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Application of forward osmosis in reusing the brackish
concentrate produced in reverse osmosis plants with
secondary treated wastewater as feed solution: a case
study
W. D. Wang, M. Esparra, H. Liu and Y. F. Xie
ABSTRACT
This study evaluated the feasibility of forward osmosis (FO) in diluting and reusing the concentrate
produced in a reverse osmosis (RO) plant in James City County, VA. Secondary treated wastewater
(STW) was used as the feed solution. Findings indicated that pH had slight effects on the water flux of
the FO membrane. As the concentration of total dissolved solids (TDS) in the concentrate was diluted
from 12.5 to 1.0 g/L or the temperature in the STW decreased from 23 to 10 WC, the membrane flux
decreased from 2.2 to 0.59 and 0.81 L/(m
2
h), respectively. The FO membrane showed a good
performance in the rejection of organic pollutants, with only a small part of the protein-like
substances and disinfection byproducts permeating to the diluted concentrate. During an 89-hour
continuous operation, water flux decline due to membrane fouling was not observed. Controlling the
TDS in the second-stage FO effluent at 1.5 g/L, approximately 8.3% of the pump energy input could
be saved. The consumption of groundwater was reduced from 22.7 ×10
3
to 10.6 ×10
3
m
3
/d. FO was
proved to be an effective method in both diluting the discharged concentrate and reducing the
energy consumption of RO.
W. D. Wang
M. Esparra
Y. F. Xie (corresponding author)
Department of Civil and Environmental
Engineering,
The Pennsylvania State University,
Middletown,
PA 17057,
USA
E-mail: yxx4@psu.edu
W. D. Wang
H. Liu
Department of Environmental and Municipal
Engineering,
Xi’an University of Architecture and Technology,
Xi’an 710055,
China
Y. F. Xie
Department of Environment,
Tsinghua University,
Beijing 100084,
China
Key words |brackish water, drinking water, forward osmosis, secondary treated wastewater
INTRODUCTION
With the increase in water demand, many utilities are turn-
ing to non-traditional water, such as brackish water,
seawater, and even wastewater, as a possible water source
in many countries and areas of the world (National
Research Council (US) ). Accordingly, reverse osmosis
(RO) is becoming more and more common as a technique
to produce drinking water because of its high efficiency in
pollutant removal (Herzberg et al. ). This is the case
for James City County in Virginia, USA, where groundwater
with a high salt content (brackish water) is the main water
source. The raw water is desalinated by a RO system. One
of the major issues that the actual process presents is dispo-
sal of the concentrate. Due to the environmental impacts of
high salinity water, the discharge of desalination
concentrated solutions is highly controlled by the regulatory
agencies. Strict regulations have been created over the last
decade, making it more difficult to build a new facility or ret-
rofit/expand existing ones.
In the RO process, hydraulic pressure is used to oppose
and exceed the osmotic pressure of a saline aqueous feed
solution. The applied pressure is the driving force for the
mass transport through a semi-permeable membrane
(Greenlee et al. ;Malaeb & Ayoub ). The membrane
allows the passage of clean water, while the salt and other
contaminants are held back (National Research Council
(US) ). Accordingly, both the energy consumed in the
RO unit and the salt content in the discharged concentrate
are high (Stoughton & Lietzke ;Lattemann & Höpner
533 © IWA Publishing 2016 Journal of Water Reuse and Desalination |06.4 |2016
doi: 10.2166/wrd.2016.097
;Semiat ;Elimelech & Phillip ). Although great
gains have been made over the last decade in the RO tech-
nology to increase its energy efficiency, energy costs still
contribute as much as 75% of the operation cost of desalina-
tion plants (Semiat ). Innovations that help to reduce
energy consumption and the amount of concentrate dis-
charged will strengthen the suitability of RO in drinking
water purification.
