Studies were conducted to develop correlation between nanofiltration (NF) membrane fouling, feed water quality, and membrane characteristics. The water quality of the three seasonal NF feed samples was characterized by UV, DOC, HPLC-0UVA-DOC, XAD, IC and ICP. Surface charge, specific flux variation at different pH and ionic strengths, molecule weight cut-off (MWCO), and functional groups of the NF 200 membrane surface were determined. The extent of fouling was significantly dependent on water quality, membrane properties, and operational conditions. Higher flux decline was observed at high DOC, high divalent cations, high alkalinity and low temperature. Temperature strongly affected the specific flux and natural organic matter (NOM) rejection. Inorganic salt precipitation was proven by FTIR, X-ray diffraction, X-ray fluorescence and SEM. NOM fouling was observed during long run pilot tests at relatively high temperature while scale fouling was observed at both high and low temperatures. Due to the decrease in solubility of CaCO3¢s) and CaSOgs) at high temperature, homogeneous crystallization occurred that led to less flux decline. At low temperature, in spite of the increased solubility, heterogeneous crystallization with NOM absorption was observed. This is due to an increase in pH and formation of nucleus components induced by a temperature decrease. Heterogeneous crystallization caused more significant flux decline than homogeneous crystallization.
Presented at the Conference on Membranes in Drinking and Industrial Water Production, Paris, France, 3-6 October 2000 International Water Association, European Desalination Society, American Water Works Association, Japan Water Works Association
144 N.G. Her et al. / Desalination 132 (2000) 143-160
1. Introduction
Nanofiltration (NF) has proven to be reliable for the removal of organic and inorganic compounds with lower operating pressures than reverse osmosis, and lower molecular weight cut-offs (MWCO) than ultrafiltration (UF) membranes. Organic and inorganic compounds are removed by steric exclusion (size exclusion), electrostatic repulsion (surface charge inter- action) and hydrophobic interaction.
The conformation of NOM is an important factor for flux decline [1,2]. Cho [3] and Speth et al. [4] determined that polysaccharides (hydro- philic neutrals) and proteins (bases) were major possible foulants on the NF membrane. The major component of naturally occurring dissolved organic matter is humic substances that are negatively charged due to carboxylic functional groups over a neutral pH range. Polyhydroxy aromatics (hydrophobic acids) such as humic substances are believed to be rejected well by negatively charged NF membranes through charge and hydrophobic interactions.
Inorganic scaling also causes a significant effect on flux decline in nanofiltration systems [5,6]. Soluble salts such as CaCO3, CaSO4, and SiO2 can precipitate on the membrane surface when feed water is over-saturated. Hetero- geneous or homogeneous crystallization of dis- solved salts leads to lateral growth of deposit or sedimentation of crystals on the membrane [7,8].
An NF prototype in France, employing an NF 200 membrane, has exhibited fouling problems during colder temperatures and/or flooding con- ditions. Recently, a full-scale NF treatment has become operational, its design based on the prototype. The river source water contains high calcium (around 100 mg/L) and a moderate DOC (around 2 mg/L) after pretreatment; how- ever, there are seasonal variations in calcium, alkalinity, sulfate, DOC, and UV absorbance. Pretreatment consists of conventional surface water treatment followed by ozonation (providing
a reduction in SDI), acid addition to pH 7.0, antiscalant addition, and cartridge filtration.
The primary objective of this research is to identify seasonal variations of potential organic and/or inorganic foulants on the NF membrane surface and to evaluate operational conditions and pretreatment (flux, pressure, and recovery pre- treatment; scale inhibitors) to minimize fouling.
2. Materials and methods
2.1. Membrane test unit
A bench-scale cross-flow unit, employing flat- sheet membrane specimens, was used to simulate the NF prototype and the full-scale NF treatment plant. A Millipore Mini-Tan system was used to perform bench-scale flux decline and rejection experiments with the NF200 membrane. The Mini-Tan system consisted of a 4-1 reservoir from which a small, variable speed gear pump provided water to the test cell (Fig. 1). The total membrane surface area in the test cell is approximately 56.27 cm 2 (5.8 cm × 9.7 cm) and the total cross flow area into the test cell is approximately 0.41 cm 2 (5.8 cm × 0.07 cm). Feed temperature is a very important factor to obtain constant specific flux. The feed water temperature was checked at point B after the pump (Fig. 1) and maintained constant using a temperature controller.
2.2. Water quality and NF 200 membrane characterization
2.2.1. Water quality parameters
Three different seasonal samples (April, July, and October 1999) of Oise River surface water for NF membrane treatment were used to compare fouling by NOM and inorganic scaling. Conventionally-treated water (coagulation, floc- culation, and sand filtration) with ozonation is used for NF filtration after pH adjustment (pH 7) and cartridge filtration (Fig. 2). The total recovery
N.G. Her et aft./Desalination 132 (2000) 143-160 145
<
Thermometer A [ ( I II
Recycle (Reten tate) Recycle (Per meate)
~ ~ h e r m o m e t e ~
F e e d I . . . . . rl mCer%V n°Wu it
~ F l o w Meter
>~'~ Back Pressure Valve
~ Pressure Gage
Fig. 1. Schematic diagram of the cross-flow filtration unit (Mini-Tan).
