PII: S0043-1354(98)00498-9
Wat. Res. Vol. 33, No. 11, pp. 2517±2526, 1999
Published by Elsevier Science Ltd
Printed in Great Britain
0043-1354/99/$ - see front matter
MEMBRANE FILTRATION OF NATURAL ORGANIC
MATTER: INITIAL COMPARISON OF REJECTION AND
FLUX DECLINE CHARACTERISTICS WITH
ULTRAFILTRATION AND NANOFILTRATION
MEMBRANES
M
JAEWEON CHO1*, GARY AMY1*
and JOHN PELLEGRINO2{
Civil, Architectural and Environmental Engineering, University of Colorado at Boulder, Boulder,
CO 80309, U.S.A. and 2Physical and Chemical Properties Division, National Institute of Standards and
Technology, MS 838.01, Boulder, CO 80303, U.S.A.
1
(First received June 1998; accepted in revised form November 1998)
AbstractÐTwo source waters containing natural organic matter (NOM) with di€erent physical and
chemical characteristics were cross¯ow-®ltered using four types of membranes having di€erent material
and geometric properties. Transport measurements of NOM rejection and ¯ux decline were made. A resistances-in-series model was used to represent and quantitatively compare membrane ¯ux decline and
recovery. As anticipated, the resistance due to speci®c adsorption depended on the concentration at the
membrane interface. For the two membranes showing evidence of NOM adsorption, reducing the initial
¯ux (which we infer to also reduce the interfacial NOM concentration) also lowered the measured resistance assigned to adsorption in our protocol. Relative molecular mass (RMM) distribution measurements (by size exclusion chromatography) were used to calculate the average RMM of the NOM and
persuasively illustrated that the nominal relative molecular mass cut-o€ (MWCO) of a membrane is not
the unique predictor of rejection characteristics for NOM compounds. Size exclusion, electrostatic
repulsion, and NOM aromaticity all in¯uenced the NOM rejection. For a given water composition
(including pH and ionic strength), membrane characteristics (such as the surface charge, hydrophobicity
and nominal MWCO) can be combined with the NOM properties (such as total dissolved organic carbon, speci®c UV absorbance at 254 nm and humic content) to provide a consistent qualitative rationale
for the transport results. Published by Elsevier Science Ltd
Key wordsÐdrinking water, ¯ux decline, MWCO, nano®ltration, natural organic matter, NOM, rejection, ultra®ltration
INTRODUCTION
We made bench scale measurements using micro®ltered surface waters and four membranes, including
NF and UF. Metrics for the characterization of the
water composition and the membranes were tabulated and the resistance-in-series model was used to
analyze the ®ltration process. Our results are consistent with those of prior researchers with regard
to the general in¯uences and mechanisms associated
with rejection and ¯ux decline during natural organic matter (NOM) ®ltration. In this report we
present initial measurements from our protocol that
may be useful for developing future correlations.
Previous studies (for example, Taylor et al., 1987;
Fu et al., 1994) have shown that NOM can be e€ec*Present address: Kwangju Institute of Science and
Technology, 1 Oryong-dong, Puk-gu, Kwangju 500712, Korea (E-mail: [email protected]).
{Author to whom all correspondence should be addressed.
[Tel.: +1-303-497-3416; fax: +1-303-497-5259; e-mail:
[email protected]]
tively rejected during ®ltration by low and medium
pressure membranes, including nano®ltration (NF)
and, to a lesser extent, ultra®ltration (UF).
Removal of NOM is important since they act as the
precursors to disinfection by-products (DBPs)
which, in turn, have recently received attention in
drinking water regulations. Also, organic matter is
often found to be a primary source of ¯ux decline
due to fouling in RO and NF systems. Heretofore,
the most popular predictor of NOM rejection by
membranes has been the nominal molecular mass
cut o€ (MWCO). Nonetheless, often di€erent relative molecular mass (RMM) rejections have been
observed for di€erent membranes with comparable
nominal MWCOs and for the same membranes
when applied to di€erent solutes, including NOM
source waters. Moreover, membrane ®ltration ¯ux
decline due to organic (NOM) fouling (and ¯ux
recovery after cleaning) is felt to be less well understood than that due to other colloidal, biological
and scale-related fouling.
2517
2518
Jaeweon Cho et al.