The amount of energy used in RO mainly depends on
the salt concentration of the feed solution, and can range
from 3.0 to 7.0 kWh/m
3
for seawater desalination (National
Research Council (US) ). For brackish water desalina-
tion, however, where the concentration of salt (1–10 g/L)
is notably lower than that of the seawater (10–30 g/L), the
energy consumption is approximately 0.5–3.0 kWh/m
3
(National Research Council (US) ). For a feed solution
with low salt content, a notable decrease in the osmotic
pressure that must be overcome with applied hydraulic
pressure can be achieved (Semiat ;Subramani et al.
). Accordingly, the RO process would require less
energy to desalinate diluted brackish water than it does for
the raw brackish water, which is the upgrading plan pro-
posed by the authors for the upgrading of the existing RO
system in James City County. The secondary treated waste-
water (STW) was used as the dilution water.
Compared with direct dilution, forward osmosis (FO)
can enable the use of impaired water to dilute the water
entering the RO plant with high pollutant rejection effi-
ciency (Cath et al. ,). In the FO unit, a low salt
concentration water source, acting as the feed solution, is
separated by a selective FO membrane from water with a
high salt concentration, which acts as the draw solution.
The two solutions are placed on opposite sides of the mem-
brane. Fresh water will move from the feed solution towards
the draw solution, leaving contaminants retained in the
membrane (McCutcheon & Elimelech ;Qin et al.
). Accordingly, FO is probably a competitive technique
in diluting the influent of the RO system.
However, the quality of the STW differs from traditional
water sources and has limited usages because of the pres-
ence of toxic organic materials (Carrara et al. ), which
pose a proven or potentially high health risk to humans. Pre-
vious studies have mainly focused on the rejection
performance of FO for a few parts of specific hazardous
materials, such as pharmaceuticals, personal care products,
and flame-retardants (Hancock et al. ;Kong et al. ,
). The application of STW as an indirect source water
in the hybrid FO/RO system and the organic rejection per-
formance of FO have not yet been studied in depth. The
objective of this work, therefore, is to investigate (i) the
water flux variation law with the dilution of the concentrate
and the variation in temperature and pH of the STW, (ii) the
organic rejection performance of the FO membrane, and
(iii) the feasibility of FO in diluting and reusing the concen-
trate produced in the RO unit using STW as the source
water.
Existing RO desalination process in James City County,
VA
James City County is a 144-square mile municipality located
at the head end of the Virginia Peninsula, between the James
and York Rivers in Virginia. The James City Service Auth-
ority operates the largest solely dependent groundwater
based water system in the Commonwealth of Virginia. The
majority of its groundwater supply system is derived from
the Potomac and Chickahominy-Piney Point Aquifers. The
water treatment facility consists of five wells drawing brack-
ish groundwater from the Middle Potomac and Lower
Potomac aquifers. To obtain potable water, RO is used to
remove salts and other pollutants (Figure 1). The removed
salts, also known as concentrate, is discharged into the
James River. For every 18.9 ×10
3
m
3
/d of potable water pro-
duced, approximately 3.8 ×10
3
m
3
/d of concentrate is
discharged.
Four RO skids are installed in the treatment facility for
the desalination of the raw water extracted from the
Lower Potomac Wells. The RO skids consist of a two-stage
membrane system. The first stage consists of 20 vessels,
and the second stage consists of 10 additional vessels. The
vessels contain six membranes each. Groundwater is
pumped into the first stage membranes of the RO skid.
The permeate is discharged to the permeate line. The con-
centrate from the first stage RO becomes the feed water
for the second stage RO. The permeate from the second
stage combines with the permeate from the first stage. The
concentrate from the second stage RO is piped to the con-
centrate line.
534 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
The raw water used in the drinking water plant is
pumped from five wells. One of them supplies approxi-
mately 3.8 ×10
3
m
3
/d groundwater with low salt
concentration (Middle Potomac Well). The other four
wells supply approximately 18.9 ×10
3
m
3
/d groundwater
with a higher salt concentration (Lower Potomac Well).
