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ELSEVIER Journal of Membrane Science 132 (1997) 159-181

Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes

Seungkwan Hong, Menachem Elimelech* Department of Civil and Environmental Engineering, School of Engineering and Applied Science, University of California, Los Angeles, CA

90095-1593, USA

Received 10 December 1996; received in revised form 25 February 1997; accepted 3 March 1997

Abstract

The role of chemical and physical interactions in natural organic matter (NOM) fouling of nanofiltration membranes is systematically investigated. Results of fouling experiments with three humic acids demonstrate that membrane fouling increases with increasing electrolyte (NaC1) concentration, decreasing solution pH, and addition of divalent cations (Ca2+). At fixed solution ionic strength, the presence of calcium ions, at concentrations typical of those found in natural waters, has a marked effect on membrane fouling. Divalent cations interact specifically with humic carboxyl functional groups and, thus, substantially reduce humic charge and the electrostatic repulsion between humic macromolecules. Reduced NOM interchain repulsion results in increased NOM deposition on the membrane surface and formation of a densely packed fouling layer. In addition to the aforementioned chemical effects, results show that NOM fouling rate increases substantially with increasing initial permeation rate. It is demonstrated that the rate of fouling is controlled by an interplay between permeation drag and electrostatic double layer repulsion; that is, NOM fouling of NF membranes involves interrelationship (coupling) between physical and chemical interactions. The addition of a strong chelating agent (EDTA) to feed water reduces NOM fouling significantly by removing free and NOM-complexed calcium ions. EDTA treatment of NOM-fouled membranes also improves the cleaning efficiency dramatically by disrupting the fouling layer structure through a ligand exchange reaction between EDTA and NOM-calcium complexes.

Keywords: Natural organic matter; Water treatment; Nanofiltration membranes; Humic substances; Divalent cations; Fouling control

1. Introduction

In recent years, membrane filtration has emerged as a viable treatment alternative to comply with existing and pending water quality regulations [1,2]. Of particular interest is the use of nanofiltration (NF) as a

*Corresponding author. Tel.: (310) 825-1774; fax: (310) 206- 2222; e-mail: [email protected].

0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI I S0376-73 8 8(97)00060-4

treatment alternative for the removal of natural organic matter (NOM), a precursor of disinfection by-products, in anticipation of more stringent regulations [3,4]. NF technology also offers a versatile approach to meeting multiple water quality objectives, such as the control of organic, inorganic, and micro- bial contaminants [5,6].

NF membranes are usually made of polymeric films with a molecular weight cutoff between 300 and 1000

160 S. Hong, M. Elimelech/Journal of Membrane Science 132 (1997) 159-181

[5]. This allows performance characteristics of both ultrafiltration and reverse osmosis, depending on molecular weight cutoff. The energy requirement for NF is much lower than reverse osmosis (RO), for NF operates at lower pressure, usually in the range of 50-150 psi. NF membranes typically reject divalent ions at a much higher rate than monovalent ions [7,8].

Successful application of NF technology, however, requires efficient control of membrane fouling. Foul- ing, often associated with accumulation of substances on the membrane surface or within the membrane pore structure, worsens membrane performance and ulti- mately shortens membrane life [2]. A wide spectrum of constituents in process waters contribute to fouling. These include dissolved and macromolecular organic substances, sparingly soluble inorganic compounds, colloidal and suspended particles, and microorgan- isms [2,9].

Dissolved naturally occurring organic substances are considered a major cause of fouling in membrane filtration of natural waters [10-12]. A major fraction of dissolved natural organic matter in aquatic environments is contributed by humic substances [13,14]. Humic substances are refractory anionic macromolecules of low to moderate molecular weight. They contain both aromatic and aliphatic components with primarily carboxylic and phenolic functional groups. Carboxylic functional groups account for 60-90% of all functional groups [14]. As a result, humic substances are negatively charged at the pH range of natural waters.

Humic substances in aquatic environments readily adsorb to mineral surfaces, such as clays and metal oxides [ 15,16]. Because of the immense importance of NOM adsorption to mineral surfaces in geochemical and technological processes, NOM adsorption mechanisms have been studied extensively in the past two decades [14]. Findings reveal that the solution chemistry of natural waters, in particular divalent cations and pH, has a significant effect on NOM adsorption [16-18]. In addition, NOM molecular weight and hydrophobicity are also found to influence NOM adsorption strongly [18,19]. Although studies dealing with NOM adsorption mechanisms can provide some insight into the adsorption mechanisms of NOM onto membrane surfaces, they cannot explain NOM fouling in membrane filtration of natural waters, because fouling mechanisms involve not only chemi-

cal interactions but also physical (hydrodynamic) interactions.

The important role of NOM in fouling of membranes processing natural source waters has been pointed out by several investigators. Mallevialle et al. [10] characterized the fouling layer formed during microfiltration (MF) and ultrafiltration (UF) of natural waters. They reported that the fouling layer was composed mostly of clay (kaolinite) and organic matter. Furthermore, the organic matter was found to be packed under the inorganic fouling layer, forming a gel-like organic matrix. The study of Bersillon [ 11 ] on ultrafiltration of natural waters also confirmed that organic matter played a critical role in cohesion of the formed fouling layer, resulting in long-term flux decline. In a more recent study, Kaiya et al. [12] indicated that organic matter was a major cause of product water flux decline during potable water treatment with MF membranes. These MF and UF studies, however, did not provide a mechanistic understanding of NOM membrane fouling. Moreover, the studies are not directly applicable to NF membrane fouling because pore adsorption and plugging are most likely major fouling mechanisms for MF and UF membranes, but much less important for NF membranes.

Although membrane fouling by organic macromolecules, such as proteins, has been studied extensively in the biochemical engineering field [20,21], findings of such studies are not directly relevant to NOM fouling because of the vast differences between NOM and proteins, and because of the very heterogeneous nature of NOM macromolecules. Conse- quently, the underlying mechanisms of NOM fouling are not as yet well understood. Generally, factors influencing NOM fouling can be classified as (i) characteristics of NOM and membranes, (ii) hydrodynamic conditions, and (iii) chemical composition of feed water. A fundamental understanding of these factors is essential for unraveling the mechanisms of NOM fouling and for developing efficient means for fouling control.

Several researchers have shown that the extent of NOM fouling is greatly influenced by the hydrophobicity of the membrane and NOM. Static adsorption experiments by Jucker and Clark [22] demonstrated that humic macromolecules adsorbed more favorably onto hydrophobic membranes. More recently, Nilson and DiGiano [23] have investigated the effect of NOM

S. Hong, M. Elimelech/Journal of Membrane Science 132 (1997) 159-181 161

properties on NOM fouling of NF membranes. In their work, aquatic NOM was fractionated into hydrophilic and hydrophobic components. Fouling tests revealed that the hydrophobic fraction of NOM was mostly responsible for permeate flux decline, whereas the hydrophilic fraction caused much less fouling. Furthermore, they also concluded that only the large molecular weight fraction of NOM contributed to the formation of a fouling layer.

Hydrodynamic operating conditions can also have a significant influence on NOM fouling. Braghetta [24] demonstrated the importance of crossflow in NF membrane fouling. In that work, it was shown that the permeate flux increases at higher crossflow velo- cities. This observation was attributed to disruption of the NOM fouling layer due to strong hydrodynamic shear at high crossflows. However, other important hydrodynamic factors, such as permeation rate, were not investigated in that study.

The role of chemical composition of feed water in NOM fouling has also been the focus of several investigations. Braghetta [24] observed a substantial decline in permeate flux at low pH and high ionic strength during NF fouling experiments with aquatic NOM. This observation was attributed to conformational changes of NOM. Lahoussine-Turcaud et al. [25] and Bonner [26] indicated that membrane fouling by humic acid was enhanced in the presence of divalent cations. The effect of divalent cations on NOM adsorption to polymeric membrane surfaces was also recognized by Jucker and Clark [22]. Their results for static adsorption of NOM, however, cannot be generalized to actual conditions of membrane filtration, especially for crossflow filtration, in which hydrodynamics may play an important role.

The objective of this paper is to investigate systematically the role of chemical and physical interactions in NOM fouling of NF membranes. Emphasis is placed on the effect of divalent cations, an important constituent of natural waters, on NOM membrane fouling. Prior to the fouling experiments, the NF membrane and NOM were carefully characterized under a wide range of solution chemistries. The characterization included measurement of membrane zeta potential, determination of NOM carboxyl acidity, and complexation of divalent cations with NOM. Fouling experiments were then carded out under controlled chemical and physical conditions in a

laboratory-scale crossflow filtration unit. Lastly, the ability of a strong chelating agent (EDTA) to lessen NOM fouling and to clean NOM-fouled membranes was also investigated. Based on the obtained results, the mechanisms of NOM fouling of NF membranes are delineated and discussed.