Table 1. E€ects of chemical and ¯ux conditions on ®ltration of humic substances (adapted from Hong and Elimelech, 1997)
Chemical conditions
Humics in solution
Humics on membrane surface
High ionic strength low pH,
or presence of divalent ions
compact, coiled
con®guration
compact, dense, thick fouling layer
severe
severe
insigni®cant
Low ionic strength high pH,
and absence of divalent ions
stretched, linear
con®guration
loose, sparse, thin fouling layer
small
severe
insigni®cant
NOM is a general descriptor for a mixture containing a variety of organic, slightly water-soluble
components. A speci®c, sequential, two-column
fractionation of a general NOM (Thurman and
Malcolm, 1981; Leenher and Noyes, 1984; Aiken et
al., 1992) separates it into (1) hydrophobic acids
(which are adsorbed on XAD-8 resin) including
strong (fulvic and humic) and weak (alkyl monoand dicarboxylic acids) acids; (2) hydrophilic acids
(which are adsorbed on XAD-4 resin) including
hydroxy and sugar acids and (3) strongly hydrophilic species (which are not adsorbed on either XAD8 or 4 resins) including polysaccharides, alkyl alcohols, amides and bases.
A variety of prior studies have addressed the
chemical and physical aspects of NOM ®ltration
and ¯ux decline with NF membranes. For the most
part model solutions of humic acids obtained from
commercial sources have been used to provide consistent measurement conditions. Fractionated NOMs
and partially treated (with micro®ltration and/or
powdered activated-carbon adsorption) natural
waters have also been used. Considering the highly
heterogeneous nature of NOM, it is not surprising
that many prior measurements (Laine et al., 1989;
Jucker and Clark, 1994; Nilson and DiGiano, 1996;
Braghetta et al., 1997) have indicated the in¯uence
of hydrophobic and charge interactions between the
membrane and NOM (in addition to MWCO) on
the ®ltration ®gures of merit: water ¯ux decline and
solute rejection. Recently, Hong and Elimelech
(1997) have studied the ®ltration of three classes of
isolated humic substances with a thin ®lm composite (TFC) NF membrane (nominal MWCO < 100)
based on crosslinked aromatic polyamide. Their
study included e€ects of divalent cation (Ca2+) concentration, pH, total ionic strength, and interfacial
concentration (controlled by changes in the permeation rate) on ¯ux decline and the mass of humic
substances adsorbed on the membrane. The e€ects
from changes in permeation rate were interpreted
by a critical ¯ux viewpoint, i.e. that upon start-up
of a ®ltration process there is an initial ¯ux below
which a decline of ¯ux does not occur (Field et al.,
1995). Hong and Elimelech's results were consistent
with much prior literature and are summarized in
the following Table 1.
It is important to keep in mind that there can
always be several sources of ¯ux decline in any application. In a broad sense we can designate ¯ux
decline due to (1) concentration polarization; (2) gel
Flux decline
Flux decline
Flux decline
(>critical ¯ux) (>>critical ¯ux) (<critical ¯ux)
(precipitate) formation, reversible with mild cleaning; (3) gel (precipitate) formation, reversible with
harsh cleaning; (4) surface adsorption, both reversible and irreversible; (5) pore adsorption, both reversible and irreversible and (6) reversible and
irreversible physical changes to the membrane (for
example, compression). Many of these sources of
¯ux decline are directly related to the solute concentration at the membrane interface and therefore can
be a€ected by permeate ¯ux, cross¯ow velocity, turbulence promoters, real-time cleaning regimes
(back¯ushing, pulsing, etc.), bulk concentrations of
solutes and membrane rejection qualities.
The general mechanistic view of NOM interactions with membranes and the ®ltration process,
including the e€ects of charge, ionic strength, pH,
chelation by divalent ions, and ¯ux is analogous to
that which has evolved for biological and other
macromolecules. The following four points are
major aspects of the ®ltration process. Similar issues
are important even if there is no ®ltration (for
example, static adsorption) but then the driving
forces for solute interactions with the membrane
material are simply the bulk concentrations.
1. The NOM mixture has an intrinsic chemical
nature (aromaticity, polarity, ionizable groups,
etc.) and molecular size. The actual charge, con®guration and chemical potential of the NOM in
solution depend on the current solution environment (pH, ionic strength, ion compositions, temperature, pressure, etc.) which varies throughout
the ®ltration process.
2. The combination of (i) the operating conditions
of the ®ltration process (transmembrane pressure
and hydrodynamic mass transfer at the membrane/feed interface); (ii) the membrane geometry (porosity and pore size distribution) and (iii)
the membrane's rejection characteristics toward
the NOM controls the NOM's concentration at
the membrane surface and in the pores.