The water quality of both types of wells is presented in
Table 1. Treatment with RO removes nearly all of the salts
in the Lower Potomac Well water. To maintain a normal
range of salt contents in the potable water, the RO effluent
is blended with the groundwater pumped from the Middle
Potomac Well as shown in Figure 1. The concentration of
Figure 1 |Current drinking water treatment process and the proposed upgrading plan based on a two-stage FO system (dashed line) in James City County.
Table 1 |Water quality of the well waters, concentrate, and the STW used in the experiment
Water quality parameters Middle Potomac Well Lower Potomac Well Concentrate STW
pH 7.9–8.2 7.6–7.8 8.3 7.5
Temperature (WC) 13 13 13 10–30
Conductivity (μS/cm) 1,500 4,500 25,430 9.1
TDS (mg/L) 1,000 2,300 12,500 280
Na
þ
(mg/L) 417 1,100 6,127.4 –
Ca
2þ
(mg/L) 6.1 20.3 105 18.0
Mg
2þ
(mg/L) 1.5 4.2 25 14.5
Iron (mg/L) 0.01 0.02 0.1 0.05
SiO
2
(mg/L) 41 29 164 –
Cl
–
(mg/L) 340 1,250 7,625 –
Alkalinity (mg/L) 320 340 1,910 52.0
Turbidity (NTU) <1.0 <1.0 <1.0 5.2
THMs (μg/L) ND ND ND 105.0
HAAs (μg/L) ND ND ND 15.5
TOC (mg/L) 0.06 0.05 0.19 11.2
–, data not available; ND, under determination limit.
535 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
total dissolved solids (TDS) in the brackish water is approxi-
mately 2,500 mg/L, which increases to approximately
13,000 mg/L after being concentrated. To avoid the dis-
charge of this highly concentrated brackish water, a two-
stage FO is suggested for the upgrading of the RO system
in James City County.
EXPERIMENTAL MATERIALS AND METHODS
Feed and draw solution of the FO system
The concentrate, which was used as the draw solution, was
obtained from the RO system in James City County, VA. The
STW, provided by a wastewater treatment plant in Middle-
town, PA, was used as the feed solution, as the plant is
conveniently located near the Harrisburg campus where
the experiments were conducted. This wastewater treatment
plant consists of a solids grinder and grit remover, primary
clarification, biological treatment with clarification, chlorine
addition for disinfection, and de-chlorination prior to dis-
charge. The quality of the concentrate and the STW are
listed in Table 1.
FO cross flow setup
A bench-scale FO system including a membrane permeation
unit, a water circuiting system, and a monitor and data
recording system (Figure 2) was constructed and operated
in the Environmental Engineering Laboratories at The
Pennsylvania State University Harrisburg campus. The
membrane permeation unit was made of acrylic plastic
and had channel dimensions of 10.5 cm long, 5.0 cm wide,
and 0.2 cm deep. The total effective membrane area was
52.5 cm
2
. The FO membrane was provided by Hydration
Technologies, Inc. (Albany, OR). The active layer made
from cellulose triacetate (CTA) is supported by an embedded
polyester screen to give the membrane asymmetry and
additional mechanical stability (McCutcheon & Elimelech
). The recommended working pH and temperature of
the FO membrane was in the ranges of 3.0–8.0 and
0–71 WC, respectively.
Feed and draw solutions were continuously circulated
between the storage tanks and the membrane cell. The
initial volumes of the feed and draw solutions were 350
mL and 1,000 mL, respectively. The cross-flow rate on
each side of the membrane was the same, and was
Figure 2 |FO setup used in the experiment.
536 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
controlled at 45 mL/min with a variable speed pump (Cole-
Parmer, Vernon Hills, IL). Changes in the weight of the
draw solution were recorded by a digital balance (TL2100,
Mettler Toledo, Germany) to determine the membrane
flux of the pure water. The active layer of the FO membrane
was facing the feed solution throughout the experiment.