2. Experimental

2.1. Preparation and characterization of natural organic matter

Three different humic acids were chosen as models for NOM: Suwannee River humic acid (SHA), peat humic acid (PHA), and commercial Aldrich humic acid (AHA). Suwannee River humic acid and peat humic acid were obtained from the International Humic Substances Society (Golden, Colorado) and commercial humic acid was purchased from Aldrich Chemicals (Milwaukee, Wisconsin). All humics were received in a powder form. Stock solutions (1 g/l) were prepared by dissolving the humic acid in deio- nized (DI) water and raising the pH to 8 through the addition of NaOH. Suwannee River humic acid and peat humic acid were used without any further purification.

2.1.1. Purification of commercial humic acid Aldrich humic acid was purified through repeated

precipitation by a strong acid to remove bound iron and decrease the ash content [27]. In this cleaning process, Aldrich humic acid was first dissolved in DI water and precipitated by dropwise addition of 1 M HC1. The pH of the resulting AHA suspension was adjusted to approximately 1 and the precipitation of AHA was continued for an additional 10 rain. The AHA suspension was then placed in six 80 ml poly- propylene centrifuge tubes and centrifuged at 6000 rpm for 10 min. Following the centrifugation, the supernatant was discarded, and the precipitate was resuspended in 1 M HC1 solution. This cleaning procedure was repeated five times, after which a dialysis against DI water was performed to purify further the acid-cleaned precipitate. In the dialysis step, the precipitate was placed in a dialysis membrane bag having a MWCO of 50 000 Da (Spectrum, Houston, Texas) and dialyzed against DI water for eight days. The


dialysis solution was replaced every day. After eight days, the conductivity of the final dialysis solution was less than 5 gS/cm. Finally, to obtain a solid substance, the dialyzed AHA suspension was freeze-dried under vacuum.

2.1.2. Measurement of carboxylic acidity The carboxyl acidity of the model humic substances

was measured by either direct or indirect titration [ 14]. In direct titration, 10 mg of humic acid was dissolved in 40 ml of 0.1 M HC1 solution. The humic and a blank solution were titrated with 0.2 N NaOH using an automatic titrator (DL21, Mettler Instrument Co., Hightstown, New Jersey). An N2 atmosphere was maintained throughout the titration. The carboxylic acidity was estimated from the net titration curve (i.e., humic titration minus blank titration) at pH 7.0.

In indirect titration, 10 mg of humic acid was first dissolved in 20 ml of 0.2 N Ca(CH3COO)2 solution. The vial was sealed after displacing the air with N2 and shaken at room temperature for 24 h. The solution was then filtered through a 0.02 gm polycarbonate syringe filter (Anotop, Whatman Inc., Cliton, New Jersey). Both the humic and a blank solution were titrated to pH 9.8 with a 0.2 N NaOH solution using the above mentioned automatic titrator. During the titration, N2 gas was purged continuously into the reaction vessel to maintain a CO2-free environment.

2.2. Preparation and characterization of NF membrane

An aromatic polyamide thin-film composite membrane, denoted as TFCS by the manufacturer (Fluid Systems, San Diego, California), was selected as a model NF membrane. According to the manufacturer, the allowable operating pH range is between 4 and 11, the allowable feed water temperature is between 1 and 45°C, and the recommended operating pressure is 80 psi (ca. 552 kPa). The membrane was provided as a fiat sheet and stored in DI water at approximately 5°C.

The permeability and selectivity of the NF membrane was determined at various feed water chemical compositions. Product water flux was measured auto- matically by a computer-interfaced digital flow meter (Humonics, Rancho Cordova, California). Selectivity of the membrane for inorganic salts and humic acids

was assessed by measurements of total dissolved solids (TDS) and total organic carbon (TOC), respectively. TDS rejection was quantified by conductivity measurements (Model 32, YSI, Yellow Springs, Ohio). TOC rejection was evaluated by a Dohrman Total Carbon Analyzer (Model 80, Xertex Co., Santa Clara, California) using ultraviolet-promoted persul- fate oxidation and infrared spectrometry.

2.3. Membrane test unit

A schematic diagram of the laboratory-scale crossflow NF membrane test unit used in the fouling experiments is shown in Fig. 1. The membrane test unit consists of membrane cells, pump/motor, feed reservoir, temperature control system, and data acquisition system. In this unit, the test solution is held in a 20-liter reservoir and fed to the membrane cells by a pump (Hydra-Cell, Wanner Engineering Inc., Min- neapolis, Minnesota), capable of providing a maximum pressure of 1000 psi and a maximum flow of 6.93x10 -5 m3/s (1.1 gpm).

The two rectangular plate-and-frame membrane cells are configured in parallel. Each cell contains a fiat membrane sheet, placed in a rectangular channel 63.5 mm long, 25.4 mm wide, and 7.0 mm high. Product water is collected from the top of each membrane cell, and the brine is recirculated back to the reservoir. The crossflow velocity in the membrane cells is controlled by bypassing a portion of the test solution using a bypass valve. The desired pressure is set by a back-pressure regulator (US Para Plate Co., San Jose, California). Temperature is controlled by circulating chilled water through a stainless-steel coil immersed in the test solution; the chilled water is provided by a refrigerated recirculating chiller (Fisher Scientific, Pittsburgh, Pennsylvania). Pressure, crossflow, and temperature are measured by an installed pressure gauge, flow meter, and thermometer, respectively.

2.4. Zeta potential measurements

Zeta potential of the membrane surface was determined using a streaming potential analyzer (BI-EKA, Brookhaven Instruments Co., Holtsville, New York). A detailed description of this instrument is given elsewhere [28]. The zeta potential of the membrane

S. Hong, M. ElimelechlJournal of Membrane Science 132 (1997) 159-181 163

Peru..ate

Membrane Cells

Feed Pump Feed , I [ ,

Pres I ~'~ Bypass Valve I I I

i Regulator

i!u,. Data Acquisition Temperature Refrigerated Chiller System

Controlled Reservoir

Fig. 1. Schematic description of the crossflow NF membrane test unit.

surface was calculated from the measured streaming potentials using the Helmholtz-Smoluchowski equation [29,30]. For the streaming potential measurements, each of the test solutions had a background electrolyte concentration of 10-ZM NaC1. In streaming potential measurements in the presence of humic acid, the concentration of each humic acid was adjusted to 10 mg/1. Seven representative calcium concentrations ranging from 10 -5 to 10-2M were investigated. All streaming potential measurements were performed at room temperature (approximately 22°C).

2.5. Calcium complexation with NOM

Potentiometric calcium titration experiments were performed at three different pH values (4, 6, and 8) to investigate interactions between NOM and divalent cations [27]. Humic solution (0.1 g/l) was prepared by dissolving humic acid in DI water. The humic acid solution contained 10-2M NaC1 as a background electrolyte. The pH of the humic solution was adjusted using HC1, NaOH, or NaHCO3 solutions. CaC12 from stock solutions of 0.05 or 0.5 M was then added to the humic solution to achieve the desired total calcium concentration. The investigated range of total calcium concentration was 10-5-10-2M. After 15 min of equilibration, the free calcium concentration was

measured by a calcium ion selective electrode (Model 93-20, Orion, Boston, Massachusetts). The complexed calcium concentration was calculated from the difference between total added calcium concentration and free calcium concentration. For each titration experiment, a CaCI2 standard with identical solution chemistry was also prepared for the concentration calibration curve.

2.6. Fouling experiments

NOM fouling experiments were performed with model feed solutions. The chemistry of the model solutions was carefully selected to reflect the composition of natural waters. In most fouling experiments, the total ionic strength of the feed solution was fixed at 10 -2 M by adjusting the NaCI concentration. Solution temperature was kept constant at 20°C, and laminar flow was maintained (crossflow velocity of 3.54 cm/s corresponding to Reynolds number of approximately 390). The desired solution pH (4 or 8) was achieved by adding concentrated HC1, NaOH, or NaHCO3.