3. The chemical nature of the NOM; its concentration at the membrane ¯uid±solid interfaces
and the chemical and geometrical nature of the
membrane (under the given solution conditions)
control the amount (and degree) of gel or precipitate formation and reversible and irreversible
adsorption that occurs.
4. The NOM's interfacial concentration; the interfacial solution's viscosity and the mass and porosity of the adsorbed layer in¯uence the
UF and NF membrane ®ltration of NOM
2519
Table 2. Feed water metrics
DOC (mg/L)
UVA254 (cmÿ1)
SUVA254 (L cmÿ1 mgÿ1)
Cond. (mS/cm)
pH
Humic (%DOC)
Ca (mg/L)
2.0
3.9
47.8
0.048
0.172
1.769
0.024
0.045
0.037
21
30
1066
6.2
6.4
7.0
43.3
56.9
60.6
8.1
ÿ
34.3
W_SL-SW
R_SL-SW
Twitchell
hydrodynamic aspects of ¯ux decline and the
change in the ®ltration process's apparent rejection of the NOM through both porous media
and physical property aspects.
Clearly, when the degree of chemical complexity
of NOM constituents is combined with the many
physico-chemical aspects of the membranes and the
®ltration process (as outlined above) it will be very
dicult to predict performance from ®rst principles.
Correlations may be a useful ®rst step in matching
water compositions, membrane properties, and ®ltration conditions.
METHODS AND ANALYSIS
Source waters
Silver Lake surface water (SL-SW) and Twitchell water
were used to perform bench-scale membrane tests. SL-SW
is a Colorado drinking water source and Twitchell water is
an agricultural drain feeding into the California State
Project water. SL-SW samples were collected in winter
(W_SL-SW) and runo€ (spring) seasons (R_SL-SW) to
obtain seasonal in¯uences.
Several analytical metrics are used for quantifying
NOM content and their potential to form disinfection
byproducts: total organic carbon (TOC), dissolved organic
carbon (DOC), ultraviolet adsorption at 254 nm (UVA254)
and the speci®c UV adsorbance (SUVA = UVA254/DOC).
The UV absorbance of NOM is ascribed exclusively to
aromatic chromophores. The SUVA is considered a
measure of the relative aromatic content of the colloidal
carbon and therefore the NOM. Additional molecular interpretations of NOM UV spectra have been presented
(Korshin et al., 1997) but were not applied in this study.
All membrane ®ltration tests were performed with
source water that had been pre®ltered using a 0.45 mm ®lter. The measurement of TOC on the permeate through
this ®lter corresponds to the working de®nition of DOC.
For this study each source water was analyzed for DOC,
UVA254, SUVA, conductivity, pH, humic content of the
NOM and Ca2+. These results are tabulated in Table 2.
High performance size exclusion chromatography
(HPSEC) was used to determine the RMM distribution of
NOM with a Waters* protein-pak column and a commercial UV spectrophotometric detector. The eluent for the
HPSEC was composed of mQ water (water that is ®ltered
with two proprietary cation-exchange mixed beds, an
anion-exchange bed, and a 0.2 mm ®lter) bu€ered with
phosphate (pH 6.8) and NaCl to increase ionic strength to
0.1 M (Chin et al., 1994). Standard solutions for the
RMM calibration curve of NOM were made with sodium
polystyrene sulfonates (PSS) (1.8, 4.6, 8.0 and 35.0 k) and
*Such identi®cation is not intended to imply recommendation or endorsement by the National Institute of
Standards and Technology, nor is it intended to imply
that the equipment identi®ed is necessarily the best
available for the purpose.
salicylic acid (RMM = 138.12) was used to extend the
lower range of the sodium-PSS calibration curve.
The RMM distributions (Fig. 1) and average RMMs
(Table 3) of winter season and runo€ SL-SW and
Twitchell water were determined by HPSEC. The humic
fractions of NOM source waters were determined by performing a DOC mass balance across an XAD-8 resin column, with the column e‚uent representing the nonhumic
fraction.
Continuous cross¯ow ¯at-sheet membrane unit
A commercial bench scale cross¯ow membrane module
was used to evaluate ¯at sheet membrane specimens. Our
system is composed of the membrane module and the
feed, permeate, recycle, and waste lines. The module accommodates 60 cm2 ¯at sheet specimens (of which
056 cm2 are active for ®ltration) under tangential feed
¯ow conditions with a channel height of 0.04 cm. Figure 2
presents a schematic of the mass transfer dimensions and
con®guration of the ®ltration cell. For measurements in
this report, the cross¯ow velocity was kept approximately
constant at 08.6 cm/s by setting up a constant feed ¯owrate of 200 ml/min. The temperature was maintained at
298 K and the transmembrane pressure was kept constant
at approximately 345 kPa (50 psi). At these conditions the
Reynolds number is decidedly laminar, nominally 36.
