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pubs.acs.org/est
Comprehensive Bench- and Pilot-Scale Investigation of Trace Organic
Compounds Rejection by Forward Osmosis
Nathan T. Hancock,† Pei Xu,† Dean M. Heil,† Christopher Bellona,‡ and Tzahi Y. Cath*,†
†
Department of Civil and Environmental Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401,
United States
‡
Department of Civil and Environmental Engineering, Clarkson University, 8 Clarkson Avenue, Potsdam, New York 13699,
United States
bS Supporting Information
ABSTRACT: Forward osmosis (FO) is a membrane separation technology that has been
studied in recent years for application in water treatment and desalination. It can best be
utilized as an advanced pretreatment for desalination processes such as reverse osmosis
(RO) and nanofiltration (NF) to protect the membranes from scaling and fouling. In the
current study the rejection of trace organic compounds (TOrCs) such as pharmaceuticals, personal care products, plasticizers, and flame-retardants by FO and a hybrid FORO system was investigated at both the bench- and pilot-scales. More than 30
compounds were analyzed, of which 23 nonionic and ionic TOrCs were identified and
quantified in the studied wastewater effluent. Results revealed that almost all TOrCs were
highly rejected by the FO membrane at the pilot scale while rejection at the bench scale
was generally lower. Membrane fouling, especially under field conditions when wastewater effluent is the FO feed solution, plays a substantial role in increasing the rejection of
TOrCs in FO. The hybrid FO-RO process demonstrated that the dual barrier treatment
of impaired water could lead to more than 99% rejection of almost all TOrCs that were identified in reclaimed water.
1. INTRODUCTION
1.1. Reclamation and Water Reuse and the Presence of
Trace Organic Compounds (TOrCs) in Water and Wastewater. A recent report to Congress highlighted the conse-
quences of climate change on water resources in the Western
and Southwestern US.1 With expected drier climate, new water
resources will be required to fulfill the needs of growing population centers and economies. Coastal utilities already use seawater
as an unconventional source of drinking water, and inland communities will have to increasingly rely on reclaimed water and
desalinated brackish groundwater for nonpotable and indirect
potable reuse.2
In the US and around the world water providers are already
treating impaired water to supplement their fresh water supplies.
Examples include the Water District Groundwater Replenishment System in Orange County, California,3 the Prairie Waters
Project in Aurora, Colorado,4 and the NeWater treatment clusters in Singapore.5 These facilities rely on processes such as
microfiltration (MF), reverse osmosis (RO), and ultraviolet
irradiation with advanced oxidation processes (UV/AOP) to
treat impaired water to a high quality. Yet, one of the major
concerns related to water reuse is currently the presence of trace
organic compounds (TOrCs) in effluents from conventional
wastewater treatment facilities and consequently their current
prevalence in many water resources.6,7 While RO, NF, UV/AOP,
carbon adsorption, and natural system (i.e., riverbank filtration
and aquifer recharge) processes demonstrated efficient removal
r 2011 American Chemical Society
of TOrCs from impaired and treated water,7 12 new treatment
processes might be required to provide multibarrier protection of
potable waters.
