegTCPP (MW 327.6) ulation for TOC (CDPH, 2007). TCEP (MW 285.5) The TMG10 and NF-90 memMe Phenacetine (MW 179.2) mb ran branes ultimately chosen for piloteT yp e scale testing were selected because of adjusted SF values that were greater than those for the TFC-HR MW—molecular weight (expressed in g/mol), TCEP—tris(2-chloroethyl) phosphate, TCPP—tris(1-chloro-2-propyl) phosphate, TDCPP—tris(1,3-dichloro-2-propyl) phosphate and ESPA2 membranes and because permeate water quality met the cri*At 80% recovery and permeate flux rate of 20–24 L/m2-h teria discussed previously. However, †Koch Membrane Systems, Wilmington, Mass. ‡Hydranautics, Oceanside, Calif. the other candidate ULPRO mem§Toray Industries (America) Inc., New York branes were similar in rejection and **Dow/Filmtec, The Dow Chemical Co., Midland, Mich. operational performance and would BELLONA ET AL | 100:9 • JOURNAL AWWA | PEER-REVIEWED | SEPTEMBER 2008


109

FIGURE 2 Summarized laboratory-scale candidate membrane rejection of ionic trace organic compounds

100 95 90 Rejection—%

85 80 75 70 65

TF C -H R ES † PA TF 2 ‡ C -U TM LP† G 10 § X ES LE ** N A 1LF ‡ TF C -S † N F90 ** N F20 0* N * F40 40 ** N F27 0* *

60

Me mb ran eT yp e

Ibuprofen (MW 206.3) Mecoprop (MW 214.7) Gemfibrozil (MW 250.3) Diclofenac (MW 296.2)

MW—molecular weight (expressed in g/mol) *At 80% recovery and permeate flux rate of 20–24 L/m2-h †Koch Membrane Systems, Wilmington, Mass. ‡Hydranautics, Oceanside, Calif. §Toray Industries (America) Inc., New York **Dow/Filmtec, The Dow Chemical Co., Midland, Mich.

FIGURE 3 Temperature-corrected specific flux values of membranes during pilot-scale testing at the full-scale facility TMG10* NF-90† TFC-HR‡

200 180

1,200

160 1,000

140 120

800

Pressure—psi

Pressure—kPa

1,400

100 600

0.291

2

Specifc Flux—L/m –hr–kPa

0.07

0.241

0.06 0.05

0.191

0.04 0.141 0.03 0.091

0.02 0.01 0

500

1,000

1,500

2,000

2,500

3,000

Specifc Flux—gfd/psi, 25°C

80

0.041 3,500

Operation—h *Toray Industries (America) Inc., New York †Dow/Filmtec, The Dow Chemical Co., Midland, Mich. ‡Koch Membrane Systems, Wilmington, Mass., at full scale

110

were operated at a system recovery of ~ 80% and a constant flux rate of 20.4 L/m2-h (11.8 gfd) and 21.2 L/m2-h (12.2 gfd), respectively. Temperature-corrected specific flux (TCSF) and pressure data for both the TMG10 and NF-90 membrane pilot-scale testing periods are shown in Figure 3. During the membrane compaction and fouling periods (~ 200 h), the TCSF of the TMG10 membrane decreased from ~ 0.04 L/m2-hkPa (0.17 gfd/psi) to 0.027–0.03 L/m2h-kPa (0.11–0.12 gfd/psi) and remained relatively stable throughout the testing period (i.e., feed pressures stabilized at around 1,140 kPa [165 psi]). The initial feed pressure increase and subsequent TCSF decrease of the NF-90 membrane occurred at a faster rate and to a greater extent (from 0.09–0.036 L/m2-h-kPa in 48 h) than for the TMG10 membrane, although the absolute required pressure was less (Figure 3). These findings were similar to results observed during the bench-scale flux-decline experiments. An additional decrease in the NF-90 membrane’s TCSF occurring between 600 and 1,300 h of operation resulted in TCSF values similar to those for the TMG10 membrane. Because of the initial rapid increase in feed pressure compared with that for the TMG10 membrane, the presumed advantage of using an NF membrane similar to the NF-90 membrane (i.e., low feed pressure and reduced energy costs) diminished fairly quickly (~ 500 h) as compaction and fouling occurred. NF-90 membrane testing was terminated after 1,300 h because it appeared that the TMG10 membrane had stabilized at a TCSF value that offered operational advantages (e.g., lower pressure than with the TFC-HR RO membrane operated at full scale, represented by a dashed line in Figure 3, ~ 0.022 L/m2-h-kPa) and the NF-90 membrane had not. The TMG10 membrane was subsequently tested at full scale using seven 8040 spiral-wound elements in a test vessel integrated into a full-scale RO train in parallel with the existing ESPA2 membrane to determine potential cost savings associated with the TMG10 membrane compared with the ESPA2 and TFC-HR membranes. The