Each new membrane was soaked in mQ water for 1 day
to clean any coatings and/or pore stabilizers. Fresh mQ
water was ®ltered through a membrane specimen until approximately constant ¯ux was obtained, then the NOM
solution was ®ltered. The NOM-containing natural waters
(stored under refrigeration) were kept at room temperature
for 1 day prior to permeation measurements to assure
thermal equilibration. The permeate ¯ow, UVA and DOC
of the permeate were measured over time. NOM rejection,
based on bulk concentration, was calculated by
Rj …bulk† ˆ
Cb ÿ Cp
,
Cb
…1†
where Cb is NOM concentration in the bulk ¯uid in the
feed channel and Cp is the NOM concentration in the
permeate.
Flux decline and adsorption tests using cross¯ow ®ltration
unit
A membrane resistances-in-series model was used to
quantify ¯ux decline by obtaining the di€erent series resistances. At any pseudo-equilibrium condition the ¯ux
can be evaluated by
Jv ˆ
DP
,
m…rm ‡ rc ‡ rg ‡ ra1 ‡ ra2 †
…2†
Table 3. Relative molecular mass parameters of the NOM in the
feed waters
W_SL-SW
R_SL-SW
Twitchell
Mw
Mn
Polydispersity
984
1282
1349
656
855
1042
1.5
1.5
1.3
Mw=mass averaged relative molecular mass. Mn=number averaged relative molecular mass. Polydispersity = Mw/Mn.
2520
Jaeweon Cho et al.
Fig. 1. Fractional relative molecular mass distribution of the NOM contained in the feed waters used in
these ®ltration tests: Twitchell drainage water (w), winter Silver Lake (q) and spring-runo€ Silver Lake
(r).
where Jv is water ¯ux through the membrane (cm/s), DP is
the transmembrane pressure (kPa), m is the dynamic viscosity (kPas), rm is the clean membranes's hydraulic resistance, rc is the resistance due to concentration
polarization (CP), rg is any gel layer resistance, ra1 is
weakly adsorbed foulant's resistance and ra2 is ``irreversibly'' adsorbed foulant's resistance (all r's have units of
cmÿ1). rc and rg are related to the osmotic pressure and
viscosity of the phases immediately proximal to the membrane surface.
The numerical value of the resistances were obtained by
using the protocol depicted in Fig. 3, described as follows:
Step 1: mQ water was ®ltered through the membrane
until a constant ¯ux was obtained.
Step 2: the NOM-containing water was introduced
and the permeate rate was monitored over time until it
reached a constant value, the permeation rate through
the fouled membrane.
Step 3: the applied pressure was released (to remove
any concentration polarization that resulted from the
membrane's rejection of NOM under forced per-
meation) and mQ water replaced the NOM-containing
water and permeation was again measured.
Step 4: the fouled membrane was then vigorously
¯ushed (volumetric ¯ow rate of 450 ml/min, cross ¯ow
velocity of 19.3 cm/s) for 10 min with mQ water so that
loosely-held gel layer (concentrated NOM) could be
removed from the membrane surface and mQ water
permeation was again measured.
Step 5: the membrane was then soaked in 0.1 N
NaOH solution for 24 h so that weakly adsorbed NOM
on the membrane surface could be desorbed, then mQ
water was again ®ltered through the chemically cleaned
membrane.
Using the ¯ux values from steps 1±5, rm, rc, rg, ra1 and
ra2 could be calculated. These resistances are speci®c to
the protocol that is used to create them (i.e. water composition and ®ltration conditions) and measure them. The gel
layer resistances are in¯uenced by any solid phase structure. The weak adsorption can be de®ned as the NOM
adsorption which can be removed with chemical cleaning
by 0.1 N NaOH, while strongly adsorbed NOM is not.
Fig. 2. Schematic of the ®ltration cell with dimensions of the available mass transfer area.
UF and NF membrane ®ltration of NOM
2521
zeta potential measurements we made and the existence of
ionizable groups (carboxylic acid) in the polymer (which is
information provided by manufacturers). Zeta potential
measurements were made with a commercial instrument
(Elimelich and Childress, 1996) using mQ water with a
conductivity of 02±3 mS/m (KCl) and pH varied between
5.5 to 8.5.
RESULTS AND DISCUSSION
Transport measurements
Fig. 3. Schematic representation of the ®ltration protocol
used to determine resistances-in-series.
