Minimization of membrane organic fouling
and haloacetic acids formation by controlling
amino sugars and/or polysaccharide-like
substances included in
colloidal NOM
Boksoon Kwon, Sangyoup Lee, Man Bock Gu, and Jaeweon Cho *
Department of Environmental Science and Engineering, Kwangju Institute of Science and
Technology (K-JIST), Oryong-dong, Buk-gu, Gwangju 500-712, Korea, *corresponding
author: Tel. 82-62-970-2443; Fax 82-62-970-2434; e-mail [email protected]
ABSTRACT
The objective of this study was to evaluate the effect of colloidal NOM on the disinfection
by-products (DBPs) formation potentials and membrane organic fouling. From various
analyses of a NOM-fouled UF membrane surface, and large amount of colloidal NOM
were found in the analyzed foulants. From FTIR spectra and pyrolysis analyses, colloidal
NOMs were found to include amino sugars and polysaccharides, as indicated by N-acetyl
groups in the FTIR spectra and from pyrolysis GC-Mass analyses. Colloidal NOMs have
two problematic aspects for membrane applications, which may induce bio-fouling due to
their low biostabilities, and relatively high DBPs reactivities. The amino sugars and
polysaccharides can be utilized by heterotrophic bacteria, and are identified as colloidal
NOM with high fractions of BDOC (approx. 39%), so, their removal is expected to involve
some form of biological pre-treatment process. Colloidal NOM also exhibit relatively high
formation potentials of DBPs, especially haloacetic acids (HAA).
KEYWORDS: Colloidal NOM, Foulant, BDOC, DBPs, membrane filtration
INTRODUCTION
Pilot tests, and actual membrane plants, utilizing source waters with natural organic matter
(NOM) have been observed to experience a significant flux decline, which is believed to be
as a result of organic- and/or bio-fouling. Some studies reported that the hydrophobic
NOM constituents are most likely responsible for this membrane fouling, while others
suggested hydrophilic NOMs (including amino sugars and polysaccharides featuring N-
acetyl groups from FTIR spectra and pyrolysis GC-Mass analyses) are the major foulants.
Two pieces of evidence support the latter; firstly, most NOM-fouled membranes obtained
from pilot- and full-scale filtration systems exhibited strong IR peaks for N-acetyl groups
(an evidence of polysaccharide-like or amino sugar substances), when compared to clean
membranes, and secondly, both the membrane surface and NOM acid constituents (both
hydrophobic and hydrophilic acids) are negatively charged, suggesting stronger
electrostatic repulsion, resulting in less fouling or organic adsorption than with other NOM
constituents (such as NOM neutrals and bases). NOM has been categorized into three
different fractions, including; hydrophobic, transphilic and hydrophilic NOM constituents; in
addition to colloidal NOM.
Colloidal NOM can be characterized in to three problematic areas: potential
membrane foulants, bio-stability, and DBPs reactivity. From previous studies, it was found
that colloidal NOM contain N-acetyl groups, typical of amino sugars and/or
polysaccharides (Leenheer et al., 2000). The neutral properties of colloidal NOM can
influence foulants during membrane filtration (Cho et al., 1998). Little research has been
carried out on the characterization of colloidal NOM with respect to their bio-stabilities and
DBPs reactivities; this information could provide a good insight to the optimization of water
treatment processes, targeting minimization of biodegradable dissolved organic carbon
(BDOC) levels and DBPs formation.
Biodegradable dissolved organic matter (BDOM) is relatively difficult to remove,
probably due to its small molecular weight and neutral properties, to minimal BDOC levels
in finished water (LeChevallier et al., 1987). BDOC levels (prior to distribution), and their
treatability (prior to chlorination), are also important aspects in the bio-stability of
distribution systems and DBP formation potentials (Mathieu et al., 1992; Van der Kooij et
al., 1992; Servais et al., 1995).
The objectives of this study was to characterize colloidal NOM in terms of DBP
formation potentials, membrane fouling and bio-stability to determine the optimum
conditions for membrane filtration (including pre-treatment processes).
MATERIALS AND METHODS
Foulants sample and NOM preparations
Fouled UF membranes (MWCO of 2500) were obtained from a pilot-scale membrane
filtration unit at in the Gwangju drinking water treatment plant, which had been operated for
6 months with Juam Lake source water. Foulants were scraped and collected from the
fouled membrane surface, poured into a 5 L flask contaning pure water, stirred for at least
5 days, and then filtered through a 0.45μm micro-filter. Filtered NOM solution was used
for all characterization analyses, which included; colloidal NOM fractionation, structure
analysis (i.e., XAD-8/4 resin isolation), functionality measurements with FTIR, and
membrane filtration tests.