1.2. Forward Osmosis As a New Process for Reclamation
of Impaired Water. Recent advancements in research and
development demonstrated that osmosis can be utilized in
engineered systems to separate contaminants from water using
the forward osmosis (FO) process.13 In FO, clean water can be
extracted from contaminated streams utilizing special semipermeable membranes that have contaminant rejection capabilities similar to those of RO membranes. In the process, water
diffuses from the feed stream, through a dense, semipermeable
FO membrane, and into a draxw solution that has a high osmotic
pressure.13 Past studies demonstrated that domestic wastewater,
anaerobic digester sludge, food and beverage products, brackish
groundwater, and even seawater could be effectively treated with
FO.14 16 A recent study also demonstrated the effective dual
barrier characteristics of the FO process when hybridized with
seawater RO for treatment of secondary and tertiary treated
domestic effluents.16 In the hybrid process, impaired water is first
treated by an FO membrane and water is drawn into a seawater
draw solution, and the diluted draw solution is desalinated by
RO to produce a stream of purified water (see Supporting
Received: May 16, 2011
Accepted: August 12, 2011
Revised:
August 2, 2011
Published: August 12, 2011
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Information, section S1). The rejection of wastewater constituents such as organic carbon, nutrients (i.e., ammonia, nitrate,
phosphate), and suspended solids by the two tight barriers was
very high (95 99.9%), the FO membranes protected the
sensitive RO membranes from fouling and scaling, and energy
saving could be achieved.16
1.3. Previous Studies That Employed FO To Remove TOrCs
from Impaired Water. An early bench-scale study by Cartinella
et al.17 demonstrated the rejection by FO of a limited number of
TOrCs from wastewater generated in life support systems. The
study investigated the removal of three hormones that were
spiked into batches of synthetic wastewater. Results revealed that
membrane fouling and the presence of surfactants in the feed
solution substantially improved rejection of TOrCs at higher
water recoveries. A more recent study by Cath et al.16 compared
TOrCs rejection by FO at the pilot- and bench-scale. A pilotscale FO-RO hybrid system was utilized, and a continuous stream
of secondary effluent was used as a feed stream to an FO plate
and frame membrane cell. Only a limited number of TOrCs was
quantified in the effluent. Bench-scale tests were performed with
water spiked with the same TOrCs. Results from short-term
experiments (7 14 days) demonstrated that rejection of TOrCs
by FO membranes and by the dual barrier FO-RO process are
very high (>95%) and can lead to potentially successful incorporation of the FO process in future water reclamation systems for
potable reuse. Although the previous studies demonstrated the high
efficiency of FO in removal of TOrCs, the experiments were too
short, with limited number of compounds, and with a custom-made
membrane cell.
Fundamental NF and RO studies8,18 20 have demonstrated
that the rejection of TOrCs is primarily governed by steric
interactions (i.e., increased rejection with increased molecular
size), electrostatic exclusion, and adsorption (often related to
solute hydrophobicity); however, other factors such as a solute’s
functional groups (e.g., hydroxyl groups, carbonyl groups, amide
groups) may play a role in TOrCs rejection.21 A similar systematic investigation is needed to better understand the rejection
mechanisms of TOrCs during FO treatment.
1.4. Objectives. The main objective of the current study was
to investigate the rejection of TOrCs by FO and by the dual
barrier FO-RO processes using mainly seawater as a renewable
source of draw solution. This includes testing for extended time
(i.e., months) of newer generation FO membranes in a novel
spiral wound commercial packaging configuration, with wastewater of different qualities (i.e., tertiary and secondary effluents),
and with a larger number of TOrCs having a broader range of
physical and chemical properties. Pilot-scale experiments were
conducted to assess the performance of commercial FO membrane modules under field conditions, and bench-scale experiments were conducted to further elucidate the mechanisms and
conditions controlling rejection of TOrCs by FO membranes.
Bench-scale tests were conducted with a flat sheet FO membrane
from the same cast, using synthetic feed solutions spiked with
similar types and concentrations of TOrCs that were observed in
the effluent of the treated wastewater, and with various draw
solutions compositions.
2. MATERIAL AND METHODS
2.1. Membranes. FO membranes made from a hydrophilic
cellulose-based polymer were acquired from Hydration Technology
Innovations (HTI) (Albany, OR). The membranes incorporate a
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cellulose triacetate (CTA) active layer, and a woven polyester
mesh is embedded in the CTA to create a flat sheet membrane.
For the bench-scale study, flat sheet FO membrane coupons
were used. For the pilot study the CTA membrane was employed
in a spiral wound packaging configuration that improves membrane-packing density (i.e., membrane active surface area per
module volume) for commercial applications (additional details
presented in Supporting Information, section S2).
Seawater RO spiral wound membranes (SW30 2540, Dow
Filmtec, Edina, MN) were used for the pilot RO system. These
membranes were chosen for their high salt rejection (>99.4%22)
that enables production of concentrated draw solution for the
FO process.
Water permeance and NaCl rejection for the fresh FO
membrane were measured during experiments conducted
in RO mode and found to be 0.78 ( 0.02 L/m2-h-bar and
93.2 ( 0.9%, respectively. Feed at a constant temperature of 25 °C
was either deionized water for permeance tests or a 2,000 mg/L
NaCl solution for rejection tests. Tests were conducted
with feed pressures of 100 and 150 psig (0.69 and 1.03 MPa).
