Mechanisms of virus removal from secondary wastewater effluent by low pressure membrane filtration

Journal of Membrane Science 409–410 (2012) 1–8

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Mechanisms of virus removal from secondary wastewater effluent by low pressure membrane filtration Haiou Huang a , Thayer A. Young a , Kellogg J. Schwab a , Joseph G. Jacangelo a,b,∗ a b

Johns Hopkins Bloomberg School of Public Health, Department of Environmental Health Sciences, 615 N. Wolfe Street, Baltimore, MD 21205, USA MWH Global, 40814 Stoneburner Mill Lane, Lovettsville, VA 20180, USA

a r t i c l e

i n f o

Article history: Received 2 December 2011 Received in revised form 29 December 2011 Accepted 30 December 2011 Available online 8 January 2012 Keywords: Disinfection Effluent organic matter Low pressure membrane Membrane fouling Pretreatment Virus removal

a b s t r a c t As available drinking water supplies are increasingly strained, use of low pressure membranes (LPMs1 ) for wastewater reuse has become more widespread. Control of viruses in reclaimed water is critical to the protection of public health. The interaction between viruses, water chemistry and membrane properties plays an important role in the organism’s removal, especially when its size is smaller than the size of reported membrane pores. Using MS2 bacteriophage as an indicator organism, the log removal value (LRV2 ) of the virus in waters containing secondary effluent organic matter increased with filtration time and concentration of high molecular weight organic foulants. The LRV increased from 2.1 to 3.0 for high fouling water, while removal in low fouling water ranged from 0.8 to 1.7. In comparison, a LRV of 1.0 was achieved in model water prepared to simulate a non-fouling condition. Addition of equal ionic strength of either sodium or calcium to model water reduced the LRV from 2.5 to 1.6 for sodium and to 0.9 for calcium. Mechanisms are proposed to explain the complexity of the observed membrane virus exclusion. The data in this study show that the use of pretreatment to reduce membrane fouling may ultimately impair virus removal efficiency. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Removal of microorganisms from water by LPMF3 is a well documented treatment method for water treatment and wastewater repurification [1–3]. Global installed capacity of low pressure membranes increased more than ten-fold between 1996 and 2006 to greater than 3 billion gallons per day, with 60% used for drinking water and 22% for wastewater applications [4]. This expansion initially was driven by outbreaks associated with microorganisms resistant to traditional drinking water disinfection methods [4]. More recently, shortages in potable water supplies have led to increased interest in LPMF for repurification of wastewater [2]. LPMF has been successfully employed for the removal of protozoa, bacteria and viruses [5–7]. The term LPMF includes both micro- and ultrafiltration, whose differentiation is crudely based on either pore size or molecular weight cutoff; however, this dis-

∗ Corresponding author at: 40814 Stoneburner Mill Lane, Lovettsville, VA 201802245, USA. Tel.: +1 540 822 5873; fax: +1 540 822 5874. E-mail addresses: [email protected] (H. Huang), [email protected] (T.A. Young), [email protected] (J.G. Jacangelo). 1 Low pressure membranes (LPMs). 2 Log removal value (LRV). 3 Low pressure membrane filtration (LPMF). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.12.050

tinction has been shown to not always be reliable in selecting a membrane for microbial removal. As such, the term LPMF is used here [8]. The removal mechanism of microorganisms is predominantly physical; however, chemical interactions may play a role as the pore size exceeds the size of the organism, especially in the case of viruses [1]. Removal of viruses depends on several factors including the physico-chemical properties of the virus and the membrane, as well as the hydraulics of the filtration process [1,9,10]. Feed water chemistry also has a substantial effect on the observed LRV4 of membranes. Organic matter and ionic composition and strength can affect the LRV of viruses during LPMF [1,11,12]. Presence of cations has been shown to increase the LRV [12]. The effects of feedwater chemistry are of particular importance to membrane filtration for water reuse. Secondary treated wastewater effluent is composed of EfOM5 relevant to membrane fouling [13,14]. Previous work has generally shown an increase in LRV with increased membrane fouling [15]; however, little is known about the fractions of EfOM that play a role in altering virus LRV, or the manner in which other water and membrane parameters interact in this process. Consequently, this study was

4 5

Log removal value (LRV). Effluent organic matter (EfOM).

2

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

Table 1 Characteristics of bacteriophage and membrane employed in this study. a. Bacteriophage MS2 characteristics (Penrod, et al. [17,18]) Equivalent diameter 27 nm Surface geometry Icosahedral, no coat Isoelectric point, pI (measured) 3.9 −12 to −15 mV Zeta potential (pH 6–8, 10 mM NaCl) −0.06 C/m2 Surface charge (calculated for pH 6–8) b. Membrane characteristics Outside-in, constant flux Operational mode 54 LMH Filtration flux Polyvinylidene fluoride Membrane material 100 nm Nominal pore size (porometer) Hollow fiber Membrane geometry

undertaken to investigate the influence of EfOM and ionic composition on removal of virus by LPMF.