Colloidal NOM was isolated using a regenerated cellulose dialysis membrane
(Spectra/Por 3) that had a molecular weight cutoff (MWCO) of 3,500 Daltons. Prior to
isolation, an appropriate length of membrane was cut to accommodate the sample volume
to be dialyzed, and the membrane washed by soaking in pure water overnight. Dialysis
was conducted with other chemical processes, as suggested by Leenher et al. (2000).
Some of the colloidal NOM was freeze-dried into powder, and was used for FTIR spectral
analysis.
Membrane characterizations
Zeta potentials, of both the clean and organic fouled membranes, were measured by an
electrophoretic method using an electrophoretic light scattering apparatus (ELS8000,
Otzca, Japan). The contact angle (an index of hydrophobicity) was measured by a sessile
drop method. An attenuated total refractive-Fourier transform infrared (ATR-FTIR)
spectroscopic method was used to determine the functional groups of both the membrane
surface and NOM solutes (either foulants or isolated powders) using a FTIR apparatus
(Perkin–Elmer IR 2000 series with a 45 degree ZnSe crystal). Table 1 lists a summary of
the membrane characterization.
Table 1. Membrane properties
Code
Material
MWCO
(mass unit)
Zeta potential
(mV) @ pH 7
Manufacturer
Contact
angle()
PW
GH
Polyethersulfone
Polyamide TFC
10,000
2,500
-29.10
-30.50
Desal.
Desal.
66.0
38.6
Measurements of disinfection by products (DBPs) formation potential
For the measurement of DBP formation potentials (i.e., haloacetic acids (HAAs) and
trihalomethanes (THMs)), chlorine was added to sample solutions containing different
NOM fractions. Chlorinated samples were stored in a 20oC incubator for three days prior to
solvent extraction and DBPs measurements. Gas chromatography, employing electron
capture detection (GC-ECD, 5890 Series Ⅱ plus, Hewlett Packard) was used to measure
DBP concentrations.
Biodegradable dissolved organic carbon (BDOC) measurement
The biodegradable organic matter (BOM) in the water samples was analyzed by the
method suggested by Joret et al. (1989). Water samples (300mL) were filtered through a
0.45m filter and poured into a bottle, containing 100g of sand (diameter 0.5mm,
uniformity coefficient 1.7), attached with heterotrophic bacteria, which were acclimatized
with Nakdong river surface water (sand-water mixtures were incubated at 25oC until the
DOC stabilized to a minimum value). The BDOC concentration was calculated from the
difference between an initial DOC (average of DOC of sample and DOC of sample
contacted with sand) and the minimum DOC obtained during the incubation period.
RESULTS AND DISCUSSION
Foulants obtained from the fouled GH membrane surface were evaluated by a flat-sheet
membrane filtration test, and for colloidal NOM and BDOC fractions and NOM structure, as
a comparison to NOM included in Juam Lake raw water. It can be hypothesized that
foulant solutions may induce a greater flux decline than the raw Juam lake water. Tested
UF membranes (GH) are negatively charged and exhibits low hydrophobicity, based on
contact angles. From these membrane properties, it is also hypothesized that hydrophilic
NOM constituents (especially hydrophobic NOM neutrals) could easily foul the membrane
surface. Thus, the constituents of fouled membranes are anticipated to contain a high
fraction of hydrophilic NOM components. These hypotheses were investigated by foulant
structural analyses, and zeta potential measurements, of both the clean and fouled
membranes. These results are summarized in Figure 1 and Table 2. No significant
differences in NOM fractions, between Juam raw water and foulants, were obtained, which
was contrary to our hypothesis. However, the fouled GH membrane exhibited low negative
zeta potentials compared to the clean GH membrane due to foulants (i.e., neutrals)
coating/screening the membrane surface. From flux-decline tests of the PW membrane
with both foulant solutions, and raw Juam river water (see Table 3); no significant
difference in the flux decline, between the two filtration tests, was observed, as shown in
Figure 2.
Juam lake
H-PHO
26.92%
H-PHIL
52.88%
T-PHIL
21.19%
Figure 1. NOM structure of Juam lake and foulant: H-PHO; hydrophobic NOM, T-PHIL: transphilic
NOM, H-PHIL: hydrophilic NOM
Table 2. Comparison of Zeta potential between clean membrane and fouled membrane
Clean GH
Fouled GH membrane not
washing
Fouled GH membrane slightly
physical washed with pure water
Zeta potential
(mV)
-30.51
-6.14
-17.95
Table 3. Properties of samples
Sample
pH
Conductivity
(S/cm)
UV254
(cm-1)
DOC
(mg/L)
Juam lake
7.23
83.4
0.0211
2.08
Foulants
7.25
83.8
0.047
2.59
Juam Lake
Foulant
60
-1
-2
-1
Permeance, (Lday m kPa )
80
40
20
0
0
500
1000
1500
2000
Time, min
Figure 2. Comparison of Flux decline trends between Juam lake and foulant (J 0/k=2.5)
FTIR spectra of the clean and fouled membranes and the foulants (as powder) are shown
in Figure 3. The clean membrane exhibits IR peaks for aromatic double bonded carbons
(around 1500 and/or 1600 cm-1), carboxylic groups (around 1250 cm-1), and a C-O bond of
either ethers or carboxylic acids (1250-1050 cm-1) (Skoog and Leary, 1992; Bellamy, 1975).