111

Rejection—%

Chloroform—µg/L

rejection values between 20 and 40%. Feedwater chloESPA2 and TMG10 membranes operated similarly durroform concentrations were greater during NF-90 meming full-scale testing, stabilizing at an SF value of ~ 0.03 brane testing (between 15 and 24 µg/L), and combined L/m2-h-kPa over the 3,000-h runtime. Historical operpermeate concentrations were between 10 and 15 µg/L ational data from the facility demonstrated that the during the testing period, which resulted in rejection TFC-HR membrane exhibited SF values between 0.007 values between 35 and 47%. Although NF-90 memand 0.02 L/m2-h-kPa when treating feedwater with the brane chloroform rejection was greater than TMG10 same quality, which is significantly lower than values achieved with the ESPA2 and TMG10 membranes. TABLE 4 Summary of pilot- and full-scale results* TOC, ammonia, and UVA rejection. Average TOC and ammonia conFull Scale centrations for the TMG10 and NF-90 Pilot Scale (January 2004 to May 2005) membranes during pilot-scale testing and for the TFC-HR and ESPA2 memParameter TMG10† NF-90‡ TFC-HR§ ESPA2** branes during full-scale testing are Recovery—% 82 82 80—85 80—85 shown in Table 4. TOC, ammonia, TOC and UVA were well rejected by all Feed TOC—mg/L 11.8 13 11.3 11.6 membranes examined at pilot and full Permeate TOC—mg/L 0.2 0.18 0.37 0.31 scale. The TMG10 and NF-90 memTOC rejection—% 98.3 98.6 96.7 97.3 branes reduced feed TOC concentraAmmonia tions to < 0.3 mg/L (> 97% rejection), Feed ammonia—mg/L N 31.6 37 33.2 31.8 ammonia concentrations to < 3 mg/L Permeate ammonia—mg/L N 1.2 2.3 1.9 1.7 N (> 93%), and UVA values to near Ammonia rejection—% 96.2 93.8 94.3 94.7 –1 the detection limit (0.01 cm ) in perUVA meate samples collected during testFeed UVA—abs/cm 0.12 0.18 NA NA ing (Table 4). Because the feedwater Permeate UVA—abs/cm 0.014 0.013 NA NA was provided by a nonnitrifying wasteUVA rejection—% 88.3 92.8 NA NA water treatment plant, neither nitrate NA—not available, TOC—total organic carbon, UVA—ultraviolet absorbance nor nitrite was detected in any of the feedwater samples. *All values are averages †Toray Industries (America) Inc., New York A comparison of pilot-scale test‡Dow/Filmtec, The Dow Chemical Co., Midland, Mich. §Koch Membrane Systems, Wilmington, Mass. ing of the TMG10 membrane with **Hydranautics, Oceanside, Calif. full-scale operation indicates that this membrane performed similar to or FIGURE 4 Rejection of chloroform by the NF-90 and TMG10 membranes better than the ESPA2 and TFC-HR at pilot scale and the TFC-HR and ESPA2 membranes at full scale membranes in terms of TOC and ammonia rejection. All three membranes achieved high rejection and Feed 40 60 Permeate low ammonia and TOC permeate Pilot scale Full scale Rejection 35 concentrations. The NF-90 mem50 TMG10* NF-90† TFC-HR‡ ESPA2§ brane achieved TOC and ammonia 30 permeate concentrations similar to 40 25 those achieved with the RO and ULPRO membranes. 20 30 DBP rejection. Chloroform rejec15 tion data generated during pilot-scale 20 testing of TMG10 and NF-90 mem10 branes and full-scale monitoring of 10 5 TFC-HR and ESPA2 membranes are summarized in Figure 4. In general, 0 0 feedwater chloroform concentrations 1 2 3 5 6 7 8 9 1 3 4 6 8 were between 5 and 22 µg/L during Sampling Campaign TMG10 and NF-90 membrane test*Toray Industries (America) Inc., New York ing. The TMG10 membrane com†Dow/Filmtec, The Dow Chemical Co., Midland, Mich. ‡Koch Membrane Systems, Wilmington, Mass. bined permeate concentrations were §Hydranautics, Oceanside, Calif. between 4 and 9 µg/L, resulting in