~ Coagulation] I Floculation /
/
~ Sulfuric acid, antiscalant ~ addition ~lCartrid~L.~ Nano- I
'~Filtration] ~ filtration [ 251~m/1011 m
first stage second stage third stage
Average 53 % 44 % 43 % recovery
Fig. 2. Schematics of water treatment process train and membrane stages.
Table 1
Water quality parameters and NOM properties
Sample name pH DOC, UVA254, SUVA,
mg/L cm -l L/m-m~ April 3rd feed 7.21 6.92 0.083 1.20
of the three membrane processes was around 82.6% (53% for fast stage, 44% for second stage, and 4% for third stage, Fig. 2).
Water quality parameters of each stage feed water are given in Table 1. The values of DOC and UV254 for the April third-stage feed were higher than any other third-stage feed samples. SUVA (representing an index of NOM aromaticity) and the humic fraction decreased with ozonation lowering hydrophobicity. During separation by the NF200 membrane, the pH of permeates was slightly more acidic than the feed water, and retentate pH increased through the membrane array.
The concentrations of major cations and anions contained in the feed waters were measured using a Liberty-Series II ICP-AES Spectrometer (Varian, TX) and a Dionex ion chromatograph with an IonPac AS-SC column. H g h concentrations of divalent cation (calcium, Ca 2+) and divalent anion (sulfate, SO42-) were found for all feed waters (Table 2). The rejection ofmonovalent ions (such as sodium and chloride) and uncharged SiO2 was smaller than those of divalent ions such as calcium, magnesium, and sulfate. Sulfate and alkalinity (measured by titration) were highly concentrated through the membrane processes.
2.2.2. M W distribution o f samples
High performance size exclusion chroma- tography (HPSEC) with both UVA and on-line TOC detection was used to determine the MW and MW distribution of NOM with a TSK 50S column and a 2 mL sample loop. A TOC detector was used instead of an UV detector for MW determination because it detects all NOM in a sample. The weight-averaged MW of the April third-stage feed (1398 Daltons) was higher than that of the other feed waters (1205 Daltons for July third-stage feed and 1174 for October third- stage feed). MWs determined by the DOC detector are slightly lower than those values measured by a UVA detector because most natural waters contain a relatively higher aliphatic fraction over low MW range that is not detected by a UVA detector.
MW distributions of NOM were obtained to identify DOC components (e.g., polysaccharides, proteins, humic substances, simple organic acids, and neutrals) that may be potential membrane foulants. Fig. 3 shows clearly the MW distribu- tions of dissolved organic matter (DOM) of the third stage feeds as a function of MW. Around 17,500Daltons, small peaks (possibly poly- saccharides) that have no double bonds were
N.G. Her et el./Desalination 132 (2000) 143-160 147
0.12
o 0
C 0 D.
0,08
0.04
-0.04 100000
150~) 710
1,6oi 9 9 - 1 0 - 3 r d Feed
9 9 - 7 - 3 r d f e e d ~ ~
DOC ~ 1~oo
~ " ~ I I I
9 9 - 4 - 3 r d f e e d
9 9 - 1 0 - 3 r d Feed
9 9 - 7 - 3 r d feed
UVA
• ! l
10000 1000 100
MW (daltons)
IIIII . i rll
IIIII
Fig. 3. UVA and DOC chromatograms of the third stage feeds (April, July and October samples).
III
10
0,4
0,3
,< >
0.2 ¢.. O Q. O tt
0.1
shown for April and October samples. The peak around 1,500 Daltons represents components that may be humic substances and aliphatic diprotic organic acids. The sharp peak around 710 MW range is likely monoprotic organic acids and other low MW organic acids including small size fulvic acids [9]. Lower SUVA peaks were observed at lower MW range.
2.3. NF 200 membrane characterization
The properties of the NF 200 membrane have been characterized by contact angle (an index of hydrophobicity), zeta potential (an indicator of surface charge), molecular weight cut-off, elemental composition and functional groups by XRF and FTIR, and specific flux at different pH, ionic strengths, and temperatures.
2.3.1. Composition of NF 200
The NF 200 membrane is composed of three layers: ultrathin top layer, microporous support,
and support (Fig. 4). The ultrathin top layer of the NF 200 is polypiperazine (thickness: 200 A) that provides the controlling properties as to its semipermeability (solvent flux and solute rejection). Polypiperazine contains carboxylic acid, ketone, and tertiary amine groups causing the relatively low hydrophobicity of the NF200. The contact angle of the NF 200 was relatively low (39.3 °) compared with NF 45, NTR 7410, and PM 10 [3]. Based on its contact angle, the NF 200 would be expected to cause less hydrophobic interactions that might lead to flux decline.
