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Trace organic solutes in closed-loop forwardosmosis applications: Influence of membranefouling and modeling of solute build-up

Arnout D’Haese a,*, Pierre Le-Clech b, Sam Van Nevel c, Kim Verbeken d,Emile R. Cornelissen e, Stuart J. Khan f, Arne R.D. Verliefde a,g

aParticle and Interfacial Technology Group, Ghent University, Coupure Links 653, B-9000 Ghent, BelgiumbUNESCO Centre for Membrane Science and Technology, UNSW Sydney, NSW 2052, Australiac Laboratory for Microbial Ecology and Technology, Ghent University, Coupure Links 653, B-9000 Ghent, BelgiumdDepartment of Materials Science and Engineering, Ghent University, Technologiepark Zwijnaarde 903,

B-9052 Zwijnaarde, BelgiumeKWR Watercycle Institute, Groningenhaven 7, Postbus 1072, 3433 PE Nieuwegein, The Netherlandsf School of Civil & Environmental Engineering, UNSW Sydney, NSW 2052, AustraliagDepartment of Sanitary Engineering, Delft University of Technology, Stevinweg 1, 2628CN Delft, The Netherlands

a r t i c l e i n f o

Article history:

Received 15 February 2013

Received in revised form

14 May 2013

Accepted 2 June 2013

Available online 12 June 2013

Keywords:

Forward osmosis

Organic micro pollutant

Membrane fouling

Biofilm

Modeling

* Corresponding author. Tel.: þ32 (0)9 264 99E-mail address: arnout.dhaese@ugent.be

0043-1354/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.watres.2013.06.006

a b s t r a c t

In this study, trace organics transport in closed-loop forward osmosis (FO) systems was

assessed. The FO systems considered, consisted of an FO unit and a nanofiltration (NF) or

reverse osmosis (RO) unit, with the draw solution circulating between both units. The

rejection of trace organics by FO, NF and RO was tested. It was found that the rejection

rates of FO were generally comparable with NF and lower than RO rejection rates. To assess

the influence of fouling in FO on trace organics rejection, FO membranes were fouled with

sodium alginate, bovine serum albumin or by biofilm growth, after which trace organics

rejection was tested. A negative influence of fouling on FO rejection was found which was

limited in most cases, while it was significant for some compounds such as paracetamol

and naproxen, indicating specific compound-foulant interactions. The transport mecha-

nism of trace organics in FO was tested, in order to differentiate between diffusive and

convective transport. The concentration of trace organics in the final product water and the

build-up of trace organics in the draw solution were modeled assuming the draw solution

was reconcentrated by NF/RO and taking into account different transport mechanisms for

the FO membrane and different rejection rates by NF/RO. Modeling results showed that if

the FO rejection rate is lower than the RO rejection rate (as is the case for most compounds

tested), the added value of the FO-RO cycle compared to RO only at steady-state was small

for diffusively and negative for convectively transported trace organics. Modeling also

showed that trace organics accumulate in the draw solution.

ª 2013 Elsevier Ltd. All rights reserved.

11.(A. D’Haese).ier Ltd. All rights reserved.

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 4 5233

1. Introduction Although previous studies have investigated TOrCs rejec-

ForwardOsmosis (FO) has gained attention in recent years as a

water treatment technology capable of handling heavily

impaired water sources and capable of concentrating solu-

tions with a high fouling potential (Cath et al., 2006; Zhao

et al., 2012). The main benefits of FO in this regard are: the

production of high quality permeate because of a high rejec-

tion of different pollutants and operation under osmotic

driving force without the need for a hydraulic pressure dif-

ference (Zhao et al., 2012). FO has been used to reclaim water

from (un)treated domestic waste water (Alturki et al., 2012;

Cath et al., 2010; Cornelissen et al., 2008; Hancock et al.,

2011; Zhang et al., 2011), anaerobic digestion concentrate

(Holloway et al., 2007), and to concentrate liquid foods

(Sant’Anna et al., 2012).

Impaired water sources, such as waste water treatment

plant (WWTP) effluents, are often contaminated with trace

organic compounds (TOrCs) (Daughton and Ternes, 1999;

Ternes et al., 2005). TOrCs are anthropogenic organic com-

pounds present inwastewater at concentrations in the ng/L to

mg/L range. This group of compounds consists of, among

others, pharmaceuticals, personal care products, flame re-

tardants and pesticides. One potential consequence of chronic

exposure to TOrCs is endocrine disruption (Markey et al.,

2002). Although some controversy remains whether endo-

crine disruption has significant effects on humans

(Giwercman, 2011; Sharpe, 2003), the effects of endocrine

disruption caused by estrogenic compounds in aquatic ver-

tebrates has been reported (Sumpter and Johnson, 2005).

Conventional waste water treatment systems, such as coag-

ulation, trickling filters, sand filters and activated sludge

remove TOrCs to varying degree, but often unsatisfactory

when waste water is to be reclaimed. Svenson et al. (2003)

found an increased removal rate of estrogenic compounds in

treatment media with an increased bioactivity. Ternes (1998)

found a TOrCs removal rate varying between 10 and almost

100% for pharmaceuticals in German WWTPs, which is the

same conclusion reached by Van De Steene et al. (2010) who

also found removal rates varying from 0 to almost 100%.