2. Experimental 2.1. Bacteriophage MS2 F-specific RNA bacteriophage MS2 has frequently been employed as a surrogate for removal of pathogenic enteric viruses by LPMF due to its similarity in size, capsid geometry and nucleic acid content [6,12,16]. MS2 has an equivalent diameter of 27 nm, an icosahedral geometry, an isoelectric point of 3.9, a calculated surface charge of −0.06 C/m2 for pH 6–8 and a zeta potential from −12 to 15 mV at pH 6–8 in 10 mM sodium chloride [17,18]. MS2 properties are summarized in Table 1a. MS2 was obtained from ATCC (15597-B1, Manassas, VA) and was assayed using the double agar layer procedure described in EPA Method 1602 [19]. The stock concentration of MS-2 was 5.9 ± 0.9 × 1011 PFU/mL.6 Repeated assays of the stock over the duration of the experiments were always within 0.2 log of the mean.

2.2. Hollow-fiber membrane All bacteriophage removal experiments were performed using a single dead-end membrane mini-module, composed of 9 fibers with an effective length of 23.3 cm and an outer membrane surface area of 54 cm2 . The hydrophobic hollow fiber membrane was constructed of PVDF7 , had a nominal pore size of 0.1 ␮m and was characterized by a slight negative charge based on streaming potential analysis (Table 1b). Fig. 1a and b shows FESEM8 image of a typical clean membrane used in this study. FESEM imaging of hollow fiber membranes was performed as previously described [20]. A membranes module fabricated with the PVDF membrane was wetted for 30 min with 30% 2-propanol and rinsed by filtering approximately 2 L of purified water from a Millipore Milli-Q Synthesis deionization system. This module was used in all filtration experiments and its pure water permeability was 436 and 449 L/m2 h-bar (LMH/bar) before and after concluding all bacteriophage removal experiments, respectively. No changes in the membrane pore structure were observed following the bacteriophage removal experiments, aside from the deposition of a limited amount of material on the outside surface (Supporting Information Fig. S1a and b).

6 7 8

Plaque forming unit (PFU). Polyvinylidene fluoride (PVDF). Field emission scanning electron microscopy (FESEM).

Fig. 1. FE-SEM images of a clean membrane (a) cross section, showing the direction of flow through the <1 ␮m thick outer layer, middle macroporous layer, and inner microporous layer. (b) Outer membrane surface showing (1) large round pores, 65–75 nm, (2) elongated cracks with smaller pores inside, and (3) slit shaped smaller pores, ∼10 × 40 nm. Inset shows a representative elongated pore; inset scale bar is 100 nm long. (c) Semi-log plot of surface pore size distribution for the longest and shortest dimension of each pore, with the diameter of a MS2 particle shown for reference.

2.3. Membrane filtration unit operation Bacteriophage removal experiments were performed using a bench-scale MFU9 equipped with the PVDF membrane module. A diagram of the MFU is shown in Fig. 2.

9

Membrane filtration unit (MFU).

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

Fig. 2. Operational diagram for the bench-scale hollow fiber low pressure membrane filtration unit, in submerged configuration.

Before bacteriophage removal experiments, the MFU was sterilized by filtration of 3 system volumes of 12 mg/L free chlorine, in the form of sodium hypochlorite (diluted from a 4–6% purified grade solution, Fisher) at caustic pH, followed by dechlorination with 5 system volumes of 2.7 mM sodium thiosulfate (Fisher) solution. The system was then rinsed with 5 hydraulic volumes of NaPi10 buffer to remove any remaining dechlorination solution. In cases where the membrane had previously been exposed to EfOM, the sodium hypochlorite solution was adjusted to 120 mg/L free chlorine, and the membrane was soaked for 30 min to oxidize any remaining foulant, followed by dechlorination and rinsing. Sodium thiosulfate and NaPi buffer were autoclaved at 121 ◦ C for a minimum of 15 min and allowed to cool to room temperature prior to use. Membrane filtration experiments were conducted using the cleaned membrane module. Transmembrane pressure and filtrate flow were recorded to determine any temporal variations in filtrate specific flux (Js ) as previously described [21,22]. Fouling curves are presented as the ratio of Js /Js0 , versus filtrate throughput which is defined as the cumulative volume of filtrate normalized to the outer membrane surface area (L/m2 ). Removal of MS2 virus was determined as log10 reduction in virus concentration after membrane filtration. The permeate sample was assayed for the analysis of MS2 viruses at time points determined based on the breakthrough time of sodium chloride and sodium nitrate tracers (SI Fig. S211 ). Membrane integrity was checked periodically using a pressure decay test in which the module lumen was pressurized to 100 kPa with compressed nitrogen, and the pressure recorded for 15 min. Integrity was maintained throughout the study period, with pressure decay never changing more than 1.5% of the original pressure. 2.4. Feed water for MS2 bacteriophage removal experiments Six types of feed waters were spiked with MS2 to prepare suspensions used in the bacteriophage removal experiments. The waters, designed to represent high, low and no fouling feedwaters consisted of: a secondary wastewater effluent (effluent), an effluent that was previously filtered through a separate module of the PVDF membrane used in this study (filtered effluent), and four NaPi-based model waters, respectively. The NaPi-based model

10 11

Sodium phosphate (NaPi). Supporting Information (SI).