The microporous support of the NF200 is polysulfone (PSf) that contains aromatic rings connected by one carbon and two methyl groups, oxygen, and sulfonic groups (Fig. 4). Support of the NF200 is non-woven polyester. Polyester is a hydrophilic material that has deloealized nega- tive charge on oxygen. It provides maximum strength and compression resistance combined with minimum resistance to permeate flow (higher water permeability).
148 N.G. Her et al. /Desalination 132 (2000) 143-160
Top layer (Polypiperazine
amide)
Microporous Support
(Polysulfone) Support
(Polyester)
olr OOOH 1
0 C H 3 II
0 C H s
0 O "
II I ~ " / n
---Top layer-front side J .,~'A W;loporo~s ~ppor l (top I~ye~.bzc~ side}
Fig. 4. Chemical structures of three layers and their FTIR spectra.
0,11 ,Q
o.o8 ~
0,05
0.02
Infrared spectroscopy is widely used to identify organic compounds that are readily distinguished from the absorption pattems of other compounds. Attenuated total reflection- Fourier transform infrared (ATR-FTIR) spectro- scopy was used, using a Nicolect 752 spectro- photometer with a 45 ° ZeSe fiat plate crystal, to identify the ionizable functional groups of NF 200.
Fig. 4 exhibits the FTIR spectra of the three faces of NF200 after splitting between support and microsupport to represent the ultrathin polypiperazine layer, the microporous poly- sulfone, and the nonwoven polyester reinforcing support. However, polypiperazine was not well detected; this layer is too thin to obtain FTIR peaks and IR light permeates the ultrathin top layer and detects the polysulfone layer. Therefore, almost all of the FTIR peaks of the top layer were the same as those of the micro- porous polysulfone except the small-broad amide (-CO-NR2) peak at 1650 cm -1. The indicative FTIR peaks of polysulfone were seen at around 1592 cm -l and 1110 cm -l (aromatic double bond- ed carbons), at 1016 cm -] (ether groups), at 1492 cm -] (methyl groups), and at 1151 and 694 cm -1 (sulfone groups). The bottom layer, indicating the polyester support, has an ester peak at 1720 cm -] and an ether peak at 1016 cm -].
X-ray fluorescence (XRF) was used to determine the elemental composition of a clean NF200 membrane. Table 3 shows the compo- sition-elements; sulfur (S) and titanium (Ti) were the main elements, except for carbon, oxygen, and hydrogen that are not detected by XRF. Titanium may be used to impart white color to the NF200 membrane.
Table 3 The main composition elements of NF200 by XRF except carbon and oxygen
S Ca Ti Mn Fe Z n
Intensity 13111 640 14331 366 111 106
% 45.7 2.2 50.0 1.3 0.4 0.4
2. 3.2. Surface charge of the NF 200
The zeta potential, as calculated from the streaming potential, of the NF200 membrane was measured by a commercial electrokinetic measurement apparatus (EKA, Brookhaven Instruments Corp., Holtsville, NY) to investigate the potential for electrostatic interactions. The NF200 membrane exhibited a high negative surface charge (-16.5 mV at neutral pH, Fig. 5). This zeta potential suggests the possibility of
N.G. Her et al. / Desalination 132 (2000) 143-160 149
0.0
> I= -5.0
<.o C
-10.0 o L",
~ . 1 5 . 0 N
-20.0
0 -5.6
"~"6~6~'6 ~ ( ~ " 17 0
I I I 5 7 9 11
pH
0.0
~ -10.0
~ -20.0 o O.
/
-1 . ~ 6 /
/ ///
/ -30.0 - / ~ -
N 0 , . 3 3 . 9
-40,0 1 I I 0 200 400 600 800
Ionic strength (uSIcm)
Fig. 5. Surface charge variation with a change ofpH and ionic strength.
electrostatic repulsion of negatively charged NOM components (e.g., humic and fulvic acids) by the NF200 membrane. The NF200 membrane surface has ionizable functional groups (carboxylic acid) that reduce contact angle and increase zeta potential (negatively).
The surface charge of the NF 200 membrane was highly influenced by pH and ionic strength (Fig. 5). It is negatively charged over the pH range of 4--10 but negative charge was signi- ficantly reduced at lower pH range due to the protonation of functional groups with ionic strength of 300 l.tS/cm (with KCI). The increase of ionic strength (adjusted with KCi at pH 7) significantly reduced negative charge.
2.3.3. Effects of temperature and ionic strength on the specific flux
To determine temperature effects on the NF 200, permeate flux was monitored at various temperatures (10°C-30°C) using a Mini-Tan cross-flow filtration unit. Permeate flux is predicted to increase with decreasing viscosity of water (increasing temperature). For this reason, more favorable permeate rates can be achieved at higher temperatures. The equation by Poland
[12], used to compensate for temperature effects on membranes, is as follows:
J r = J25 "1.03 (r-zs)
Where Jr is a permeation flux at an arbitrary temperature (7) and J25 is the permeate flux at a reference temperature of 25°C. The specific flux (J/AP) of NF200 was 9.1 L/cm2.d.psi at 25°C. Specific flux decreased continuously with de- creasing temperature. The permeate flux of the NF200 membrane was more significantly affected by temperature than that predicted by Poland's equation (Fig. 6).
