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Surface shear stress and retention of emerging contaminants during ultrafiltration for drinking water treatment
Separation and Purification Technology 122 (2014) 183–191
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Surface shear stress and retention of emerging contaminants during ultrafiltration for drinking water treatment Heather E. Wray a,⇑, Robert C. Andrews a, Pierre R. Bérubé b a b
Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, ON, Canada Department of Civil Engineering, University of British Columbia, 6250 Applied Science Lane, Vancouver, BC, Canada
a r t i c l e
i n f o
Article history: Received 14 September 2013 Received in revised form 31 October 2013 Accepted 1 November 2013 Available online 15 November 2013 Keywords: Ultrafiltration Air sparging Pharmaceuticals EDCs Fouling Natural organic matter
a b s t r a c t This study investigated the impact of surface shear stress, to represent air sparging employed for fouling control, on the retention of organic micropollutants during ultrafiltration for drinking water treatment. The retention of 16 different pharmaceutically-active and endocrine disrupting compounds was examined during ultrafiltration of three natural surface waters (two lake, one river) under four different surface shear stress regimes: no shear stress, low peak shear stress (representative of continuous coarse bubble sparging), sustained peak shear stress (representative of intermittent coarse bubble sparging), and high peak shear stress (representative of large pulse bubble sparging). Results indicate that surface shear stress does impact the retention of emerging contaminants; however, it is dependent on water matrix and compound properties. The greatest retention of micropollutants was observed in waters with a higher concentrations of organic matter, and for conditions where no surface shear stress was applied (average 32% retention), and under conditions representative of large pulse bubble sparging (average 34% retention). The observed retention under conditions of no shear stress was likely due to a heavy fouling layer that altered the membrane selectivity and was able to entrap organic micropollutants of larger molecular weight. Under conditions that mimicked air sparging, increasing the shear stress (quantified as the root mean square applied shear) resulted in increased retention of organic micropollutants, particularly those that are neutral and hydrophobic in nature. This may be related to solute–solute complexes, which are kept in solution when shear stress is applied, or related to modification of the fouling layer by the shear stress induced onto the membrane surface. The results suggest that there may be value added with respect to removal of organic micropollutants, such as pharmaceuticals, when employing air sparging as a fouling control strategy during ultrafiltration. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In North America, several classes of emerging organic micropollutants including pharmaceutically active compounds (PhACs) and endocrine disrupting compounds (EDCs), have been measured at trace concentrations (ng/L) in wastewater effluents [1,2], their receiving surface waters, and in treated drinking waters [3,4]. Concerns exist regarding organic micropollutants with respect to their ecotoxicologic effects on the aquatic environment [5], and their unknown human health effects when considering lifetime low-levels of exposure through drinking water [6]. Although these compounds are not yet regulated in drinking water, the consensus is to remove or reduce them if possible. Thus, it is important to understand if existing drinking water treatment practices offer any added value with respect to the removal of organic micropollutants. ⇑ Corresponding author. Tel.: +1 416 946 0486. E-mail address: [email protected] (H.E. Wray). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.11.003
The use of ultrafiltration (UF) membranes for drinking water treatment has increased in the past 15 years for both new and recently upgraded treatment plants [7]. Retention of organic micropollutants during ultrafiltration is typically reported to be less than 30% due to the large pore size of UF membranes (10– 100 kDa) relative to the size of many PhACs and EDCs (<1 kDa) [8,9]. Retention of organic micropollutants that does occur during ultrafiltration has mainly been attributed to adsorption on the membrane during the early stages of filtration, but this may decrease once equilibrium is established [10,11], or be limited when natural organic matter (NOM) competes with micropollutants for binding sites on the membrane [10]. Organic micropollutants may also be rejected during UF via interactions with the membrane fouling layer, and/or interactions with dissolved NOM in solution [12]. When organic matter is present, several studies have demonstrated that retention of organic micropollutants is enhanced [3,13–15]. The enhanced retention is attributed to interactions between the micropollutants and the organic matter [12,16], which are retained together by the membrane. The fouling layer may also
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lead to increased retention of micropollutants as it may essentially act as a more selective secondary membrane [11,14] due to partitioning or adsorption of contaminants [6,17,18]. Interactions between NOM and organic micropollutants may vary depending on the compound properties [19], the chemistry of the water matrix [20], and the nature of the organic matter, including the type and concentration [3,21]. Studies have suggested that NOM of a higher molecular weight (>20 kDa), including biopolymers (e.g., proteins and polysaccharides), have a greater impact on micropollutant retention than lower molecular weight NOM [8] and are also the main foulants of UF membranes [22– 24]. A number of studies have been undertaken to assess different fouling control strategies. However, the impact of UF fouling control strategies on the retention of organic micropollutants in natural water matrices is largely unknown. Air sparging is one strategy employed for fouling control during UF. Typically, during drinking water treatment, air is applied during backwash only, to aid in the removal of foulants from the membrane surface. Previous research has demonstrated that air sparging during filtration may also be a cost effective means of controlling fouling in wastewater [25], as well as synthetic [26] and natural water matrices [27]. In addition, air sparging may be optimized (bubble size, frequency) to allow fouling control. Numerous studies have identified large (>100 mL) ‘pulse’ bubbles as an effective air sparging regime for UF fouling control [26–29]. In addition, pulse bubble sparging is also more energy-efficient compared to traditional coarse bubble sparging [28]. Air sparging during filtration reduces fouling by inducing shear stress at the membrane surface which acts to back-transport foulants (biopolymers). Based on the interactions between organic micropollutants and NOM, both in solution and within the fouling layer, the surface shear stress induced by air sparging may impact contaminant retention during UF of natural waters. The specific objectives of this study were to: 1. determine if there is value added with respect to emerging contaminant retention when applying shear stress conditions to UF membranes treating natural water matrices, and 2. relate compound and water matrix properties to contaminant retention by UF under different shear stress conditions. 2. Materials and methods 2.1. Water matrices Three natural surface waters located in Ontario Canada: Lake Ontario (Lorne Park Water Treatment Facility, Mississauga, ON), Lake Simcoe (Barrie South Surface Water Treatment Plant, Barrie, ON), and the Otonabee River (Peterborough Water Treatment Plant, Peterborough, ON) were considered. These waters were selected because they serve as drinking water sources for millions of people, and also represent a range of organic matter concentrations (Table 1). Both Lake waters are commonly treated via ultrafiltration for the production of drinking water. Raw, unchlorinated water was collected at the intakes of the treatment facilities, stored at 4 °C and allowed to warm to room temperature (20 ± 2 °C) prior to use (within 1 week of collection). 2.2. Membrane filtration apparatus A bench-scale apparatus was used to vary shear stress conditions at the surface of a single ultrafiltration membrane hollow fiber. These shear stress conditions were representative of different air sparging regimes. Briefly, the membrane fiber was mounted onto a rubber backing with epoxy and fixed to the inner side of a cylindrical tank. Impeller blades of different geometries rotated
Table 1 Selected characteristics of the surface waters examined (± standard deviation for samples that were replicated).
DOC (mg/L) Hydrophobic DOC (mg/L) Hydrophilic DOC (mg/L) Biopolymers (mg/L) Humic substances (mg/L) Building blocks (mg/L) LMWa neutrals (mg/L) LMW acids (mg/L) pH UV254 (cm1) SUVA (L/mg m) Mean particle surface charge (mV) a b
Lake Ontario
Lake Simcoe
Otonabee River
2.08 0.20 1.89 0.23 1.06 0.33 0.26 n.q.b 8.35 (±0.01) 0.005 1.77 5.29 (±0.15)
4.06 0.42 3.64 0.33 2.01 0.85 0.33 0.12 8.14 (±0.1) 0.008 1.99 3.43 (±0.46)
5.83 0.44 5.39 0.45 3.24 1.02 0.51 0.18 8.36 (±0.1) 0.018 3.05 7.23 (±0.56)
Low molecular weight. Not quantified.