The RMM distribution of the NOM foulants removed by
0.1 N NaOH was analyzed for comparison with the RMM
distributions of XAD-8 isolation solutions.
Contact angle measurements
The water contact angle on the membranes was
measured with the sessile drop method (Adamson and
Gast, 1997) using a goniometer to measure the contact
angle between the water droplet, the membrane surface
and air. Each membrane was cleaned of any coating materials by ¯oating it skin-side down in a container of mQ
water for 24 h, changing the water three times. The rinsed
membranes were dried in a closed desiccator for 24 h and
stored in closed petri dishes before measurements.
Membrane samples were cut into small pieces and
mounted on glass supports. A 2 mL mQ water droplet is
placed on the sample and the contact angle is immediately
measured. A low contact angle is associated with high
water anity.
Membranes
Four di€erent membranes, including NF45, YM3, GM
and PM10, were used for transport measurement tests.
NF45 and GM membranes are made of crosslinked, polyamide thin-®lm-composite (TFC), YM3 membrane is
made of regenerated cellulose and PM10 membrane is
made of polyethersulfone. The nominal MWCOs of the
membranes were provided by the manufacturers. Table 4
provides a detailed listing of the characteristics and identi®cation code for the membranes.
According to the contact angle measurement it could be
concluded that YM3 membrane is relatively hydrophilic,
while NF45, GM and PM10 membranes are relatively
hydrophobic. The solute-free water permeability (PWP) of
the PM10 membrane is especially high compared with the
other membranes. NF45 and GM membranes are thought
to have a signi®cant negative surface charge based on the
W_SL-SW and Twitchell water permeation was
done with all four membranes to determine DOC
rejection. As shown in Table 2, W_SL-SW represents relatively low DOC, low RMM and low
aromaticity, while Twitchell represents relatively
high DOC, high RMM and high aromaticity. NF45
membrane exhibits signi®cantly high rejection of
DOC in both waters, because it has low MWCO
and negative charges. YM3 and PM10 membranes
do not reject DOC in W_SL-SW, but reject some of
the larger RMM fraction in Twitchell water. Even
though GM membrane has larger MWCO than
YM3 and almost the same MWCO as PM10, it
exhibits relatively high DOC rejections in both
waters compared with YM3 and PM10. This can be
rationalized by the fact that GM membrane has a
negative charge (like the NOM macromolecules) so
that there are charge repulsions between the membrane surface and NOM. YM3 membrane does not
have any signi®cant negative charge on its surface
(which is consistent with its nominal cellulose composition). The PM10 membrane has the highest permeability (or PWP) and nominal MWCO and is
expected to allow NOM macromolecules to freely
pass through the membrane pores.
E€ective MWCO determination of GM membrane
The charged GM membrane had higher NOM
rejection than expected based on the manufacturer's
speci®cation MWCO of 8000. We postulate that the
e€ective MWCO for a charged membrane with a
charged solute (e.g. NOM) will be di€erent than
what is obtained in measurements with neutral
(uncharged) solutes. We determined the NOM fractional rejection (Mulder, 1991) using
Table 4. Nominal transport and physical characteristics of the membranes
DOC rejection (% m)a
Membrane ID
NF45
YM3
GM
PM10
W_SL-SW, low DOC,
low RMM, low aromaticity
Twitchell, high DOC,
high RMM, high aromaticity
85.2
0
37.6
0
97.7
51.1
60.0
37.1
MWCOb
Contact angle
(H2O anity)
Ionizable groupsc
400
3000
8000
10000
45.5 (low)
13.3 (high)
54.7 (low)
61.7 (v. low)
yes (ÿ)
no
yes (ÿ)
no
d
PWP
2.6
4.5
8.0
150.0
a
DOC rejection is not equally sensitive to all NOM fractions. For example, a decrease in UVA254 can be observed without a similar
change in DOC.
b
Manufacturer's data.
c
Determined with zeta potential measurements.
d
PWP (L dÿ1 mÿ2 kPaÿ1) is water permeability determined with ®ltered, deionized water.
2522
Jaeweon Cho et al.
Fig. 4. Relative molecular mass distributions of R_SL-SW before and after ®ltration through GM
membrane.
RMi ˆ
WMi …feed † ÿ WMi …perm†…1 ÿ Roverall †
,
WMi …feed †
…3†
where RMi is fractional rejection for a certain
RMM ``i'', WMi is the mass fraction of that RMM
in the speci®c stream and Roverall is overall NOM
rejection by the membrane (based on DOC
measurements).