To examine the effect of pH on the flux of water,
0.1 mol/L NaOH was added to the feed solution to increase
its pH from 7.5 to 8.5. The total running time was controlled
at approximately 15 min. To determine the effects of the
water temperature on the flux of the FO membrane, the
temperature of the feed solution was adjusted between 3
and 30 WC in a constant-temperature incubator. To assess
the effects of membrane fouling on water flux, the variation
of the water flux of the FO membrane was monitored in 89 h
of continuous running. The pH and temperature of the draw
solution were maintained at 8.0 and 23 WC, respectively.
Water quality and membrane structure analysis
methods
The trihalomethanes (THMs) and haloacetic acids (HAAs)
were analyzed by gas chromatography with an electron cap-
ture detector (Agilent 7890A, Santa Clara, USA) after liquid-
liquid extraction and methylation (for HAAs only) accord-
ing to the standard method of EPA551.1 and EPA552.3,
respectively. Standard solutions of four THMs, including
CHCl
3
, bromodichlorobromomethane (CHCl
2
Br), chlorodi-
bromomethane (CHBr
2
Cl), and bromoform (CHBr
3
), and
six HAAs, including monochloroacetic acid, monobromoa-
cetic acid, dichloroacetic acid (DCAA), bromochloroacetic
acid (BCAA), dibromoacetic acid, and trichloroacetic acid
(TCAA), were purchased from Sigma-Aldrich (Germany).
pH and temperature were monitored on-line using a
Logger Pro 3.8.6 with corresponding sensors. Before
measuring, all sensors were calibrated with standard sol-
utions. TDS was measured according to standard method
2540c (APHA et al. ). The concentration of organic mat-
ters was measured by a total organic carbon (TOC) analyzer
(TOC-VCPH, Shimadzu, Japan) and a spectrometer (T6,
Puxi, China) at 254 nm.
Fluorescence measurements were conducted using a
spectrofluorometer (FP-6500, Jasco, Japan) equipped with
a 150 W xenon lamp at 23 WC. A 1.0 cm quartz cuvette
with four optical windows was used for the analyses. An
emission scan was conducted from 250 to 600 nm at a wave-
length step of 5 nm and excitation wavelengths from 220 to
450 nm at a 5 nm interval. The detector was set to high sen-
sitivity, and the scanning speed was maintained at
2,000 nm/min. To observe the surface characteristics of
the FO membrane, scanning electron microscopy (SEM)
(JSM-6490LV, JEOL Ltd, Japan) was applied. The SEM
samples were sputter-coated with gold before conducting
scanning.
RESULTS AND DISCUSSION
Effects of TDS in the concentrate on the water flux of
the FO membrane
The experiments were conducted for 15 min to avoid the
effects of membrane fouling. As shown in Figure 3, the aver-
age water flux of the FO membrane was approximately
2.2 L/(m
2
h) when the TDS concentration in the concen-
trate (draw solution) was 12.5 g/L. This would benefit the
application of FO in the dilution and reuse of the concen-
trate. Without membrane fouling, the water flux could be
maintained above 2.0 L/(m
2
·h). Decreasing the concen-
tration of TDS in the draw solution to 6.0 g/L, the water
flux of the FO membrane decreased notably to 1.3 L/(m
2
·h).
Figure 3 |Variation of water flux with the concentration of TDS in the draw solution.
537 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
Unlike the draw solutions with high TDS, for the draw sol-
utions with TDS below 2.0 g/L, the variation of TDS
showed fewer effects on the membrane flux (Figure 3).
Decreasing the concentration of TDS in the draw solution
further to 2.0 g/L and 1.0 g/L, the membrane fluxes were
similar, at 0.72 L/(m
2
·h) and 0.59 L/(m
2
·h), respectively.
