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Effect of pore structure of membranes and module configuration on virus retention
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of MembraneScience 115 (1996) 21-29
Effect of pore structure of membranes and module configuration on virus retention Taro Urase, Kazuo Yamamoto, Shinichiro Ohgaki Department of Urban Engineering, University of Tolt3'o, 7-3-1 Hongo, Bunk~'o-kn, Tokyo 113, Japan
Received 31 July 1995:accepted 3 October 1995
Abstract We measured virus retention by many types of membranes including microfiltration membranes, ultrafiltration men> branes and nanofiltration membranes. We succeeded in evaluating quantitatively virus retention in a very high retention range by employing coliphage Q/3 and T4 as model viruses. Q/3, which is the smaller virus in this study, penetrated all tested pieces of ultrafiltration membranes and nanofiltration membranes, though the retention was very high such as in the range of 99-99.9999%. The analysis of the polyethylene glycol retention data has shown that the leakage of viruses is caused by abnormally larger pores which are not included in the main pore size distribution. The diameter of the abnormal pores was estimated from the results of retention of different coliphages. The leakage of Q/3 was also observed in the case of inorganic ceramic ultrafiltration membranes, but T4, which is larger than Q/3, cannot penetrate them. Some types of microfiltration membrane have shown higher retention than ultrafiltration membranes and nanofiltration membranes. This suggests the possibility that we can develop high virus retention membranes with low filtration resistance. Kevwords: Pore size distribution:Virus retention:Water treatment:Membrane structure: Ultrafiltrationmembrane
1. Introduction Membrane processes are widely used in water and wastewater treatment including wastewater reuse systems. Chlorination has been a most popular technique for quality control of treated water in terms of microbiological safety. However, chlorination has several disadvantages in by-products and in controlling dosage in small scale plants. Moreover, viruses are generally more resistant to chlorine than bacteria. Virus retention by membrane processes without disinfection is attractive when we apply them to drinking water purification and wastewater reclamation. Water treated by membrane processes is thought to contain no suspended solids and consequently no pathogenic bacteria. Pathogenic viruses would also
be removed by the membrane processes because some of the viruses are adsorbed onto suspended solids in the environment [1]. When we want to discuss retention of viruses which are not adsorbed onto suspended solids, we must examine the virus retention performance of membrane processes experimentally. On the basis of nominal cutoff size of membranes, viruses should be completely retained by ultrafiltration membranes or nanofiltration membranes. But some reports have shown incomplete retention of viruses by these membranes [2,3]. Our earlier research also indicated that ultrafiltration membranes cannot be complete barriers for viruses because they have abnormally larger pores although the number of the abnormal pores is very small [4].
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0376-7388(95)00269-3
22
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
In this study, we investigated virus retention by many types of membranes ranging from microfiltration to nanofiltration membranes. Anopore membranes, Nuclepore membranes and inorganic membranes, which are said to have narrow pore size distribution, were also tested. Various module configurations were also tested such as flat sheet membranes, tubular membranes, monolith membranes, disk holder membranes and hollow fiber membranes.
branes appear last in the table. We selected many types of membranes such as track etched membranes, anodic oxidation membranes, sol-gel membranes as well as phase inversion membranes and composite membranes which are the popular structure of ultrafiltration membranes and nanofiltration membranes. The module configuration was different according to the shape of the membrane used. All membranes were tested in crossflow conditions except for the holder type membranes.
2. Experimental
2.2. Viruses
2.1. Membranes
In order to discuss quantitatively the virus retention even at very high level of retention, we seeded coliphages to the water. Coliphages are viruses which can attack Escherichia coli. We used two types of coliphages; one is Q/3 which is 23 nm in diameter and the other is T4 which is 80 nm in diameter [5].
The membranes used in this study are listed in Table I. The order in the table corresponds to the nominal cutoff size of the membranes. Microfiltration membranes appear first and nanofiltration mere-
Table 1 List of the membranes used in this study Membrane Name
Nominal cutoff
Manufacturer
Material
Preparation
Module
IRIS-3065 Nuclepore A1-AO VSWP Anopore A1-AO Nuclepore CPI 0-1010 TS- 1000 UK-200 IRIS-3065 Cefilt USY-5 IRIS-3065 NTU-OZK IR1S-3038 IRIS-3026 NTU-3030 NTU-2020 Cefilt UP-20 UK- 10 USY- 1 NTR-7410 NTR-7250 NTR-729HF NTR-759HR
100 nm 50 nm 37 nm 25 nm 20 nm 16 nm 15 nm 10 nm 1 000 000MW 200 000MW 200 000MW 50 000MW 50 000MW 40 000MW 30 000MW 20 000MW 20 000MW 20 000MW 20 000MW 20 000MW 20 000MW 10 000MW l 0 000MW 20% Desalin. 65% Desalin. 93% Desalin. 99% Desalin.