RMM distributions of the R_SL-SW (spring runo€) feed and permeate through the GM membrane
are shown in Fig. 4. The e€ective MWCO (de®ned
as the RMM when RMi =0.90) of the GM membranes for R_SL-SW and Twitchell water NOMs
was determined using equation 3 and are listed in
Table 5. The lower (than nominal) e€ective
MWCOs of the GM membrane for R_SL-SW and
Twitchell water is attributed to charge and hydrophobic interactions between the membrane surface
and NOM.
The data in Table 5 were obtained in a separate
set of measurements than those listed in Table 4.
The DOC rejection for the Twitchell water is consistent
between
the
two
measurements.
Interestingly, the DOC rejection for the R_SLSW>Twitchell>W_SL-SW, even though the nominal DOC molecular mass distribution is similar for
both R_SL-SW and Twitchell water. The main
di€erence is that R_SL-SW has the highest
Table 5. E€ective relative molecular mass cut o€ (MWCO) of GM
membrane against representative surface waters at their natural
conditions of pH and ion content (see Table 2)
Water
R_SL-SW
Twitchell
Overall DOC rejection (%)
E€ective MWCO of GM
72.1
56.3
1520
2220
SUVA254 and a low ionic strength. Its high molecular mass species therefore have greater aromaticity
and can maintain an elongated con®guration (due
to low ionic strength) thus enhancing exclusion
from the membrane's pores. Some self aggregation
and/or adsorption at the membrane surface is possible as the interfacial concentration increases which
would further increase the apparent rejection.
Flux decline (W_SL-SW water)
Flux declines from ®ltration of W_SL-SW water,
which has relatively low DOC, low RMM and low
aromaticity, using NF45, YM3, GM and PM10
membranes were monitored over time. These data
are presented in Fig. 5. Table 6 lists the calculated
resistances for each of the series resistances discussed in the context of equation 2. The presence
and magnitude of these resistances support the following interpretations.
As a general observation, NF45 and YM3 membranes did not exhibit any signi®cant ¯ux decline
over the time period of our measurements. The
NF45 experiences minimal concentration polarization (CP) probably because of the low permeation
rate and low feed DOC concentration. Also, apparently no adsorption on the NF45 occurs over the
time scale of these measurements. YM3 does not
reject any signi®cant amount of DOC (therefore no
CP) and similarly does not experience any apparent
adsorption of the NOM from this water. The GM
apparently rejects enough of the NOM in this water
(probably due to the membrane surface charge) to
create a weakly adsorbed gel layer that can be
cleaned by water ¯ushing and NaOH cleaning. The
PM10 is a high ¯ux membrane and resistance builds
up quickly even though there is no discernible DOC
rejection during the measurement period.
UF and NF membrane ®ltration of NOM
2523
Fig. 5. Relative ¯ux decline (and recovery for GM and PM10) experienced during ®ltration of W_SLSW by YM3 (w), NF45 (q), GM (+) and PM10 () using protocol in Fig. 3.
DOC measurements are not as speci®cally sensitive to the aromatic components as the UVA254
measurement. UVA254 measurements were made on
the overall feed and permeate reservoirs after the ®ltration tests were performed and are presented in
Table 7. (The NOM UV spectrum in the area of
this peak is broad so we used the value of absorbance at the speci®c wavenumber.) All the membranes provided some rejection of aromatic
components. The PM10 membrane probably
rejected aromatic NOM components by adsorption
on both the outer surface and in the pores. We can
speculate that since the largest pores carry the most
¯ow, they become the most quickly fouled (blocked)
and signi®cant relative ¯ux decline occurs when the
membrane's pore size distribution is broad (or
bimodal). The PM10 may have this attribute.
Adsorption of aromatic components by the GM
may also occur but the mechanism is likely to be
more complex due to the GM membrane's intrinsic
charge and the presence of the other (charged)
rejected NOM components at the interface.
Flux decline (Twitchell water)
Figure 6 presents the ¯ux declines using the
Twitchell water, which has relatively high DOC,
high RMM, and high aromaticity, with the four
di€erent membranes. The calculated resistances are
listed in Table 8.
The NF45 and YM3 still exhibit insigni®cant CP.
This is probably because of the low permeation
rate. Apparently, no adsorption occurs over the
time scale of these measurements. GM exhibits
somewhat faster ¯ux decline kinetics when compared to the results with W_SL-SW, which is probably due to the di€erence in DOC feed
concentration. PM10 also exhibits faster and larger
¯ux decline than W_SL-SW and the magnitudes of
all resistances are greater. The high aromatic content of the feed NOM in the Twitchell water and
the hydrophobicity of PM10 (according to contact
angle measurements) likely combine to cause greater
driving force for adsorption of mass on the PM10.