The water flux of the FO membrane is positively pro-
portional to the content of TDS in the draw solution. To
obtain a high water flux,alargedifferenceinTDSbetween
the feed and draw solution is desired. As described in Equation
(1) (Cath et al. ),thegreaterthedifferenceinosmotic
potential, the faster water moves through the membrane. Mean-
while,theosmoticpressureishighfortherawconcentrateand
keeps decreasing with the permeation of pure water through
theFOmembrane.Asthedifferenceinosmoticpressuregot
lower, the water flux decreased with operation time:
Jw¼A(σΔπΔP) (1)
where J
w
is the water flux, A is the water permeability con-
stant of the membrane, σis the reflection coefficient, and
ΔPis the applied pressure (Cath et al. ). For FO, ΔPis
zero, making the water flux directly proportional to the
difference in osmotic pressure.
Effects of pH and temperature in the STW on the water
flux of the FO membrane
The water flux of the FO membrane increased gradually as
the solution pH increased. The highest value of approxi-
mately 2.45 L/(m
2
·h) was obtained at pH 8.5 (Figure 4(a)).
The membrane used in this study was made from CTA
with embedded polyester screen support. Under basic con-
ditions, the number of deprotonated hydroxyl groups (with
negative charge) in the membrane matrix increased notably,
which probably forced adjacent polymers apart, thus
increasing water permeability (Braghetta et al. ;You
et al. ). However, because osmotic pressure is the driving
force in FO, and this pressure was relatively stable under
different pH conditions, the overall variation of membrane
flux with pH was slight.
Compared with pH, temperature showed a more signifi-
cant effect on the water flux through the FO membrane. As
shown in Figure 4(b), the average water flux was
approximately 2.75 L/(m
2
·h) when the temperature of the
draw solution was 30 WC, corresponding to the STW temp-
erature in summer, and decreased notably to
approximately 0.81 L/(m
2
·h) at 10 WC, corresponding to the
STW temperature in winter. A linear relationship was
found between the membrane flux and the solution tempera-
ture. The slope and intercept of the best fit line were
calculated to be 0.102 and –0.372 respectively, which was
close to the values (0.064 and –0.227) obtained by Wang
et al. ()using rainwater and cooling water as the feed
and draw solutions, respectively.
However, based on the van’t Hoff equation (You et al.
), the osmotic pressure of the feed solution decreases
with the decrease of its temperature, which will increase
the net osmotic pressure for driving the water in the FO
Figure 4 |Effects of pH (a) and temperature (b) in the STW on the water flux of the FO
membrane.
538 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
process. Besides net osmotic pressure, both the absolute and
kinematic viscosities of the feed solution increased notably
with the decrease of feed solution temperature (Phuntsho
et al. ). Kim et al. ()determined that as the solution
temperature decreased from 50 WCto20
WC, osmotic coeffi-
cients and diffusivity decreased by 6.6% and 48.3%
respectively, which increased the internal concentration
polarization (ICP). The increased ICP and solution viscosity
was probably the major reason that inhibited the permeation
of pure water at low temperatures.
Organic rejection performance of the FO membrane
One of the challenges in using the STW as a potable water
source is whether the FO membrane could effectively
remove the organic matter existing in the STW. The
amount of organic matter transported to the diluted concen-
trate was evaluated using both UV
254
and TOC. The UV
254
of the draw solution increased slightly from 0.195 to 0.211,
corresponding to an increase in TOC from 0.44 to 0.65 mg/L
in 8 h of operation (Figure 5(a)). This indicated that the
membrane used in this study provided a good organic
matter rejection performance. The amount of organic
pollutants entering the influent of the RO unit from the
STW was few.
Besides UV
254
and TOC, a 3D excitation–emission
matrix fluorescence spectrum was used to investigate the
type of organic matter that passed through the FO membrane.