Water permeance and NaCl rejection for the SW30 membrane
(1.31 L/m2-h-bar and 99.4%, respectively) were obtained from
the manufacturer.22
2.2. Test Systems. 2.2.1. Bench-Scale FO Apparatus. The FO
bench-scale apparatus utilized a membrane cell constructed with
symmetric flow channels on both sides of the membrane that
facilitated parallel, cocurrent flow along the 455-cm2 membrane.
A programmable logic control (PLC) system was developed to
maintain constant experimental conditions (i.e., feed volume of
3 L, draw solution concentration of 30 g/L, and system temperature of 20 °C) and to collect data during the experiments. Feed
and draw solution were continuously circulated between their
respective tanks and the membrane cell at a flow rate of 1.6 L/min,
and feed and draw solution samples were intermittently drawn
for analysis. Details about the design and operating conditions and control of the system are provided in our previous
publication.23
2.2.2. Pilot-Scale FO-RO System. A pilot-scale FO-RO hybrid
system was utilized for the long-term testing of the spiral wound
commercial FO membrane. Details about the design and operating conditions and control of the system are provided in our
previous publication16 and in the Supporting Information, section S3. In the current study the system was deployed at a
wastewater treatment research facility on the campus of the
Colorado School of Mines (Golden, CO). Approximately 7000
gallons per day (26.5 m3) of domestic wastewater are treated at
the facility by a demonstration scale sequencing batch membrane
bioreactor (SBMBR) system (Aqua-Aerobic Systems, Inc., Rockford, IL). Feed to the FO membrane was a continuous side
stream from the SBMBR operated with an ultrafiltration (UF)
membrane (Puron, KMS, Wilmington, MA). To simulate secondary treated effluent water quality, activated sludge from the
SBMBR was dosed into the SBMBR permeate at different rates.
2.3. Water Chemistry. 2.3.1. Draw Solutions. Synthetic sea
salt (Instant Ocean, Mentor, OH) was used for preparation of
draw solution for all pilot- and two bench-scale experiments.
During most of the pilot-scale experiment (approximately three
continuous months) the draw solution was maintained at a
concentration of approximately 30 g/L sea salt by the RO subsystem. For a limited time, the RO subsystem produced concentrated draw solution of 60 g/L sea salt. In one bench-scale
experiments, ACS grade sodium chloride (Fisher Scientific, Atlanta,
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Table 1. Average Concentrations of Major Constituents and Water Quality Parameters Measured in SBMBR Permeate and
Composition of the Feed Solution for the FO Bench-Scale Tests
pilot-scale FO feed
bench-scale FO feed
constituent
mg/L
mg/L
bicarbonate
boron
54.3 ( 1.7
0.08 ( 0.04
54
-
O-phosphate
potassium
pilot-scale FO feed
bench-scale FO feed
mg/L
mg/L
12.3 ( 2.5
11.7 ( 1.7
barium
0.06 ( 0.02
-
pH
7.1 ( 0.1
bromide
0.06 ( 0.01
-
silica
10.6 ( 1.1
5
7.0
-
calcium
36.1 ( 3.6
35
sodium
54.5 ( 3.6
56
chloride
72.0 ( 5.6
97
strontium
0.20 ( 0.02
-
3.9 ( 0.7
4a
sulfate
79.8 ( 5.4
magnesium
10.3 ( 1.0
10
nitrate
37.3 ( 15.4
SUVA, L/mg 3 m
-
TDS
DOC
a
constituent
2.4 ( 0.1
325.0 ( 22.7
74
332
5 mg/L humic acid sodium salt (Aldrich, St. Louis, MO) was added to the feed.
GA) was used as draw solution at 30 g/L to elucidate draw
solution chemistry effects on TOrCs rejection.
2.3.2. Feed Solutions. SBMBR permeate was the feed to the
pilot-scale FO membrane and was continuously pumped into the
feed side of the FO spiral wound module at a rate of 12 L/min.
The quality of the feedwater (SBMBR effluent with or without
activated sludge dosing) to the pilot FO fluctuated throughout
the testing period because of the decentralized nature of the
SBMBR facility. Fluctuation in the number of users, precipitation
events, and constantly changing conditions in the SBMBR system
may contribute to varying feedwater quality. Average SBMBR
permeate water quality based on 23 samples taken over a 38 day
period is summarized in Table 1. Data indicate that the dominant
ions in the SBMBR permeate are sulfate, chloride, sodium,
calcium, and magnesium. Variable nitrate concentration observed in samples may result from a variety of factors associated
with the SBMBR system operation (e.g., aeration cycles and
duration) and influent water quality. The concentrations of
major constituents that were added to the feedwater during the
bench-scale tests are also summarized in Table 1.