Specific flux continuously decreased with increasing ionic strength with KCI at pH 7 (Fig. 6). This is attributable to the fact that the pore size of the membrane was reduced due to the decreased double layer of the membrane surface by the increased ionic strength.
2.3.4. Effects of pH on the specific flux at different temperatures
Specific flux was measured at different pH and temperatures to determine the effects of water viscosity and membrane charge (zeta potential) on the pure water permeability. Fig. 7
150 N.G. Her et aL / Desalination 132 (2000) 143-160
12
= 6 u r_
0 NF 200 Y = 0.025x + 0.08 ~ i ._.=.12 Z~ The oretical e xpectstlon y = 0.018x + 0.26 O ~ i ~a
i . o10 - i ¢,i
i < i E
~ 8 i =
; u 6
5 10 15 20 25 30 35 Tam perature (Celsius)
C o n c e n t r a t i o n o f NaCl (M) 0 0.03 0.06 0.09
i r i
20 C
300 600 900
C o n d u c t l v l t y ( u S l c m )
O 9 0 p s i 0 7 0 p s i ~ 5 0 p s i
1200
Fig. 6. The effects of temperature and ionic strength on the specific flux.
7.2
~ 6 . 2
E 3 5,2
o 4 .2
o e,, U)
3.2
10 *C - )l~Di, f lo.Ts 90 psi
. , 0 . i
I °,.. \
2 4 6 8 10 pH
1 2
8 . 0
o .
> , 6 . 8 t~
~ 5 . 5
o 4.3
3 . 0
Fig. 7. The effects ofpH on the specific flux at different temperatures.
20"C
~ ~ D i ; f " 0 - 9 ~ ¢ ~ Opsi
70 psi
50 psi
2 4 6 8 10 12 pH
shows an increase in specific flux at low pH for both temperatures (10°C and 20°C). The zeta potential of the membrane depends highly on the pH of water and becomes less negative at low pH ( -16 .6mV at pH8 and - 5 . 6 m V at pH4, Fig. 5).
The charge of the membrane surface induces an increase in the viscosity of the permeating water near the pore wall (electroviscous effects). This originates through interactions between the permeating water and solvated water molecules in the diffuse layer near the pore walls. As a
result, an additional resistance to flow is imparted on water as it moves through the membrane pores. The increase of specific flux at lower pH was induced by the decrease of negative charge on the membrane surface that could reduce electroviscous effects [13].
The differences in specific flux (maximum specific flux - minimum specific flux) over pH were not the same at different pressures. At lower temperature, the membrane is stiffened and is more resistant to high pressure, causing a lower variation of specific flux with changing
N.G. Her et al. / Desalination 132 (2000) 143-160 151
pH. In contrast, the variation of specific flux with changing pH was greater at 20°C due to the moderated membrane structure.
2.3.5. MWCO for N F 200
A range of polyethylene glycols (PEGs) from 200 to 600 Daltons was used for membrane MWCO determinations using a dead-end stirred- cell filtration unit operated under 60psi. A 20 mg/L concentration as DOC was used for each PEG rejection experiment with Milli-Q water. The NF200 membrane has a MWCO value of approximately 360 Daltons.
The charged NF200 membrane would be expected to reject NOM smaller than its MWCO (360 Daltons), as determined by PEG rejection tests. It can be envisioned that there is a certain MWCO for a (negatively) charged membrane interacting with (negatively) charged NOM com- ponents. This effective MWCO was determined using NOM fractional rejection with the MW distribution of the membrane permeate [10].
The effective MWCO of the NF200 mem- brane with July third-stage feed was around 290Daltons, based on 90% rejection. The effective MWCO (290 Daltons) obtained by NOM fractional rejection is smaller than the
MWCO (360 Daltons) obtained by non-charged PEGs due to charge interactions between the negatively charged surface of the NF200 and negatively charged NOM components.
3. Results and discussion
3.1. Results o f f lux decline
Fig. 8 shows the results of flux decline tests at 10°C and 20°C for the three seasonal samples of third-stage membrane feed waters. Even though the April third-stage feed contained the lowest amount of inorganics among the tested third-stage feed waters, flux decline for the April sample was the most significant due to the higher pH (pH 7.2 for April sample, pH 6.9 for July sample, and pH 6.95 for October sample), higher alkalinity (374 ktS/cm for April sample, 325 ktS/cm for July sample, and 340 IxS/cm for October sample), and higher DOC (6.9 mg/L for April sample, 4.5 mg/L for July sample, and 6.3 mg/L for October sample). Flux decline at 10°C was more significant compared to 20°C for all samples. This trend may be caused by changes in the solubility of salts (see later discussion on precipitation). The bulk concentrations of organic and inorganic components are higher at the
T e m p e r a t u r e : 10 C ~ [ ] 99-July-3rd i "~ 0 99-October-3rd " 40
20
0
T e m p e r a t u r e : 20 C ~ - - 99.ApriF3rd
[ ] 99-July-3rd
- - - ~ 99-October-3rd
10 20 30 40 50 60 10 20 30 40 50
T i m e (h ra ) T i m e (h rs )
60
Fig. 8. Flux-decline of third stage feed samples at 10°C and 20°C; recovery: 10-15%, operating pressure: 60 psi for April/July samples and 90 psi for October sample.