3

Fig. 3. Reproducibility of removal for triplicate runs for MS2 virus spiked in NaPi.

waters were designed to elucidate the effect of ion composition on virus removal. Selected properties of the waters used, the parameters tested in each condition and identifying condition numbers are presented in Table 2. The effluent employed for the bacteriophage removal experiments was collected from the Scottsdale Arizona wastewater treatment plant, prefiltered using 1.2 ␮m glass fiber GF/C filters (Whatman, Maidstone, UK) and stored at 4◦ C until use. Size fractionation of the effluent was characterized by SEC-DOC12 by the method of Lee et al. [23]. Calcium concentration in the effluent was determined using a Perkin Elmer Analyst 100 atomic absorption spectrometry with air acetylene flame. Conductivity and pH measurements, corrected to 25 ◦ C were performed using an Accumet AR50 pH/ion/conductivity meter (Fisher Scientific). Additional water quality parameters for the effluent include: DOC13 5.73 mg/L (Shimadzu TOC-VCSN ), ultraviolet absorbance at 254 nm 0.292 cm−1 (Hach DR-4000), and specific UV absorbance 5.09 L/mg m. Concentrations of magnesium and aluminum were found to be 32.3 mg/L and 1.93 mg/L, respectively, by inductively coupled plasma mass spectrometry (ICP-MS) using a Perkin Elmer Sciex Elan DRC II. 2.5. Bacteriophage removal experiments For each trial, 1.2 L of BPS14 was prepared by seeding ambient temperature (21–23 ◦ C) feed water with 4.8 × 106 ± 1.6 × 106 PFU/mL of MS2. The BPS was then filtered for 3 h using the MFU at ambient temperature. Filtrate was collected for 1 min, into 15 mL single use sterile polypropylene centrifuge tubes, at the following time points: 6, 30, 60, 120, and 180 min. The 6 min time point corresponds to 3.1 hydraulic retention times (SI Fig. S2). In all experiments, the BPS was sampled before and after the filtration period. All samples were immediately placed on ice, and stored for no more than 4 h before assaying. The inactivation of MS2 in the samples under the testing condition was less than 0.5 log as found in our preliminary study (SI Fig. S4). Following enumeration, the LRV of MS2 was calculated for each time point as the log of the average of the before and after filtration BPS concentrations, divided by the concentration in the appropriate filtrate sample. Reproducibility of the LRV was validated in selected runs as shown in Fig. 3.

12 Size exclusion chromatography with online dissolved organic carbon detection (SEC-DOC). 13 Dissolved organic carbon (DOC). 14 Bacteriophage suspension (BPS).

4

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

Table 2 Properties of feedwaters used to prepare BPS and parameter evaluated in each group of experiments. Condition number

Initial membrane condition Feedwater to which MS2 was spiked

Parameter tested

pH Conductivity (␮S/cm)

Calcium (mg/L)

Feedwater fouling potential

1 2 3

Clean Clean Clean

58.2a 59.1a 60b

High Low None

Fouled Fouled

6.7 10 7.7 1990

0b 58.2a

None High

6

Clean

0.1 mM NaPi Buffer

7 8

Clean Clean

0.1 mM NaPi Buffer, +Na 0.1 mM NaPi Buffer, +Ca

High PS foulant Low PS foulant Simulated effluent without organic foulants Preexisting foulant layer Preexisting foulant layer and high PS foulant No calcium and low ionic strength High ionic strength high ionic strength with calcium

7.7 1990 7.7 2040 7.7 2040

4 5

Effluent Filtered Effluent 0.1 mM NaPi Buffer, +Ca +Na 0.1 mM NaPi Buffer Effluent

a b

10

0b

None

7.0 7.0

573 372

0b 60b

None None

Measured by atomic absorption spectroscopy. Calculated concentration.

2.6. Pre-existing foulant layer bacteriophage removal experiments In these experiments, the membrane was fouled for 3 h with effluent, before introducing a BPS prepared in either NaPi or effluent. For NaPi (condition 4), the feed water column was drained and refilled with BPS, before beginning the removal experiment as previously described. For the effluent BPS (run 5), after the initial fouling period the feed tank was switched to the BPS, eliminating any disruption to the foulant layer on the membrane. For this experiment, the first time point was taken at 20 min based on tracer test results (SI Fig. S3). 2.7. Virus adsorption experiments Static sorption of MS2 to the membrane was evaluated at ambient temperature (21◦ C) for the same duration as the bacteriophage removal experiments (180 min). Sorption experiments were conducted in triplicate in polypropylene centrifugal tubes. Membranes with a surface area equivalent to that used for filtration experiments were thoroughly rinsed, cut into 1-cm length, and added in three 50-mL centrifugal tubes. A NaPi BPS with approximately 6 × 105 PFU/mL of MS2 was then added into these tubes. Afterwards, the membrane fibers and the BPS were mixed at 20 rpm15 on a 35 cm diameter rotating mixer. Three centrifugal tubes containing BPS were also mixed under similar conditions without a membrane as a control. A 0.5-mL aliquot of the suspension was sampled from each centrifugal tubes at 30 and 180 min, respectively. MS2 concentration in these samples were immediately determined after the sorption experiments. 3. Results 3.1. Membrane structure and pore size distribution Cross-sectional FESEM images show that the membrane employed in this study had three major layers (Fig. 1a). The thin outer layer (0.3–1.0 ␮m) comprises the major physical barrier to filtration for this outside-in hollow fiber membrane. The surface of this outer layer has pores with sizes ranging from approximately 9–37 nm in the shortest dimension and 9–75 nm in the longest dimension for each pore (Fig. 1c). The distribution was determined from 5 randomly selected 1 ␮m2 regions of Fig. 1b. Each region was transformed using a 55% threshold from grayscale to black and white (Graphic Converter v. 6.1, Lemkesoft), and the resulting