2.4. Analytical Methods for TOrCs. TOrCs were quantified
using a liquid chromatography tandem mass spectrometry (LCMS/MS) system assembled with an HPLC system (1200 Series,
Agilent Technologies Inc., Santa Clara, CA) coupled to a tandem
mass spectrometer (3200 Q Trap, Applied Biosystems, Carlsbad,
CA) according to the matrix suppression abating method
described in Vandeford and Snyder.24 To alleviate issues induced by matrix suppression of environmental samples, an
isotope of each TOrC was spiked into each sample at a known
concentration. The absolute recovery of the isotope was used to
adjust the concentration of the native TOrC. Solid phase
extraction was used to preconcentrate TOrCs from the samples
prior to LC-MS/MS analysis. The volume of sample used for
solid phase extraction varied depending on the dissolved organic
carbon (DOC) concentration in each sample. Volumes processed for RO product water, draw solution, and feed samples
were 500, 250, and 100 mL, respectively; however, draw solution
samples with total dissolved solids (TDS) exceeding 30 g/L were
diluted prior to solid phase extraction to abate low isotope
recovery.
Samples were analyzed using both ESI+ and ESI modes24 to
quantify 32 ionic and nonionic TOrCs. Of the 32 TOrCs, 23
were detected in samples and are summarized in Table 2. The
concentrations of TOrCs in the synthetic feed during benchscale experiments are also summarized in Table 2. Previous
research8,12 designated hydrophobic nonionic TOrCs as
those with log Kow values above 2. Ten of the 14 nonionic
TOrCs in this study were hydrophobic with log D values
(log Kow calculated at pH 6) above 2. Classification, structure,
and charge of each TOrC were determined using ChemIDplus
Advanced (United States National Library of Medicine, Bethesda,
MD), and the octanol water distribution coefficient (log Kow)
at solution pH of 6 (log D) was obtained from ACD/Laboratories software version 8.14 (ACD/LABORATORIES, Toronto,
Canada).
2.5. Experimental Procedures. 2.5.1. Bench-Scale Experimental Procedures. Three bench-scale experiments were performed. For each experiment, 3 L of feed solution was prepared
with deionized water dosed with the major ions and TOrCs
identified in the SBMBR permeate (Tables 1 and 2, respectively).
In the second and third experiments approximately 5 mg/L of
humic acid (Aldrich, St. Louis, MO) was added to the feed for a
targeted DOC concentration of 4 mg/L to examine the effect of
organic matter on solute rejection. One liter of draw solution was
prepared and transferred to the draw solution tank at the
beginning of each experiment. In the first and second experiments the draw solution was seawater (Instant Ocean) at 30 g/L
TDS, and in the third experiment the draw solution was sodium
chloride at 30 g/L to compare the impact of salt composition on
TOrCs permeation. Draw solution and feed concentrations were
maintained constant throughout the experiments using a PLC
system.23 Feed and draw solution samples were drawn for
analysis at the beginning of each experiment, after 1 L of water
permeated the membrane from the feed into the draw solution,
and after a second liter of water permeated into the draw solution
(end of the experiment). Water flux and reverse salt flux from the
draw solution to the feed were monitored and recorded throughout each experiment. TOrCs concentrations and water flux were
used to conduct mass balance and calculate TOrCs rejection by
the FO membranes (Section S5 in the SI). Membrane integrity
tests were conducted before and after the set of experiments to
confirm that the rejection of ions by the membrane did not
change during the experiments. Before each experiment, the feed
solution was recirculated on the feed side of the membrane for
12 h to ensure that adsorption of TOrCs to the membrane did
not affect mass transport and mass balance calculations. Following the 12 h stabilization period, each experiment lasted 4 5 h.