Table 2 Membrane characteristics and operating conditions for this study. Material
Polyvinylidene difluoride (PVDF)
Operation mode Nominal pore size (lm) Length of fiber used (cm) Available membrane surface area (cm2) Target permeate flux (L nm2 h1) Permeation time (h) Backwash frequency
Outside-in 0.04 20 6 30 12–14 No backwashing applied
close to the membrane surface in the cylindrical tank to generate desired shear stress profiles. A detailed description of the apparatus can be found in Chan et al. [26]. The surface shear stress profiles considered were shown to be representative of air sparging regimes measured at full scale [29,30]. Four different surface shear stress regimes were examined: low peak (representative of continuous coarse bubble air sparging), sustained peak (representative of intermittent coarse bubble air sparging), high peak (representative of large pulse bubble air sparging), and a control where no shear was applied. A peristaltic permeate pump (Masterflex 7553-80) with a PTFE tubing head was used to draw permeate by vacuum. Shear stress was quantified as the root mean square (RMS) of the time variable shear stress (results not presented). The RMS was calculated to be 0.42, 0.46, and 0.50 Pa for conditions representative of continuous coarse bubble, intermittent coarse bubble, and large pulse bubble sparging, respectively. A hollow-fiber membrane (ZeeWeedÒ-500, GE Water and Process Technologies, Oakville, ON) was used for this study (Table 2). Virgin membrane fibers were initially cleaned soaking in a 750 mg/ L sodium hypochlorite solution for 16 h, and then stored in a 50 mg/L sodium hypochlorite solution until use. Prior to filtration using natural waters, fibers were rinsed by filtering distilled water for 2 h. Experiments were conducted for a period of 10–12 h. Water was continuously filtered with no backwashing during this period. This length of time was chosen to allow fouling of the membrane surface and also to allow the production of enough permeate to perform water analyses, including measuring the concentration of organic micropollutants. Membrane integrity testing was conducted at the start and end of each experiment using a bubble test at a pressure of 5 psi (34.5 kPa). If air bubbles escaped during the test the membranes were discarded.
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H.E. Wray et al. / Separation and Purification Technology 122 (2014) 183–191 Table 3 Compound properties of organic micropollutants studied.
a b c d e
Compound
MW (g/mol)
log Kow
pKa
Chargea
Hydrophobic/Hydrophilicb
Class/Use
Acetaminophen Bisphenol A Clofibric Acid Diclofenac Diethylstilbestrol Estriol Estrone Gemfibrozil 17 b-estradiol Carbamazepine Ketoprofen Naproxen Pentoxifylline Sulfamethoxazole Sulfamethizole Sulfachloropyridazine
151 228 214 296 268 288 270 250 272 236 254 230 278 253 270 285
0.23 3.86 2.76 4.29 5.33 2.45 3.46 4.39 4.01 2.23 2.54 3.01 0.48 0.86 0.54 0.68
10.2 10.3 4 4.3 9.7 10 10.3 4.9 10.3 14.3 4.3 4.3
Neutral Neutral Ionic Ionic Neutral Neutral Neutral Ionic Neutral Neutral Ionic Ionic Neutral Ionic Ionic Neutral
Hydrophilic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophilic Hydrophilic Hydrophilic Hydrophilic
PhACc, NSAIDd EDCe, plasticizer EDC, herbicide PhAC, NSAID EDC, estrogen replacement EDC, natural hormone EDC, 17b estradiol metabolite PhAC, lipid regulator EDC, natural hormone PhAC, antiepileptic PhAC, NSAID PhAC, NSAID PhAc, vasodilator PhAc, antibiotic PhAc, antibiotic PhAc, antibiotic
5.7 5.3 6.1
Ionic if pKa < 6. Hydrophobic if log Kow > 2. Pharmaceutically active compound. Non-steroid anti-inflammatory drug. Endocrine disrupting compound.
2.3. Analysis of emerging contaminants Sixteen emerging contaminants were studied, representing both PhACs and EDCs (Sigma–Aldrich, Oakville, ON, and CDN Isotopes Inc., Pointe–Claire, QC, Canada) covering a wide range of physical and chemical characteristics (Table 3). In addition, many of the selected compounds have been detected in North American surface waters [4,31]. Collected source waters for this study were spiked to achieve a target concentration of 1000 ng/L for the emerging contaminants of interest and mixed for 24 h prior to filtration to allow equilibration with NOM to occur [10,32]. Compounds were analyzed in spiked feed and permeate waters using a liquid chromatography – tandem mass spectrometry (LC–MS/MS) method, adapted from one developed by the Ontario Ministry of the Environment [33]. Solid phase extraction (SPE) incorporating 6 cc 200 mg Oasis Hydrophilic–Lipophilic Balance (HLB) cartridges (Waters, Mississauga, ON) was used to extract compounds from a 400 mL sample volume. Extracted samples were analyzed using an Agilent 500 Ion Trap system with a Model 212 LC, equipped with a Pursuit XRs-C18 guard column (MetaguardÒ, 2.0 mm ID 3 mm) and a Pursuit XRs Ultra 2.8-C18 analytical column (2.0 mm ID 100 mm, 2.8-lm particle size) (Agilent Technologies, Mississauga, ON, Canada). Method detection limits (MDLs) for the compounds ranged from 6 to 87 ng/L. Compound retention via ultrafiltration was calculated using Eq. (1):
Retention ð%Þ ¼ 1 C p =C f 100%
ð1Þ
where Cf is the concentration of the compound in the feed water and Cp is the concentration in the permeate. A blank run was conducted by filtering unspiked Milli-QÒ water through the apparatus; none of the target analytes were found to be present.
et al. [34]. Data was processed using ChromCALC (DOC-LABOR, Karlsruhe, Germany). The impact of water matrix and surface shear stress conditions on compound retention was compared using a two factor analysis of variance (ANOVA) (Design Expert 8, StatEase, Minneapolis, MN). Post hoc t-tests were used to determine significant differences between levels of the main factors including source water and shear stress condition. Multiple linear regression Ò analysis (SPSS 21, IBM Corp., NY) was applied to investigate relationships between compound retention and surface water and compound characteristics. Fouled membrane surfaces were examined using Field Emission Scanning Electron Microscopy (FESEM, Supra™ 40, Carl Zeiss Ltd.) after coating with 1 nm of iridium. Organic foulant analysis was conducted on select samples with Fourier Transform Infrared Spectroscopy (FTIR, Spectrum One, Perkin Elmer Inc.) equipped with a germanium crystal. Prior to analysis, membrane samples were dried in a dust-free cabinet for a minimum of 24 h. FTIR data was normalized over the range of wavenumbers 800–1000 where two strong PVDF peaks are present; it is assumed that PVDF material does not change. 3. Results and discussion 3.1. Adsorption of organic micropollutants to the membrane surface Ò
Milli-Q water spiked to a target concentration of 1000 ng/L with the analytes of interest was filtered through the UF membrane to determine if organic micropollutants were retained by the membrane. Retention values for this test were consistently low (<5%) (Fig. 1). Given the small size of the compounds (MW < 300 g/mol) relative to the pore size of the membrane (0.04 lm), it is unlikely that the observed removal was due to size exclusion, but rather due to adsorption to the membrane.
2.4. Water and fouling characterization 3.2. Impact of surface shear stress on membrane fouling Raw feed water samples and membrane permeate were characterized using liquid chromatography–organic carbon detection (LC–OCD) to determine the concentration of total dissolved organic carbon (DOC) as well as DOC fractions, including biopolymers, humic substances, building blocks, low molecular weight acids, and low molecular weight neutrals as defined by Huber et al. [34]. LC–OCD analysis was conducted on filtered water samples (0.45 lm, Gelman Sciences, Ann Arbor, MI) at the University of Waterloo, Ontario, Canada, following methods described by Huber
For the three water matrices studied, application of surface shear stress during filtration resulted in lower fouling rates, relative to conditions where no shear stress was applied (Fig. 2). For all shear conditions tested, high peak (representative of large pulse bubble sparging) offered the largest reduction in fouling rates for all waters. These results were consistent with previous work assessing the impact of surface shear stress on membrane fouling in Lake Ontario, Lake Simcoe, and Otonabee River waters [27]. This
H.E. Wray et al. / Separation and Purification Technology 122 (2014) 183–191
Retention (%)
20
15
10
5
A
6 No shear Low peak Sustained peak High peak
5
Resistance (x1012m-1)
186
4 3 2 1
0 0
2
4
6
8
10
12
Permeation time (h) 6
3.3. Impact of water source on compound retention during ultrafiltration Specific water sources significantly impacted the retention of organic micropollutants (p < 0.0001). Lake Ontario had significantly lower retention (average 14%) than Lake Simcoe (average 30%) or Otonabee River (average 36%) waters (Figs. 3–5). However, the overall retention of micropollutants was not significantly different between Lake Simcoe and Otonabee River waters (p = 0.06). The retention observed in natural waters was likely related to interactions of contaminants with organic matter, a phenomenon that has been observed in other studies [15–17]. The higher retention observed for Lake Simcoe and Otonabee River water is likely related to the higher concentrations of organic matter measured in these waters (4.06 and 5.83 mg/L DOC, respectively) compared to that in Lake Ontario (2.08 mg/L DOC). Neale and Schäfer [12] reported an increase in interactions between steroidal hormones and dissolved organic matter in solution, with increasing organic carbon concentrations during ultrafiltration of humic acid-spiked water. In addition, Dalton et al. [20] observed an increased removal of organic micropollutants at high organic matter to micropollutant ratios for nanofiltration membranes. Binding of pharmaceutical compounds has been reported to be greater for dissolved organics that had more carboxylic and phenolic functional groups [32]. This suggests that micropollutants interact with aromatic compounds, of which there is a higher concentration, as indicated by the specific UV absorbance (SUVA) (Table 1), in Otonabee River and Lake Simcoe waters.