15

7.0

Revolutions per minute (RPM).

black points, were measured in their longest and shortest dimension. This procedure yielded 814 pores in the 5 ␮m2 survey area. The central layer of the membrane is composed of a macroporous region 40–140 ␮m deep with void chambers up to 10 ␮m wide. The innermost microporous layer is 40–80 ␮m deep with pores typically 0.2–1 ␮m in width. 3.2. Effect of EfOM on membrane fouling Membrane fouling occurred during the filtration of secondary wastewater effluent due to the accumulation of EfOM on the membrane surface (pore plugging) or in the membrane matrix (pore constriction). As shown in Fig. 4a, effluent-based BPS (condition 1) caused a 61% decrease in normalized specific flux, filtered effluent BPS (condition 2) caused a 14% decrease, and NaPi pH 7 BPS (condition 3) caused a negligible loss in specific flux. These results demonstrate that effluent, filtered effluent and the NaPi based BPSs are representative of high, low and no fouling waters, respectively. Comparison of the SEC–DOC chromatograms for effluent and filtered effluent shows the removal of a single major component, a high molecular weight fraction, from the effluent by the membrane during filtration (Fig. 4b). This fraction was previously determined to be primarily composed of polysaccharide, protein and organic colloids (PS) [22,23]. The PS fraction was also recovered in a highly concentrated form in the membrane backwash fluid (Fig. 4b), which was associated with restoration of membrane permeability (Fig. 4a). These results support PS as the major hydraulically reversible foulant and are consistent with previous findings [21,23]. 3.3. Effect of membrane fouling on bacteriophage removal Levels of virus removal varied with membrane fouling. Fig. 5a shows the LRV of MS2 in the effluent BPS, the filtered effluent BPS, and a model water BPS, conditions 1, 2 and 3 respectively, over a 3 h filtration period. Virus removal using model water BPS, condition 3, ranged between 0.9 and 1.3 logs. The LRVs observed are consistent with other studies [12,24]. The LRV for the filtered effluent, condition 2, was similar to that of the model water BPS, condition 3, ranging from 0.8 to 1.7. In comparison, the LRV using effluent BPS, condition 1, increased monotonically from 2.1 to 3.0. The slopes of the removal for effluent and filtered effluent experiments were similar, with the removal employing effluent maintaining a 1.4 ± 0.1 log higher removal over the experimental period. Fig. 5b shows the effect of a preexisting fouling layer on virus removal. In these experiments, the membrane was prefouled by filtration of the effluent without MS2 added for 3 h before initiating a bacteriophage removal experiment with model water BPS

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

5

Fig. 4. Characteristics of EfOM-containing water. (a) The fouling capacity of different bacteriophage suspensions (BPSs) during filtration at 54 LMH, preceded by the determination of baseline permeability by filtration of NaPi. (b) SEC-DOC chromatogram showing removal of the PS fraction from the effluent by filtration, and its recovery by backwashing.

and effluent BPS, conditions 4 and 5, respectively. The removal of MS2 using effluent BPS on a non-prefouled membrane, condition 1, is also shown for comparison purposes. The data show a similar pattern of removal for conditions 4 and 5. The LRVs ranged from 3 to 4 regardless of sampling time. The initial decrease in LRV in these runs was probably due to depletion of the sorption sites on the membrane surface. By comparison the LRV of condition 1 was similar to those under prefouled conditions after 3 h of filtration.

Fig. 5. Effect of (a) EfOM, (b) prefouling and (4) ion composition on virus removal. Condition number is shown in parenthesis in legend. See Table 2 for detailed conditions.

3.4. Effect of selected electrolytes on bacteriophage removal A series of experiments were conducted with NaPi based model waters to investigate the influence of selected electrolytes on LRV, the results of which are shown in Fig. 5c. Equal ionic strength of two salts, 1.5 mM calcium chloride (+Ca2+ ), and 4.5 mM sodium chloride (+Na+ ) were compared for their impact on the baseline condition LRV of NaPi BPS at pH 7.0. The LRV for the NaPi BPS + Na+ experiment, condition 7, was 0.9 ± 0.1 log lower than the NaPi baseline condition, condition 6, over the time course; for NaPi BPS + Ca2+ , condition 8, the LRV was 1.6 ± 0.2 log lower (Fig. 5c). These data demonstrate the importance of calcium ions in LPM16 removal of viruses in the absence of membrane fouling.

16

Low pressure membrane (LPM).

Fig. 6. Static sorption of MS2 onto the membrane at sorption times of 30 min and 180 min. Experiments were conducted in triplicate, with and without the membrane. Temperature = 21 ◦ C, p values for unpaired t-tests are shown together with percentage decrease in statistical mean from control to sorption sample.

3.5. Adsorption of MS2 virus on membrane surface Adsorption of MS virus on membrane surface was observed in the static adsorption experiment (Fig. 6). BPS in the presence of the membrane was found to have 17% or 27% less phage than

6

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

Fig. 7. Five potential mechanisms for virus removal. (i) Sorption to the membrane, (ii) physical sieving, (iii) electrostatic repulsion, (iv) large-EfOM induced removal, v small EfOM induced removal.