Table 7. UVA254 (aromatic components) measurements after cross¯ow ®ltration tests (arbitrary units)
Table 6. Resistance-in-series from ®ltration of W_SL-SW water
rm
rc
rg
ra1
ra2
NF45 (cmÿ1)
YM3 (cmÿ1)
GM (cmÿ1)
PM10 (cmÿ1)
286.7
163.0
106.8
7.9
0.9
4.2
9.4
1.4
3.1
30.8
W_SL-SW water
NF45
YM3
GM
PM10
Twitchell water
feed reservoir
permeate
feed reservoir
permeate
0.046
0.042
0.040
0.042
0.004
0.023
0.014
0.037
1.799
1.786
1.776
1.769
0.018
0.778
0.653
1.131
2524
Jaeweon Cho et al.
Fig. 6. Relative ¯ux decline (and recovery for GM and PM10) experienced during ®ltration of Twitchell
water by YM3 (w), NF45 (q), GM (+), and PM10 () using protocol in Fig. 3.
General discussion of measurement uncertainties
Based on systematic uncertainties from resolution
of the mass balance, volumetric standards and timing devices we estimate the uncertainties in the
permeate ¯ux measurements to be 0.2±1% of the
reported value. The lower uncertainties are for the
initial periods when the permeation rates are higher.
Carrying this uncertainty into the calculation of the
tabulated resistances-in-series, we estimate an uncertainty of between 1 and 10% of the reported value,
again depending on its magnitude. The expanded
uncertainties (coverage factor of 2) due to random
and systematic e€ects (based on replicate analyses)
in the reported NOM rejections are 22 to 5% when
based on DOC and 20.5 to 2% when based on
UVA254. The expanded coverage on the RMM
values are estimated to be 260 based on the variance of peak times observed for replicate measurements with the RMM standards.
Concentration boundary layer mass transfer e€ects
Solute rejection by the membrane leads to CP,
that is a higher concentration of solute nearer to
the membrane surface than in the bulk ¯uid (which
has a more uniform concentration). This concentration gradient provides for back mass transfer of
rejected solute away from the membrane surface.
This mass transfer coecient (k) in the concentration boundary layer for our experimental protocol was estimated after assuming an NOM di€usion
coecient (D) of 5 10ÿ7 cm2/s. This value is in the
range reported (Cussler, 1984) for a variety of medium RMM proteins in aqueous solution at 298 K.
The classical (Porter, 1972) mass transfer coecient
correlation for laminar ¯ow in a channel,
ub D2 1=3
k ˆ 1:177
,
…4†
hL
(with h = half channel height, L = length of channel and ub=average bulk velocity) was used to
determine that k = 0.3 10ÿ3 cm/s in the measurements presented in the previous sections. If we
assume D = 3 10ÿ6 cm2/s (a preliminary value for
humic acids measured by a collaborator) then
k = 1.0 10ÿ3 cm/s.
These values of k may be compared to the initial
pure water ¯uxes (in Table 9) for the membranes.
The lack of signi®cant CP and adsorbed-layer formation during ®ltration with the NF45 and YM3
Table 8. Resistance-in-series from ®ltration of Twitchell water
rm
rc
rg
ra1
ra2
NF45 (cmÿ1)
YM3 (cmÿ1)
GM (cmÿ1)
PM10 (cmÿ1)
324.2
192.9
149.6
10
20.6
13.8
54.6
4.3
5.3
28.9
Table 9. Initial pure water ¯ux, J0
Membrane
NF45
YM3
GM
PM10
W_SL-SW, Jo103 (cm/s)
Twitchell, Jo103 (cm/s)
0.6
1.1
1.6
21.9
0.5
0.9
1.2
17.2
UF and NF membrane ®ltration of NOM
2525
Fig. 7. Relative ¯ux decline (and recovery) for GM (+) and PM10 () experienced during ®ltration of
Twitchell water at two di€erent transmembrane pressures (TMP) using protocol in Fig. 3. (Larger symbols are at lower TMP.)
membranes is consistent with the estimate that
solute mass transfer away from the membrane interface is very close to the ¯ux toward the membrane.
In the case of the GM membrane, the initial ¯ux
may have been 050% higher than the back transport. Therefore, early CP could have led to some
adsorption and subsequent ¯ux decline. This decline
could then have retarded further CP, in such a way
that our resistance-in-series analysis did not detect
it. The PM10 membrane probably had high concentration polarization and susceptibility to surface
and pore adsorption. This led to its severe ¯ux
decline and contributions from all the resistances
included in our analysis.