No shear Low peak Sustained peak High peak
5 4 3 2 1
0
2
4
6
8
10
12
10
12
Permeation Time (h)
C Resistance (x1012m-1)
previous study further identified the biopolymer fraction of organic matter (as determined by LC–OCD) as the main membrane foulant and provided evidence that shear stress minimized membrane fouling via back-transport of biopolymers from the membrane surface. In addition, the impact of surface shear stress on fouling was more pronounced for Lake Simcoe and Otonabee River waters, which is not surprising given that these waters have a higher concentration of biopolymers (>0.3 mg/L) to be back-transported from the membrane.
6
No shear Low peak Sustained peak High peak
5 4 3 2 1
0
2
4
6
8
Permeation time (h) Fig. 2. Resistance increase over time for different shear stress conditions applied during ultrafiltration of (A) Lake Ontario, (B) Lake Simcoe, and (C) Otonabee River water.
100 90
No shear stress Low peak
Retention (%)
Fig. 1. Average UF retention (±standard deviation) of compounds when filtering analyte-spiked (to 1000 ng/L) Milli-QÒ water.
Resistance (x1012m-1)
B
80
Sustained peak
70
High peak
60 50 40 30 20 10 0
nr nr
nr
nr nrnr
3.4. Impact of surface shear stress on retention of organic micropollutants Surface shear stress conditions significantly impacted compound retention (p < 0.0001). The average retentions for all compounds under conditions representative of continuous coarse, intermittent coarse, and pulse bubble sparging were 18%, 22%, and 34%, respectively. Under control conditions where no shear stress was applied average retention was 32% (Figs. 3–5). Com-
Fig. 3. Average compound retention (%, ± standard deviation) in Lake Ontario water for a range of surface shear stress conditions; nr = no retention.
pound retention under conditions representative of pulse bubble sparging was not significantly different than control conditions of no shear stress (p = 0.6). However, these two conditions resulted
H.E. Wray et al. / Separation and Purification Technology 122 (2014) 183–191 100 No shear stress Low peak Sustained peak High peak
90 80 Retention (%)
70 60 50 40 30 20 nr
10
nr
nr
nr
0
Fig. 4. Average compound retention (%, ±standard deviation) in Lake Simcoe water for a range of surface shear stress conditions; nr = no retention.
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(p > 0.3) and compound retention was consistently low (<40%) under all test conditions (Fig. 3). For Lake Simcoe water, low peak shear stress (representative of continuous coarse bubble air sparging) resulted in significantly lower compound retention than all other shear stress conditions (p < 0.03), which were all not significantly different from each other (p > 0.1) (Fig. 4). For Otonabee River water, retention was not significantly different between conditions of no shear stress and high peak shear stress (representative of pulse bubble sparging) (p = 0.24) (Fig. 5). These two conditions resulted in significantly higher retention of compounds than sustained peak shear stress (p < 0.01) and low peak shear stress (p < 0.0001). High retention of micropollutants for both the highest fouling (i.e., no shear stress) and lowest fouling (i.e., high peak shear stress) conditions suggests different mechanisms may govern the retention of micropollutants depending on whether shear stress is applied or is absent. 3.5. Impact of fouling and NOM-micropollutant interactions on micropollutant retention
100 90
No shear stress Low peak
Retention (%)
80
Sustained peak
70
High peak
60 50 40 nr
30 20 10 0
nr nr
Fig. 5. Average compound retention (%, ±standard deviation) in Otonabee River water for a range of surface shear stress conditions; nr = no retention.
in significantly higher compound retention than conditions representative of continuous and intermittent coarse bubble sparging (p < 0.002), which were not significantly different from each other (p = 0.09). The impact of surface shear stress on the retention of organic micropollutants was water matrix-specific and varied from compound to compound. This indicates that retention is likely related to compound properties as well as the impact of surface shear on the characteristics of the fouling layer, and the back-transport of biopolymers (and biopolymer-micropollutant complexes) away from the membrane surface. For Lake Ontario water, the type of surface shear stress applied did not significantly affect the retention of any micropollutants
3.5.1. Micropollutant retention in the absence of surface shear stress Under conditions where no shear stress was applied, retention of organic micropollutants during UF of Lake Simcoe and Otonabee River waters can likely be attributed to the formation of a dense fouling layer. Many studies have observed increased retention of organic micropollutants due to organic fouling in nanofiltration (NF) and reverse osmosis (RO) membranes [18,35,36]. Higher molecular weight NOM, such as biopolymers, tend to foul UF membranes by forming a hydraulically reversible cake layer [37,38], which has been shown to increase micropollutant retention (up to 40% versus lower molecular weight humic substances) [8]. The cake fouling layer may serve as a more selective ‘‘secondary membrane’’ to retain organic micropollutants, the impact of which increases proportionately with the level of fouling [8]. In the present study, a fouling layer on the membrane surface was observed during continuous permeation (no backwash), without application of shear stress for the three water matrices (Fig. 6). Images of the membrane surface illustrate the increased extent of surface fouling in waters with a higher organic content. When filtering Lake Ontario water, a more porous fouling layer developed, whereas when filtering Lake Simcoe and Otonabee River waters, a denser fouling layer was formed (Fig. 6). Increased compound retention in Otonabee River and Lake Simcoe water (compared to Lake Ontario water) may be attributed to this fouling layer. Under these high fouling conditions (i.e., no shear stress applied), there also appears to be a relationship between compound properties and retention. Hydrophobic neutral compounds were rejected to a higher degree than anionic compounds (Fig. 7A). This may be attributed to interactions between neutral micropollutants and negatively-charged organic matter [12,16] that is retained by the membrane.
Fig. 6. FESEM images (magnification 50,000) of membranes surfaces fouled with different source waters after continuous permeation for 12 h (no backwashing).
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H.E. Wray et al. / Separation and Purification Technology 122 (2014) 183–191 100
A 100
Neutral compounds 90
Anionic compounds
80
60
y = 0.08x + 0.14 R² = 0.47
70
20
y = -0.001x + 0.30 R² = 0.0005 0 0
1
2
3
4
5
6
Compound Log Kow
B
60 50 40 30
100
20
Neutral compounds 80
Retention (%)
Bisphenol A Diethylstilbestrol Estriol Estrone 17 B-estradiol Carbamazepine
40
Retention (%)
Retention (%)
80
10
Anionic compounds
0 0.42
60
y = 0.003x -0.39 R² = 0.36
0.44
0.46
0.48
0.5
RMS Shear (Pa)
40
y = 0.002x -0.17 R² = 0.42
20
0 150
200
250
300
350
Fig. 8. Retention of hydrophobic (log Kow > 2), neutral micropollutants with increasing shear stress, i.e., from low peak to high peak, quantified as the root mean square (RMS) of the applied shear stress. Values represent the average retention during filtration of Lake Simcoe and Otonabee River waters ± standard deviation; lines illustrate overall trends between measurements.
Molecular weight (g/mol) Fig. 7. Relationship between neutral and anionic compound retention with (A) compound log Kow, and (B) compound molecular weight under conditions where no shear stress was applied.