152 N.G. Her et al. / Desalination 132 (2000) 143-160
membrane surface due to concentration polariza- tion. Concentration polarization, an increase in the concentration of rejected solutes in the membrane near-field, is affected by flux recovery; thus, more concentrated conditions will exist near the membrane surface at high flux recovery.
3.2. Results of NOM rejection
Temperature also affected NOM rejection. The increase (around 5 %) of NOM rejection at 10°C may be compared to that at 20°C (Fig. 9) may be caused by membrane pore size com- paction. As discussed earlier at Fig. 6, specific flux decreased around 39% at 10°C compared to that at 20°C (at 90 psi). Lower temperature may decrease the pore size of the NF200 (decrease the effective MWCO) and affect NOM rejection.
3. 3. Effects of antiscalants
Antiscalants can control calcium carbonate and calcium sulfate precipitation, and allow the operation at higher pH without forming a deposit with calcium or magnesium ions in water, thus minimizing acid use and corrosion. To reduce alkaline scales such as calcium carbonate by acidification, sulfuric acid is being used at many
water treatment plants to maintain a pH below 7.5. However, low pH can cause corrosion, other precipitates such as calcium sulfate have lower solubility at lower pH, and the fouling potential (adsorbability) of NOM increased in water containing high concentrations of divalent cations at lower pH.
Several scale-inhibitors were evaluated to reduce fouling: a strong chelating agent (EDTA), Hypersperse, and Permatreat were assessed for fouling and scaling control. Fig. 10 shows the effects of antiscalants on flux decline and NOM rejection. Two different doses of Hypersperse/ Permatreat and ethylene diamine tetraacetic acid (EDTA) were added to October third-stage feed. Flux decline was reduced around 15% by the addition of these antiscalants, without a change in permeate-water quality. The reductions in flux decline by different doses of antiscalants (2.1 g/m 3 Hypersperse and 1.2 g/m 3 Permatreat, 4.2 g/m 3 Hypersperse and 2.4 g/m 3 Permatreat) were similar.
While the Hypersperse formulation disperses particles away from each other and away from the membrane surface, EDTA forms a chelate with metal ions (such as calcium or magnesium) and inhibits the formation of inorganic precipitates. The addition of 10 mg/L of EDTA
N.G. Her et al. / Desalination 132 (2000) 143-160 153
10080 ..................................................................................................................................................................... 0 '= 93°/o i ~-~ 8o I T
oJ
60 O 60 - ~ i - - -O~ Without antiscalants i
O Without an~calants - - - B - - 2 1 rng/L Hypersperse, 1 2 rnglL Permatreat i - -B - - - 2.1 molL Hypersperse, 1.2 mg/L Permatreat i ---Ik.-- 4 2 mg/L Hypersperse, 2 4 m0/L Permatreat
,Ig 4.2 molL Hypersperse, 2.4 mg/L Permatreat ~ EDTA 10 molL i e EDTA 10 mg/L I
> 40 40
0 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80
Time ( h r $ ) Time (hrs)
Fig. 10. Comparison of flux decline and NOM rejection with/without antiscalants.
reduced flux decline by about 15% without a change in permeate-water quality.
3.4. Analysis o f membrane foulants
3.4.1. By FTIR
The FTIR spectra of the clean membrane and fouled membranes with the three seasonal third- stage feeds through the Mini-Tan system are compared in Fig. 11. The IR absorbance intensity of the fouled membranes was reduced or dis- appeared for all of the peaks compared to that of the clean membrane. The membrane specimen fouled with April and October third-stage feeds at 10°C and 20°C showed significant differences in FTIR spectra. The membrane specimen fouled with April third-stage feed at 10°C showed several functional groups of foulants. The peaks at 1403 cm -1 and 873 cm -1 are indicative of car- bonates [14,15]. The C-O from alcohols in poly- saccharides is seen at 1029 cm -~. These peaks imply the precipitation of a calcium-organic complex, co-precipitation of organics with calcite, or adsorption of organics onto calcite.
In the case of the FTIR spectrum of the fouled membrane with October third-stage feed at 20°C, only the high absorbance intensity of
carbonate peaks at around 1410 c m - l and 871 -1 cm was observed indicating that there was not
enough sorption of organic matter onto the calcium carbonate. The precipitation of calcium carbonate without sorption of organic matter did not show significant flux decline (see Fig. 8).