The protocol developed for NOM fouling experiments is summarized in Table 1. The membrane was first stabilized and equilibrated in the membrane test unit with NOM-free solution for 45 h. This step was necessary to achieve stable water flux and salt rejection, and to produce reproducible results during the


Table 1 Experimental protocol for NOM fouling tests

Time (h) Description

0-45 Stabilization and equilibration with model solution a

45-48 Adjustment of initial permeation rate Baseline performance test

48-72 or 120 Addition of NOM Fouling Experiment b

Operating conditions: transmembrane pressure = 85 psi (586 kPa), temperature = 20°C, crossflow = 3.54x 10 -2 m/s. b Operating conditions: temperature= 20°C, crossflow = 3.54x 10 -2 m/s, and transmembrane pressure varies (depending on initial permeate flux).

ensuing fouling tests. The initial permeation rate was then adjusted to the desired value by changing transmembrane pressure. Following the adjustment of initial permeate flux, the performance of the membrane was characterized in terms of permeate flux and TDS rejection. These performance data served as a baseline for the subsequent fouling experiments. Foul- ing was initiated by adding a concentrated stock solution of humic acid to the feed solution to achieve the desired humic concentration (10 mg/1). Changes in membrane performance were assessed throughout the fouling test by continuous measurements of permeate flux and solute rejection.

At the end of the fouling experiment, the deposition of NOM on the membrane surface was quantified. The NOM-fouled membrane was gently removed from the cell and placed into a basic (0.1 M NaOH) solution. Deposited NOM was then dissolved into the basic solution. After complete removal of NOM from the membrane surface, the resulting solution and a blank (NOM-free) solution were placed in an oven and dried at 105°C for 24 h. The deposited NOM mass was measured by subtracting the dry weight of the blank solution from the dry weight of the solution containing the removed NOM.

2.7. EDTA pre- and post-treatment

The use of strong chelating chemicals was investigated as a means to lessen NOM fouling. Disodium EDTA (NazC1oHtaOsN2-2H20) was selected as a model chelating agent. In these experiments, the

membrane was first stabilized and equilibrated for 45 h with feed water containing 10-3M CaC12. NOM was then introduced to the feed water, imme- diately following the addition of 10-3M EDTA. Product water flux was measured throughout the ensuing 24 h.

The effect of chelating chemicals on the cleaning efficiency of NOM-fouled membranes was also investigated. The ability of a cleaning solution to recover product water flux was assessed using DI water, basic (pH 10) solution, and EDTA (10 -3 M). At the end of the fouling experiment, the NOM-fouled membrane was flushed with DI water for 1 h in the closed-loop membrane testing unit. The DI water was then replaced by freshly made NOM-free feed solution with a chemical composition identical to that used during fouling. Product water flux was measured for the following 30 min. The membrane was further cleaned with basic and EDTA solutions by repeating the above procedure. The recovery of product water flux was evaluated at each stage of the cleaning process.

3. Results and discussion

3.1. NOM and NF membrane characteristics

The interaction of NOM with polymeric NF membranes is markedly influenced by the physical and chemical characteristics of NOM and the membrane surface [22,24]. Hence, a thorough characterization of NOM and membrane surface is critical for understanding the mechanisms of NOM fouling. In this section, the major characteristics of the model humics and NF membrane are presented.

3.1.1. NOM properties The carboxylic acidity and molecular weight of the

three model humic acids are summarized in Table 2. Two different methods, direct and indirect titrations, were used to determine the carboxylic acidity [14]. The phenolic acidity was not determined, since the fouling experiments were performed mostly below pH 8 at which phenolic groups are not dissociated. It is to be noted that the carboxylic acidity of AHA was measured by indirect titration (calcium acetate method), while the carboxylic acidities of PHA and

S. Hong, M. Elimelech/Journal of Membrane Science 132 (1997) 159-181

Table 2 Properties of model natural organic matter

165

Humic acid Carboxylic acidity (meq/g) Molecular weight (Da)

Suwannee river humic acid 3.9 a (4.1 [36], 4.9 [34]) 1000-5000 [34] Peat humic acid 3.7 a (4.8 [33]) 10000-30000 [33] Aldrich humic acid 3.4 b (3.3 [31]) >50000 c

a Measured by the direct titration method with a strong base (NaOH). b Measured by the indirect titration (calcium acetate) method. c Lower molecular weight fraction does exist as discussed in text.

SHA by direct titration with a strong base. The calcium acetate method was not used with PHA and SHA because of the difficulty involving complete separation of these humic substances from the supernatant by the 0.02 ~m filter. According to Table 2, the measured carboxylic acidity of AHA is in fairly good agreement with that reported by Kim et al. [31]. The measured carboxylic acidities of PHA and SHA, however, are slightly smaller than those published elsewhere [32-34]. This discrepancy may be explained by the fact that the starting pH in the direct titration performed in our investigation is 3.6, which is slightly higher than the starting pH (approximately 3) employed in published direct titrations [14].

The average molecular weight of the three model humic acids increases as AHA > PHA > SHA. Gel permeation chromatography performed by Grasso et al. [35] showed that AHA consists mainly of two fractions: one with a size larger than 67 000 Da and the second with a size smaller than 20 000 Da. The molecular weight of a significant fraction of the AHA used in this research is most likely greater than 50 000 Da since it was purified by a dialysis membrane with a MWCO of 50000, Kim et al. [31] also reported that 94% of the purified AHA has a molecular weight greater than 50 000 Da. However, as shown in the next subsection, incomplete rejection of AHA by the NF membrane may indicate that low molecular weight macromolecules are not completely removed by the dialysis process. Using ultrafiltration, Amirbah- man and Olson [33] have recently reported that the majority of PHA has a molecular weight ranging from 10000 to 30000 Da. As for SHA, the average molecular weight is often reported to be around 1100 Da [32,34]. Liang and Morgan [36], however, indicated that SHA has a molecular weight ranging from 3000 to 5000 Da. Jucker and Clark [22] reported an even

higher molecular weight for SHA (10 000- 30 000 Da). This wide distribution in published results may be attributed to errors associated with the various techniques for molecular weight determination [37].

3.1.2. Membrane permeability and selectivity

Prior to the fouling experiments, the permeability (permeate flux) and selectivity (solute rejection) of the NF membrane were evaluated. Before characterization, the membrane was allowed to stabilize and equilibrate for 45 h at the desired chemical conditions. During this period, water flux gradually reached a steady state value following a substantial decline in the first few hours. The performance data obtained after 45 h are used as a baseline for the following fouling experiments. The measured steady state product water flux is 8 .24 i0 .24x10-6m/s (17.5-+-0.5 gfd) under experimental conditions of 85 psi (586 kPa), 20°C, pH 8, and 10-2M NaCI. A recent study with the same NF membrane reported a comparable specific flux of 1.36× 10 -8 m/(s kPa) (0.2 gfd/psi) [4].

The NF membrane exhibits salt rejection typical of charged membranes [38]. As expected, divalent ions are rejected better than are monovalent ions. Experi- ments show that NaC1 and CaCI2 rejections are in the range of 70-95% and 83-98%, respectively, depending on the experimental conditions (Fig. 2(a)). It is further observed that TDS (mostly NaC1) rejection decreases with decreasing pH and increasing divalent cation (Ca 2+) concentration. These observations can be explained by the so-called Donnan exclusion mechanism of charged porous membranes [8,38]. Changes in membrane pore size, caused by electrostatic repulsion within the membrane pore structure resulting from variations in solution chemistry, may also be partially responsible for the observed results [24].


100

90

80

.~ 70

~ 60

I1~ 50

40

30

(a)

4

+ > ¢m'4 )

: O> : Z>

N 8

TDS

i 4

Ca2+

x × ×

e 4 ,

O: (Ji rO>l

8 4 8 pH

100

90 A

"~ 80 .9 "6 70

"$' 6o

0 50 I..,-

4o

30

(b)

4

AHA

8 4 8

SHA

4 8 4 8 pH

Fig. 2. Rejection properties of the NF membrane under various solution chemistries: (a) inorganic (TDS and Ca 2+) and (b) NOM (AHA and SHA). Total ionic strength of the feed solution is fixed at 10 -2 M by varying NaCI concentration. Measurements are made after 45 h of stabilization and equilibration• TDS rejection is measured by electric conductivity, Ca 2+ by ion selective electrode, and NOM by TOC. Experimental conditions: initial permeate flux=8.24x10 -6m/s, crossflow=3.54x10 2m/s, and temperature = 20°C.

NOM removal by the NF membrane was quantified by TOC rejection. The NF membrane exhibits more than 80% rejection of TOC, depending on solution chemistry and NOM source (Fig. 2(b)). It is observed that NOM is removed better at pH 8 than pH 4; this phenomenon is much more pronounced with SHA than AHA. Two different NOM removal mechanisms, namely size exclusion and electrostatic repulsion, may explain the NOM rejection behavior. At low pH,

humic macromolecules have a smaller macromolecular configuration due to reduced interchain electrostatic repulsion [39] and, thus, they pass more easily through the membrane pores. In addition, the lower membrane surface charge at pH 4 (see next section) results in reduced electrostatic repulsion between NOM and membrane surfaces and, subsequently, lower NOM rejection. Similar effects of solution pH on NOM rejection have been reported elsewhere [241.