Chloroform has been shown to be poorly removed by most RO and NF membranes because of adsorption to, and partitioning across, membrane materials (Xu Feed et al, 2006). Permeate Pilot scale Full scale Rejection 30 100 In contrast with the chloroform 90 results, haloacetic acid rejection TMG10* NF-90† TFC-HR‡ ESPA2§ 25 was > 90% for all four membranes 80 evaluated (Figure 5). TCAA con70 20 centrations in membrane perme60 ate samples were < 1 µg/L during 15 50 testing of the NF-90 and TMG10 40 membranes, and similar results 10 30 were observed for DCAA (results not presented). Although DCAA 20 5 and TCAA have molecular weights 10 and molecular sizes similar to ND ND 0 0 those of chloroform (128 and 163 1 2 3 4 1 3 g/mol versus 117 g/mol), both are Sampling Campaign negatively charged in the operaND—no data, TCAA—trichloroacetic acid tional pH range (6.3–6.5), which *Toray Industries (America) Inc., New York results in greater rejection because †Dow/Filmtec, The Dow Chemical Co., Midland, Mich. of electrostatic and steric exclu‡Koch Membrane Systems, Wilmington, Mass. sion (Ozaki & Li, 2002). §Hydranautics, Oceanside, Calif. Sampling campaigns for NDMA analysis were conducted, FIGURE 6 Feed and permeate concentrations of NDMA and calculated NDMA with samples collected from the rejection values for four membranes pilot skid (TMG10 and NF-90) and two full-scale treatment trains Feed (train 3, TFC-HR and train 4, 100 70 Permeate Pilot scale Full scale Rejection ESPA2). Results from the NDMA 90 TMG10* NF-90† TFC-HR‡ ESPA2§ 60 sampling campaigns are summa80 rized in Figure 6. Feedwater sam50 70 ples collected from the pilot skid 60 after 24 h of operation and again 40 50 after 2,500 h of TMG10 mem30 brane operation exhibited NDMA 40 concentrations between 20 and 60 30 20 ng/L. Feedwater sampling events 20 during pilot-skid NF-90 mem10 10 brane testing (after ~ 500 and 0 0 1,300 h of operation) revealed 1 2 3 4 1 2 NDMA concentrations between Sampling Campaign 10 and 25 ng/L, which were lower NDMA—N-nitrosodimethylamine than those observed during pilotscale TMG10 membrane testing. *Toray Industries (America) Inc., New York †Dow/Filmtec, The Dow Chemical Co., Midland, Mich. TMG10 pilot-skid permeate con‡Koch Membrane Systems, Wilmington, Mass. centrations were ~ 50% of feed §Hydranautics, Oceanside, Calif. concentrations and varied between 10 and 20 ng/L. These concentrations are equal to or greater than the CDPH drinking and full-scale TFC-HR and ESPA2 membrane chlorowater notification level of 10 ng/L for NDMA. NDMA form rejection, chloroform permeate concentrations rejection by the NF-90 membrane was similar to rejection were similar among all four membranes evaluated. Chloby the TMG10 membrane and was ~ 45% for both samroform therefore was poorly (20%) to moderately (40%) ples (Figure 6). The TFC-HR and ESPA2 membranes rejected by all four membranes investigated in this study. 112

Rejection—%

NDMA—ng/L

Rejection—%

TCAA Concentration—µg/L

FIGURE 5 Rejection of TCAA by the NF-90 and TMG10 membranes at pilot scale and the TFC-HR and ESPA2 membranes at full scale



PP af fe in e Ib up ro G f em en fib ro zi l TD C PP N ap ro xe Ke n to pr of en D ic lo fe na Pr c i m C id ar on ba e m az ep B i ne is ph en ol -A C