A relationship also likely exists between compound molecular weight and retention, as higher retention was observed for larger compounds, irrespective of their charge (Fig. 7B). This may indicate entrapment by the fouling layer, via either adsorption and/or a sieving mechanism by a more selective fouling layer formed without any application of shear stress. Filtration of Lake Ontario water resulted in minimal fouling due to the low concentration of organic matter and biopolymers, therefore fewer NOM-micropollutant and foulant-micropollutant interactions likely occurred. In contrast, for Lake Simcoe and Otonabee River, the higher concentration of organic matter and biopolymers allowed more micropollutant-foulant and micropollutant-dissolved NOM interactions which likely contributed to compound retention. 3.5.2. Micropollutant retention under varying surface shear stress conditions When the magnitude of the surface shear stress increased, retention of hydrophobic, neutral compounds increased (Fig. 8) and membrane fouling decreased. Several studies with NF and RO membranes have reported a reduction in micropollutant retention in the presence of membrane fouling [18,35,39], which was attributed to a reduction in back diffusion of the micropollutants due to the fouling layer and thereby an increased concentration gradient, leading to diffusion through the membrane and into the permeate [35]. This ‘‘cake enhanced concentration polarization’’ has also been observed in hydrophilic UF membranes for ibuprofen, which has a relatively low affinity for organic matter binding [8]. However, stirring or other hydraulic impacts to the membrane surface such as backwashing or air sparging would decrease the concentration polarization [40], and subsequently increase the retention of micropollutants [8]. Cake enhanced concentration polarization may be a factor leading to decreased retention under conditions representative of continuous coarse bubble air sparging (lowest RMS of shear). In this case, a cake foulant still forms during the course of filtration, albeit at a slower rate than during condi-
tions of no shear stress [27]. The cake layer formed under low peak shear stress conditions is likely of intermediate thickness when compared to conditions of no shear and sustained and high peak shear stress, and therefore may not behave in the same manner as previously discussed. It is possible that the shear stress applied during low peak treatment is not sufficient to facilitate adequate back-diffusion of the organic micropollutants (or micropollutantNOM complexes) to prevent the formation of a concentration gradient across the membrane [35]. Concentration polarization is higher if there is a higher mass flux to the membrane surface. Under conditions representative of pulse bubble sparging (greatest RMS of shear), the mass flux of material to the membrane surface is reduced leading to a lower degree of concentration polarization. In Lake Ontario water there was no observed impact of surface shear stress on compound retention, likely due to a lower mass transfer of foulants to the membrane surface. The nature of the membrane foulants may also be slightly different when surface shear stress is applied. Analysis of the membrane surface via Fourier transform infrared spectroscopy (FTIR) post-filtration for Otonabee River water under conditions of no surface shear stress and high peak shear stress (Fig. 9) confirms the presence of humic substances under conditions of high peak shear, as indicated by peaks identified as CAO carboxylic acids and C@C aromatic ring stretching [41] and the absence, or lower concentration of these compounds when no surface shear stress is applied. Back transport of biopolymers away from the membrane surface may have opened up membrane sites for adsorption by humic substances which may also have aided in the retention of organic micropollutants. At a pH of 8.0, the carboxylic functional groups of humic acids are deprotonated [32]. At this pH, neutral micropollutant species may interact with organic matter via van der Waals forces or interactions between polar functional groups, and the more negative charges available (from organic matter) provide greater the affinity to the pharmaceuticals [32]. In addition to the impact of the fouling layer on micropollutant retention, the biopolymers and micropollutants may first interact to form complexes, and subsequently be retained by the membrane [8]. In a study by Neale and Schäfer [12], solute–solute interactions between hormones and organic matter were identified as a
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Average Retention (%)
A 100 Neutral compounds 80
Anionic compounds
60
y = 0.084x + 0.05 R² = 0.68
40
20
y = 0.009x + 0.20 R² = 0.03
0 0
1
2
3
4
5
6
Compound Log Kow
Fig. 9. FTIR spectra for Otonabee River fouled membranes under conditions where no shear stress was applied, and under conditions where high peak shear stress was applied. Spectra have been subtracted from spectra of epoxy (used to affix membrane to apparatus).
mechanism for retention, because they did not observe significant membrane fouling or adsorption to the membrane material. An increase in the retention of hydrophobic, neutral compounds was observed in the present study for conditions where shear stress was applied when filtering Lake Simcoe and Otonabee River waters (Fig. 10A); this relationship was not observed for compound molecular weight (Fig. 10B). This indicates that during conditions of low fouling (high peak surface shear stress), retention of compounds may still be possible due to interactions with organic matter in the water. Application of surface shear stress may facilitate the back transport the biopolymers (and their associated organic micropollutants) away from the membrane surface and keep them in solution. 3.6. Predicting compound retention Multiple linear regression (MLR) models were developed to predict retention of organic micropollutants based on compound and water characteristics. Because the relationship between surface shear and compound retention was not linear, separate models were developed for conditions of no surface shear stress and conditions where surface shear stress was applied. The dependent variable considered for the models was compound retention, whereas independent variables considered were related to compound properties (log Kow, pKa, molecular weight) and water characteristics (DOC, pH, turbidity, and concentrations of various organic matter fractions). For the model developed under conditions where surface shear stress was applied, the root mean square of the shear stress (sRMS) was used to represent surface shear stress as an independent variable [27,29,30]. Models developed indicate that source water DOC and compound hydrophobicity (log Kow) are significant factors when predicting compound retention. Under conditions of applied shear stress, the root mean square of the shear applied was also a significant factor. Under conditions where no shear stress was applied, compound molecular weight was a significant factor. Based on the regression analysis, predictive equations were developed to determine compound retention under conditions of no surface shear stress (Eq. (2)) and under conditions where surface shear stress was applied (Eq. (3)):
Average Retention (%)
B 100 80
Neutral compounds Anionic compounds
60
y = 0.002x - 0.27 R² = 0.25
40
y = 0.0003x + 0.14 R² = 0.01 20
0 150
200
250
300
350
Molecular weight (g/mol) Fig. 10. Relationship between neutral and anionic compound retention with (A) compound log Kow, and (B) compound molecular weight, under conditions where shear stress (low peak, sustained peak, and high peak) was applied during filtration of Lake Simcoe and Otonabee River waters (points represent average values for all shear stress conditions).
where DOC is the concentration of source water dissolved organic carbon (mg/L), log Kow is the compound hydrophobicity, MW is the compound molecular weight (g/mol), and sRMS is the root mean
A
B
Retention ð%Þ ¼ 67:64 þ 8:56ðDOCÞ þ 2:69ðlog K ow Þ þ 0:23ðMWÞ
ð2Þ
Retention ð%Þ ¼ 105:7 þ 5:13ðDOCÞ þ 5:07ðlog K ow Þ þ 208:45ðsRMS Þ
ð3Þ
Fig. 11. Predicted and observed compound retention under conditions of (A) no surface shear and (B) application of surface shear stress.
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square of the surface shear stress condition applied (0.42, 0.46, and 0.5 for low peak, sustained peak and high peak shear stress, respectively). A comparison of predicted and observed compound retention (Fig. 11) indicates a significant relationship, both for conditions of no surface shear stress (R2 = 0.57, p < 0.001) and conditions where surface shear stress was applied (R2 = 0.44, p < 0.001). These models suggest that UF retention of organic micropollutants may be enhanced under conditions where high peak shear stress (representative of pulse bubble air sparging) is applied for fouling control, particularly for target compounds that are neutral and hydrophobic in nature.