The organic matter contained in the STW (feed solution) was
rich in humic acid-like substances (Figure 5(b)), associated
with the peak of Ex/Em ¼420–440/235–245 nm in the fluor-
escence spectrum (Henderson et al. ). Compared with
the STW, the concentrate (draw solution) was low in organic
pollutants, and no peak associated with organic matter was
observed (Figure 5(c)). After 8 h of operation, only one peak
(Ex/Em ¼375/350 nm) contributed by protein-like
Figure 5 |Variations of UV
254
and TOC in the draw solution (concentrate) with operation time (a), and the 3D excitation–emission matrix of the feed solution (STW) (b) and draw solution
before (c) and after (d) 8 h of operation.
539 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
substances (Reynolds ;Liu et al. ) relating to the
activity of bacteria was observed. This further proved that
the FO membrane used in this study had a high capacity to
remove organic matter (Figure 5(d)).
Besides the protein-like substances, part of the disinfec-
tion byproducts (DBPs) corresponding to the killing of
bacteria might also permeate to the draw solution. Maintain-
ing water temperature at approximately 23 WC, the system
was run for 8 h at pH 6.5, 7.5, and 8.5, respectively. The con-
centration of THMs in the STW was 105 μg/L (Figure 6(a)),
which was much higher than that of HAAs (15.5 μg/L).
However, the total amounts of THMs and HAAs permeating
through the FO membrane was similar under different pH
conditions. The concentrations of both THMs and HAAs
that permeated into the draw solution were in the range of
5.2–8.4 μg/L, indicating that the FO membrane was effective
in DBP removal.
The removal rates for THMs were above 95%, higher
than that of HAAs (50–60%) under the selected pH con-
ditions (Figure 6(a)). The low rejection rate of HAAs was
probably related to its low initial concentration in the STW
used in our study (Xie ). These rejections are comparable
to the values reported for NF membranes (56–100%) and RO
membranes (86–94% for HAAs) (Agus & Sedlak ). We
speculate that the mechanisms underlying the removal of
THMs and HAAs might be size exclusion and charge repul-
sion, similar to the NF/RO membranes (Bellona et al. ;
Kong et al. ). Unlike HAAs, all of the THMs were
highly volatile organic materials (Lee et al. ). As the
hydraulic retention time of the STW in the feed solution
tank increased, part of the THMs might enter the air under
the effect of volatilization. Further study showed that the con-
centration of THMs mainly existing in the forms of CHCl
2
Br
and CHCl
3
decreased notably from 101 to 6.7 μg/L after 4
days of retention (without stirring) in the feed solution tank
(Figure 6(b)). Compared with THMs, the concentration of
HAAs, mainly existing in the forms of DCAA, TCAA, and
BCAA, was relatively stable (Figure 6(c)).
Feasibility analysis of FO in brackish water desalination
system upgrading
Compared with RO, the water flux of FO was relatively
small, especially for the draw solution with low salt content.
To guarantee the average water flux, a two-stage FO in
which the TDS in the concentrate was diluted from 12.5
to 1.5 g/L in two series-connection draw solution tanks is
Figure 6 |Contents of DBPs permeating from the feed solution (STW) to the draw sol-
ution (concentrate) after running for 8 h (a), and the effects of hydraulic
retention time on the residual concentrations of THMs (b) and HAAs (c) in the
feed solution.
540 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
suggested for the upgrading of the existing RO system in
James City County, as shown in Figure 1. STW was used
as a feed solution. By controlling the retention time of the
concentrate in the draw solution tank, the TDS in the first-
stage FO effluent was maintained at 6.0 g/L, corresponding
to an increase in the volume from 3.8×10
3
to 7.6 ×10
3
m
3
/d.
Sixty per cent of the first-stage FO effluent was blended with
9.2 ×10
3
m
3
/d of the STW directly before it was discharged
to the James River. In the second-stage FO, 40% of the first-
stage FO effluent was further diluted four times. The efflu-
ent, with 1.5 g/L of TDS, was reused as the influent of the
RO unit. The consumption of the groundwater could be
reduced from 22.7 ×10
3
to 10.6 ×10
3
m
3
/d.