2.5.2. Pilot System Experimental Procedure. The pilot-scale
system was initially tested with SBMBR permeate as FO feed
continuously for 650 h (27 days). Subsequently, the FO feed was
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Table 2. TOrCs Detected by LC-MS/MS in ESI+ and ESI Modes and Select Physicochemical Properties
TOrC
classification
charge
log D @ pH 6a
pilot FO feed,b ng/L
MW
bench FO feed,c ng/L
ESI+
amitriptyline
antidepressant
positive
1.45
277.40
3.0 ( 1.1
atenolol
beta-blocker
positive
2.73
266.34
26.7 ( 10.2
30
benzophenone
preservative
neutral
3.18
182.21
90.1 ( 22.0
100
caffeine
stimulant
neutral
0.13
194.19
32.2 ( 18.0
50
carbamazepine
anticonvulsant
neutral
2.67
236.27
388 ( 301
500
DEET
insect repellent
neutral
1.96
191.27
38.8 ( 22.3
50
diazepam
benodiazepine tranquilizer
neutral
2.96
284.70
0.63 ( 0.1
-
dilantin
diphenhydramine
antiepileptic
antihistamine
neutral
positive
2.52
1.07
252.27
255.36
133 ( 28
132 ( 45
150
150
fluoexetine
antidepressant
positive
1.03
309.33
18.3 ( 6.0
20
hydrocodone
analgesic
neutral
0.61
299.37
14.3 ( 12.4
20
oxybenzone
ingredient in sunscreens
neutral
3.63
228.24
25.9 ( 8.3
30
-
primidone
anticonvulsant
neutral
0.40
218.25
1.6 ( 0.4
-
sulfamethoxazole
anti-infective
negative
0.49
253.28
185 ( 175
250
TCEP
flame retardant
neutral
2.67
250.19
366 ( 152
400
trimethoprim
anti-infective
positive
0.42
290.32
21.6 ( 17.6
40
bisphenol A
plasticizer
neutral
3.43
228.29
89.6 ( 26.8
100
diclofenac
anti-inflammatory agent
negative
2.23
296.15
38.4 ( 33.1
50
ibuprofen
anti-inflammatory agent
negative
2.12
206.29
34.1 ( 11.3
40
methylparaben
personal care product
neutral
1.86
152.15
23.7 ( 9.0
30
naproxen
anti-inflammatory agent
negative
1.81
230.26
74.1 ( 33.9
100
triclosan
antimicrobial
neutral
5.17
289.54
90.5 ( 23.7
100
triclocarban
antimicrobial
neutral
5.74
315.58
267 ( 41
300
ESI
a
Obtained from ACD/Laboratories software version 8.14 (ACD/LABORATORIES, Toronto, Canada). b Feed concentration of TOrCs measured in
grab samples drawn prior to FO membrane during intervals B through E. c Concentration of TOrCs in the feed solution for the FO bench-scale tests.
Table 3. Summary of Operating Conditions during the Pilot
Studyc
days of
DS concentration
DS flow rate
TSS concentration
operation
mg/L
L/min
mg/L
0 14 (A)
30,000 ( 600 (est.)
1.6 ( 0.3
0
14 21 (B)a
22 27 (C)a
54,500 ( 1300
27,600 ( 300
2.0 ( 0.3
2.3 ( 0.2
0
0
27 31 (D)
29,500 ( 350
2.2 ( 0.1
5.3 ( 1.6
31 37 (D)b
28,700 ( 1200
2.4 ( 0.3
15.9 ( 2.0
37 39 (E)b
29,600 ( 1300
2.3 ( 0.1
50.2 ( 6.3
a
Chemical cleaning of the RO subsystem performed between intervals B
and C. b Feed flow reversal performed on FO spiral wound membrane
for one hour prior to increasing TSS concentration during interval E.
c
Letter indices enumerated in the days of operation column are used to
correlate operation intervals identified in Figure 2.
constantly dosed with activated sludge from the SBMBR for an
additional 300 h (12 days) to study the effects of membrane fouling
on water productivity and on rejection of nutrients and TOrCs.
Activated sludge was dosed into the FO feed stream before the feed
pump and a 50-mesh strainer. Operating conditions, including
draw solution concentration, draw solution flow rate, and feed
total suspended solids (TSS) concentration for specific time
intervals during the pilot study are summarized in Table 3.
Samples for TOrCs analysis were collected during intervals B
through E. During 960 h (40 days) of continuous operation for
TOrCs rejection investigation the hybrid process produced approximately 7,700 L of RO product water and the FO membrane
processed more than 650,000 L of wastewater effluent.