The FTIR spectra of the other fouled membranes exhibit the low intensity of organic peaks. The broad peak at 1647 cm -1 indicates fouling by an amide group (-CO-NHz). The broad peak and increased peak between 1010- 1080 are peaks of polysaccharide or poly- saccharide-like substances.
As a consequence of FTIR spectra of fouled membrane specimens for the bench scale tests, these results suggest that precipitation of pure calcium carbonate (less flux decline, October third-stage feed at 20°C), precipitation of a calcium-organic complex, co-precipitation of organics with calcite, or adsorption of organics onto calcite (significant flux decline, April third- stage feed) may be the most probable foulants.
The FTIR peaks of the membrane fouled with the April third-stage feed at 70% recovery, 20°C, and pH 7.2 show strong carbonate peaks at 1450cm -1 and 871 cm -1 (Fig. 12). On the contrary, there was no evidence of precipitation of inorganic compounds on the membrane at 10%
o.3o~ Fouled with A I April 3th feed I~ J . * [ "=
1647 1587 f l It fl f l .¢11 <
"- , , , , 0.00
1650 1450 1250 1050 850 650
Wavelength (cm-1)
recovery under the same conditions (see Fig. 11). Flux recovery significantly affected inorganic scaling on the NF200 membrane. The FT1R spectrum of the physically removed foulants and fouled membrane from prototype pilot testing was completely different than results of bench tests due to the relatively long operating time for the pilot test causing a fouling by NOM (compare with FTIR of the fouled membrane with July third-feed at 20°C in Fig. 11). The small/broad peak around 1640 cm -] indicates the presence of amide groups and the peak at 1060cm -1 represents polysaccharides. Operating time, flux recovery, temperature, and pH are likely responsible for the fouling potential.
3. 4. 2. By X-ray diffraction (XRD) and X-ray fluorescence (XRF)
X-ray diffraction and fluorescence analysis
were also used to characterize foulants on the membrane surface. Physically removed foulants from the membrane were used to obtain XRD spectra to eliminate the background peaks of the membrane itself. The spectrum of the foulants from the April third-stage feed clearly showed the presence of calcium carbonate and calcium sulfate that have peaks at 29.50/48.6 ° for calcium carbonate and at 11.7°/20.8 ° for calcium sulfate hydrate [14,15].
The elemental contents of foulants from the three fouled membranes produced by the Mini- Tan system were compared by XRF. Sulfur and titanium associated with the clean membrane (see Table 3) were eliminated for comparison, and carbon and oxygen were not included because XRF cannot detect elements having atomic numbers less than 9. The membrane fouled by the April third-stage feed at 10°C that contained higher alkalinity (higher bicarbonate)
N.G. Her et al. / Desalination 132 (2000) 143-160 155
1640 ~ , 103~0 _ " 871
!10°
0.05
.. . . Physical ly removed foulants (from Fouled m e m b r a n e during Pilot test)
0.03a~ t j c
.~[~ --Fouled membrane during Pilot test
0.02 ~
. . . . Fouled m e m b r a n e a t70% recovery (April 3rd at 20 C)
0.00 1730 1580 1430 1280 1130 980 830 680
Wavelength (cm-1)
Fig. 12. FTIR spectra of the foulants, fouled membranes and precipitates. Frencn pilot (prototype) testing: from June 17 to Sep-tember 6, 1999 at pH 6.7 and 21°C.
Table 4 Major elements of foulants on the membrane by XRF
Int., intensity of XRF; %, relative percentage of intensity
had a high absolute amount (intensity) and relatively high percentage amount (96.7%) of calcium. The intensity of calcium decreased with the membrane fouled by the July third-stage feed that contained lower alkalinity (lower bicarbonate) at 10°C. As temperature increased, the amount of calcium on the membrane also decreased (Table 4). Almost the same amount of iron deposited on the membrane, showing no relation to temperature.
3.4.3. By HPLC-UVA-DOC for MW distri- bution of and cleaning efficiency for, organic foulants
A 15 cm 2 specimen of fouled membrane from pilot tests (France) was cut and put into 50 mL of
pH 2, pH 7, and pH 12 solutions adjusted with H2804 and NaOH. After stirring for 24 h, they were filtered through a 0.45 ~tm filter and the pH was adjusted to 7. Chromatograms based on UV254 and DOC (Fig. 13) were obtained after adjusting pH and ionic strength to those of the eluent of the HPSEC.
Caustic solution (pH 12) most efficiently removed organic foulants through all MW ranges. However, acidic solution efficiently removed higher MW components (around 18,000 Daltons) showing high SUVA values (relatively high UVA compared to DOC). Fractions corresponding to a range of under 1800 were not re-moved by neutral solution (pH 7). As a consequence all fractions were not removed by one solution.
156 N.G. Her et al. / Desalination 132 (2000) 143-160
0 . 6
o o
w 0 . 1 ¢ o o .