The role of interfacial solute concentration on the
®ltration resistances was further illustrated by ®ltering Twitchell water with new samples of GM and
PM membranes, but using a transmembrane pressure (TMP) such that the initial pure water ¯uxes
would be approximately equal to each other and
closer to the estimated mass transfer coecient for
the concentration boundary layer. The average velocity in the membrane test cell was kept the same
as in the previous measurements. Figure 7 presents
a comparison of the ¯ux decline measurements for
both sets of GM and PM10 membranes and
Twitchell water and Table 10 lists the initial water
¯uxes, TMP, %DOC rejection and calculated resistances-in-series.
The lower value of J0 corresponds to very small
or nonexistent initial concentration polarization at
the membrane interface. Therefore the dependency
of the relative ¯ux decline and the resistances-inseries on the concentration of NOM at the membrane interface is illustrated by the two levels of J0.
Both membranes exhibit qualitatively similar ¯ux
decline responses but the magnitudes are signi®cantly lower when the NOM concentration at the
membrane interface is lower. The main contribution
to ¯ux decline remains weakly adsorbed NOM but
Table 10. Mass transfer e€ects from ®ltration of Twitchell water by GM and PM10 membranes
GM
PM10
TMP (kPa)
J0 (cm/s 103)
% DOC rejection
345
1.2
60.0
345
207
17.2
0.8
37.1
48.5
ÿ1
Resistances-in-series (cm )
rm
rc
rg
ra1
ra2
149.6
5.3
28.9
10.0
20.6
13.8
54.6
4.3
GM
136.1
4.0
15.4
4.6
PM10
14
0.9
11.1
7.7
1.3
0.5
10.7
1.5
2526
Jaeweon Cho et al.
both its apparent adsorption rate and its overall
¯ow resistance are less when J0 (and presumably
interfacial NOM concentration) is lower.
The average DOC rejection was also lower when
J0 was set closer to k (the concentration boundary
layer mass transfer coecient) and did not signi®cantly change over the course of the ®ltration. The
¯ux decline results (for both initial ¯ux cases) seems
to support a hypothesis that signi®cant pore blockage occurs quickly. The continued gradual ¯ux
decline is unlikely to be due to buildup of a gel or
surface layer, more likely to be the gradual narrowing and closing of further pores. This observation
might suggest that NOM adsorption at the largest
pores can occur very quickly to restrict both the
convective ¯ow and further NOM entry, thus
increasing rejection and ¯ux decline. Note that the
individual resistances for the PM10 membrane are
less than those of the GM membrane at the lower
J0 condition, even though the ¯ux decline is greater.
The net ¯ux decline is given by the ratio rm/
(rm+rc+rg+ra1+ra2), which is larger for the PM10
membrane. Additional measurements, to create a
larger database obtained under a consistent protocol, will provide us with a basis for better coupling
of the underlying mechanisms to the observed ¯ux
resistances.
CONCLUSIONS
Considering the literature already cited and our
own measurements, NOM rejection, based on
DOC, is clearly controlled by size exclusion, electrostatic repulsion and aromaticity/hydrophobicity interactions between NOM and the membrane
surface and pores. Feed NOM concentration and
NOM aromaticity were less important factors in
¯ux decline with relatively low ¯ux membranes such
as NF45 and YM3 at the permeate rates we used.
However, the ¯ux decline of relatively high ¯ux
membranes (for example UF) can be in¯uenced by
NOM aromaticity and membrane hydrophobicity.
We suggest that a tabulation of e€ective resistances in cross¯ow ¯ux decline measurements should
be obtained using a consistent means of controlling
the initial interfacial concentration. Correlations for
boundary layer mass transfer coecients are signi®cant approximations but using them allows us to
more consistently compare measurements on membranes with di€erent permeabilities and operating
under di€erent conditions than not using them.
Currently, measures of the solutes' and membrane's physical and chemical properties only provide
a qualitative means of rationalizing the transport
measurements. In the case of the natural surface
waters we studied, the combination of total DOC,
SUVA and % humic were consistent indicators of
the NOM ®ltration properties when coupled with
the available data on membrane material properties.
In the future we are optimistic that correlations
may be developed that will facilitate improved
quantitative predictions for ®ltration of complex
mixtures, such as NOM in drinking waters.
AcknowledgementÐThis work is being supported by the
American
Water
Works
Association
Research
Foundation.
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