BPS in the absence of membrane after 30 min and 3 h of exposure, respectively. Two-tailed, unpaired t-tests were conducted to determine the statistical significance of the difference between the sorption (with the membrane) and the control (without the membrane) samples. Statistical difference for the 30-min samples was extremely low (p = 0.1299); however, this difference was highly statistically different, after 180-min of contact time (p = 0.0020), It is noteworthy that, due to the absence of convective virus transport associated with filtration, the adsorption of MS2 virus on the membrane surface is expected to be less efficient in static adsorption than in direct-flow filtration. Also, this sorption experiment was conducted with a NaPi suspension at pH 7, corresponding to Condition 6 employed in the filtration experiments. More efficient adsorption is anticipated for NaPi + Na (Condition 7) and NaPi + Ca (Condition 8) as a result of the increase in monvalent or divalent electrolyte concentrations [25]. 4. Discussion The experimental results obtained in this study demonstrate the importance of EfOM and aquatic ion composition when using LPMs for virus removal during water treatment. Based on the findings of this and previous studies, five potential mechanisms for virus removal are presented as illustrated in Fig. 7. Their relevance to the filtration experimental results is summarized in Table 3.

for microbial removal by LPMs [6]. Langlet et al. showed a narrow size distribution for MS2 centered near 27 nm, for 1 mM sodium nitrate at pH 7.2, using dynamic light scattering [26]. Mylon et al. found that MS2 was stable with respect to aggregation in solutions containing greater than 1.0 M of monovalent electrolytes or lower than 10 mM of divalent electrolytes [27]. Therefore, at solution chemical conditions employed in this study, little aggregation of MS2 particles was expected to occur. Further, FESEM imaging suggested that the majority of membrane pores are smaller than the icosahedral virus in at least one dimension (Fig. 1b and c). In the absence of other removal mechanisms at pH 7.0, physical sieving accounted for a baseline of 0.9 LRV, which was similar to that observed for the calcium containing NaPi based BPS (condition 8). In this case, additional virus removal contributed by other removal mechanisms was minimized. It is noteworthy that membrane pores with a shortest dimension that is still larger than the dimension of MS2 virus only account for about 0.2% of the total number of pores (Fig. 1c). However, a greater portion of the permeate would flow through the larger pores according to the Hagen–Poiseulle law expressed below [28]: J=

2 P fdpore

32ım

(1)

where J (m/s) is the permeate flux, f is the fraction of open pore area, P (Pa) is the transmembrane pressure,  (Pa s) is the water viscosity,  (dimensionless) is the pore tortuosity factor, ım is the effective thickness of the membrane (symmetric membranes) or the skin layer (asymmetric membranes). Assuming other viariables in Eq. (1) as constants, permeate flux will be proportional to the square of dpore and the permeate flowrate will be proportional to J × d2 pore or the 4th power of dpore . As a result, the distribution of permeate flowing through pores larger than MS2 virus was greater than the overall fraction of these pores, leading to the passage of approximately 10% of the total MS2 virus through the membrane in condition 8. 4.3. Electrostatic repulsion at the pore entrance

4.1. Sorption to the membrane Viruses may adsorb to membrane matrices and be removed even though the membrane pore size is larger than the virus. In all of the experimental conditions performed using NaPi based BPS, the LRV decreased 0.6 ± 0.1 between the 6 and 30 min time points. The high LRV observed at 6 min is not attributable to dilution as this time point corresponds to >99% of the original concentration achieved during a nitrate tracer study (SI Fig. S3); instead it can be described by virus adsorption. Virus adsorption has previously been found by Voorthuizen et al. [12] when filtering a water spiked with 103 PFU/mL of MS2 bacteriophage by a hydrophobic PVDF membrane. The investigators showed that the adsorption effect dissipated at a permeate volumetric throughput of approximately 1250 L/m2 due to the saturation of membrane surface sites for virus binding. Because a relatively high virus feed concentration (∼106 PFU/mL) was used in this study, it is probable that adsorption of MS2 bacteriophage occurred over a shorter filtration time of 6–30 min, or at a smaller permeate volumetric throughput of 27 L/m2 as compared to 1250 L/m2 in the previous study [12]. The observed removals in the sorption experiments (Fig. 6) support the notion of virus adsorption to the membrane surface. 4.2. Physical sieving Physical sieving, virus removal mechanism (ii), occurs when virus particles are excluded by membrane pores that are smaller in size than the target organisms; this mechanism is well established