EP

TC

TC

Sa llc yl lc

ac i

d

Concentration—ng/L

TCEP and TCPP were both detected and quantified in operating at full scale rejected ~ 30% of the NDMA, numerous permeate samples collected during testing of the which resulted in permeate concentrations between 15 TFC-HR and TMG10 membranes but were also detected and 40 ng/L. The low rejection (< 40%) of NDMA reveals and quantified in two blank samples. Because the occurthe limitation of RO membranes in removing low-molerence of these compounds in permeate samples could not cular-weight nonionic polar solutes (74 g/mol for NDMA). be verified, they are not included in Table 5. However, a The NF-90 membrane rejected NDMA at levels similar to recent study by Snyder et al (2006) at full-scale membrane those for the ULPRO membranes TMG10 and ESPA2 facilities reveals that the detection and quantification of and the RO membrane TFC-HR. TCEP in RO permeate samples are in the concentration Rejection of unregulated organic contaminants. Durrange of 1–10 ng/L. ing pilot-scale testing at the full-scale facility (~ 4,500 h), On the basis of concentrations of the trace organic samples were collected for quantification of the unregucompounds quantified in feedwater samples collected lated trace organic compounds that are listed in Table 2, during testing (Figure 7) and on the number of permeate generating a large amount of data. A box-and-whisker samples with no trace organic contaminant detections, plot generated using feedwater trace organic compound the TMG10 and TFC-HR membranes exhibited very high data is shown below in Figure 7. organic solute removal efficiencies. No permeate detecFeedwater concentrations of the quantified trace tions were made during sampling campaigns with the organic compounds varied over the testing period and ESPA2 membrane, indicating that RO and ULPRO memranged from below quantification for certain compounds branes were effective barriers to unregulated organic to the low microgram-per-litre range for the chlorinated solutes quantified in the feedwater of the full-scale facilflame-retardant TCPP and the pharmaceutical residues ity. Of the solutes quantified in the feedwater, gemfibrozil, ibuprofen and gemfibrozil. Median concentrations of the pharmaceutical residues were between 50 and 400 ng/L, with salicylic acid, ibuprofen, primidone, FIGURE 7 Summarized feedwater trace organic compound detections and carbamazepine most frequently found in the feedwater samples. 3,000 Concentrations of the pharmaceutical residues ibuprofen, gemfibrozil, Sample number Maximum 12 and naproxen were highly variable, concentration 15 possibly because of fluctuations in 2,500 wastewater treatment and/or usage 75th percentile patterns. The chlorinated flameMedian retardants TCEP, TCPP, and TDCPP concentration were detected in more than 90% of 2,000 22 25th percentile the feedwater samples, with median Minimum concentration concentrations increasing in the folMethod reporting limit lowing order: TDCPP, TCEP, and 1,500 TCPP. Bisphenol-A, a plasticizer that 22 is also considered an endocrine disruptor, was infrequently quantified in feedwater samples in concentra15 1,000 22 tions between 20 and 250 ng/L. Summarized results from moni15 22 toring the TMG10 and NF-90 (pilot22 scale) and TFC-HR and ESPA2 (full500 22 15 scale) permeate streams for the select 15 trace organic compounds are shown 22 22 in Table 5. Bisphenol-A was detected 0 in permeate samples collected from the TMG10, TFC-HR, and NF-90 membranes. Ibuprofen was detected in one sample and quantified in the low nanogram-per-litre range in perOrganic Compound meate samples collected from the TCEP—tris(2-chloroethyl) phosphate, TCPP—tris(1-chloro-2-propyl) phosphate, TMG10 membrane at pilot scale. TDCPP—tris(1,3-dichloro-2-propyl) phosphate The chlorinated flame-retardants BELLONA ET AL | 100:9 • JOURNAL AWWA | PEER-REVIEWED | SEPTEMBER 2008


113

• category 2 membranes, characterized by lower pressure than the category 1 membranes, effectively rejected Pilot Scale Full Scale TOC and were selective for most trace TMG-10* NF-90† TFC-HR‡ ESPA2§ organic contaminants but were limited Samples—ng/L 14 10 6 5 in their removal of monovalent ions Compound including nitrate and ammonium (NFSalicylic acid

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