For the practical implementation of FO, maintaining a high

rejection of TOrCs under different operating conditions is a

key challenge, especially if FO is used to produce reclaimed

water from WWTP in- or effluent. Alturki et al. (2012) noted

that rejection of TOrCs by FO in an osmotic MBR (OMBR), of

which the sludge was conditioned to the tested TOrCs, was

consistently high for the solutes with a molecular weight

above 266 g/mole, while the rejection of smaller solutes

appeared to relate to their biodegradation susceptibility. This

could indicate that the actual rejection rates of the smaller

soluteswere quite low. Hancock et al. (2011) have tested TOrC-

rejection in both bench- and pilot-scale installations, using

MBR-treated domestic waste water as a feed. The rejection of

hydrophobic nonionic compounds by FO appeared to improve

with increasing TSS concentration in the feed, which could

indicate sorption of these compounds onto the TSS and sub-

sequent filtration of the TSS. The rejection of mainly nega-

tively charged organic solutes was consistently high,

regardless of the TSS concentration.

tion by FO membranes, there is still relatively limited knowl-

edge on the actual transport mechanisms of these solutes in

FO, which contrasts the transport of TOrCs in pressure-driven

processes such as NF and RO (Bowen et al., 1997; Kim et al.,

2007; Ramon et al., 2012; Verliefde et al., 2009a). In addition,

although the influence of fouling by several water matrices on

rejection of TOrCs has been investigated in practice, most

studies have not tried to identify underlying mechanisms of

influence of fouling on rejection. In contrast to studies on NF/

RO (Botton et al., 2012), the influence of different model fou-

lants and of biofilm formation on rejection of TOrCs in FO has

not been systematically investigated.

When FO is used to reclaim water from impaired sources,

the effect of draw solution regeneration in a closed-loop sys-

tem on the TOrCs concentration in the final product water has

not been investigated yet. Hancock et al. reported a build-up of

TOrCs in the draw solution (Hancock et al., 2011) when using

RO to regenerate the FO draw solution (consisting of NaCl) in a

closed-loop configuration. Cath et al. (2010) made a similar

observation in a closed loop FO-RO configuration. Although

both groups reported a total rejection of TOrCs by the com-

bined FO-RO system in the order of 99%, the statement of FO-

RO being a double barrier against micropollutants has not

been thoroughly assessed in closed-loop systems. Both groups

reported that the build-up of TOrCs in the draw solution was

caused by a higher rejection of TOrCs by RO than by FO (Cath

et al., 2010; Hancock et al., 2011). A TOrCs build up in the draw

solution might negatively impact the TOrCs concentration in

the final permeate. It is therefore imperative to investigate the

fate of TOrCs when FO is used in a closed loop system.

In this study, different model foulants were used to foul FO

membranes and effects on FO rejection of 20 pharmaceuticals

was studied. In addition, long-term biofouling experiments

were carried out, in which the biofouled membrane was

extensively characterized and again the effect on the FO

rejection of pharmaceuticals was investigated. The build-up

of TOrCs in the draw solution is systematically studied and

modeled in closed-loop FO-RO/NF applications, and the po-

tential implications for potable water production are

discussed.

2. Materials and methods

2.1. FO setup and filtration protocols

2.1.1. FO setupThe FO membranes used in this study, were commercial cel-

lulose tri-acetate (CTA) membranes produced by Hydration

Technology Innovations (HTI) (Albany, Oregon, USA). Mem-

brane properties are shown in supplementary information.

The membrane orientation in this study was in FO mode

(active layer facing the feed solution). The membrane cell was

a transparent polycarbonate cell, with a flow channel length

of 250 mm, a width of 50 mm and a membrane surface area of

124 cm2. The membrane cell was oriented horizontally, with

the feed channel on top. Feed and draw solution were deliv-

ered to the membrane module in counter-current mode, both

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 45234

at a cross-flow velocity of 0.20 m/s, by peristaltic pumps

(ColeeParmer Metrohm, Belgium). The volume and salt con-

centration of the draw solutionwere controlled using a Biostat

B (Sartorius, Germany) which pumped out excess draw solu-

tion and dosed a concentrated salt solution to restore the draw

solution’s salt concentration. Salt leakage was checked by

measuring the conductivity of the feed solution. The Biostat

algorithms were controlled using a conductivity probe by

Consort (Turnhout, Belgium), measuring the conductivity of

the draw solution. The water flux was measured by logging

the weight of the excess draw solution using an OHaus 5000

Xtreme scale which was logged using a LabVIEW script. The

flux was corrected for the dosed salt solution and the draw

solution density. A scheme of the FO setup is given in Fig. 1.

2.1.2. Streaming potential measurementsThe streaming potential of the FOmembrane was determined

using a polymethylmethacrylate clamping cell, which has a

flow channel of 76.2 mm by 25.4 mm, the dimensions of a

microscopy glass slide. For each test, 2 membrane samples

were fixed to microscopy glass slides with the same mem-

brane side facing the test liquid. They were inserted in the cell

with a polypropylene membrane spacer sandwiched in be-

tween them, to ensure liquid flow past themembrane surface.

The streaming potential was calculated using the Helmholtze

Smoluchowski equation:

DE$m$KL

DP$ε$ε0¼ z (1)

in which DE is the measured potential difference, m is the

liquid viscosity, KL is the specific conductivity of the bulk

liquid, DP is the applied pressure difference, and ε0 and ε are

the electrical permittivity of vacuum and the relative

permittivity of the liquid, respectively. The test liquid was a

10 mM KCl solution at pH 7. The specific conductivity of this

solution was measured, the relative permittivity and viscosity

of the dilute solution were assumed to be equal to those of

pure water.

2.1.3. Pressure-driven membrane systemsThe FO performance was compared with two pressure-driven

membrane processes, NF and RO (DOW Filmtec NF 270 and

ESPA4 by Hydranautics respectively). Both membranes are

composite thin-film polyamide membranes. The experi-

mental setup has been described in an earlier publication

Fig. 1 e Scheme of the FO setup. The feed is tap water (10), which

in a buffer vessel (8). Prior to entering the membrane module (6

The draw solution is recirculated (5), the volume and salinity ar

NaCl solution (3) and pumping out excess draw solution (2). Th

(Botton et al., 2012). The NF and RO membranes were com-

pacted at 15 and 25 bar respectively, both for 1 h. Pharma-

ceutical rejection was measured after 72 h, when the

membranes were operated at a flux of 20 L/(m2*h).