According to a model simulation conducted by Kim and Zydney [29], electrostatic interactions may prevent particles from entering the pores of LPMs and, depending upon hydrodynamic forces and Brownian forces, may result in particle rejection. Electrostatic repulsion comprises virus removal mechanism (iii). Since the experiments of this study were conducted at similar hydrodynamic conditions, the variations in virus removal observed in the presence of sodium or calcium ions (Fig. 5c, conditions 7 and 8 respectively) were probably a consequence of changes to electrostatic repulsion between the virus and like-charged membrane surfaces around the pores as the surface of both the membrane [22] and virus [17] are negative at the pH condition employed. The difference between sodium and calcium ions in LRVs (Fig. 5c) indicated that, at similar ionic strengths, divalent calcium ions were more effective than monovalent sodium ions in suppressing the electrostatic repulsion between the membrane and viruses, which decreased the contribution of electrostatic repulsion to virus removal. This trend was, in general, consistent with the prediction of classical DLVO (Derjaguin and Landau, Verwey and Overbeek) theories on the effects of mono- and divalent counter ions [30]. From this perspective, the LRV of 0.9, determined in presence of calcium ions (Fig. 5c condition 8) probably had the least contribution from electrostatic repulsion, and therefore, was closest to the removal solely by physical sieving (mechanism (ii)). Comparing the results obtained in the presence of Ca2+ , condition 8, to that in the absence of Na+ or Ca2+ , condition 6, an additional removal of 1.6 logs was found for electrostatic interactions (Fig. 5c).

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

7

Table 3 Potential mechanisms for the removal for various BPS chemistries and the experimentally determined LRV. Condition number

1 2 3 4 5 6 7 8 a

Water chemistry

Effluent (pH = 7.7) Filtered Effluent (pH = 7.7) NaPi + Ca + Na (pH = 7.7) NaPi pH 7.0 on prefouled membrane Effluent on prefouled membrane NaPi pH 7.0 NaPi pH 7.0 + Na NaPi pH 7.0 + Ca

Removal mechanisma

LRV

Sorption

Sieving

Electrostatic repulsion

Large EfOM

Small EfOM

− − + −a − + + +

+ + + + + + + +

− − − + − + − −

+ − − + + − − −

+ + − + + − − −

2.7 1.3 1.0 3.3 3.4 2.5 1.6 0.9

Likelihood a mechanism is involved: +high, −low.

Contrary to the results shown here, some investigators previously found that addition of calcium or sodium increases the LRV of MS2 [11,12,31]; this discrepancy may be attributable to the difference in membrane pore structure. These previous studies were performed with membranes with larger pores. The nominal pore size in studies by both Farahbakhsh and Smith [11] and van Voorthuizen et al. [12] was 0.2 ␮m. The membrane used in Jacangelo et al. [31] had a nominal pore size of 0.1 ␮m (similar to the membrane employed in this study), but electron microscopy of the membrane [20] shows a symmetrical porous membrane with surface pores near 1 ␮m in size and narrower pore throats inside the “sponge-like” membrane matrices. These large surface pores allowed viruses to enter membrane matrices regardless of solution chemistry and interact with membrane internal surfaces. Therefore, as electrostatic repulsion is reduced by increased ionic strength or presence of calcium, more MS2 viruses would be adsorbed onto membrane surface and LRV would be expected to increase. In comparison, the current study employed a commercially available LPM with pore sizes much closer to the size of the virus particles (Fig. 1c); as such, electrostatic repulsion is expected to play a role in preventing the virus from entering the pores. 4.4. PS-EfOM induced removal Two potential mechanisms are proposed involving EfOM. The first focuses on the accumulation of large-sized EfOM on the membrane surface increasing rejection of virus. For this study, large-sized EfOM can be operationally defined as the EfOM fraction that was removed by a single pass through the membrane and was likely predominantly comprised of the high molecular weight PS fraction (Fig. 5b). The PS EfOM increases virus removal by forming a cake layer, causing pore blocking, or by adsorbing virus before it can pass through the membrane. For effects associated with pore blocking, the presence of EfOM rapidly elevates the LRV as large membrane pores are quickly blocked and viruses in the BPS are forced by the bulk water to flow towards small pores. Fig. 1c illustrates that there are relatively few pores large enough for the virus to pass through, which supports the idea that they could be blocked rapidly. Mechanism (iv) is observed in the 1.4 log higher initial LRV of the effluent compared to filtered effluent (Fig. 5a conditions 1 and 2 respectively). The effects of mechanism (iv) on virus removal were substantially reduced in the filtered effluent due to the removal of PS-EfOM. The effects associated with cake layer formation may be prolonged. When BPS was introduced onto a preexisting foulant layer, the viruses were likely bound to the surfaces in the EfOM cake layer that had been accumulated on the membrane surface; this increased the log removal until the adsorption capacity was