Considering the concentration of organic matter perme-
ated into the diluted concentrate, part of which was reused
as the influent for the RO, was limited based on accumulat-
ive values in 8 h, the effects of organic matter in the STW on
the quality of the treated drinking water could be ignored.
Before upgrading, the pump pressure needed in the RO
unit was approximately 0.6 MPa. Based on the water quality
shown in Table 1, NaCl and NaHCO
3
were assumed to be
contributing to the osmotic pressure generated by the brack-
ish water. Calculation results showed that after upgrading
the osmotic pressure existing between the two sides of the
RO membrane would decrease from 0.18 to 0.13 MPa so
as the salt content in the influent decreased from 2.3 to
Figure 7 |Variation in water flux in an 89-hour operation (a), and the morphological characteristics of the FO membrane before (b) and after (c) running for 89 h.
541 W. D. Wang et al. |Application of FO in reusing the brackish concentrate produced in RO plants Journal of Water Reuse and Desalination |06.4 |2016
1.78 g/L. Maintaining a water purification capacity of 22.7 ×
10
3
m
3
/d, the reduced pump pressure was determined to be
0.05 MPa, so approximately 8.3% of the input energy of the
pump station could be saved.
Besides, the fouling process of the FO membrane was
slow. As shown in Figure 7(a), as the operation time
increase, the water flux of the FO membrane decreased
from 2.3 to approximately 1.0 L/(m
2
·h). This decrease was
mainly caused by the dilution of the draw solution. Repla-
cing the diluted concentrate (draw solution) with the
concentrate taken from the pilot, the water flux of the FO
membrane returned to 2.3 L/(m
2
·h) even after running for
88 h. The above phenomenon indicated that the fouling pro-
cess of the FO membrane was slow or membrane fouling
showed little effect on the water flux. From the morphologi-
cal characteristics of the FO membrane (Figure 7(b) and
7(c)), the development of an obvious fouling layer, which
mainly contributed to the water flux decline (Lee et al.
), was not observed on the surface of the active side.
The low fouling rate of the FO membrane might be related
to the low osmotic pressure of the feed solution, which lim-
ited the transport of foulants to the surface of the membrane
(Tang et al. ).
CONCLUSIONS
Over the last decades, strict regulations have been estab-
lished to protect water bodies by limiting the amount of
TDS that are discharged into waterways. In this study,
the implementation of FO was considered to treat and
reuse the concentrate from the RO process. Solution pH
showed a slight effect on the flux of both pure water and
DBPs. With the decrease in water temperature, especially
in winter, the water flux of the FO membrane decreased
notably. Although part of the protein-like substances per-
meated into the draw solution, the FO membrane used in
this study showed a good performance in the rejection of
organic pollutants. More than 98% of TOC, 95% of
THMs, and 50–60% of HAAs were removed from the
feed solution. Considering that these removals were
based on accumulative values in 8 h, the detrimental
effect of using STW as source water on the quality of the
produced drinking water could be ignored. Furthermore,
during an 89-hour operation, there was no observed flux
decline due to membrane fouling. By controlling the efflu-
ent concentration of TDS in the second stage of FO at
1.5 g/L, the consumption of groundwater would decrease
to 10.6 ×10
3
m
3
/d. Approximately 8.3% of the pump
energy input was saved. Meanwhile, the environmental
impact caused by the discharge of concentrate into the
James River could be minimized.
ACKNOWLEDGEMENTS
This research was supported by the National Natural
Science Foundation of China (No. 21007050), the Science
and Technology Nova Program of Shaanxi (No.
2014KJXX-66), and the open funds of Zhejiang Provincial
Key Laboratory of Water Science and Technology.
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First received 5 July 2015; accepted in revised form 4 January 2016. Available online 10 February 2016
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