3.2. Effect o f divalent cations on membrane surface charge

Membrane zeta potential, calculated from streaming potential measurements, characterizes the surface charge of the NF membrane. The change in membrane zeta potential, under solution chemistries similar to those employed in the fouling tests, is reported in this section.

3.2.1. Membrane zeta potential in the presence o f divalent cations

Thin-film composite polyamide NF membranes are usually synthesized by the interfacial polymerization of aromatic amine with acyl chloride [38,40]. This reaction results in amphoteric surface characteristics of polyamide membranes originating from surface functional groups such as carboxyl and amine [28,29]. Additives used in the manufacturing process may also contribute to membrane surface charge. Recently, Childress and Elimelech [28] have investigated electrokinetic properties of RO and NF membranes at various solution chemistries. Their study revealed that the NF membrane used in our investigation becomes less negatively charged with decreasing pH. They also reported the isoelectric point of this membrane to be around pH 3; that is, below pH 3, the membrane is positively charged, while above pH 3, the membrane is negatively charged.

The surface charge of the NF membrane is also greatly influenced by divalent cations. Fig. 3 shows changes in membrane zeta potential versus concentration of divalent cations typically found in natural waters (Ca z+ and Mg2+). In this investigation, 10 -2 M NaC1 was used as a background electrolyte, and the pH was fixed at 4 or 8. At pH 8, the zeta potential of the NF membrane becomes less negative

-4

-6

-8

-24

-26

-28

-10

.~ -14

-16

O -18

t~ -20

N -22 r-i ~ _ _

- -m - - Ca2+ (pH 8)

- - r l - - Mg2+ (pH 8)

• " " 1 . . . . . . . . I • " " . . . . = l • • " " . . . . !


l p . I l i l l . . . . I . . . . . l i d • • • , , , , . l

0 -5 10 -4 10 -3 10 -2

Cation Concentration (M)

Fig. 3. Zeta potential of the NF membrane as a function of divalent cation (Ca 2+ and Mg 2+) concentration. Solution contains 10 -2 M NaC1 as a background electrolyte. Divalent cations are introduced to the solution as CaCI2 and MgCI2.

A

>

E

m . I

c

O 0,.

N

-4

-6

-8

-10

-12

-14

-16

-18

-20

-22

-24

-26

-28

• = "1 . . . . " = " l • • " ' ' ' " 1 . . . . . . . . I

0 ~ 0 0 - - - - . - . 0 / 0 - 0 / 0

I " cp. ,I

--41,-- PHA (p H 8)

--V'--SHA (pH 8) l o l l . . . . . . . . I . . . . . . . . I . . . . . . . . I

10 -s 10 -4 10 -3 10 -2

Ca 2* Concentration (M)

Fig. 4. Zeta potential of the NF membrane as a function of calcium (Ca 2+) concentration in the presence of three model humic acids (AHA, PHA, and SHA). The concentration of each humic acid is 10mg/1. Solution contains 10-2M NaC1 as a background electrolyte. Calcium ions are introduced to the solution as CaC12.

with increasing divalent cation concentration. At pH 4, however, the effect of divalent cations on the zeta potential is less pronounced, since a large portion of the surface functional groups is already protonated. The decrease in the negative charge of the membrane is attributed to effective screening of the membrane surface charge by divalent cations [28,41]. Specific adsorption of divalent cations to membrane surface functional groups may also, in part, be responsible for the decrease in the negative charge of the membrane. Finally, it is noted that the effect of magnesium ions on the membrane zeta potential is identical to that of the calcium ions, implying that both divalent cations may have a similar impact on NOM fouling.

3.2.2. Membrane zeta potential in the presence of calcium ions and NOM

The influence of Ca 2+ ions on zeta potential of the NF membrane was also examined in the presence of three different humic acids. The results are presented

in Fig. 4. It is shown that the membrane surface becomes more negatively charged in the presence of humic acids; the increase in the negative charge of the membrane surface is much more pronounced at pH 4 than pH 8. The increase in negative charge is attributed to adsorption of the humic acids to the membrane surface [28,29]. Humic acid readily adsorbs to the membrane surface and dominates the surface charge of the membrane. The marked increase in the negative charge of the membrane surface at pH 4 can be explained by greater humic adsorption than at pH 8. The adsorption of humic acids is more favorable at pH 4, since the membrane surface charge is only slightly negative and the humic macromolecules are more hydrophobic. Despite the adsorption of humic acids, the surface charge of the membrane becomes less negative as calcium concentration increases, similar to the results discussed in the previous subsection (Fig. 3). The increased negative charge of the NF membrane surface in the presence of humic sub-

168 s. [long, M. Elimelech/Journal of Membrane Science 132 (1997) 159-181

stances is similar to observations in natural waters where NOM controls the charge characteristics of mineral surfaces [16,42].

It is also interesting to note that, at pH 8, the increase in the negative charge of the membrane surface is directly related to the carboxylic acidity of the humic acids which increases as SHA >- PHA > AHA (Table 2). However, this direct relation to carboxylic acidity is not seen at pH 4. This observation may be understood on the basis of differences in humic carboxylic groups contributing to the charge of the membrane surface at each pH. In common prac- tice, the carboxylic acidity is determined by titrating NOM with a strong base, typically from pH 3 to 8 [43]. This method often excludes carboxylic groups ionized below pH 3 and results in underestimation of the carboxylic acidity. These unaccounted carboxylic groups constitute a significant portion of the total carboxylic acidity in many humic substances [43]. The observed increase in the negative charge of the membrane surface at pH 4 comes most likely from a large fraction of the functional groups not accounted for in the measured carboxylic acidity. At pH 8, on the other hand, most of the carboxylic groups are deprotonated and contribute to the negative charge of the membrane surface. Thus, the relationship between the carboxylic acidity and the extent of negative charge enhancement is observed more clearly at pH 8.

The experimental observation discussed above may also be explained by the conformational changes of the humic macromolecules. At pH 8 and low Ca 2+ concentration, the adsorbed NOM macromolecules are stretched into the solution due to electrostatic repulsion between NOM functional groups. Conse- quently, the electrokinetic plane of shear is extended farther into the solution, and the resulting zeta potential becomes less negative [44]. The extension of the shear plane is more pronounced with increasing molecular weight. At pH 4, however, the adsorbed humic macromolecules adopt a flatter configuration, and the displacement of the shear plane is probably not as significant as that at pH 8. In addition, AHA and PHA adsorb more effectively than SHA to the membrane surface (see next section). Consequently, the zeta potential of the membrane surface at pH 4 is not as clearly related to the molecular weight of the NOM macromolecules as it is at pH 8.

3.3. Interaction be tween divalent cations and N O M

Unlike indifferent monovalent cations, divalent cations interact specifically with NOM and form metal-humic complexes [45]. The complexation of divalent cations with NOM changes the electrokinetic properties of the NOM and, therefore, the potential for NOM fouling as well. The extent of complex formation is strongly dependent on the solution chemistry and on the characteristics of the NOM [33,46]. Poten- tiometric calcium titration is useful to quantify such interactions between NOM and divalent cations.

Fig. 5 illustrates the effect of pH on calcium complexation with AHA. As shown, the extent of complexation increases with increasing pH due to increased availability of carboxyl functional groups of NOM at higher pH. Similar pH dependency was found with the other humic substances used [46].

2.5

2.0

E 1.5 +

(J "O

1.0 X (9

E 0 0.5

. . . . . ZX- . . . . . . . . . . . . . . ¢X ZX.

/ Q - - O . . . . . <3- . . . . . O - . . . . . . . . . 0

'J [ ]

~ . - . . . . . . . . . [ ] . . . . . . . . . 43 D "

s []

-- /k-- pH8

- -O-- pH6 ---El--- pH 4

0 . 0 I I I I I I

0 2x10-3 4x10-3 6x10-3 8x10-3 lx10-2

Total Ca 2* (M)

Fig. 5. Humic acid (AHA) complexed Ca 2+ as a function of total added calcium concentration at three different pH values (4, 6, and 8). Concentration of humic acid is 0.1 g/1 and solution contains 10 -2 M NaCI as a background electrolyte. Complexed calcium is determined by the difference between added calcium and measured dissolved calcium.


Calcium adsorption isotherms for the model humic acids used in this research have been reported by other investigators [27,33].

It is noteworthy that calcium complexation increases initially with increasing calcium concentration and gradually reaches a pseudo-maximum value. According to Fig. 5, the maximum calcium complexation for AHA corresponds to a total added calcium of approximately 3×10 -3 M. Based on the measured carboxylic acidity and the observed maximum complexed Ca 2+ at pH 8, it is suggested that each Ca 2+ ion binds to two humic carboxylic groups. It is also observed that NOM becomes insoluble at high calcium concentration when maximum complexation is attained. This phenomenon is attributed to charge neutralization and subsequent precipitation of humic macromolecules. Amirbahman and Olson [33] reported this critical added calcium concentration for PHA to be approximately 2.5×10 -3 M. Based on the calcium titration results, most of the fouling experiments were performed at calcium concentrations below the critical value, since humic precipitation may change NOM feed concentration.