exhausted. This behavior was also observed for BPS introduced in both NaPi and effluent (Fig. 5b). 4.5. Small-EfOM induced removal The second EfOM associated mechanism involves small-EfOM induced removal. Small EfOM can be operationally defined as the EfOM fraction which was not removed by a single pass through the membrane and was comprised of other smaller membrane foulant, other than the high molecular weight PS fraction. Removal was likely caused by small-EfOM that adhered to the membrane matrix, causing pore constriction, thereby preventing virus from passing through the pores. This effect constitutes virus removal by mechanism (v). Small-EfOM induced removal would be expected to be a long-term process that continues to accumulate over time and therefore LRV will also increase with time. Shirasaki, et al. found the gradual accumulation of hydraulically irreversible fouling to be the primary cause of virus removal by the LPM they employed [15]. As small-EFOM is expected to adsorb to the membrane, it is more likely to be a contributor to irreversible fouling. Indeed, the gradual increase in virus removal over the entire filtration period for both the effluent and filtered effluent BPSs, conditions 1 and 2, may be attributable to mechanism (v). The observation that the LRV for the effluent was consistently 1.4 log greater than that for the filtered effluent (range 1.3–1.5 log) over the 3 h filtration period (Fig. 5a, conditions 1 and 2 respectively) supports the probability of mechanism (v), as the small foulant was present in both feedwaters. Mechanism (v) would continue to exert its influence on virus removal over time eventually leveling off as the binding sites for the smaller EfOM become saturated. Evidence of saturation is presented here in the prefouled then effluent + MS2 experiment (Fig. 5b condition 5), in which the membrane was fouled initially for 3 h with effluent and MS2 was introduced after that time. In this case, the increase in LRV had mostly leveled off after 3 h of filtration. It is noteworthy that small EfOM may also decrease the initial virus removal by LPMs. The rapid decrease in LRV in the first hour of filtration that was associated with membrane sorption, mechanism (i), was not observed in EfOM foulant conditions performed on the clean membrane (Fig. 5a conditions 1 and 2); it was, however, observed when the virus suspension was introduced in effluent onto a preexisting layer of foulant (Fig. 5b conditions 4 and 5). These results suggest that smaller EfOM present in both effluent and filtered effluent may condition the surface of clean membranes and decrease the adsorption of virus, resulting in the lower log removal during the initial stage of filtration. In a previous study, Pieper, et al. observed decreased retention of viruses in an aquifer in the presence of EfOM, suggesting a role for EfOM in altering porous media and their virus adsorption capacity [32].

8

H. Huang et al. / Journal of Membrane Science 409–410 (2012) 1–8

5. Conclusions This study has two major implications to LPMF of viruses in wastewater effluents. Pretreatment is often employed for increasing membrane productivity by reducing membrane fouling [11]. The current study has shown, however, that removing membrane foulants can have a negative impact on virus removal. Removal of large EfOM by prefiltration caused the virus removal to decrease by approximately 1.4 logs. Therefore, passage of viruses through full-scale LPMF systems should be carefully monitored during the implementation of membrane fouling control measures. The findings of this study also manifest the complexity associated with the determination of virus removal by commercially available LPMs. Since removal of virus can be influenced by the composition of the influent, it is important to employ similar solution chemical conditions when the virus removal efficiencies of different LPMs are compared. On the other hand, it is essential to consider case-by-case water treatment conditions and the potential impact of EfOM and simple electrolytes (e.g., calcium), when a LPMF system is considered in wastewater treatment or reuse for its ability to remove viruses. Acknowledgments The authors would like to thank James Lozier and the Scottsdale Wastewater Treatment Plant for providing the wastewater effluent sample used in this study. The help of Dr NoHwa Lee, who performed the SEC-DOC analysis of the water sample in the laboratory of Dr. Gary Amy was critical to the understanding of water composition and fouling characteristics. The assistance of Dr. Jean Philippe Croué and his laboratory were invaluable to understanding the characteristics of the membrane used here. Thanks are also due to Siemens for kindly providing the membrane fibers used in this study. Mark Kontz kindly provided assistance in performing the FESEM. The Water Research Foundation is acknowledged for partial funding of this study; Alice Fulmer served as project officer. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.memsci.2011.12.050. References [1] S.S. Madaeni, A.G. Fane, G.S. Grohmann, Virus removal from water and wastewater using membranes, J. Membr. Sci. 102 (1995) 65–75. [2] J.G. Jacangelo, R.R. Trussell, M. Watson, Role of membrane technology in drinking water treatment in the United States, Desalination 113 (1997) 119–127. [3] J.M. Laîné, D. Vial, P. Moulart, Status after 10 years of operation – overview of UF technology today, Desalination 131 (2000) 17–25. [4] D. Furukawa, NWRI Final Project Report for Project #07-KM-008: A Global Perspective of Low Pressure Membranes, National Water Research Institute, Fountain Valley, California, 2008, pp. 13. [5] T. Hirata, A. Hashimoto, Experimental assessment of the efficacy of microfiltration and ultrafiltration for Cryptosporidium removal, Water Sci. Technol. 38 (1998) 103–107. [6] J.G. Jacangelo, S.S. Adham, J.M. Laine, Mechanism of Cryptosporidium, Giardia, and MS2 virus removal by Mf and Uf, J. Am. Water Works Assoc. 87 (1995) 107–121. [7] L. Vera, R. Villarroel-L. Ûpez, S. Delgado, S. Elmaleh, Cross-flow microfiltration of biologically treated wastewater, Desalination 114 (1997) 65–75.