4. Conclusions The impact of varying surface shear stress conditions on the retention of organic micropollutants during ultrafiltration of natural water matrices was investigated. The results indicated that retention was influenced by the specific water matrix characteristics, with increased retention in waters with higher concentrations of organic matter, including biopolymers. Under high fouling conditions (no applied surface shear stress), there is evidence that the fouling layer may act as a more selective secondary membrane capable of retaining larger molecular weight and hydrophobic micropollutants. Under conditions where shear stress is applied, the rate of membrane fouling decreases, and compound retention increases with the magnitude of shear stress applied. It is possible that under these conditions, conditions of low shear stress may not adequately facilitate back-diffusion of organic micropollutants away from the membrane surface, resulting in a cake enhanced concentration polarization leading to decreased compound retention. Under higher shear stress conditions, compound retention improves for water matrices with higher concentrations of organic matter and biopolymers, even though fouling is reduced under these conditions. Potentially, interactions between organic micropollutants (particularly hydrophobic, neutral compounds) and biopolymers in solution act to enhance retention as the biopolymer-micropollutant complexes are kept in solution via back-transport away from the membrane surface by application of high shear stress. The research provides evidence that there may be added value with respect to the removal of organic micropollutants when implementing air sparging as a UF fouling control strategy. However, more research is required to identify specific mechanisms of micropollutant retention by ultrafiltration under conditions where shear stress (air sparging) is applied. Acknowledgements This work was funded in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) Chair in Drinking Water Research at the University of Toronto and the Canadian Water Network. The authors thank Dr. Monica Tudorancea at the University of Waterloo for performing the LC-OCD analysis and Magdalena Jaklewicz at GE Water and Process Technologies for FESEM analysis. We would also like to thank the personnel at the Lorne Park water treatment facility, Barrie surface water treatment plant, and the Peterborough Utilities Commission for providing water samples. References [1] T.A. Ternes, M. Meisenheimer, D. McDowell, F. Sacher, H.J. Brauch, B. HaistGulde, G. Preuss, U. Wilme, N. Zulei-Seibert, Removal of pharmaceuticals during drinking water treatment, Environ. Sci. Technol. 38 (17) (2002) 3855– 3863.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Surface shear stress and retention of emerging contaminants during ultrafiltration for drinking water treatment Heather E. Wray a,⇑, Robert C. Andrews a, Pierre R. Bérubé b a b
Department of Civil Engineering, University of Toronto, 35 St. George Street, Toronto, ON, Canada Department of Civil Engineering, University of British Columbia, 6250 Applied Science Lane, Vancouver, BC, Canada
a r t i c l e
i n f o
Article history: Received 14 September 2013 Received in revised form 31 October 2013 Accepted 1 November 2013 Available online 15 November 2013 Keywords: Ultrafiltration Air sparging Pharmaceuticals EDCs Fouling Natural organic matter
a b s t r a c t This study investigated the impact of surface shear stress, to represent air sparging employed for fouling control, on the retention of organic micropollutants during ultrafiltration for drinking water treatment. The retention of 16 different pharmaceutically-active and endocrine disrupting compounds was examined during ultrafiltration of three natural surface waters (two lake, one river) under four different surface shear stress regimes: no shear stress, low peak shear stress (representative of continuous coarse bubble sparging), sustained peak shear stress (representative of intermittent coarse bubble sparging), and high peak shear stress (representative of large pulse bubble sparging). Results indicate that surface shear stress does impact the retention of emerging contaminants; however, it is dependent on water matrix and compound properties. The greatest retention of micropollutants was observed in waters with a higher concentrations of organic matter, and for conditions where no surface shear stress was applied (average 32% retention), and under conditions representative of large pulse bubble sparging (average 34% retention). The observed retention under conditions of no shear stress was likely due to a heavy fouling layer that altered the membrane selectivity and was able to entrap organic micropollutants of larger molecular weight. Under conditions that mimicked air sparging, increasing the shear stress (quantified as the root mean square applied shear) resulted in increased retention of organic micropollutants, particularly those that are neutral and hydrophobic in nature. This may be related to solute–solute complexes, which are kept in solution when shear stress is applied, or related to modification of the fouling layer by the shear stress induced onto the membrane surface. The results suggest that there may be value added with respect to removal of organic micropollutants, such as pharmaceuticals, when employing air sparging as a fouling control strategy during ultrafiltration. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction In North America, several classes of emerging organic micropollutants including pharmaceutically active compounds (PhACs) and endocrine disrupting compounds (EDCs), have been measured at trace concentrations (ng/L) in wastewater effluents [1,2], their receiving surface waters, and in treated drinking waters [3,4]. Concerns exist regarding organic micropollutants with respect to their ecotoxicologic effects on the aquatic environment [5], and their unknown human health effects when considering lifetime low-levels of exposure through drinking water [6]. Although these compounds are not yet regulated in drinking water, the consensus is to remove or reduce them if possible. Thus, it is important to understand if existing drinking water treatment practices offer any added value with respect to the removal of organic micropollutants. ⇑ Corresponding author. Tel.: +1 416 946 0486. E-mail address: [email protected] (H.E. Wray). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.11.003
The use of ultrafiltration (UF) membranes for drinking water treatment has increased in the past 15 years for both new and recently upgraded treatment plants [7]. Retention of organic micropollutants during ultrafiltration is typically reported to be less than 30% due to the large pore size of UF membranes (10– 100 kDa) relative to the size of many PhACs and EDCs (<1 kDa) [8,9]. Retention of organic micropollutants that does occur during ultrafiltration has mainly been attributed to adsorption on the membrane during the early stages of filtration, but this may decrease once equilibrium is established [10,11], or be limited when natural organic matter (NOM) competes with micropollutants for binding sites on the membrane [10]. Organic micropollutants may also be rejected during UF via interactions with the membrane fouling layer, and/or interactions with dissolved NOM in solution [12]. When organic matter is present, several studies have demonstrated that retention of organic micropollutants is enhanced [3,13–15]. The enhanced retention is attributed to interactions between the micropollutants and the organic matter [12,16], which are retained together by the membrane. The fouling layer may also
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lead to increased retention of micropollutants as it may essentially act as a more selective secondary membrane [11,14] due to partitioning or adsorption of contaminants [6,17,18]. Interactions between NOM and organic micropollutants may vary depending on the compound properties [19], the chemistry of the water matrix [20], and the nature of the organic matter, including the type and concentration [3,21]. Studies have suggested that NOM of a higher molecular weight (>20 kDa), including biopolymers (e.g., proteins and polysaccharides), have a greater impact on micropollutant retention than lower molecular weight NOM [8] and are also the main foulants of UF membranes [22– 24]. A number of studies have been undertaken to assess different fouling control strategies. However, the impact of UF fouling control strategies on the retention of organic micropollutants in natural water matrices is largely unknown. Air sparging is one strategy employed for fouling control during UF. Typically, during drinking water treatment, air is applied during backwash only, to aid in the removal of foulants from the membrane surface. Previous research has demonstrated that air sparging during filtration may also be a cost effective means of controlling fouling in wastewater [25], as well as synthetic [26] and natural water matrices [27]. In addition, air sparging may be optimized (bubble size, frequency) to allow fouling control. Numerous studies have identified large (>100 mL) ‘pulse’ bubbles as an effective air sparging regime for UF fouling control [26–29]. In addition, pulse bubble sparging is also more energy-efficient compared to traditional coarse bubble sparging [28]. Air sparging during filtration reduces fouling by inducing shear stress at the membrane surface which acts to back-transport foulants (biopolymers). Based on the interactions between organic micropollutants and NOM, both in solution and within the fouling layer, the surface shear stress induced by air sparging may impact contaminant retention during UF of natural waters. The specific objectives of this study were to: 1. determine if there is value added with respect to emerging contaminant retention when applying shear stress conditions to UF membranes treating natural water matrices, and 2. relate compound and water matrix properties to contaminant retention by UF under different shear stress conditions. 2. Materials and methods 2.1. Water matrices Three natural surface waters located in Ontario Canada: Lake Ontario (Lorne Park Water Treatment Facility, Mississauga, ON), Lake Simcoe (Barrie South Surface Water Treatment Plant, Barrie, ON), and the Otonabee River (Peterborough Water Treatment Plant, Peterborough, ON) were considered. These waters were selected because they serve as drinking water sources for millions of people, and also represent a range of organic matter concentrations (Table 1). Both Lake waters are commonly treated via ultrafiltration for the production of drinking water. Raw, unchlorinated water was collected at the intakes of the treatment facilities, stored at 4 °C and allowed to warm to room temperature (20 ± 2 °C) prior to use (within 1 week of collection). 2.2. Membrane filtration apparatus A bench-scale apparatus was used to vary shear stress conditions at the surface of a single ultrafiltration membrane hollow fiber. These shear stress conditions were representative of different air sparging regimes. Briefly, the membrane fiber was mounted onto a rubber backing with epoxy and fixed to the inner side of a cylindrical tank. Impeller blades of different geometries rotated
Table 1 Selected characteristics of the surface waters examined (± standard deviation for samples that were replicated).
DOC (mg/L) Hydrophobic DOC (mg/L) Hydrophilic DOC (mg/L) Biopolymers (mg/L) Humic substances (mg/L) Building blocks (mg/L) LMWa neutrals (mg/L) LMW acids (mg/L) pH UV254 (cm1) SUVA (L/mg m) Mean particle surface charge (mV) a b
Lake Ontario
Lake Simcoe
Otonabee River
2.08 0.20 1.89 0.23 1.06 0.33 0.26 n.q.b 8.35 (±0.01) 0.005 1.77 5.29 (±0.15)
4.06 0.42 3.64 0.33 2.01 0.85 0.33 0.12 8.14 (±0.1) 0.008 1.99 3.43 (±0.46)
5.83 0.44 5.39 0.45 3.24 1.02 0.51 0.18 8.36 (±0.1) 0.018 3.05 7.23 (±0.56)
Low molecular weight. Not quantified.