3.4. Chemical aspects o f NOM fouling

12

111

1 o I

? 8

,-r 6

4

3

[] 10 -3 M NaCI

O 10 -2 M NaCI

Z~ 10 "t MNaCI

2 i i i . i , i i i

0 10 20 30 40 50 60 70

T i m e (hr)

26

24

22

20

16 "11

12~" E

10 ~-~

8

6

4

Fig. 6. Effect of ionic strength on NOM (AHA) fouling. The ionic strength of the feed solution is adjusted by varying NaC1 concentration. Experimental conditions: AHA concentration=10mg/1, initial permeate flux = 1.04xl0-Sm/s, crossflow = 3.54x 10 -2 m/s, temperature -~ 20°C, and pH = 7.955:0.1.

The role of solution chemistry (ionic strength, divalent cations, and pH) and humic characteristics in NOM fouling is systematically investigated in this section. Results from previous sections on membrane and NOM characterization are used to elucidate the observed fouling behavior. Fouling is studied under fixed hydrodynamic conditions (permeate flow and crossflow), so that only chemical factors influencing fouling behavior are evaluated.

3.4.1. Effect o f ionic strength To investigate the influence of ionic strength on

NOM fouling, fouling experiments were performed at three different ionic concentrations of an indifferent salt (10 -1, 10 -2, and 10-3M NaCI). Fig. 6 shows clearly that NOM fouling becomes more severe as the ionic strength of the feed solution increases. This can be explained by an increase in the hydraulic resistance of the fouling layer which is caused by an increase in ionic strength, as described below.

The hydraulic resistance of the NOM fouling layer is determined mainly by its thickness and compact-

ness. At high ionic strength, the charges of the membrane surface and humic macromolecules are significantly reduced, due to double layer compression and charge screening, leading to a decrease in electrostatic repulsion between the membrane surface and NOM. Consequently, NOM deposition onto the membrane surface is greatly enhanced, which in turn leads to a thick deposit layer. Furthermore, due to reduced interchain electrostatic repulsion at high ionic strength, humic macromolecules become coiled and spherical in shape [39], forming as a result a more compact fouling layer. The resulting fouling layer provides a significant hydraulic resistance to water flow, causing a severe reduction in permeate flux. At low ionic strength, on the other hand, strong electrostatic repulsion between the membrane surface and NOM hinders NOM deposition. NOM also forms a much looser fouling layer, since humic macromolecules have a flat linear configuration at low ionic strength. In addition, the long range of double layer repulsion at low ionic strength prevents the formation

170 s. Hong, M. Elimelech/Journal of Membrane Science 132 (1997) 159-181

of closely packed NOM fouling layer. As a result, the decline in product water flux is not as significant as it is at high ionic strength.

Close inspection of Fig. 6 reveals that, at the lower ionic strengths, permeate flux is slightly enhanced at the initial stage of the fouling experiments. This phenomenon is attributed to hydrophilization of the membrane surface by adsorbed NOM. Support is provided by the zeta potential measurements, which demonstrate that humic adsorption increases the negative charge of the membrane surface and thus its hydrophilicity. As time progresses, the thickness of the humic adsorbed layer and hence its hydraulic resistance to permeate flow increase; consequently, the permeate flux decreases.

3.4.2. Effect o f divalent cations The effect of divalent cations (Ca 2+) on NOM

fouling is illustrated in Fig. 7 for AHA and PHA. In these experiments, total ionic strength is kept constant (10 -2 M) by varying the NaC1 concentration. The results clearly demonstrate that product water flux

decreases dramatically as calcium concentration increases. Explanations for these results are more complicated than observations with indifferent monovalent cations (Fig. 6), since calcium ions interact specifically with NOM. Unlike Na ÷, Ca 2+ binds specifically through complex formation with the acidic functional groups (predominantly carboxylic) of NOM (Fig. 5) [45]. Thus, in the presence of calcium ions, the charge of the NOM is reduced significantly not only due to effective charge screening but also due to complex formation. The substantial decrease in NOM charge in the presence of calcium ions, as well as the NF membrane charge (Figs. 3 and 4), results in an increased deposition rate of NOM on the membrane surface. Reduced interchain electrostatic repulsion of NOM due to calcium complexation also results in the formation of small, coiled humic macromolecules; subsequently, a more compact NOM fouling layer forms. Lastly, bridging between NOM macromolecules mediated by calcium complexation may also contribute to the formation of a dense fouling layer.

{

, T

. . . . , . . . . . " ' ' ' , . . . . , . . . . - 2 0

( a ) A H A 19

18

17 8

1 5 ~ ~. 7

1,* ~

1 2 ~ "

[] No Ca 2+ 11 ~ ~" ~

Z~ 10 -4 M Ca 2+ 10

10 3 M Ca 2+ 4

r . . . . . . . . . . . | . . . . i . . . .

5

. . . . , . . . . , . . . . , . . . . , . . . . . 2 0

( b ) P H A 19

18

17

16

, 1 5 ~

1 4

, [] No Ca 2+ -J 11 ~

' l = • • i . . . i I I . . . . I m • • • m I " • . j 8

10 15 20 25 0 5 10 15 20 25

T i m e (hr) T i m e (hr)

Fig. 7. Effect of calcium concentration on NOM fouling: (a) AHA and (b) PHA. The total ionic strength of the feed solution is fixed at 10-2M by varying NaC1 concentration (i.e. feed solution with no Ca 2÷ contains 10-2M NaC1, feed solution with 10-4M Ca 2+ contains 9.7×10-3M NaC1, and feed solution with 1 0 - 3 M Ca 2÷ contains 7.0×10 3 M NaC1). Experimental conditions: humic acid concentration=10 mg/1, initial permeate flux = 8.24× 10 _6 m/s, crossflow = 3.54× 10 z m/s, temperature = 20°C, and pH = 7.95±0.1.


0 Z "o

o

G) 0

18 (a)

16

14

12

10

8

6

4

• No Ca 2+

• 10 -3 M Ca 2+

14 • , i • ,

13 (b)

12

11

I " I I " I

- E - No Ca 2+

- - ~ - - 10 -a M Ca 2+

t~

4

3

2 i . i i . i n . l n ,

5 10 15 20 25 30 0 2 4 6 8 10 12 14 16 Time (hr) Deposited NOM (g/m 2)

Fig. 8. Quantification of deposited NOM (AHA) during membrane fouling: (a) deposited NOM mass per unit membrane area versus time and (b) water flux versus deposited mass. Total ionic strength of the feed solution is fixed at 10 -2 M by varying NaC1 concentration (see caption of Fig. 7). Data are collected from different fouling tests over operation periods of 6, 12, and 24h. Experimental conditions: AHA concentration=10 mg/1, initial permeate flux = 1.04× 10 -5 m/s, crossflow = 3.54x 10 -2 re]s, temperature = 20°C, and pH ----- 7.95±0.1.

The enhancement of NOM deposition by calcium ions has been experimentally verified by measuring the mass of NOM deposited on the membrane surface (Fig. 8(a)). It should be noted that, in this experiment, a higher permeation flux (1.04 × 10 -5 m/s) was used to enhance NOM deposition rate. A dramatic increase in deposited AHA mass is observed in the presence of calcium ions (10-3M CaC12, 10-2M total ionic strength). Under the given hydrodynamic conditions, the deposition of NOM increases by almost threefold compared to deposition without calcium, but at identical ionic strength (10 -2 M NaC1). Furthermore, the decline rate of product water flux per unit mass of deposited NOM is much higher in the presence of calcium ions (Fig. 8(b)), proving that the presence of calcium ions causes the formation of a more compact NOM fouling layer.