[8] J.G. Jacangelo, A. Madec, K.J. Schwab, D.E. Huffman, C.S. Mysore, Advances in the use of low-pressure, hollow fiber membranes for the disinfection of water, Water Sci. Technol.: Water Supply (2005) 109–115. [9] J. Langlet, L. Ogorzaly, J.-C. Schrotter, C. Machinal, F. Gaboriaud, J.F.L. Duval, C. Gantzer, Efficiency of MS2 phage and Q[beta] phage removal by membrane filtration in water treatment: Applicability of real-time RT-PCR method, J. Membr. Sci. 326 (2009) 111–116. [10] G. Herath, K. Yamamoto, T. Urase, The effect of suction velocity on concentration polarization in microfiltration membranes under turbulent flow conditions, J. Membr. Sci. 169 (2000) 175–183. [11] K. Farahbakhsh, D.W. Smith, Removal of coliphages in secondary effluent by microfiltration – mechanisms of removal and impact of operating parameters, Water Res. 38 (2004) 585–592. [12] E.M. van Voorthuizen, N.J. Ashbolt, A.I. Schäfer, Role of hydrophobic and electrostatic interactions for initial enteric virus retention by MF membranes, J. Membr. Sci. 194 (2001) 69–79. [13] A.J. Hamilton, F. Stagnitti, R. Premier, A.M. Boland, G. Hale, Quantitative microbial risk assessment models for consumption of raw vegetables irrigated with reclaimed water, Appl. Environ. Microbiol. 72 (2006) 3284–3290. [14] L. Fan, T. Nguyen, F.A. Roddick, J.L. Harris, Low-pressure membrane filtration of secondary effluent in water reuse: pre-treatment for fouling reduction, J. Membr. Sci. 320 (2008) 135–142. [15] N. Shirasaki, T. Matsushita, Y. Matsui, K. Ohno, Effects of reversible and irreversible membrane fouling on virus removal by a coagulation-microfiltration system, J. Water Supply: Res. Technol. – AQUA 57 (2008) 501–506. [16] B.S. Oh, H.Y. Jang, Y.J. Jung, J.W. Kang, Microfiltration of MS2 bacteriophage: effect of ozone on membrane fouling, J. Membr. Sci. 306 (2007) 244–252. [17] S.L. Penrod, T.M. Olson, S.B. Grant, Deposition kinetics of two viruses in packed beds of quartz granular media, Langmuir 12 (1996) 5576–5587. [18] S.L. Penrod, T.M. Olson, S.B. Grant, Whole particle microelectrophoresis for small viruses, J. Colloid Interface Sci. 173 (1995) 521–523. [19] USEPA, in: U.S. Environmental Protection Agency Office of Water (Ed.), Method 1602: Male-specific (F+) and Somatic Coliphage in Water by Single Agar Layer (SAL) Procedure, U.S. Environmental Protection Agency Office of Water, 2001. [20] H. Huang, R. Spinette, C.R. O’Melia, Direct-flow microfiltration of aquasols. I. Impacts of particle stabilities and size, J. Membr. Sci. 314 (2008) 90–100. [21] H. Huang, N. Lee, T. Young, A. Gary, J.C. Lozier, J.G. Jacangelo, Natural organic matter fouling of low-pressure, hollow-fiber membranes: effects of NOM source and hydrodynamic conditions, Water Res. 41 (2007) 3823–3832. [22] J. Lozier, L. Cappucci, G. Amy, N. Lee, J.G. Jacangelo, H. Huang, T. Young, C. Mysore, C. Emeraux, J. Clouet, J.P. Croue, B. Heijmann, in: A.R. Foundation (Ed.), Natural Organic Matter Fouling of Low-Pressure Membrane Systems, AWWA Research Foundation, 2008. [23] N. Lee, G. Amy, J.P. Croué, H. Buisson, Identification and understanding of fouling in low-pressure membrane (MF/UF) filtration by natural organic matter (NOM), Water Res. 38 (2004) 4511–4523. [24] V. Gitis, R.C. Haught, R.M. Clark, J. Gun, O. Lev, Nanoscale probes for the evaluation of the integrity of ultrafiltration membranes, J. Membr. Sci. 276 (2006) 199–207. [25] J. Lukasik, T.M. Scott, D. Andryshak, S.R. Farrah, Influence of salts on virus adsorption to microporous filters, Appl. Environ. Microbiol. 66 (2000) 2914–2920. [26] J. Langlet, F. Gaboriaud, J. Duval, C. Gantzer, Aggregation and surface properties of F-specific RNA phages: implication for membrane filtration processes, Water Res. 42 (2008) 2769–2777. [27] S.E. Mylon, C.I. Rinciog, N. Schmidt, L. Gutierrez, G.C.L. Wong, T.H. Nguyen, Influence of salts and natural organic matter on the stability of bacteriophage MS2, Langmuir 26 (2010) 1035–1042. [28] M. Wiesner, P. Aptel, Mass transport and permeate flux and fouling in pressuredriven processes, in: J. Mallevialle, P.E. Odendaal, M. Wiesner (Eds.), Water Treatment Membrane Processes, McGraw-Hill, New York, 1996. [29] M.M. Kim, A.L. Zydney, Effect of electrostatic, hydrodynamic, and Brownian forces on particle trajectories and sieving in normal flow filtration, J. Colloid Interface Sci. 2 (69) (2004) 425–431. [30] W. Stumm, J.J. Morgan, Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters, 3rd ed., Wiley-Interscience, New York, 1996. [31] J.G. Jacangelo, N.L.P. Brown, A. Madec, K.J. Schwab, D. Huffman, G. Amy, C. Mysore, J. Leparc, A. Prescott, in: A.R. Foundation (Ed.), Micro- and Ultrafiltration Performance Specifications Based on Microbial Removal, AWWA Research Foundation, 2006. [32] A.P. Pieper, J.N. Ryan, R.W. Harvey, G.L. Amy, T.H. Illangasekare, D.W. Metge, Transport and recovery of bacteriophage PRD1 in a sand and gravel aquifer: effect of sewage-derived organic matter, Environ. Sci. Technol. 31 (1997) 1163–1170.