Table 2 Membrane characteristics and operating conditions for this study. Material
Polyvinylidene difluoride (PVDF)
Operation mode Nominal pore size (lm) Length of fiber used (cm) Available membrane surface area (cm2) Target permeate flux (L nm2 h1) Permeation time (h) Backwash frequency
Outside-in 0.04 20 6 30 12–14 No backwashing applied
close to the membrane surface in the cylindrical tank to generate desired shear stress profiles. A detailed description of the apparatus can be found in Chan et al. [26]. The surface shear stress profiles considered were shown to be representative of air sparging regimes measured at full scale [29,30]. Four different surface shear stress regimes were examined: low peak (representative of continuous coarse bubble air sparging), sustained peak (representative of intermittent coarse bubble air sparging), high peak (representative of large pulse bubble air sparging), and a control where no shear was applied. A peristaltic permeate pump (Masterflex 7553-80) with a PTFE tubing head was used to draw permeate by vacuum. Shear stress was quantified as the root mean square (RMS) of the time variable shear stress (results not presented). The RMS was calculated to be 0.42, 0.46, and 0.50 Pa for conditions representative of continuous coarse bubble, intermittent coarse bubble, and large pulse bubble sparging, respectively. A hollow-fiber membrane (ZeeWeedÒ-500, GE Water and Process Technologies, Oakville, ON) was used for this study (Table 2). Virgin membrane fibers were initially cleaned soaking in a 750 mg/ L sodium hypochlorite solution for 16 h, and then stored in a 50 mg/L sodium hypochlorite solution until use. Prior to filtration using natural waters, fibers were rinsed by filtering distilled water for 2 h. Experiments were conducted for a period of 10–12 h. Water was continuously filtered with no backwashing during this period. This length of time was chosen to allow fouling of the membrane surface and also to allow the production of enough permeate to perform water analyses, including measuring the concentration of organic micropollutants. Membrane integrity testing was conducted at the start and end of each experiment using a bubble test at a pressure of 5 psi (34.5 kPa). If air bubbles escaped during the test the membranes were discarded.
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H.E. Wray et al. / Separation and Purification Technology 122 (2014) 183–191 Table 3 Compound properties of organic micropollutants studied.
a b c d e
Compound
MW (g/mol)
log Kow
pKa
Chargea
Hydrophobic/Hydrophilicb
Class/Use
Acetaminophen Bisphenol A Clofibric Acid Diclofenac Diethylstilbestrol Estriol Estrone Gemfibrozil 17 b-estradiol Carbamazepine Ketoprofen Naproxen Pentoxifylline Sulfamethoxazole Sulfamethizole Sulfachloropyridazine
151 228 214 296 268 288 270 250 272 236 254 230 278 253 270 285
0.23 3.86 2.76 4.29 5.33 2.45 3.46 4.39 4.01 2.23 2.54 3.01 0.48 0.86 0.54 0.68
10.2 10.3 4 4.3 9.7 10 10.3 4.9 10.3 14.3 4.3 4.3
Neutral Neutral Ionic Ionic Neutral Neutral Neutral Ionic Neutral Neutral Ionic Ionic Neutral Ionic Ionic Neutral
Hydrophilic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophobic Hydrophilic Hydrophilic Hydrophilic Hydrophilic
PhACc, NSAIDd EDCe, plasticizer EDC, herbicide PhAC, NSAID EDC, estrogen replacement EDC, natural hormone EDC, 17b estradiol metabolite PhAC, lipid regulator EDC, natural hormone PhAC, antiepileptic PhAC, NSAID PhAC, NSAID PhAc, vasodilator PhAc, antibiotic PhAc, antibiotic PhAc, antibiotic
5.7 5.3 6.1
Ionic if pKa < 6. Hydrophobic if log Kow > 2. Pharmaceutically active compound. Non-steroid anti-inflammatory drug. Endocrine disrupting compound.
2.3. Analysis of emerging contaminants Sixteen emerging contaminants were studied, representing both PhACs and EDCs (Sigma–Aldrich, Oakville, ON, and CDN Isotopes Inc., Pointe–Claire, QC, Canada) covering a wide range of physical and chemical characteristics (Table 3). In addition, many of the selected compounds have been detected in North American surface waters [4,31]. Collected source waters for this study were spiked to achieve a target concentration of 1000 ng/L for the emerging contaminants of interest and mixed for 24 h prior to filtration to allow equilibration with NOM to occur [10,32]. Compounds were analyzed in spiked feed and permeate waters using a liquid chromatography – tandem mass spectrometry (LC–MS/MS) method, adapted from one developed by the Ontario Ministry of the Environment [33]. Solid phase extraction (SPE) incorporating 6 cc 200 mg Oasis Hydrophilic–Lipophilic Balance (HLB) cartridges (Waters, Mississauga, ON) was used to extract compounds from a 400 mL sample volume. Extracted samples were analyzed using an Agilent 500 Ion Trap system with a Model 212 LC, equipped with a Pursuit XRs-C18 guard column (MetaguardÒ, 2.0 mm ID 3 mm) and a Pursuit XRs Ultra 2.8-C18 analytical column (2.0 mm ID 100 mm, 2.8-lm particle size) (Agilent Technologies, Mississauga, ON, Canada). Method detection limits (MDLs) for the compounds ranged from 6 to 87 ng/L. Compound retention via ultrafiltration was calculated using Eq. (1):
Retention ð%Þ ¼ 1 C p =C f 100%
ð1Þ
where Cf is the concentration of the compound in the feed water and Cp is the concentration in the permeate. A blank run was conducted by filtering unspiked Milli-QÒ water through the apparatus; none of the target analytes were found to be present.
et al. [34]. Data was processed using ChromCALC (DOC-LABOR, Karlsruhe, Germany). The impact of water matrix and surface shear stress conditions on compound retention was compared using a two factor analysis of variance (ANOVA) (Design Expert 8, StatEase, Minneapolis, MN). Post hoc t-tests were used to determine significant differences between levels of the main factors including source water and shear stress condition. Multiple linear regression Ò analysis (SPSS 21, IBM Corp., NY) was applied to investigate relationships between compound retention and surface water and compound characteristics. Fouled membrane surfaces were examined using Field Emission Scanning Electron Microscopy (FESEM, Supra™ 40, Carl Zeiss Ltd.) after coating with 1 nm of iridium. Organic foulant analysis was conducted on select samples with Fourier Transform Infrared Spectroscopy (FTIR, Spectrum One, Perkin Elmer Inc.) equipped with a germanium crystal. Prior to analysis, membrane samples were dried in a dust-free cabinet for a minimum of 24 h. FTIR data was normalized over the range of wavenumbers 800–1000 where two strong PVDF peaks are present; it is assumed that PVDF material does not change. 3. Results and discussion 3.1. Adsorption of organic micropollutants to the membrane surface Ò
Milli-Q water spiked to a target concentration of 1000 ng/L with the analytes of interest was filtered through the UF membrane to determine if organic micropollutants were retained by the membrane. Retention values for this test were consistently low (<5%) (Fig. 1). Given the small size of the compounds (MW < 300 g/mol) relative to the pore size of the membrane (0.04 lm), it is unlikely that the observed removal was due to size exclusion, but rather due to adsorption to the membrane.
2.4. Water and fouling characterization 3.2. Impact of surface shear stress on membrane fouling Raw feed water samples and membrane permeate were characterized using liquid chromatography–organic carbon detection (LC–OCD) to determine the concentration of total dissolved organic carbon (DOC) as well as DOC fractions, including biopolymers, humic substances, building blocks, low molecular weight acids, and low molecular weight neutrals as defined by Huber et al. [34]. LC–OCD analysis was conducted on filtered water samples (0.45 lm, Gelman Sciences, Ann Arbor, MI) at the University of Waterloo, Ontario, Canada, following methods described by Huber
For the three water matrices studied, application of surface shear stress during filtration resulted in lower fouling rates, relative to conditions where no shear stress was applied (Fig. 2). For all shear conditions tested, high peak (representative of large pulse bubble sparging) offered the largest reduction in fouling rates for all waters. These results were consistent with previous work assessing the impact of surface shear stress on membrane fouling in Lake Ontario, Lake Simcoe, and Otonabee River waters [27]. This
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3.3. Impact of water source on compound retention during ultrafiltration Specific water sources significantly impacted the retention of organic micropollutants (p < 0.0001). Lake Ontario had significantly lower retention (average 14%) than Lake Simcoe (average 30%) or Otonabee River (average 36%) waters (Figs. 3–5). However, the overall retention of micropollutants was not significantly different between Lake Simcoe and Otonabee River waters (p = 0.06). The retention observed in natural waters was likely related to interactions of contaminants with organic matter, a phenomenon that has been observed in other studies [15–17]. The higher retention observed for Lake Simcoe and Otonabee River water is likely related to the higher concentrations of organic matter measured in these waters (4.06 and 5.83 mg/L DOC, respectively) compared to that in Lake Ontario (2.08 mg/L DOC). Neale and Schäfer [12] reported an increase in interactions between steroidal hormones and dissolved organic matter in solution, with increasing organic carbon concentrations during ultrafiltration of humic acid-spiked water. In addition, Dalton et al. [20] observed an increased removal of organic micropollutants at high organic matter to micropollutant ratios for nanofiltration membranes. Binding of pharmaceutical compounds has been reported to be greater for dissolved organics that had more carboxylic and phenolic functional groups [32]. This suggests that micropollutants interact with aromatic compounds, of which there is a higher concentration, as indicated by the specific UV absorbance (SUVA) (Table 1), in Otonabee River and Lake Simcoe waters.