- 0 . 4
18000 18o01030i~ii~
c l e a n e d b y
1 0 0 0 0 0 1 0 0 0 0 1 0 0 0 1 0 0
0 10
MW (daltons)
2 . 8
<
g 1.4 w
m
Fig. 13. UVA/DOC spectra of foulants after cleaning fouled membrane during pilot tests.
3.4. 4. By scanning electron microscope (SEM)
An ISI-SX-30 scanning electron microscope was employed to determine the deposit morpho- logy of fouled membranes. Fig. 14 shows SEM photographs of membranes fouled during bench- scale cross-flow tests. The SEM photographs in Fig. 14a shows clusters and stacks of calcium complexes that may be calcium carbonate and calcium sulfate (proven by XRF). The major difference in structure and morphology of calcium carbonate deposit between the membrane fouled with April third-stage feed and the membrane with July third- stage feed is in density and size.
Even though the size of calcium carbonate crystals of the April sample (a) were relatively small (around 15 lam), permeate flux was greatly declined due to the dense packing of calcium carbonate and calcium sulfate on the membrane. Flux decline was due to the blockage of the membrane pores by the heterogeneous crystallization (lateral growth of deposit on the membrane surface) [7].
The SEM photograph in Fig. 14b indicates the presence of calcite. It was not densely packed but well developed (large size, around 300 wn), indicating the limitation of nuclei-formation for calcium carbonate crystallization.
Very small size (around 1 kun) calcium carbonate particles stacked on the membrane were observed in the membrane fouled by the October sample at 20°C (Fig. 14c). Calcium carbonate precipitation should be formed in the bulk solution due to strong over-saturation with higher temperature (homogeneous crystalli- zation) and the crystals deposit on the membrane surface [6].
3.4. 5. Concentration polarization model
A concentration polarization model was used to predict the concentration of solutes (e.g. Ca 2+)
Fig. 14. SEM photographs of membrane fouled through bench-scale cross-flow tests employing flat-sheet membrane: a, 33 h deposition with April third-stage feed at 10°C; b, 55 h deposition with July third-stage feed at 10°C; c, 70 h deposition with October third-stage feed at 20°C.
N.G. Her et al. / Desalination 132 (2000) 143-160 157
at the membrane surface (Cm) from the bulk (feed) water concentration (Cb) and operating conditions (e.g. recovery). Equilibrium models were used to determine whether the solubilities of inorganic scalants (e.g. CaCO3) are being exceeded at the membrane surface.
Concentration polarization is an important factor contributing to scale formation at the membrane surface. As permeate passes through the membrane, the rejected ions in the water accumulate near the membrane surface where their concentration is higher than that in the bulk fluid far from the membrane. The calculation of equilibrium and precipitation potentials by the equilibrium software program, MINEQL +, ac- counted for concentration polarization effects.
Inorganic scale formation on the membrane surface may occur when salts in the raw water are concentrated beyond their solubilities. As a result, precipitative fouling is an important consideration in the operation of membrane processes. The potential for precipitation and the distribution of chemical species in the aqueous system can be calculated by MINEQL +. This computer program provides the solubilities and precipitation potentials of components in the system. It can be applied to predict the formation of inorganic scale that may cause fouling in membrane filtration.
The concentration of solute at the membrane surface (Cm) depends upon the bulk con- centration of solute (Cb), flUX through the membrane (V), the diffusion coefficient of the solute (D), and the approximate thickness of the concentration boundary layer (5). The relation- ship between those factors can be expressed by Cm = Cb exp (D5/V).
The ratio of concentration in the boundary layer to that in the bulk solution is termed the concentration-polarization factor, PF. The value of PF can be estimated as an exponential function of recovery, r: PF = exp (Kr) where, K is a semiempirical constant. The concentration of
ions at the membrane surface, Cm, Can be calculated from the concentrations in the bulk solution, Cb, using the polarization factor: Cm = Cb PF = Cb exp (Kr).
A K value of 0.8 and a flux recovery of 15% were used in calculation of Cm. The concen- trations of solute at the membrane surface (Cm) were concentrated around 12%-13% (Table 5). The latter values were input to the MINEQL + program.
The precipitation potentials of the inorganic compounds were simulated by MINEQL + for July third-stage feed. The results in Fig. 15 show that CaCO3(s), CaSO4(s), and MgCO3(s) are possible major scalants in the system. The precipitation of CaCO3 and MgCO3 increase with increasing pH. The most likely potential for CaSO4 precipitation is shown over the pH range of 3-9.
The degree of saturation can be found by using the solid saturation index (SI): SI = log (Q/Ksp) where Q is ion activity product and Ksp is solubility product of solid. An SI value of zero indicates equilibrium. Values of SI are negative when solids are under-saturated and positive when over-saturated. Calcite (CaCO3), aragonite (CaCO3), dolomite (Ca Mg (CO3) 2), barite (BaSO4), and quartz (SiO2) may be precipitated around pH 7.5 with April third-feed (Fig. 15).