3. RESULTS AND DISCUSSION
3.1. Bench-Scale Performance: TOrCs Rejection. Benchscale FO tests were performed to investigate the effect of draw
solution composition and organic matter on the rejection of
TOrCs in a relatively simple matrix for ease of analysis. TOrC
rejection during the three bench-scale experiments is summarized in Figure 1. Because several compounds (i.e., bisphenol A,
diclofenac, benzophenone, and triclosan) were not quantified in
one or more draw solution samples, rejection could not be
accurately determined and were not included in Figure 1.
Bench-scale FO rejection of TOrCs was between 40 and 98%
and depended primarily on molecular size and charge. Rejection
of positively and negatively charged TOrCs was greater than
80%, while the rejection of the nonionic TOrCs was more
variable, between approximately 40 and 90%. With the exception
of TCEP, the rejection of the nonionic TOrCs tended to increase
with increasing molecular weight, which is to be expected based
on hindered diffusion. The chlorinated flame retardant TCEP,
however, exhibited lower rejection than expected based on size.
Although TCEP was demonstrated to be highly rejected by RO and
NF membranes,25 rejection of TCEP in our past FO bench scale
study was consistently low.26 While ionic TOrC rejection was greater
than 80%, rejection of these compounds was lower than expected
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Figure 1. Rejection of select TOrCs by the FO membrane during the
three bench-scale experiments. In the first experiment feed contained
major ions and TOrCs and draw solution was seawater at 30 g/L. In the
second experiment feed contained major ions, TOrCs, and humic acid,
and draw solution was seawater at 30 g/L. In the third experiment feed
contained major ions, TOrCs, and humic acid, and draw solution was
NaCl at 30 g/L. Numbers in parentheses represent molecular weight of
the compound. ** designates questionable results and rejection could
not be calculated. Error bars represent standard deviation derived from
three sampling events in each of the experiments.
Figure 2. Compiled water flux data from the pilot-scale system. Letters
marked on the graph indicate intervals of interest during the pilot study
and are defined in Table 3.
based on previous experiments demonstrating sodium chloride rejection of approximately 94%. This less than optimal rejection of ionic
compounds could be explained by the relatively low water flux
across the membrane (approximately 5 L/m2-h) and the relatively low surface charge of the cellulose acetate FO membrane
(zeta potential lower than 10 mV at pH range of 5 to 9).
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The addition of humic acid to the feedwater when operating
with seawater draw solution had minimal effect on the rejection
of TOrCs by the FO membrane. Generally, the type of draw
solution employed did not substantially affect rejection of TOrCs
during bench-scale experiments. Methylparaben was the only
compound that exhibited a substantial increase in rejection with
the NaCl draw solution compared to the simulated seawater
draw solution (oxybenzone exhibited a statistically insignificant
increase as well); however, there is no apparent explanation for
this behavior. DEET was detected at a higher concentration in
the draw solution than in the feed in samples from the NaCl draw
solution experiment, indicating that the sample may have been
contaminated during processing for LC/MS-MS analysis.
3.2. Pilot-Scale Performance. 3.2.1. Water Flux. Water flux
was continually monitored during the pilot study. Water flux as a
function of operation time is shown in Figure 2. Letter indices
represent operation intervals enumerated in Table 3. During
interval A the pilot system was operated for approximately 250 h
with SBMBR permeate as FO feed before reaching a steady-state
water flux. This was likely due to interaction of the membrane
with various constituents in the feed that changed the surface
properties of the virgin FO membrane. Also, a film of organic and
biological foulants accumulated on the FO membrane surface
during this time interval and further contributed to the flux
decline. Membrane cleaning in similar experiments was not able
to restore the initial water flux of a new FO membrane.27 During
the last 100 h of interval A the water flux was nearly constant.
During interval B the draw solution concentration was
doubled, and although the driving force for the process (i.e.,
the osmotic pressure of the draw solution) was doubled during
this interval, water flux did not increase proportionally. This is
attributed to internal concentration polarization phenomenon in
FO.28 Water flux during this interval was almost constant, and
additional flux decline from membrane fouling was not observed.
Chemical cleaning of the RO subsystem and draw solution
channels in the FO spiral-wound membrane was performed
between interval B and C to remove mineral scale within the
RO subsystem. During interval C, draw solution concentration
was reduced to approximately 30 g/L sea salt. An equivalent
water flux to the end of interval A was observed during this time
period. During interval D activated sludge was dosed into the FO
feed stream to achieve 5 and later 16 mg/L TSS, and a decrease in
water flux was observed. During interval E the dosing rate of
activated sludge was increased to achieve approximately 50 mg/L
TSS in the FO feed, and additional water flux decline was
observed.