Rejection was calculated as follows:

R ¼ 1� cpcf

(2)

with cp and cf being the permeate and feed concentration

respectively.

2.1.4. Fouling protocol2.1.4.1. Model foulants. Sodium alginate and Bovine Serum

Albumine (BSA) were used as model foulants in this study. FO

membrane fouling was induced by filtration of a solution

containing 200 mg/L of either sodium alginate or BSA. The

draw solution was a 3 M NaCl solution, the duration of the

fouling run was 10 h. The flux was on average 9.3 L/(m2*h) for

the cleanmembrane test, and for the BSA and alginate fouling

tests 8.8 L/(m2*h) and 8.5 L/(m2*h) respectively.

2.1.4.2. Long term biofouling and analysis. Biofouling was

induced by incubating the feed spacer for 24 h in a solution of

10 mL settled activated sludge in 800 mL of water. The sludge

was acquired from themunicipal wastewater treatment plant

Ossemeersen, Ghent. After incubation, the spacer was gently

rinsed to remove any particulate matter. The feed during the

biofouling experiments was tap water, which was dechlori-

nated by dosing sodium metabisulfite (Na2S2O5) in a buffer

vessel at a concentration of 1.5 mg/L, residence time in the

buffer vessel was 20 min. The feed was also spiked with so-

dium acetate at a concentration of 100 mg/L after the buffer

vessel outlet, which was chosen as an easily assimilable

organic compound (AOC) as a substrate for biofilm growth.

The draw solution was a 0.5 M sodium chloride solution. The

feed solution was single use, except during the TOrCs rejec-

tion tests. The draw solution was continuously recirculated; it

was never discharged and refilled.

The growth of the biofilm was monitored visually and by

measuring the pressure drop over both the feed and draw

spacer channels. The pressure drop increasewas calculated as

a ratio of the pressure drop compared to the initial pressure

drop, in which the initial pressure drop is an average of the

pressure drop measured during the first two days of the

experiment.

is dechlorinated using sodiummetabisulfite (9) and stored

), sodium acetate is spiked (7), the feed is not recirculated.

e kept constant by a Biostat B (4), by dosing a concentrated

e excess draw solution is weighed and logged (1).

Table 1 e List of tested pharmaceuticals.

Name Molecular weight(g/mole)

Log D at pH 7.4

Negatively charged

Salicylic acid 138.12 �1.14

Ibuprofen 206.28 0.58

Clofibric acid 214.65 �1.20

Naproxen 230.26 0.35

Gemfibrozil 250.33 1.69

Sulfamethoxazole 253.28 �0.54

Ketoprofen 254.28 �0.16

Diclofenac 296.15 1.44

Bezafibrate 361.82 �1.09

Neutral

Paracetamol 151.16 0.47

Phenazone 188.23 0.44

Caffeine 194.14 �0.63

Primidone 218.25 0.83

Carbamazepine 236.27 1.90

Pentoxifyline 278.31 �0.15

Positively charged

Methformin 129.16 �3.01

Propranolol 259.34 0.79

Atenolol 266.34 �1.76

Metoprolol 267.36 �0.47

Trimethoprim 290.32 0.47

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 4 5235

2.1.5. Trace organic compounds rejection protocolA pharmaceutical stock solution was prepared by dissolving

20mg of each pharmaceutical in 10 L of deionizedwater. If the

pharmaceutical was obtained in ionized form, care was taken

that 20 mg of the active compound was added. The stock so-

lution was stored in a glass bottle in the dark and refrigerated

and was used within one month.

During the pharmaceutical FO rejection tests, the feed was

changed to a 10 L solution of deionized water spiked with

10 mL pharmaceutical stock, yielding a feed solution with a

pharmaceutical concentration of 2 mg/L. The rejection of

pharmaceuticals by the FO membrane was determined after

recirculating the feed solution for 24 h, which ensured suffi-

cient adsorption of solutes to the membrane or tubing to

equilibrate (Kimura et al., 2003). The rejection rate was

calculated as stated above (Eq. (1)), taking into account the

volume of permeate produced, the volume of draw solution

initially present, the volume of draw solution replenishment

was dosed and the average TOrCs concentration in the feed

during the experiment. This yields the following expression

for the permeate concentration, cp:

cp ¼cd$

�Vp þ Vd þ Vb

�Vp

(3)

with cd being the TOrCs concentration in the draw solution at

the end of the experiment, Vp, Vd and Vb being the volume of

the permeate, initial draw solution and brine dosed to rec-

oncentrate the draw solution. The pharmaceuticals rejection

protocols for NF and RO have been described previously

(Botton et al., 2012).

2.2. Chemicals and analysis

The draw solute used in the FO experiments was technical

sodium chloride (98%) obtained from VWR (Leuven, Belgium).

Sodium metabisulfite was obtained from SigmaeAldrich

(Diegem, Belgium) at ACS reagent quality (98e100%). Sodium

alginate and BSA were obtained from SigmaeAldrich at FDA

certified and 96% quality respectively.

The pharmaceuticals used in this study are listed in Table

1, along with their molecular weight and distribution coeffi-

cient and charge at pH 7.4. All pharmaceuticals were obtained

from SigmaeAldrich at purity of 98% or above.

Pharmaceutical concentrations were determined using

solid phase extraction (SPE) on Oasis HLB cartridges (Waters,

USA), followed by HPLC tandem MS. The sample preparation

analytical methods were previously published by Sacher et al.

(2001).

2.3. Foulant characterization

2.3.1. SEM-EDXMembrane samples were desiccated during 24 h prior to SEM

analysis. SEMmicrographsof biofouledFOmembranesamples

were made using a Philips XL30 SEM microscope using a

tungstenfilamentandaSUTWdetector. Samplesweresputter-

coatedwithgoldusinganSCD005CoolSputterCoater (Bal-Tec,

Germany) at a current of 25 mA for 30e90 s. Both secondary

electron and back scattered electron detection were used.