No shear Low peak Sustained peak High peak
5 4 3 2 1
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2
4
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12
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Permeation Time (h)
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previous study further identified the biopolymer fraction of organic matter (as determined by LC–OCD) as the main membrane foulant and provided evidence that shear stress minimized membrane fouling via back-transport of biopolymers from the membrane surface. In addition, the impact of surface shear stress on fouling was more pronounced for Lake Simcoe and Otonabee River waters, which is not surprising given that these waters have a higher concentration of biopolymers (>0.3 mg/L) to be back-transported from the membrane.
6
No shear Low peak Sustained peak High peak
5 4 3 2 1
0
2
4
6
8
Permeation time (h) Fig. 2. Resistance increase over time for different shear stress conditions applied during ultrafiltration of (A) Lake Ontario, (B) Lake Simcoe, and (C) Otonabee River water.
100 90
No shear stress Low peak
Retention (%)
Fig. 1. Average UF retention (±standard deviation) of compounds when filtering analyte-spiked (to 1000 ng/L) Milli-QÒ water.
Resistance (x1012m-1)
B
80
Sustained peak
70
High peak
60 50 40 30 20 10 0
nr nr
nr
nr nrnr
3.4. Impact of surface shear stress on retention of organic micropollutants Surface shear stress conditions significantly impacted compound retention (p < 0.0001). The average retentions for all compounds under conditions representative of continuous coarse, intermittent coarse, and pulse bubble sparging were 18%, 22%, and 34%, respectively. Under control conditions where no shear stress was applied average retention was 32% (Figs. 3–5). Com-
Fig. 3. Average compound retention (%, ± standard deviation) in Lake Ontario water for a range of surface shear stress conditions; nr = no retention.
pound retention under conditions representative of pulse bubble sparging was not significantly different than control conditions of no shear stress (p = 0.6). However, these two conditions resulted
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90 80 Retention (%)
70 60 50 40 30 20 nr
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nr
nr
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Fig. 4. Average compound retention (%, ±standard deviation) in Lake Simcoe water for a range of surface shear stress conditions; nr = no retention.
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(p > 0.3) and compound retention was consistently low (<40%) under all test conditions (Fig. 3). For Lake Simcoe water, low peak shear stress (representative of continuous coarse bubble air sparging) resulted in significantly lower compound retention than all other shear stress conditions (p < 0.03), which were all not significantly different from each other (p > 0.1) (Fig. 4). For Otonabee River water, retention was not significantly different between conditions of no shear stress and high peak shear stress (representative of pulse bubble sparging) (p = 0.24) (Fig. 5). These two conditions resulted in significantly higher retention of compounds than sustained peak shear stress (p < 0.01) and low peak shear stress (p < 0.0001). High retention of micropollutants for both the highest fouling (i.e., no shear stress) and lowest fouling (i.e., high peak shear stress) conditions suggests different mechanisms may govern the retention of micropollutants depending on whether shear stress is applied or is absent. 3.5. Impact of fouling and NOM-micropollutant interactions on micropollutant retention
100 90
No shear stress Low peak
Retention (%)
80
Sustained peak
70
High peak
60 50 40 nr
30 20 10 0
nr nr
Fig. 5. Average compound retention (%, ±standard deviation) in Otonabee River water for a range of surface shear stress conditions; nr = no retention.
in significantly higher compound retention than conditions representative of continuous and intermittent coarse bubble sparging (p < 0.002), which were not significantly different from each other (p = 0.09). The impact of surface shear stress on the retention of organic micropollutants was water matrix-specific and varied from compound to compound. This indicates that retention is likely related to compound properties as well as the impact of surface shear on the characteristics of the fouling layer, and the back-transport of biopolymers (and biopolymer-micropollutant complexes) away from the membrane surface. For Lake Ontario water, the type of surface shear stress applied did not significantly affect the retention of any micropollutants
3.5.1. Micropollutant retention in the absence of surface shear stress Under conditions where no shear stress was applied, retention of organic micropollutants during UF of Lake Simcoe and Otonabee River waters can likely be attributed to the formation of a dense fouling layer. Many studies have observed increased retention of organic micropollutants due to organic fouling in nanofiltration (NF) and reverse osmosis (RO) membranes [18,35,36]. Higher molecular weight NOM, such as biopolymers, tend to foul UF membranes by forming a hydraulically reversible cake layer [37,38], which has been shown to increase micropollutant retention (up to 40% versus lower molecular weight humic substances) [8]. The cake fouling layer may serve as a more selective ‘‘secondary membrane’’ to retain organic micropollutants, the impact of which increases proportionately with the level of fouling [8]. In the present study, a fouling layer on the membrane surface was observed during continuous permeation (no backwash), without application of shear stress for the three water matrices (Fig. 6). Images of the membrane surface illustrate the increased extent of surface fouling in waters with a higher organic content. When filtering Lake Ontario water, a more porous fouling layer developed, whereas when filtering Lake Simcoe and Otonabee River waters, a denser fouling layer was formed (Fig. 6). Increased compound retention in Otonabee River and Lake Simcoe water (compared to Lake Ontario water) may be attributed to this fouling layer. Under these high fouling conditions (i.e., no shear stress applied), there also appears to be a relationship between compound properties and retention. Hydrophobic neutral compounds were rejected to a higher degree than anionic compounds (Fig. 7A). This may be attributed to interactions between neutral micropollutants and negatively-charged organic matter [12,16] that is retained by the membrane.
Fig. 6. FESEM images (magnification 50,000) of membranes surfaces fouled with different source waters after continuous permeation for 12 h (no backwashing).
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A 100
Neutral compounds 90
Anionic compounds
80
60
y = 0.08x + 0.14 R² = 0.47
70
20
y = -0.001x + 0.30 R² = 0.0005 0 0
1
2
3
4
5
6
Compound Log Kow
B
60 50 40 30
100
20
Neutral compounds 80
Retention (%)
Bisphenol A Diethylstilbestrol Estriol Estrone 17 B-estradiol Carbamazepine
40
Retention (%)
Retention (%)
80
10
Anionic compounds
0 0.42
60
y = 0.003x -0.39 R² = 0.36
0.44
0.46
0.48
0.5
RMS Shear (Pa)
40
y = 0.002x -0.17 R² = 0.42
20
0 150
200
250
300
350
Fig. 8. Retention of hydrophobic (log Kow > 2), neutral micropollutants with increasing shear stress, i.e., from low peak to high peak, quantified as the root mean square (RMS) of the applied shear stress. Values represent the average retention during filtration of Lake Simcoe and Otonabee River waters ± standard deviation; lines illustrate overall trends between measurements.
Molecular weight (g/mol) Fig. 7. Relationship between neutral and anionic compound retention with (A) compound log Kow, and (B) compound molecular weight under conditions where no shear stress was applied.