3.4. 6. Precipitation o f calcium carbonate
Calcium carbonate deposition can be realized in two ways: temperature-induced deposition due to changes in calcium carbonate solubility, or a pH change in a calcium salt solution resulting in CaCO3 precipitation. The equilibrium between CaCO3 and an aqueous solution is given by the solubility product and dissociation constants of carbonic acid. Therefore, the growth rate depends on the ion activity product [Ca 2+] [HCO3-] and the saturation pH. CaCO3 is formed by a reaction between Ca 2÷ and HCO3- via
158 N.G. Her et al. / Desalination 132 (2000) 143-160
Fig. 15. LogC-pH precipitation diagram for Ca and Mg species and saturation index at different pH and 20°C.
intermediate C a l i C O 3 + ion pairs and dissociated by a reaction with IT [ 11 ].
If the surface of the solid substrate matches well with the crystal, the interracial energy between the two solids is smaller than the interfacial energy between the crystal and the solution, and nucleation may take place at a lower saturation ratio on a solid substrate surface (heterogeneous crystallization) than in solution (homogeneous crystallization). Inorganic crys- tals, clays, sand and biological surfaces can serve as suitable substrate. Aluminum oxide is an excellent substrate for the nucleation of calcite. Calcium (Ca 2÷) binding to aluminum oxide increases with pH and CO3-2/I--ICO3 - also binds specifically to the A1203 substrate surface; correspondingly, the tendency to form CaCO3 nuclei is favored by a slightly alkaline pH-range.
Temperature affects both the equilibrium position of the precipitation reaction and the reaction rate. Specifically, the equilibrium con- stants of calcium carbonate and calcium sulfate decreases as temperature increases (Table 6). The equilibrium constant for CaCO3(s)+ H+ = Ca 2+ + HCO3- at 10°C is approximately 1.4 times larger than at 20°C. However, the solubilities of possible substrates for the nucleation such as A1203. SIO2, or Na2SOn.10H20 decrease with decreasing temperature, indicating more supply of nuclei at lower temperature.
The precipitation of calcite depends highly on pH. pH changes with temperature and membrane separation. Table 7 shows the variation of pH values at different temperatures. The pH of a neutral solution changes from pH 7.08 at 20°C to pH 7.28 at 10°C.
N.G. Her et al./ Desalination 132 (2000) 143-160 159
Table 6 Equilibrium constants for CaCO3
Equilibrium constant, K 5°C 10°C 15°C 20°C 25°C 40°C
Source: H.S. Hamed and B.B. Owen, The physical chemistry of electrolyte solution, 3rd ed., Reinhold, New York, 1958.
During separation by the NF200 membrane, the pH of the permeate was slightly more acidic than the feed water, and the pH of retentate was more basic than the original feed water. Table 1 shows the variation of pH values through the membrane array. In the case of the October samples, the pH of the first-stage feed was 6.71 but the pH of the third-stage retentate was 7.31. The pH of the combined permeate from the 1 st, 2nd, and 3rd stages was 6.52.
4. C o n c l u s i o n s
The extent of membrane fouling is dependent on water quality, membrane properties, and operational conditions. The feed waters studied contained high calcium concentrations and high alkalinities (all third-stage samples were over- saturated, as indicated by Langelier index) with a significant fouling potential. However, fouling potential by organic materials, including algae,
may also be significant. Through an analysis of membrane properties, the NF200 membrane can be operated to effectively reject hydrophobic NOM due to its high negative charge and low contact angle (hydrophilicity).
Two important metals that can potentially cause fouling are iron and aluminum. However, these metals were not significantly detected by various analytical methods. The possible foulants under low temperature conditions were calcium carbonate, calcium-organic complexes or adsorption of organics onto calcite, as verified by SEM morphology, X-ray diffraction, X-ray fluorescence, and FTIR. Specially, FTIR spectrum of the membrane fouled with April third-stage feed showed the co-precipitation of polysaccharides with CaCO3.
pH is the most important factor in influencing anti-scaling with temperature. However, pH does not remain constant through the membrane array. Due to the higher rejection of negatively charged ions by the negatively charged NF200
160 N.G. Her et al. / Desalination 132 (2000) 143-160
membrane, an increase in pH was observed through each step of the membrane array. Also, pH polarization can contribute to a higher pH existing near the membrane surface.
Inorganic salts can be deposited through temperature- or pH-induced deposition due to variations in solubility. The solubility product constants of CaCO3(s) and CaSO4(s) decrease with increasing temperature causing a homogeneous crystallization (October third-stage feed, Fig. 11). However, the change of solubility induced by temperature variation was minor compared to the solubility change induced by pH variation. Fig. 15 shows the prediction of precipitate amounts for CaCO3(s) and CaSOn(s) (July third- stage feed) as a function of pH. At low temperature, in spite of the increased solubility, heterogeneous crystallization with NOM absorption was observed. This is due to an increase in pH and formation of nuclei such as A1~O3 and SiO2 to initiate precipitation for an over-saturated solution. Heterogeneous crystalli- zation caused more significant flux decline than homogeneous crystallization.
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