2.3.2. ATP content of the biofouling layerThe bioactivity of the fouling layer of the long-term FO fouling,

induced by inoculation and spiking with sodium acetate, was

assessed by quantifying the ATP content of the fouling layer.

This was done using the BacTiter-Glo microbial viability assay

kit (Promega, USA). Samples were taken from (i) the feed and

draw solution, (ii) the membrane surface at the feed solution-

side (three 4 cm2 samples at the inlet, middle and outlet), and

(iii) themembrane surface at the draw solution side (one 4 cm2

sample at the draw solution inlet). The feed solution sample

was taken from the buffer vessel. The membrane samples

were taken immediately after harvesting the fouled mem-

brane and were stored at �20 �C until analysis two days later.

Biomass was harvested by scraping the fouling layer off of the

membrane samples, followed by mild sonication during 30 s

to disrupt the biofilm structure. two different ATP extraction

methods were used, one using cell wall hydrolyzing enzymes

and one using both enzymatic means and dimethylsulfoxide

(DMSO), at a concentration of 2% (vol/vol), with the DMSO-

enhanced extraction yielding better results (Hammes et al.,

2010).

The blanks in this assay were samples of PCR water (Sig-

maeAldrich). Dilution series of a standard ATP solution were

made spanning six orders ofmagnitude, from 0.1 pM to 10 nM,

one series for the purely enzymatic method and one for the

DMSO-enhanced extraction method. The detection limit was

defined as the mean light detector output of the blanks plus

three times the standard deviation of the blanks. The detec-

tion limit corresponded to an ATP concentration of 10 pM,

thus the data for 0.1 and 1 pMwas ignored in the calculation of

the calibration curves.

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 45236

2.4. Modeling of TOrCs build-up in closed loopapplications

To investigate the effect of this build-up on the final TOrC

concentration in the produced potable water, the concentra-

tion of trace organics in the draw solution of the FO and in the

RO permeate were modeled as a function of cycle time.

For a more elaborate description of the derivation of the

below mentioned equations and assumptions made in this

model, the reader is referred to the supplementary

information.

Solute transport through a densemembrane can occur due

to diffusion, convection, or a combination of both, as will be

clearly shown in the results below. For all transport mecha-

nisms, the FO permeate solute concentration is defined as:

cp ¼< Js > =< Jw >with<Js> and<Jw> being the average solute

and water flux respectively.

For diffusive transport, the solute flux is given by Fick’s law

of diffusion:

Js ¼ �Dm;s$dcdx

(4)

with �Dm,s the solute diffusion coefficient in the membrane

phase. Integration across the membrane and rearranging

leads to the following equation for the solute concentration in

the FO permeate, cp:

cp ¼B�cf � csð1� yÞ�Jw þ B$y

(5)

in which B ¼ Dm;s$ε=Dx is the solute mass transfer coefficient,

cf and cs the initial solute concentration present in the feed

and in the draw solution prior to dilution by FO respectively,

and y the fraction of draw solution originating from permeate.

In the case of convective transport, the solute concentra-

tion in the permeate is given by:

cp ¼ Kc;s$cf (6)

When both convection and diffusion contribute to solute

transport, the FO permeate solute concentration is given by:

cp¼ Js;diffusionþ Js;convectionJw

¼B�cf �csð1�yÞ�JwþB$y

þKc;s$cf (7)

In this model, the RO rejection rate was assumed to be

constant, regardless of the solute concentration in the draw

solution. The solute concentration in the RO permeate is then

given by:

cp;RO ¼ ð1� RROÞ$cd (8)

with cp,RO and cd the solute RO permeate and draw solution

concentration respectively, and RRO the solute RO rejection

rate. The solute concentration in the RO concentrate is given

by:

cconc ¼ RRO$cd1� y

(9)

The solute concentration in the RO concentrate, cconc, of

cycle t is then substituted in equation (4) or 6 as cs of cycle tþ 1.

The model was solved recursively in Excel 2010 and as a

steady-state in Matlab 7.13.

3. Results

3.1. TOrCs rejection by clean FO, NF and RO membranes

The draw solution in FO processes can be reconcentrated

using NF (when divalent salts or nanoparticles are the draw

solute) or RO membranes. This implies that the trace

contaminant concentration in the final product water is not

only determined by FO rejection, but by the reconcentration

system rejection as well. For this reason, the TOrCs rejection

was compared between FO, NF and RO.

TOrCs rejection in NF and RO is governed by different

processes, which can be broadly divided in steric, electric and

dielectric phenomena. Steric hindrance is an important

mechanism, as has been demonstrated in multiple studies

(Fujioka et al., 2012; Nghiem et al., 2010). Steric hindrance is

dependent on both solute size and pore size, as well as on

solute geometry. A second rejection mechanism is electro-

static repulsion, which is of importance for polyamide mem-

branes, as their surface contains ionizable functional groups.

Charged solutes are rejected by a charged membrane either

due to the solute and membrane having the same charge

(Nghiem et al., 2006), or by Donnan exclusion. Electrostatic

repulsion also influences steric hindrance, because the charge

of functional groups at the membrane surface can alter the

conformation of polymer chains. Electrostatic repulsion is

believed to cause shrinking of the membrane pores by some

authors (Childress and Elimelech, 2000), due to polymer

chains adopting an extended conformation, while other

studies have found increased pore sizes when the active layer

became charged (Bellona et al., 2004). Solute-membrane af-

finity also influences rejection by changing the partitioning of

the solute in the membrane matrix, as was noted and incor-

porated in models in several studies (Fujioka et al., 2012;

Verliefde et al., 2009a, 2009b). As most polyamide mem-

branes are moderately hydrophobic, as shown by their water

contact angle, hydrophobic solutes are absorbed in the

membrane matrix. Very hydrophobic solutes (log Kow > 4)

show a high rejection initially, due to absorption, followed by

a breakthrough as the membrane becomes saturated with the

solute (Bellona et al., 2004).