All the peaks of the clean membranes had reduced absorbance intensities following
organic fouling. The aromatic and carboxylic groups of the clean membrane surface almost
disappear due to foulant coatings. The fouled membrane and foulants exhibit different IR
spectra trends. N-acetyl peaks (at 1045 cm-1) was found in the IR spectra of the fouled
membrane, which is evidence of fouling caused by neutral constituents of NOMs such as
amino sugars and polysaccharides. Foulant powers, in comparison to the clean membrane,
exhibited a strong peak near 1100 cm-1; indicative of alcohol groups in carbohydrates
(probably from amino sugars and polysaccharides). Broad peaks near 1400 (hydrophobic
neutral fractions) and 1600 cm-1 (carboxylate groups for the colloid) were also present with
the fouled membrane.
120
8
clean membrane
100
foulant
6
fouled membrane
80
7
A 60
5
4 A
3
40
2
20
1
0
700
0
900
1100
1300
1500
1700
1900
2100
-1
cm
Figure 3. FTIR spectra of clean membrane, fouled membrane, and foulants powder
For the rigorous characterization of membrane foulants, colloidal versus non-colloidal
NOM foulants, were evaluated. Foulants were identified as containing a relatively high
fraction of the colloidal NOM (62%), compared to the non-colloidal NOM (38%), as shown
in Figure 4. Colloidal NOM consists mostly of neutral hydrophilic NOM with a relatively
high molecular weight (≥ 3500 daltons), causing organic-fouling of the membrane due to
the neutral properties of the amino sugars and polysaccharides included in the colloidal
NOM (Figure 5). Moreover, these amino sugars and polysaccharides can be utilized by
heterotrophic bacteria, suggesting the potential for bio-fouling; the colloidal NOM consisted
of a high fraction of BDOC (approx. 39%: measured by aeroboc BDOC tests with activity
and inhibition controls). FTIR spectra showed an amide (1655 cm–1), a methyl (1382 cm–1)
peaks, and a broad C-O (1045cm-1) peak for the colloid fractions, which are all indicative
of N-acetyl amino sugars. From the flat-sheet membrane filtration tests with the UF
membrane, the colloidal NOM exhibited a significant flux decline due to organic fouling, as
opposed to fouling by other NOM constituents (see Figure 6). Colloidal NOM had relatively
high DBP formation potential reactivities in comparison to the other NOM fractions (see
Figure 7).
Colloidal NOM
62%
Non-colloidal
NOM+Missing
NOM
38%
Figure 4. Fraction of colloidal NOM and non-colloidal NOM in foulants from fouled membrane surface
A
600
800
1000
1200
1400
1600
1800
2000
2200
2400
-1
cm
Figure 5. FTIR spectra of colloidal NOM in foulants
40
P e rm e a n c e , L m -2 d a y -1 kP a -1
R aw
HP
CD
pure w ater
30
20
10
S t a rt in g p o in t s o f
a d s o rp t io n d o m in a n t
0
s tag e
0
300
600
T im e , m in
900
1200
1500
Figure 6. Flux-decline trends of UF membrane with different NOM solutions
25
Reactivity(μg/mg)
Based on DOC
HAA Reactivity
THM Reactivity
20
15
10
5
0
Raw(SW)
CD(Raw)
CD(H-PHO) CD(T-PHIL) CD(H-PHIL) NCD(Raw)
NCD(HPHO)
NCD(TPHIL)
NCD(HPHIL)
Figure 7. The reactivities of DBPs of various different NOM constituents
CONCLUSIONS
From the analyses performed; FTIR spectrum, BDOC measurements, membrane filtration
tests, NOM and DBPs characterization, colloidal NOM exhibited many problematic
inferences: high organic/bio-fouling potential, low bio-stability, and high DBPs reactivity,
giving rise to some questions; how can the colloidal NOM be removed efficiently prior to
membrane filtration, chlorination, and the distribution system? These answers should be
determined in conjunction with the neutral property, and the relatively high molecular
weight and biodegradability of colloidal NOM.
ACKNOWLEDGEMENTS
This work was supported by the Korea Science and Engineering Foundation (KOSEF)
through the Advanced Environmental Monitoring Research Center (ADEMRC) at Kwangju
Institute of Science and Technology (K-JIST).
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Presenter Information:
Name: Boksoon Kwon (Student)
Affiliation: Department of Environmental Science and Engineering, K-JIST
Phone: +82-62-970-2449
Fax: +82-62-970-2434
e-mail: [email protected]