3.2.2. Pilot-Scale Performance: TOrCs Rejection. Samples
were drawn from the feed, draw solution, and product water of
the RO pilot system during interval B through E and analyzed by
the isotope dilution LC-MS/MS method. Isotope recoveries for
TOrCs in each stream are summarized in the Supporting
Information, section S4. All isotope recoveries exceeded minimum thresholds for accurate TOrC concentration quantification
based on previous research by Vanderford and Snyder.24 Of the
23 compounds detected in the SBMBR permeate (FO feed), 14
are nonionic, four are negatively charged, and five are positively charged at solution pH between 6 and 7 (representative of the draw solution and feed pH, respectively). Nine
TOrCs were analyzed for but were not detected in the samples.
These include acetaminophen, atrazine, cimetidine, gemfibrizol,
ketoprofen, propylparaben, meprobamate, 4-n-nonylphenol, and
norfluoxetine.
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Figure 3. Rejection of TOrCs by the hybrid process. TOrCs are divided into classifications based on charge and hydrophobicity. Nonionic hydrophobic
compounds are those with log D values greater than two. TOrC molecular weight is indicated in parentheses after the name of each compound.
Operation intervals defined in Table 3 are shown above each column of results. (†) corresponds to TOrC concentrations measured below the detection
limit for the instrument in the draw solution (red) or RO product water (blue) and rejection cannot be calculated. (X) corresponds to instances where
TOrCs or their isotopes produced poor chromatographs and a nondetect of that TOrC is recorded in the draw solution (red) or RO product water
(blue). (*) designates assumption of 100% rejection because the compound was detected only in the feed and not in the draw solution or RO product
water. (cluster of four black diamonds) designates samples where TOrC concentration was measured in the feed and the RO product water but the draw
solution sample registered a nondetect. Error bars represent standard deviation derived from four sampling campaigns during interval B, seven sampling
campaigns during interval C, nine sampling campaigns during interval D, and three sampling campaigns during interval E.
Average feed concentration of TOrCs with their associated
standard deviation is summarized in Table 2. Concentration of
TOrCs detected in the feed varied over the 25 day sampling
campaign following fluctuations in usage patterns (e.g., diurnal
variation29) or wastewater treatment.25 TOrC concentrations
varied substantially between different constituents. Dilantin,
diphenhydramine, sulfamethoxazole, TCEP, and triclocarban
were routinely detected at concentrations exceeding 100 ng/L.
Recent studies demonstrated that these compounds are routinely detected in wastewater treatment plant effluents at concentrations significantly greater than LC-MS/MS quantification
limits.30 Other TOrCs, such as amitriptyline, diazepam, and
primidone, were detected at very low concentrations (i.e., below
10 ng/L) likely due to their low usage by the residents. Rejection
of several TOrCs that were poorly quantified during bench-scale
experiments (i.e., bisphenol A, diclofenac, benzophenone, and
triclosan) was well characterized during the pilot-scale investigation. There was negligible difference in TOrC concentrations
between SBMBR permeate and SBMBR permeate dosed with
activated sludge.
Rejection of TOrCs by the hybrid FO-RO process was
calculated for all 23 compounds detected in the SBMBR permeate. Rejection of nonionic, nonionic hydrophobic, negatively
charged, and positively charged TOrCs by the hybrid process is
summarized in Figure 3. TOrCs are listed in the order of
increasing molecular weight. Three rejection values are calculated for each TOrC; these include rejection by the FO membrane, rejection by the RO membrane, and total rejection of the
hybrid FO-RO process. Rejection measurements for each group
of TOrCs are also divided into three discrete operation intervals,
including intervals B, C, and combined D and E (Table 3).
Across all TOrC groups, the RO membrane generally
provided a similar rejection of contaminants to the FO
membrane. However, a few exceptions were observed: the
RO system provided significantly better rejection than the FO
system for bisphenol A during intervals B and C and the FO
system provided better rejection for methylparaben (all intervals), oxybenzone (intervals C, D, and E), amitriptyline (all
intervals), and triclosan (Interval B). It is worth noting that
because low feed concentrations were quantified for many
compounds, and often near the detection limit of the LC-MS/
MS, rejection can be a poor metric for comparison. Also,
reduced apparent rejection of constituents may be expected
under conditions of low water flux;31 in the current study the
RO flux was adjusted to produce permeate at the same flow
rate of the FO module.