3.4.3. Effect of solution pH The influence of pH on NOM fouling has been

investigated in the absence (10-z M NaC1) and pre-

sence (10 -3 M CaC12, 10 -2 M total ionic strength) of divalent cations. Results for AHA and SHA at two different pH values (4 and 8) are presented in Fig. 9. A more significant decline in product water flux is observed at pH 4 in both the absence and presence of calcium ions. This behavior is attributed to charge reduction of the membrane and humic macromolecules at low pH. As discussed in the previous section, the surface charge of the NF membrane becomes less negative with decreasing pH and is neutralized at pH 3, the isoelectric point of the membrane. In addition, more carboxylic groups of NOM become protonated with decreasing pH, resulting in a reduction in charge of the humic macromolecules. Thus, the electrostatic repulsion between the membrane surface and NOM and between NOM in solution and deposited NOM is reduced at pH 4 and, as a result, the deposition rate of NOM on the membrane surface increases. In addition, NOM has a smaller macromolecular configuration at pH 4, due to reduced electrostatic repulsion between neighboring functional groups, and thus forms a den-


. . . . , . . . . , . . . . , . . . . , . . . . • 2 0

9 (a) .&,HA 1 9 9

18 • ,..... 81 17 , ,_8 I

~ 7 14 1 5 ~ ~v7

"11

B pH 8 (10 "3 M Ca 2+)

[] p, H4(1013MCa2 I)

m n n m . . . . . . . . . . . . i . . . .

. . . . . . . . . . . . . . , . . . . , . . . . . 2 o

(b) SHA 19

18

1 6

14

O pH8(NoCa 2+) 11 O pH 4 (No Ca 2+) 10 [] pH8(10 -3MCa 2+) [] pH4(10 "3MCa 2+) 9

8 . . . . I . . . . m . . . . n . . . . | . . . .

5 10 15 20 25 5 10 15 20 25 T i m e ( h r ) T i m e ( h r )

Fig. 9. Effect of solution pH on NOM fouling in the absence and presence of calcium ions: (a) AHA and (b) SHA. The total ionic strength of the feed solution is fixed at 10-2M by varying NaC1 concentration (see caption of Fig. 7). Experimental conditions: humic acid concentration=10 rag/l, initial permeate flux = 8.24x 10 - 6 no's, crossflow = 3.54× 10 -2 m/s, and temperature = 20°C.

ser fouling layer. Similar trends of increased NOM adsorption onto membrane surfaces at lower pH were observed by Jucker and Clark [22] and Bonner [26].

Lastly, it is to be noted that for SHA fouling (Fig. 9(b)), the initial sharp decline in permeate flux at low pH may be attributed to pore adsorption and plugging by humic macromolecules. This speculation is supported by the decrease in TOC rejection at low pH (Fig. 2(b)), suggesting that a fraction of SHA deposits within the membrane pores.

3.4.4. Comparison o f three humic acids The fouling behavior with the humic acids used in

this investigation is compared in Fig. 10. In the absence of calcium ions (10-2M NaC1), product water flux decreases only slightly, regardless of the type of humic acid. In the presence of calcium ions (10 -3 M CaC12, 10 -2 M total ionic strength), on the other hand, a substantial decline in product water flux is observed with all three humic acids. Results show that SHA causes less fouling than PHA and AHA; PHA and AHA exhibit an almost identical pattern of permeate flux decline.

The fouling behavior observed in Fig. 10 is attributed to differences in properties of the humic acids. SHA is an aquatic NOM with the lowest molecular weight and the highest carboxylic acidity of these humic acids. Hydrophobic interactions between NOM macromolecules and between NOM and the NF membrane are smaller for low molecular weight NOM. In addition, stronger electrostatic repulsion is expected for SHA since it is more negatively charged under given chemical conditions. Reduced hydrophobicity and increased electrostatic repulsion lead to a lower deposition rate of SHA onto the membrane surface and thus less fouling.

Several recent studies have focused on identifying which fraction of NOM contributes most to membrane fouling [22,23]. The general consensus is that the hydrophobic fraction of NOM tends to adsorb more favorably to membrane surfaces than the hydrophilic fraction. Typically, the hydrophobicity of NOM increases with increasing molecular weight and decreasing acidity. Nilson and DiGiano [23] observed increased NOM fouling with increasing molecular weight. Jucker and Clark [22] found that Suwannee


20

19

18

8 | " 17

? 7 1 5 ~

E.. 1 2 ~ ' ~ PHA (No Ca~*) ~ 1 1 E.

I':1 SHA (10-a M Ca;Z+) 10 A PHA (10 -3M Ca 2+)

4 O AHA (10 .3 M Ca 2+)

• • m • m . . . . i . . . . m . . . . ~ . . . .

5 10 15 20 25 T i m e (hr)

Fig. 10. Comparison of fouling behavior with three model humic acids (ALIA, PHA, and SHA). Total ionic strength of the feed solution is fixed at 10-2M by varying NaC1 concentration (see caption of Fig. 7). Experimental conditions: humic acid concentration=10 mg/l, initial permeate flux = 8.24× 10 -6 m]s, crossflow = 3.54x 10 -2 m/s, temperature = 20°C, and pH = 7.954-0.1.

River humic acid adsorbed much more than fulvic acid. They also reported that the effect of calcium ions on NOM adsorption was less pronounced with fulvic acid than with humic acid.

3.5. Physical aspects of NOM fouling

The hydrodynamics of membrane modules have an important effect on the mass transfer, separation, and fouling behavior of membrane systems. In typical crossflow membrane modules, the flow field is composed of a convective flow perpendicular to the membrane (permeate flow) and a shear flow parallel to the membrane surface (crossflow) [47]. NOM is transported to the membrane surface by the permeate flow and swept away from the membrane surface by the crossflow.

While the influence of crossflow on NOM fouling has recently been explored [24], the effect of permeate flow on NOM fouling has not as yet been examined systematically. In this section, we present results of

NOM fouling experiments performed at three different initial permeate fluxes. The results point out the dramatic role of permeate flow in NOM fouling.

3.5.1. Hydraulic resistance of the NOM fouling layer Permeate flux decline due to membrane fouling by

organic macromolecules is a rather complex phenomenon involving adsorption of macromolecules to the membrane surface and within membrane pores, and formation of a gel-like cake layer [48]. Concentration polarization, that is, the accumulation of suspended macromolecules above the membrane surface or cake layer, can also result in flux decline [47]. The effect of concentration polarization, however, is negligible for such dilute solutions of macromolecules as those used in this investigation. Furthermore, concentration polarization occurs within a much shorter time scale (minutes) than does fouling (hours and days). Because of the difficulties in distinguishing between adsorption and cake formation, particularly for a complex mix- ture of organic macromolecules such as NOM, the term deposition will be used in this paper to describe the combined effect of these two processes.

Several models have been employed to explain organic matter fouling, including gel polarization, osmotic pressure, and hydraulic resistance models [49,50]. However, because of the heterogeneous nature of NOM, development of a fundamental model describing NOM fouling is rather complicated. In this paper, a hydraulic resistance model, based on classical filtration theory, is adopted to analyze NOM fouling.

When transmembrane pressure is applied, organic macromolecules are transported to the membrane and accumulate near the membrane surface. A fraction of these macromolecules deposits on the membrane surface, resulting in additional resistance to water fow. The decline in water flux due to this fouling layer can be described by Darcy's law [20]:

A p j - ( 1 )

#(Rm + Rd)

Here, J is the permeate flux, P is the transmembrane pressure drop,/z is the feed water viscosity, Rm is the membrane resistance, and Rd is the resistance of the fouling layer. Rearrangement of Eq. (1) yields

J0 - J Rd - - - - (2)

J Rm


where Jo is the permeate flux of the clean membrane ( = A P / # R m ) .

The additional resistance, Rd, is related to the total mass of organic macromolecules deposited on the membrane surface as follows:

rdMd Rd -- (3)

Am

where rd is the specific resistance of the deposited fouling layer, Md is the total mass of deposited organic macromolecules, and Am is the effective membrane surface area. In dead-end filtration, Md is equal to the total mass of the organic macromolecules transported to the membrane surface. In crossflow filtration, however, only a fraction of the organic macromolecules transported to the membrane deposits on the membrane surface, depending on chemical and physical (hydrodynamic) conditions [20]. Thus, Md is modified for crossflow filtration by introducing an attachment efficiency, a, so that

Md = aCb V = CtCbAm J dt (4)

Here, c~ is the ratio of the organic macromolecules deposited on the membrane surface to those transported to the membrane surface, Vis the total volume of permeate, Cb is the bulk concentration of organic macromolecules, and t is the filtration time. Using Eqs. (3) and (4), Eq. (2) can be expressed as

J ° j J - (rdC~Cb~v (5) ~,Amgmf

This equation eliminates the effect of mass transfer by normalizing the results in terms of the accumulated permeate volume.