The water flux of the FO was set at 9.3 L/(m2*h), while the

NF and RO flux were set at 20 L/(m2*h). The higher flux in the

NF and RO influences rejection, but it was deemed realistic in

practical applications, because a diluted draw solution should

not contain any appreciable amount of natural organicmatter

or inorganic foulants. Thus, a higher NF and RO flux would be

feasible and would limit capital expenditure when con-

structing an FO-MBR.

The results of TOrCs rejection by clean FO, RO and NF

membranes are shown in Fig. 2. RO clearly has the highest

rejection rate of the three processes in this study, with a

pharmaceutical rejection that is consistently 96% or higher.

NF and FO have comparable rejection rates which are mark-

edly lower than RO rejection. Negatively charged compounds

are consistently rejected at a rate of 95% or more, and are

rejected to a higher degree by both NF and RO than by FO. This

could be due to the composition of the active layer: the FO

membrane in this study is composed of cellulose tri-acetate,

Fig. 2 e Rejection of pharmaceuticals by a clean NF, FO and RO membranes. Compounds are grouped by charge, from left to

right positively charged, neutral and negatively charged.

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 4 5237

which is uncharged. Streaming potential measurements of

the FO membrane revealed a small negative charge of

�6.6 mV, possibly originating from anion adsorption. The

TOrCs rejection by FO can be increased by using TFC mem-

branes (Jin et al., 2012) TheNF and ROmembranes on the other

hand, have a negatively charged polyamide active layer at

neutral pH. Unbound carboxylic acid groups on these active

layers can create negative charges on the membrane surface

by deprotonation at neutral pH (Hurwitz et al., 2010). For

positively charged or neutral compounds, the NF and FO

rejection is more variable and lower, varying for FO from 45%

for paracetamol to 91% for primidone. The lower rejection by

NF compared to RO is likely caused by decreased steric hin-

drance in NF due to larger pore size. There is a weak correla-

tion between rejection and molecular weight, indicating that

aside from steric hindrance also solute-membrane in-

teractions influence rejection. Indeed, models incorporating

solute-membrane affinity based on the Gibbs free energy of

interaction have been shown to predict rejection in NF and RO

better than models based only on steric interactions (Botton

et al., 2012; Verliefde et al., 2009a). Developing such models

for FO will be the subject of future work.

Although solute transport and rejection in FO is likely to

sharemany characteristics with the pressure driven processes

NF and RO, due to the similarity in pore size and membrane

material, the high salinity of the draw solution and reverse

permeation of the draw solute however could alter transport

mechanisms and rejection of different solutes, as noted by Xie

et al. (2012). However, more research is needed to systemati-

cally study the influence of draw solute type, concentration

and reverse permeation rate on trace organic rejection.

3.2. Influence of model foulants in FO TOrCs rejection

The influence of organic fouling on the FO TOrCs rejectionwas

tested using a draw solution of 3 MNaCl. The average fluxwas

9.3 L/(m2*h) in the clean membrane test, and it declined by 5%

due to BSA fouling to 8.8 L/(m2*h) after 10 h, and by 9% to 8.5 L/

(m2*h) after 10 h due to alginate fouling.

The difference in rejection between the clean membrane

and the membrane fouled by either BSA or alginate, i.e. the

influence of fouling on rejection, is shown in Fig. 3. The

rejection does not seem to be significantly altered by BSA

fouling, there appears to be only a slight declining trend. The

standard deviation was determined based on the calibration

series, and significant in this test implies a difference greater

than twice the standard deviation.

When the membrane is fouled by alginate however, the

rejection of some solutes is lowered more significantly, but the

change in rejection is marginal for the majority of the tested

compounds. For those compounds whose rejection was

impactedbyalginate fouling, the loweredrejectionappears tobe

independent of molecular weight or charge of the compounds

(e.g. sulfamethoxazole, naproxen). Also, there is no clear link

with the dominant transport mechanism (convection and/or

diffusion, see below). This suggests that solute-membrane and

solute-foulant specific interactions are responsible for the low-

ered rejection. This will be the subject of further study.

The main conclusion from the model fouling experiments

is that in most cases, rejection is not influenced significantly

by fouling by the model components. When rejection is

affected, however, the trend always shows a clear decrease in

rejection compared to the clean membrane. A possible

explanation is that the model foulants form a cake that is

relatively porous in comparison with the FO membrane, thus

contributing little to TOrC rejection. Hindered TOrC diffusion

back to the bulk feed solution within the foulant layer then

induces cake-enhanced concentration polarization, causing

reduced apparent rejection (Hoek and Elimelech, 2003; Ng and

Elimelech, 2004). This contrasts earlier findings for NF and RO,

in which both increases as well as decreases in rejection due

to fouling by model compounds have been observed (Fujioka

et al., 2013; Nghiem and Hawkes, 2009). Decreases in rejec-

tion of TOrCs by fouled FO membranes will have a significant

impact in closed-loop FO applications: since the reconcen-

tration system will most likely not be influenced by fouling

(given the (near) total rejection of bacteria, colloidal particles

and multi-valent ions by FO), rejection of TOrCs in these

-40

-30

-20

-10 0 10

Rejection (%)

BSAAlginate

Fig. 3 e Change in FO rejection after membrane fouling by

alginate or BSA, compared to clean FO rejection. A negative

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 45238

reconcentration systems will be constant. If rejection values

of the FO decline, build-up of TOrCs in the draw solution will

be even higher, leading to an evenmore compromised “double

barrier” system.