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During intervals C, D, and E, the rejection of nonionic
compounds (Figure 3) by the RO membrane generally increased
with increasing molecular weight in agreement with findings in
previous research.8 Compounds exhibiting average RO rejection less than 90% during one or more operation intervals
included methylparaben, caffeine, benzophenone, oxybenzone,
bisphenol A, triclosan, and amitriptyline. Several of these compounds including methylparaben, bisphenol A, and triclosan
have phenolic functional groups, which were demonstrated to
have lower than expected rejection based on molecular size due to
adsorption to and partitioning through membrane materials.32 34
Benzophenone has been quantified in RO permeate samples collected from water reuse facilities, which is likely due to hydrophobic
interactions with membrane materials.35
Rejection of TOrCs during pilot-scale experiments was significantly greater than observed for bench-scale experiments
under all conditions evaluated (Figures 1 and 3). While the exact
reason for this disparity is unclear, membrane compaction, the
establishment of a fouling layer, and optimized hydrodynamic
conditions in the pilot-scale system may result in more efficient
rejection. Furthermore, the FO membrane achieved substantial
removal of all constituents except for caffeine during interval B
and C. It is possible that the relative hydrophilicity of caffeine (log
D equal to 0.13) and its relatively small molecular weight
contribute to its ability to partition into the relatively hydrophilic
FO membrane and to diffuse through the active layer.
Hydrophobic nonionic compounds demonstrated lower correlation with molecular weight (e.g., benzophenone and triclosan
rejection by RO, and bisphenol A rejection by FO); yet, with the
exception of oxybenzone during interval B, high rejection of these
constituents by the hybrid process was maintained (Figure 3).
Bisphenol-A rejection by the FO membrane was consistently the
lowest of all TOrCs among all groups. A relatively high hydrophobicity (log D equal to 3.43) and small molecular weight may
partially explain the ability of bisphenol A to diffuse through the
membrane. It is also possible for the phenolic structure of
bisphenol A to more easily associate with the acetylated hydroxyl
groups of the CTA membrane, thus enhancing its transport
through the membrane by increasing its concentration at the
membrane surface.33 Enhanced rejection of bisphenol A was
observed during intervals D and E, when small doses of activated
sludge were injected into the FO feed stream. Previous research19
suggests that a fouling layer on the membrane surface could
separate and inhibit the interaction of the bisphenol A molecule
with the membrane surface, thus increasing rejection.
Rejection of charged TOrCs was generally high (Figure 3) and
is in agreement with previous research.8,36,37 Negatively charged
constituents may be rejected by electrostatic exclusion arising
from the negative surface charge of the CTA and SW30
membranes. The mechanism for rejection of positively charged
compounds is not completely understood; yet, results from
previous studies36 39 indicate that constituent rejection in excess
of 90% is possible.
The dual barrier FO-RO process was able to achieve consistently high removal of TOrCs from the wastewater feed stream.
Accumulation of TOrCs in the draw solution closed loop16
(Supporting Information, section S3) was observed; however, a
mass balance of the water and TOrCs diffusing through the
membrane between sampling intervals revealed that a high rejection of these constituents was maintained. TOrC accumulation
in the draw solution also challenged the SWRO membrane
with greater concentration of these constituents than would be
ARTICLE
expected in an open loop system (Supporting Information,
section S1). Despite the shortcomings associated with operating
the system with the draw solution in a closed loop, total system
rejection of TOrCs generally exceeded 99%.
’ ASSOCIATED CONTENT
bS
Supporting Information. Details on the FO-RO dual
barrier system for high efficiency water purification, novel spiral
wound FO membrane modules, pilot-scale FO-RO system, and
TOrCs analysis and rejection calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: (303) 273-3402. Fax: (303) 273-3413. E-mail: tcath@
mines.edu.
’ ACKNOWLEDGMENT
The authors acknowledge the support of California Department of Water Resources (Grant 46-7446-R-08) and the AWWA
Abel Wolman Fellowship. Special thanks to Aqua-Aerobic Systems, Inc. and to Hydration Technologies Innovations for
providing membranes, systems, and Supporting Information
for this study and to Monte Dipalma for his analytical support.
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