3.5.2. Role of permeation rate in NOM fouling The effect of initial permeation rate on NOM foul-

ing has been investigated under various solution chemistries. The results are shown in Fig. 11. At the higher permeation rate (3.11 x 10 -5 m/s), a dramatic decline in product water flux is observed regardless of solution chemistry. The rate of permeate flux decline at high permeation rate also increases with increasing ionic strength. At the lower permeation rate (0.47x 10 -5 m/s), on the other hand, no flux decline is observed at low (10 -3 M) and moderate (10 -2 M)

ionic strengths, and only a slight flux decline is seen in fouling experiments at high ionic strength (10 -1 M). At the intermediate permeation rate investigated (1.04xl0-Sm/s), the permeate flux also declines more quickly as ionic strength increases, but the effect of ionic strength is much smaller than that perceived at the high permeation rate. Finally, by comparing Fig. l l (b) and (d), it is observed that the decrease in permeate flux is much more pronounced in the presence of calcium ions (10 -3 M) at all initial permeation rates, although total ionic strength is kept identical for both sets of experiments.

The above fouling results, analyzed by Eq. (5), are presented in Fig. 12. Fig. 12(a) clearly shows increasing slope of the curves with increasing ionic strength at high permeation rate. At low permeation rate, however, the slope of the curves is close to zero for low and moderate ionic strengths, indicating no NOM fouling. These observations suggest that the hydraulic resistance of the fouling layer (Rd) increases with increasing permeation rate and ionic strength, since the left-hand side of Eq. (5) describes the ratio of fouling layer resistance to membrane resistance. In this analysis, internal fouling, such as pore blockage and pore adsorption, is assumed to be negligible, implying that Rm does not change with time, Fig. 12(b) presents the effect of permeation rate in the absence and presence of Ca z+ ions. The slope of the curves increases markedly in the presence of calcium ions, indicating a substantial increase in the hydraulic resistance of the fouling layer.

Increased hydraulic resistance of the NOM fouling layer can be explained by an increase in both rd and a. As the initial permeation rate increases (i.e., higher transmembrane pressure), the NOM fouling layer becomes more compressed. Compressibility of organic fouling layers has been studied extensively, particularly in the area of protein fouling. Chudacek and Fane [51] investigated the properties of three types of deposits (colloid, protein, and chain polymer) formed in unstirred ultrafiltration. In their work, a simplified filtration model incorporating the compressibility of the deposited layer was employed to inter- pret the results. All three substances exhibited increasing specific resistance of the deposit layer with increasing transmembrane pressure. A similar trend was also observed by Opong and Zydney [52], who studied the hydraulic permeability of a protein (BSA)

35

0

3~ A

25 ? o 20

n-

| | - i - | • , • i |

~ [ J ~ t i J ~ ( l / ( ~ [ / ~ ( [ r t r ~ u ' i ' [ u . n . i , ¢ [ i ' n [ I l k . . , . , r e . u ~ | i l l l r f t t j [ [ n ~ L ~ m ¢ [ m ~

• i , | , i , e . J . | , i

10 20 30 40 50 60 70 Time (hr)

35 70

40 ,~ ~

° i 20 ,~ 10<

10 5:

35

• | . | • | | - | . | • !

4o _

/ • i ' - ' - = , I I = I ] 0

10 20 30 40 50 60 70 Time (hr)

3d

i

20

15

1(~

5,

351: . . . . . . . . . . . . .] 35~ , ],0 ; (c) 10- M NaCI 6O

t,o 1 5 '

t 2 o E ~ 1

o ~ . . . . . . . . . . . . . ~o 0 10 20 30 40 50 60 70 0

Time (hr)


• . . : . : , . | . |

i • i • | - i • | - t - | /

(d) 10 -3 M Ca2* 1 70

(10 -2 M Total Ionic Strength) 1 60

4o. 3o ~

2o~ |, ~ ,o Urn=== = =Io

. : . : | , ,

10 20 30 40 50 60 70 Time (hr)

Fig. 11. Effect of initial permeate flux on NOM (AHA) fouling for different solution chemistries: (a) 10 -3 M NaC1, (b) 10 -2 M NaC1, (c) 10-1M NaCl, and (d) 10-3M CaCl2 and 7×10-3M NaC1 (i.e., 10 2M ionic strength). Initial permeate fluxes are as follows: ([S]) 3.11× 10 -5 m/s, (O) 1.04× 10 -5 m/s, and (A) 0.47x10 -5 m/s. Experimental conditions: AHA concentration=10 rag/l, crossflow = 3.54× 10 -2 m/s, temperature = 20°C, and pH = 7.95+0.1. (Note: due to mechanical problem with the computer-interfaced flow meter, flux data for the low permeate flow run in (c) were taken manually after 24 h).

layer deposited during ultrafiltration. They reported that the permeability of the protein deposit layer decreased as transmembrane pressure increased. It was also suggested that the porosity of the deposit layer was determined primarily by a balance between the electrostatic repulsion between the protein macromolecules and the compressive pressure exerted by permeate flow.

In addition to an increase in the specific resistance of the fouling layer, the deposition of NOM macromolecules is also enhanced at high permeation rates• Because differences in the convective particle transfer rate toward the membrane (due to differences in permeate flow) are accounted for when fouling results are presented in terms of accumulated permeate

volume (Fig. 12) rather than time, one concludes that a larger fraction of NOM transported to the membrane deposits on the membrane surface as the initial permeate flux increases (i.e., c~ increases with increasing permeate rate). It is suggested that the deposition of NOM is determined by an interplay between permeation drag and electrostatic repulsion• Permeation drag induced by permeate flow refers to the hydrodynamic force that acts perpendicular to the membrane surface and carries NOM to the membrane surface [53]. The permeation drag is opposed by electrostatic repulsion, which prevents NOM from attaching to the membrane surface. At high permeate rate, permeation drag is much greater than electrostatic repulsion and, as a result, NOM deposition takes place rapidly. At low


3

? O

"-"2

• • • • I • " • • I . . . . I

--laq-- 0.47x10-5 m/s (10 -1 M NaCI)

- - e - - 1.04x10 -5 rn/s (10 -1 M NaCI)

--=~--3.1 lx10 -s m/s (10 -1 M NaCI)

--[3--0.47x10 -s m/s (10 -2 M NaCI)

- -O- - 1.04x10 -s m/s (10 -2 M NaCI)

--Z~-- 3.1 l x l 0 -5 m/s (10 .2 M NaCI)

--FI-- 0.47x10-5 m/s (10 .3 M NaCI)

--E)-- 1.04x10 -s m/s (10 .3 M NaCI)

- - A - - 3.11 xl 0 -s m/s (10-3 M NaCI)

(a)

0 ~ ' ~ - ~ t - ] K : , i ( - ] - , t~ t . } l { . l ~ t -u r ,a -= - ] t l T t ' l T R t ' ~ l t - 1 [ - l ' [ T ] ' t ~ - ] l u

t - - 1 l l i I l l i • • • •

0.0 0.5 1.0 1.5 2.0 Accumulated Permeate Volume (L)

4

3 A

O

v

2

. . . . I • • • | I . . . . I . . . .

--I1]-- 0.47xl 0 -s m/s (10 .3 M Ca 2+) ( b )

- ~ ) - 1.04x10 "s m/s (10 "3 M Ca 2+)

--~,-- 3.1 lx10 -s m/s (10 -3 M Ca 2.)

--r-I-- 0,47x10 -s m/s (No Ca 2.)

- -O- - 1,04x10 -s m/s (No Ca 2*) .~t~=~=.

- - A - - 3.11 x l0 -s m/s (No Ca2*) u 4 t ~ t r '

/

- I . . . . i I . . . . i . . . .

0.0 0.5 1.0 1.5 2.0

Accumulated Permeate Volume (L)

Fig. 12. Role of initial permeate flux in NOV (AHA) fouling: (a) effect of ionic strength and (b) effect of divalent calcium ions. The ratio of the hydraulic resistances of the NOV (AHA) fouling layer to the membrane, (Jo-J)/J, is plotted against the accumulated permeate volume. Experimental conditions: AHA concentration=10 mg/1, crossflow = 3.54× 10 2 m/s, temperature = 20°C, and pH = 7.954-0.1.

permeation rate, however, permeation drag is not strong enough to overcome electrostatic repulsion, leading to less NOV fouling. The latter is observed for fouling experiments with low ionic strength, low permeation rate, and no calcium ions. The important role of permeate flux in membrane fouling by colloidal materials has recently been addressed by Song and Elimelech [53] and Palecek and Zydney [54].

Several investigators have reported the existence of a critical flux, defined as the permeate flux below which a decline in product water does not occur. Cohen and Probstein [55] observed a critical permeate flux during colloidal fouling of RO membranes. This finding was attributed to the strong electrostatic repulsion between colloidal particles and the membrane surface at very low ionic strength. The theoretical investigation of Song and Elimelech [53] also showed a sharp increase in particle deposition at a certain permeate flux above which permeation drag over- comes electrostatic repulsion and controls the rate of particle deposition. Bacchin et al. [56] reported that, for colloids with sizes ranging from 0.1 to 10 ~tm, the critical flux is determined mainly by colloidal and

hydrodynamic interactions. The effectiveness of membrane processes operating below the critical flux has recently been discussed by Field et al. [57] and Howell [58].