3.3. Biofouling in FO

3.3.1. Influence of biofouling on flux and flow channelpressure dropResults of the accelerated biofouling experiment are shown

below in Fig. 4. During the biofouling experiments, the flux

remained stable at around 5.49 � 0.40 L/(m2*h) (0.5 M NaCl

draw solution). The flux deviated for one 3 day interval, be-

tween days 35 and 37, in which the concentrated salt solution

was dosed at an incorrect rate to the draw solution due to

software malfunction. However, the flux appeared not to be

reduced due to the accumulating biofouling over the course of

this experiment. The pressure drop across the membrane

spacers on the other hand, increased from 0.18 bar at the start

and peaked at 0.40 bar, a doubling of the initial pressure drop,

and maintained a plateau just under 0.40 bar. The pressure

build-up occurred simultaneous in the feed and draw

compartment of the membrane cell, although biofouling

visually only occurred on the feed side (this was later

confirmed by ATP measurements, see below). The increase in

the pressure drop in the draw solution is probably due to the

flexible nature of the FO membrane, making it capable of

conducting pressure: although only the spacer channel of the

feed of the FO got blocked by biofouling, there was also a

narrowing of the draw solution spacer channel due to the

expansion of the feed spacer side.

A sudden decrease of the pressure drop is noted at day 51.

This drop was caused by accidental entrapment of air bubbles

in the buffer tank of the tap water that were entrained to the

feed spacer channel, resulting in scouring of the biofilm. A

degasser was subsequently installed on the feed tubing to

prevent this from occurring. Apparently, the pressure drop is

significantly reduced by the scouring event, to a pressure drop

equal to or even below the initial pressure drop. Good cleaning

efficiency for biofouling by air scouring has been shown for NF

and RO membranes in previous studies (Cornelissen et al.,

2009, 2007). Despite the scouring event however, three days

later, the pressure drop increased again and reached the pre-

scouring level. This indicates that air scouring can mitigate

biofilm growth in FO, but that regrowth under favorable con-

ditions is fast. This contrasts earlier findings for NF and RO

(Cornelissen et al., 2009). The biofilm appeared to have

reached a steady-state between growth and erosion, given

that the pressure drop increase, caused by biofilm regrowth,

again stabilizes around 0.40 bar.

3.3.2. Influence of biofouling on the rejection of trace organicsThe rejection of pharmaceuticals by the biofouled FO mem-

brane was tested twice with the same biofouled membrane.

The first rejection test was after 8 days of biofilm growth. At

number indicates lowered rejection by fouled FO

membranes. Compounds are grouped by charge, from left

to right positively charged, neutral and negatively charged.

Fig. 4 e Flux and pressure drop increase across feed and

draw spacers caused by biofilm growth.

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 4 5239

that time the biofilm formation was still ongoing, as is shown

by the increasing pressure drop, and the biofilm thicknesswas

hypothesized to still be relatively small. The second rejection

test was performed 49 days after the start of the experiment.

At that point, biofilm formation had almost reached a steady

state, as discussed above. The results of the first test are given

in Fig. 5 relative to the difference in rejection with the clean

membrane (a negative rejection indicating a decrease in

rejection compared to the cleanmembrane). The results of the

second test are given in supplementary information. The

presence of a developing biofilm appears to slightly alter

TOrCs rejection, leading to a slightly higher rejection of

neutral solutes and a lower rejection of some negatively

charged solutes. For positively charged compounds, no clear

trend can be seen. A fully developed biofilm caused a more

pronounced reduction in the rejection of positively charged

compounds and a less clear trend in the rejection of negatively

charged compounds compared to the developing biofilm.

Differences in TOrCs rejection behavior of the two biofilm

growth stages is likely due to differences in the composition of

the extracellular matrix and bacterial cell walls. The relatively

minor differences in TOrCs rejection, lead to the conclusion

that biofilm formation only has a limited impact on FO

rejection, which appears to corroborate findings from NF and

ROmembranes (Botton et al., 2012). The difference in rejection

by a clean membrane and rejection by a biofouled membrane

is for the majority of compounds 10% or less. Hancock et al.

(2011) also noticed an increased TOrCs rejection by an FO

membrane fouled by activated sludge in the case of non-ionic

compounds. The high rejection of charged compounds by FO

was confirmed in this study as well.

3.3.3. Biofilm characterizationThe ATP concentration of the feed and draw solution are

shown in Table 2a, the ATP concentration of the biofilm

samples are shown in Table 2b. The biofilmwas sampled upon

harvesting, 67 days after the start of the experiment. A num-

ber of studies have correlated ATP measurements in fresh

water and drinkingwater environments to cell concentrations

(Berney et al., 2008; Hammes et al., 2010; van der Wielen and

van der Kooij, 2010), these estimates range from about

1.7*10�17 to 13.7*10�17 g ATP/cell. Possible cell concentrations

are calculated based on this range.

van der Wielen and van der Kooij (2010) studied ATP con-

centrations in unchlorinated drinking water in the

Netherlands, and reported ATP concentrations ranging from

0.32 ng/L to 28.0 ng/L Berney et al. (2008) reported ATP con-

centrations of 0.016e0.055 ng/L in unchlorinated drinking

water in Switzerland. These concentrations are considerably

lower than the feed ATP concentration of this test. A possible

explanation is that the feed in this test was potable fresh

water, but it was stored in a large underground buffer during

two days prior to use. Furthermore, the feed was sampled in

the dechlorination buffer vessel of the experimental setup.

Additional microbial growth in this vessel could further in-

crease the amount of active microbial biomass. Growth of

biomass in the feed lines was visually confirmed.

Sodium acetate was chosen as a model AOC and was

spiked in the feed as a nutrient for biofilm development. The

ATP concentration in the biofilm samples show that the ATP

content of the biofilm lowers as the distance to the sodium

acetate solution inlet increases. This indicates that the AOC

was being metabolized as the feed passes over the biofilm.