A relationship also likely exists between compound molecular weight and retention, as higher retention was observed for larger compounds, irrespective of their charge (Fig. 7B). This may indicate entrapment by the fouling layer, via either adsorption and/or a sieving mechanism by a more selective fouling layer formed without any application of shear stress. Filtration of Lake Ontario water resulted in minimal fouling due to the low concentration of organic matter and biopolymers, therefore fewer NOM-micropollutant and foulant-micropollutant interactions likely occurred. In contrast, for Lake Simcoe and Otonabee River, the higher concentration of organic matter and biopolymers allowed more micropollutant-foulant and micropollutant-dissolved NOM interactions which likely contributed to compound retention. 3.5.2. Micropollutant retention under varying surface shear stress conditions When the magnitude of the surface shear stress increased, retention of hydrophobic, neutral compounds increased (Fig. 8) and membrane fouling decreased. Several studies with NF and RO membranes have reported a reduction in micropollutant retention in the presence of membrane fouling [18,35,39], which was attributed to a reduction in back diffusion of the micropollutants due to the fouling layer and thereby an increased concentration gradient, leading to diffusion through the membrane and into the permeate [35]. This ‘‘cake enhanced concentration polarization’’ has also been observed in hydrophilic UF membranes for ibuprofen, which has a relatively low affinity for organic matter binding [8]. However, stirring or other hydraulic impacts to the membrane surface such as backwashing or air sparging would decrease the concentration polarization [40], and subsequently increase the retention of micropollutants [8]. Cake enhanced concentration polarization may be a factor leading to decreased retention under conditions representative of continuous coarse bubble air sparging (lowest RMS of shear). In this case, a cake foulant still forms during the course of filtration, albeit at a slower rate than during condi-
tions of no shear stress [27]. The cake layer formed under low peak shear stress conditions is likely of intermediate thickness when compared to conditions of no shear and sustained and high peak shear stress, and therefore may not behave in the same manner as previously discussed. It is possible that the shear stress applied during low peak treatment is not sufficient to facilitate adequate back-diffusion of the organic micropollutants (or micropollutantNOM complexes) to prevent the formation of a concentration gradient across the membrane [35]. Concentration polarization is higher if there is a higher mass flux to the membrane surface. Under conditions representative of pulse bubble sparging (greatest RMS of shear), the mass flux of material to the membrane surface is reduced leading to a lower degree of concentration polarization. In Lake Ontario water there was no observed impact of surface shear stress on compound retention, likely due to a lower mass transfer of foulants to the membrane surface. The nature of the membrane foulants may also be slightly different when surface shear stress is applied. Analysis of the membrane surface via Fourier transform infrared spectroscopy (FTIR) post-filtration for Otonabee River water under conditions of no surface shear stress and high peak shear stress (Fig. 9) confirms the presence of humic substances under conditions of high peak shear, as indicated by peaks identified as CAO carboxylic acids and C@C aromatic ring stretching [41] and the absence, or lower concentration of these compounds when no surface shear stress is applied. Back transport of biopolymers away from the membrane surface may have opened up membrane sites for adsorption by humic substances which may also have aided in the retention of organic micropollutants. At a pH of 8.0, the carboxylic functional groups of humic acids are deprotonated [32]. At this pH, neutral micropollutant species may interact with organic matter via van der Waals forces or interactions between polar functional groups, and the more negative charges available (from organic matter) provide greater the affinity to the pharmaceuticals [32]. In addition to the impact of the fouling layer on micropollutant retention, the biopolymers and micropollutants may first interact to form complexes, and subsequently be retained by the membrane [8]. In a study by Neale and Schäfer [12], solute–solute interactions between hormones and organic matter were identified as a
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Average Retention (%)
A 100 Neutral compounds 80
Anionic compounds
60
y = 0.084x + 0.05 R² = 0.68
40
20
y = 0.009x + 0.20 R² = 0.03
0 0
1
2
3
4
5
6
Compound Log Kow
Fig. 9. FTIR spectra for Otonabee River fouled membranes under conditions where no shear stress was applied, and under conditions where high peak shear stress was applied. Spectra have been subtracted from spectra of epoxy (used to affix membrane to apparatus).
mechanism for retention, because they did not observe significant membrane fouling or adsorption to the membrane material. An increase in the retention of hydrophobic, neutral compounds was observed in the present study for conditions where shear stress was applied when filtering Lake Simcoe and Otonabee River waters (Fig. 10A); this relationship was not observed for compound molecular weight (Fig. 10B). This indicates that during conditions of low fouling (high peak surface shear stress), retention of compounds may still be possible due to interactions with organic matter in the water. Application of surface shear stress may facilitate the back transport the biopolymers (and their associated organic micropollutants) away from the membrane surface and keep them in solution. 3.6. Predicting compound retention Multiple linear regression (MLR) models were developed to predict retention of organic micropollutants based on compound and water characteristics. Because the relationship between surface shear and compound retention was not linear, separate models were developed for conditions of no surface shear stress and conditions where surface shear stress was applied. The dependent variable considered for the models was compound retention, whereas independent variables considered were related to compound properties (log Kow, pKa, molecular weight) and water characteristics (DOC, pH, turbidity, and concentrations of various organic matter fractions). For the model developed under conditions where surface shear stress was applied, the root mean square of the shear stress (sRMS) was used to represent surface shear stress as an independent variable [27,29,30]. Models developed indicate that source water DOC and compound hydrophobicity (log Kow) are significant factors when predicting compound retention. Under conditions of applied shear stress, the root mean square of the shear applied was also a significant factor. Under conditions where no shear stress was applied, compound molecular weight was a significant factor. Based on the regression analysis, predictive equations were developed to determine compound retention under conditions of no surface shear stress (Eq. (2)) and under conditions where surface shear stress was applied (Eq. (3)):
Average Retention (%)
B 100 80
Neutral compounds Anionic compounds
60
y = 0.002x - 0.27 R² = 0.25
40
y = 0.0003x + 0.14 R² = 0.01 20
0 150
200
250
300
350
Molecular weight (g/mol) Fig. 10. Relationship between neutral and anionic compound retention with (A) compound log Kow, and (B) compound molecular weight, under conditions where shear stress (low peak, sustained peak, and high peak) was applied during filtration of Lake Simcoe and Otonabee River waters (points represent average values for all shear stress conditions).
where DOC is the concentration of source water dissolved organic carbon (mg/L), log Kow is the compound hydrophobicity, MW is the compound molecular weight (g/mol), and sRMS is the root mean
A
B
Retention ð%Þ ¼ 67:64 þ 8:56ðDOCÞ þ 2:69ðlog K ow Þ þ 0:23ðMWÞ
ð2Þ
Retention ð%Þ ¼ 105:7 þ 5:13ðDOCÞ þ 5:07ðlog K ow Þ þ 208:45ðsRMS Þ
ð3Þ
Fig. 11. Predicted and observed compound retention under conditions of (A) no surface shear and (B) application of surface shear stress.
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square of the surface shear stress condition applied (0.42, 0.46, and 0.5 for low peak, sustained peak and high peak shear stress, respectively). A comparison of predicted and observed compound retention (Fig. 11) indicates a significant relationship, both for conditions of no surface shear stress (R2 = 0.57, p < 0.001) and conditions where surface shear stress was applied (R2 = 0.44, p < 0.001). These models suggest that UF retention of organic micropollutants may be enhanced under conditions where high peak shear stress (representative of pulse bubble air sparging) is applied for fouling control, particularly for target compounds that are neutral and hydrophobic in nature.
4. Conclusions The impact of varying surface shear stress conditions on the retention of organic micropollutants during ultrafiltration of natural water matrices was investigated. The results indicated that retention was influenced by the specific water matrix characteristics, with increased retention in waters with higher concentrations of organic matter, including biopolymers. Under high fouling conditions (no applied surface shear stress), there is evidence that the fouling layer may act as a more selective secondary membrane capable of retaining larger molecular weight and hydrophobic micropollutants. Under conditions where shear stress is applied, the rate of membrane fouling decreases, and compound retention increases with the magnitude of shear stress applied. It is possible that under these conditions, conditions of low shear stress may not adequately facilitate back-diffusion of organic micropollutants away from the membrane surface, resulting in a cake enhanced concentration polarization leading to decreased compound retention. Under higher shear stress conditions, compound retention improves for water matrices with higher concentrations of organic matter and biopolymers, even though fouling is reduced under these conditions. Potentially, interactions between organic micropollutants (particularly hydrophobic, neutral compounds) and biopolymers in solution act to enhance retention as the biopolymer-micropollutant complexes are kept in solution via back-transport away from the membrane surface by application of high shear stress. The research provides evidence that there may be added value with respect to the removal of organic micropollutants when implementing air sparging as a UF fouling control strategy. However, more research is required to identify specific mechanisms of micropollutant retention by ultrafiltration under conditions where shear stress (air sparging) is applied. Acknowledgements This work was funded in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) Chair in Drinking Water Research at the University of Toronto and the Canadian Water Network. The authors thank Dr. Monica Tudorancea at the University of Waterloo for performing the LC-OCD analysis and Magdalena Jaklewicz at GE Water and Process Technologies for FESEM analysis. We would also like to thank the personnel at the Lorne Park water treatment facility, Barrie surface water treatment plant, and the Peterborough Utilities Commission for providing water samples. References [1] T.A. Ternes, M. Meisenheimer, D. McDowell, F. Sacher, H.J. Brauch, B. HaistGulde, G. Preuss, U. Wilme, N. Zulei-Seibert, Removal of pharmaceuticals during drinking water treatment, Environ. Sci. Technol. 38 (17) (2002) 3855– 3863.
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