3.6. Pre- and post- treatment by chelating chemicals

The discussion in the preceding sections has shown that the NF membrane is extremely susceptible to NOV fouling, especially in the presence of divalent cations. Thus, the feasibility of NF membrane technology would be strongly dependent on the effectiveness of pretreatment processes designed to reduce NOM fouling. Furthermore, cleaning of NOV-fouled membranes may be necessary to maintain membrane performance at acceptable levels. In the following subsections, we demonstrate the effectiveness of a strong chelating agent (EDTA) for preventing NOM fouling and cleaning of NOM-fouled membranes.

3.6.1. Effect of EDTA on N O V fouling The successful operation of membrane processes

generally requires development of an appropriate

S. Hong, M. Elirnelech/Journal of Membrane Science 132 (1997) 159-181 177

pretreatment strategy [9]. It is obvious that reducing NOM concentration in feed water may be the first choice for pretreatment to lessen NOM fouling. The fouling results discussed earlier suggest that reducing the concentration of divalent cations in feed water may also be a reasonable pretreatment to alleviate NOM fouling. Reduction of divalent cation concentration can be achieved through complexation with metal- chelating agents. Examples of synthetic metal chelators include ethylenediaminetetraacetate (EDTA), nitrilotriacetate (NTA), and polyelectrolytes such as polyacrylic acid (PAA) and polyethylenimine (PEI). Volchek et al. [59] demonstrated that divalent cations were successfully removed from feed water by a hybrid separation process involving an MF membrane and polyelectrolyte chelation. Reduction in membrane organic (protein) fouling by metal chelators has also been reported by Kelly and Zydney [60]. In their study, increased protein (BSA) fouling of MF membranes was attributed to BSA aggregation induced by divalent cations. The addition of metal chelators (EDTA and citrate) effectively reduced the aggregation of BSA and subsequent BSA fouling.

The introduction of a strong chelating agent (EDTA) to feed water is found very effective for improving membrane performance (Fig. 13). Only a slight decrease in permeate flux is observed for both AHA and SHA in the presence of E D T A (10 - 3 M ) .

The EDTA lessens NOM fouling by reducing the concentration of free calcium ions through complexation. In addition, a ligand exchange reaction mediated by EDTA removes calcium from NOM-calcium complexes. The removal of free and complexed calcium ions results in increased electrostatic repulsion between the NOM and the membrane surface and among the humic macromolecules, leading to a decrease in NOM deposition onto the membrane surface.

3.6.2. Effect of EDTA on cleaning of NOM-fouled membrane

Effective membrane cleaning is often just as important as pretreatment processes for efficient operation of NF membrane systems [9]. The effectiveness of cleaning is greatly influenced by the choice of cleaning solutions. Typical cleaning solutions for organic matter fouling contain a variety of chemicals such as sodium dodecyl sulfate, phosphate, and sodium hydro-

. . . . ' . . . . ' . . . . ' . . . . ' . . . . . 2 0

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1 8

8 ! " 1 7

~ 7 , 1 5 ~

' 1 4

I,. ' 1 2 ~ "

~: : , 11 ~

. . . . . . li e 0 5 1 0 1 5 2 0 2 5

T i m e ( h r )

Fig. 13. Effect of a strong chelating agent (10 3M EDTA) on NOM (AHA and SHA) fouling. Experimental conditions: humic acid concentration=10 mg/1, initial permeate flux = 8.24x 10 6 m/ s, crossflow = 3.54x10 2 m/s, temperature = 20°C, pH = 7.95-4- 0.1, CaC12 = 10 3M, and N a C I = 7 x l 0 - 3 M (i.e. 10-2M ionic strength).

xide [61]. Metal chelators, often known as sequester- ing agents, are also included in cleaning solutions to enhance cleaning efficiency [62]. The pH of the cleaning solutions is usually raised to 11 or 12. Finally, it should be noted that selection of cleaning solutions depends greatly on membrane materials; membrane compatibility with cleaning solutions should be considered carefully in the selection process [61].

Fig. 14 demonstrates that EDTA treatment of NOM-fouled membranes markedly improves cleaning efficiency. In these experiments, three different model cleaning solutions are utilized to recover product water flux. They include DI water, basic (pH 10) solution, and EDTA (10 -3 M) solution. As shown, cleaning by both DI water and basic solution results in a small improvement in product water flux. EDTA treatment, however, completely recovers the product water flux. This striking result can be explained by disruption of the fouling layer structure through a ligand exchange between NOM-calcium complexes and EDTA [45]. As a result, interchain electrostatic


m m m l m m

I0

EDTA (I0-3 M)

Initial Flux 9 ~ + . 0 . 0 , O

J

Basic Solution (pH=10)

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5'

4 , I

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22 10

20

9

18 .-.. ~ a

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DI Water \ ,

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4 I i i l i 8 I i

40 60 80 100 120 140 0 20 40 Time (rain)

, , , 2 2 1 (10 -3 M) t EDTA

0 0 • 0120 t

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i i i l 8

60 80 100 120 141 Time (min)

Fig. 14. Effect of a strong chelating agent (10 -3 M EDTA) on cleaning of the NOM-fouled NF membrane: (a) AHA and (b) SHA. Fouling experimental conditions: humic acid concentration=10mg/1, initial permeate flux = 8 . 2 4 × 1 0 - 6 m / s , c r o s s f l o w = 3 . 5 4 x 1 0 - 2 m / s , temperature = 20°C, pH = 7.954-0.1, CaC12 = 10 -3 M, and NaCl = 7x 10 -3 M (i.e., 10 -2 M ionic strength). (Note: rejection of EDTA by the NF membrane is more than 97%).

Chemical Conditions NOM in Solution NOM on Membrane Surface

High ionic strength, low pH, or presence of divalent cations

Coiled, compact configuration

Compact, dense, thick fouling layer

Severe permeate flux decline

Low ionic strength, high pH, and absence of divalent cations

Stretched, linear configuration

Loose, sparse, thin fouling layer

Small permeate flux decline

Fig. 15. Schematic description of the effect of solution chemistry on the conformation of NOM macromolecules in the solution and on the membrane surface, and the resulting effect on membrane permeate flux. The NOM fouling described in the diagram is applicable for permeation rates above the critical flux. The difference between the two chemical conditions shown becomes less clear at very high permeate flux. At low permeate flux (below the critical flux), no significant fouling is observed for both conditions.

s. Hong, M. Elimelech/Journal of Membrane Science 132 (1997) 159-181 179

repulsion among humic macromolecules increases, leading to favorable conditions for the desorption of NOM from the fouling layer. The resulting NOM fouling layer is easily removed by hydrodynamic shear. A l igand exchange reaction of humic substances with synthetic chelating agents, analogous to the reaction discussed above for NOM fouling, is often observed in natural aquatic systems [45].

4. Conclusion

The chemical composit ion of feed water greatly influences the fouling rate of NF membranes by natural organic matter. The role of solution ionic strength, pH, and divalent cations in N O M membrane fouling, as well as the fouling mechanisms involved, are summarized in Fig. 15. Of these chemical factors, the presence of divalent cations, such as calcium and magnesium, has a marked effect on NOM fouling. Severe NF membrane fouling is expected when processing natural waters with moderately elevated levels of hardness cations.

In addition to chemical factors, permeate flux plays a critical role in NOM fouling. Rapid membrane fouling occurs at high permeation rates, even under chemical conditions not favorable for fouling, such as low ionic strength, low levels of divalent cations, and high pH. Moreover, for given chemical conditions, a critical permeate flux exists below which the fouling rate is very low. These observations, along with theoretical analyses, suggest that the extent of NOM membrane fouling is controlled by an interplay between permeation drag and electrostatic double layer repulsion. This coupling between physical and chemical factors has important implications for the operation of N F membrane systems. Control of permeate flux (or t ransmembrane pressure) can be an important strategy for coping with membrane fouling.

For many source waters, pretreatment may be necessary to prevent N O M membrane fouling. Clearly, for hard waters, the removal of calcium and magnesium ions substantially reduces NOM fouling. Alternatively, when pretreatment is not employed, frequent cleaning of NOM-fouled membranes may be necessary. Our results demonstrate that cleaning with strong chelating agents, such as EDTA, most

effectively removes the fouling layer and restores permeate flux.

Acknowledgements

We acknowledge the State of California, Depart- ment of Water Resources, for financial support and Fluid Systems Corporation, a member of Anglian Water Group, for providing the NF membrane. We also thank Satoshi Tanaka for his help on characterization of N O M and NF membranes.

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