The draw side of the membrane remained devoid of any

visible biofilm formation during the entire experiment, and

the ATP concentration of the draw solution and the draw so-

lution side membrane sample confirm the low microbial ac-

tivity. The low microbial activity can be due to a lack of

nutrients, a lack of suitable inoculum, or both. The initial draw

solution consisted of a 0.5 MNaCl solution whichwasmade in

deionized water, a solution which can be considered low in

both nutrients and inoculum.

SEM micrographs showed a fully developed biofilm, in

which a large numbers of ellipsoidal particles of approxi-

mately 10 mm length are embedded (Fig. 6). It was suspected

that the particles were eukaryotic organisms, possibly di-

atoms. DAPI staining, in which DAPI preferentially binds to

double-stranded DNA, and DICmicroscopy confirmed that the

particles were indeed eukaryotic organisms (micrograph

given in supplementary information). EDX analysis results

showed a high iron and silica content of the biofilm, spectra

are also shown in supplementary information.

3.4. Transport mechanisms and draw concentrationmodeling

3.4.1. Diffusive versus convective transportTo investigate FO transport mechanisms and differentiate

between diffusive and convective transport of pharmaceuti-

cals, an experiment was carried out in which pharmaceuticals

were only spiked in the feed solution in one run, and then in

both the feed and draw solutions in similar concentrations in

the next run. In case solute transport would only be occurring

by diffusive transport, there would be no net mass transfer of

the solute from the feed to the draw solution when the solute

is present in equimolar quantities in both solutions. This fol-

lows fromFick’s law of diffusion, inwhich the driving force for

mass transfer is a gradient in concentration, as in Equation (1).

In the case transport is purely convective, there would be

mass transfer of the solute in the same direction as the solvent

flux in both cases (i.e., when the feed concentration is higher or

equal to than the draw solution concentration). The results of

themass transfer tests are plotted in Fig. 7. From the figure, it is

-20

-10 0 10 20 30

Difference in rejection (%)

Fig. 5 e Difference in rejection between a virgin FO

membrane and a membrane fouled by a developing

Table 2a e ATP concentration in the feed and drawsolution.

ATPconcentration

(pM)

ATPconcentration

(ng/L)

Calculated cellconcentration

(cells/L)

Draw solution 24.27 12.31 0.09e0.72*109

Feed solution 76.69 38.90 0.28e2.29*109

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 45240

apparent that for most solutes, mass transfer drops to almost

zerowhenthesolute concentration in thedrawsolution isequal

to the concentration of the feed. This indicates that for the

majority of solutes, diffusive transport dominates. For caffeine,

convective transport is clearlysignificant, but solute transport is

not equallyhigh inbothcases, indicating that solute transport is

probably a combination of convection and diffusion. For sulfa-

methoxazole, on the other hand, transport is similar in both

cases, indicating that transport is dominated by convective

transport. Sulfamethoxazole is the only charged solute in this

test.We suspect that convective transport dominateswhen the

affinity of the solute for water (expressed as a negative free en-

ergy of interaction) is larger than the affinity for themembrane

matrix. For sulfamethoxazole, this seemsplausible, given that it

is negatively charged at neutral pH and has a predicted logD

coefficientof�0.54, and that thecellulose tri-acetatemembrane

matrixhasahighelectron-donatingsurface tensioncomponent

(given in supplementary information).

The results clearly show that the dominant transport

mechanism of TOrCs is not only dependent on the membrane

and water flux, but also largely depends on the solute prop-

erties. The relative contributions of diffusion and convection

to solute mass transfer relate to the solute diffusion co-

efficients in the membrane, as well as the ratios of the solute

size to the membrane pore size, and also solute-membrane

affinity (Kim et al., 2007). More research is needed to exactly

identify the different contributions and be able to construct a

predictive transport model for organic solute transport in FO.

This will be the subject of further studies.

3.4.2. Modeling of solute accumulation in draw solution andconcentration in product waterWhen FO is used to reclaim water and RO is used to re-

concentrate the draw solution, the higher rejection of trace

organics by RO than by FOwill lead to an accumulation of trace

organics in the draw solution. This system of FO-RO in closed

loop was modeled for different solute transport mechanisms:

diffusive, convective andmixed solute transport. Graphsof the

modeling results are given in supplementary information.

For solutes transported diffusively, the solute concentra-

tion in the draw solution at steady state is given by:

Cd ¼y$B$Cf

y$Bþ ð1� RROÞ$JW (10)

In the case of convective solute transport, the concentration

again reaches a steady state. Assuming there is no diffusive

flux from the draw solution to the FO feed, accumulation will

biofilm. Compounds are grouped by charge, from left to

right positively charged, neutral and negatively charged.

Table 2b e ATP concentration of solubilised biomassextracted from membrane surface samples.

ATPconcentration(pmol/cm2)

ATPconcentration

(ng/cm2)

Calculatedcell

density(cells/cm2)

Feed side,

front

41.36 20.97 0.15e1.2*109

Feed side,

middle

32.44 16.45 0.12e1.0*109

Feed side,

rear

11.57 5.87 0.04e0.3*109

Draw side,

front

0.71 0.36 2.6e21.1*106

-500 0

500

1000

1500

2000

2500

Mass transport (ng/l)

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 4 5241

continue until the RO solute flux equals the FO solute flux. The

solute concentration at steady state is given by:

Cd ¼ Cf$Kc;s$y

ð1� RROÞ (11)

At the steady-state, the solute RO permeate concentration is

determined by the FO rejection of the solute. This implies that

RO rejection influences only the steady state solute concen-

tration in the draw solution, but not the solute concentration

in the RO permeate. This is shown by the derivation of the

steady state RO permeate solute concentration, which is:

CP;RO ¼ Cd$ð1� RROÞ ¼ Cf$Kc;s$y (12)

The solute concentration in the RO permeate of the FO draw

solution was compared with RO permeate if the impaired

water was subjected to RO treatment directly, without an FO

barrier in between. This was calculated for diffusive and

convective transport (Table 3). A high rejection was modeled

as 98%, and a low rate as 50%. In all cases, the dilution ratio of

the FO draw solution equals 0.5. A higher dilution ratio causes

solutes to accumulate to higher levels.

The added value of FO is defined as:

Added Value ð%Þ ¼ 100$

�1� Cp;FO�RO

Cp;RO

�(13)

Fig. 6 e SEM micrograph of the biofouled FO membrane,

showing both the membrane surface and the biofilm. The

membrane surface was exposed due to the dried biofilm

peeling off.

c in draw solution = 0

c in draw solution = c in feed

Fig. 7 e Mass transport of pharmaceuticals in case of a

concentration difference across the membrane and water

flux (hatched bars) and in case of only water flux (full bars).

with Cp,FO�RO the solute concentration in the RO permeate of

the combined FO�RO installation, and Cp,RO the solute con-

centration in the RO permeate if the feed would be subjected

to RO treatment directly.

Table 3 e Modeling results of solute concentration in thefinal product water in function of FO and RO rejection.Graphs of cases 1, 2, 5 and 6 are given in supplementaryinformation.

Case# RFO RRO Cp (% of Cf) ofFO-RO system

Cp (% of Cf)only RO

Addedvalue of FO

Diffusive solute transport

1 98% 98% 0.67% 2% 66.5%

2 50% 98% 1.89% 2% 5.66%

3 50% 50% 20.0% 50% 60.0%

4 98% 50% 0.99% 50% 98.0%

Convective solute transport

5 98% 98% 1% 2% 50%

6 50% 98% 25% 2% �1150%

7 50% 50% 25% 50% 50%

8 98% 50% 1% 50% 98%

wat e r r e s e a r c h 4 7 ( 2 0 1 3 ) 5 2 3 2e5 2 4 45242

It was found that for diffusive transport, if FO and RO

rejection is equal (cases 1 and 3), the double barrier causes an

additional removal of roughly 60% of the TOrCs. This would be

more or less the case for most charged compounds tested in

this study. If, however, FO rejection is lowwhile RO rejection is

high (case 2), the added value of the double barrier ismarginal.

This would be the case for paracetamol, caffeine and carba-

mazepine. Incase3, rejectionbyboth theFOandROmembrane

is low, leading to on the one hand a higher added value of the

double barrier, but simultaneously also to a relatively high

contaminant concentration in the product water. This could

for instance be the case if NF instead of RO is used to re-

concentrate the draw solution. Given the higher water

permeability of NF compared to RO membranes, using NF

would require less membrane surface area and less pumping

power. The draw solutewouldhave to bemodifiedhowever, as

mostNFmembranes showapoor rejectionofmonovalent ions,

leading to another drawback of using NF for reconcentration:

multivalent ions or largemolecular weight solutes also show a

decreased osmotic potential and low diffusion coefficient,

making for a less efficient draw solution (Achilli et al., 2010). In

case 4, FO rejection is higher than that of the RO membrane.

This was not the case for any compound tested in this study,

however, if such an FO membrane would be developed, this

would lead to a high added value of the double barrier.

For convective transport, there is again an added value of

FO in cases 5, 7and 8. However, in case 6 (low FO and high RO

rejection), the added value of FO is negative. This case is the

most common based on the experimental rejection values of

this study, as is illustrated by sulfamethoxazole. Sulfameth-

oxazole was transported exclusively through convection in

this study. The rejection by FO and RO was 95.4 and 99.8%,

respectively. Assuming a concentration of 2 mg/L in the FO

feed and a FO dilution ratio of 0.5, the RO permeate concen-

tration of the FO-RO system is expected at steady state to be

46 ng/L, in contrast, the RO permeate concentration is ex-

pected to be 4 ng/L if the feed was subjected to RO directly.

In the case of a combination of convective and diffusive

solute transport, intermediate results are obtained. In this

case, at steady state, diffusion and convection across the FO

membrane counteract each other, and a steady-state is

reached, which is higher than the feed concentration, but not

as high as in pure convective FO transport.

Given that convectively transported contaminants accu-

mulate to a concentration much higher than the FO feed

concentration, FO-RO can definitely not be considered a dou-

ble barrier system and recovery of the FO draw solution by RO

would require an additional removal mechanism for trace

contaminants to ensure the production of high quality RO

permeate. It also has to be mentioned that this evaluation is

not limited to trace organics. A similar concentration build-up

will occur for nutrients and heavy metals, as long as the

rejection by FO is lower than that by RO.

4. Conclusions

� Trace organics rejection by CTA FO membranes is compa-

rable to NF but lower than RO rejection for the compounds

tested in this study. The FO unit was operated at a lower flux

than the NF and RO, which would be realistic in a FO-MBR

process.

� Model foulants caused a slight decrease in rejection formost

compounds (10% drop or less), while the rejection of some

were significantly negatively impacted. The water flux

decreased by 10%.

� Fouling by long-term biofilm growth caused FO rejection to

vary in function of biofilm age, although overall biofilm in-

fluence was limited.

� Neutral soluteswerepredominantly transported by diffusion.

� Modeling revealed an undesirable build-up of trace organics

in a FOe RO closed loop system, which limits the usefulness

of FO when trace organics are transported diffusively. When

compounds are transported convectively, trace organics

build-upeven leads toahigherTOrCsconcentration in theRO

product water compared to water production with only RO.

Acknowledgments

HTI is acknowledged for providing FO membrane samples.

Arne Verliefde wishes to acknowledge the Netherlands

Organisation for Scientific Research (NWO), as this work is

part of the research programme Veni 10654, which is financed

by NWO. Sam Van Nevel is supported by the project grant no.

G.0808.10 of the FWO Flanders.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.watres.2013.06.006.

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