Mitsui Petrochem Costar Nakao Lab. Millipore Whatman Nakao Lab. Costar Toray Toso Advantech Mitsui Petrochem Nihon Gaishi Advantech Mitsui Petrochem Nitto Denko Mitsui Petrochem Mitsui Petrochem Nitto Denko Nitto Denko Nihon Gaishi Advantech Advantech Advantech Nitto Denko Nitto Denko Nitto Denko Nitto Denko
Polyvinylidene fluoride Polycarbonate Aluminum oxide Cellulose acetate Aluminum oxide Aluminum oxide Polycarbonate Polyacrylonitrile Polysulfone Polysulfone Polyvinylidene fluoride Ceramic Poly sulfone Polyvinylidene fluoride Polysulfone Polyacrylonitrile Sulfonated polysulfone Polysulfone Polyolefine Ceramic Aromatic polyamide Polysulfone Polysulfone Sulfonated polysulfone Polyvinyl alcohol Polyvinyl alcohol Aromatic polyamide
Phase inversion Track-etch Anodic oxidation Phase separation Anodic oxidation Anodic oxidation Track-etch Phase inversion Phase inversion Phase inversion Phase inversion Sol-gel Phase inversion Phase inversion Phase inversion Phase inversion Phase inversion Phase inversion Phase inversion Sol-gel Phase inversion Phase inversion Phase inversion Composite Composite Composite Composite
Flat sheet(U) Flat sheet25 Flat sheet45 Flat sheet25 Holder Flat sheet45 Flat sheet25 Hollow Fiber Flat sheet62 Flat sheet62 Flat sheet(U) Monolith Holder Flat sheet(U) Hollow Fiber Flat sheet(U) Flat sheet(U) Tubular Tubular Monolith Flat sheet62 Flat sheet62 Holder Flat sheet(R) Flat sheet(R) Flat sheet(R) Flat sheet(R)
MW: Molecular weight cutoff size (Dalton).
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29 The size and structure of Q/3 closely resembles enteric viruses which are most important in considering water reclamation. The concentration of coliphages was measured by the double layer agar method [1] with the host cell of Escherichia coil K I 2 A / A (F +) as to Q/3, and Escherichia coli B as to T4. 2.3. Experimental set-up and sampling In real environmental samples the presence of suspended solids increases the retention of viruses due to the gel-layer effect [6]. In this paper viruses were suspended in 1 m M phosphate buffer solution to avoid the gel-layer effect. W e used pure water for 1 h filtration before virus retention tests started. Virus retention tests were conducted in the crossflow conditions except for the holder type membranes. Crossflow velocity was typically 0.8 to 1.2 m / s . Applied pressure was typically controlled at 2 0 - 5 0 kPa with the exception of 98 kPa for VSWP, A1-AO, UK-10, UP-20, TS-1000, UK-200 membranes, 196 kPa for Nuclepore membranes and 500 kPa for nanofiltration membranes. The pressures were determined based on the low flux operation which is usual in water and wastewater treatment. 2.4. Poh,ethvlene glycol retention tests In order to know the retention performance of the membranes, we examined the retention of polyethylene glycols (PEGs) whose molecular weights are 20 000, 4 000 and 600. The measurements were carried out in 0.5 g / l solution under the same operating conditions as the measurement of retention of viruses. The concentrations of PEGs were measured by HPLC with a RI detector. 2.5. Sah retention tests Salt retention tests were carried out for nanofiltration membranes by using composite salt solutions of K H 2 P O 4 44 r a g / l , NaNOz 99 mg/1, KNO 3 144 mg/1, NaCl 50 m g / 1 and M g S O a • 7 H 2 0 77 m g / 1 under the same operating conditions as the measurement of retention of viruses. Salt retention was characterized by anion retention. Anion concentrations were measured by HPLC by the indirect UV method.
23
2.6. Resistance q[ membranes The filtration resistances of the membranes were calculated by pure water permeability as follows. AP R
Ill
- -
r/J,
where R m is the filtration resistance of the membrane, ,.IP the applied pressure, r 1 is viscosity and J , is volume flux. 2. Z El'aluation o f retention &tta The permeate and bulk solution were sampled 1 h after the virus retention test started. The logarithmic retention coefficient was defined by the following equation in order to evaluate quantitatively the very high retention level [7]. Cb qb = log I~.-~4
where Cb is the concentration in the bulk, Cp the concentration in the permeate and qb is the logarithmic retention coefficient. The concentration of the viruses in the seeded bulk solution was 107--10 s P F U / m l . We succeeded in measuring a retention as high as 7 of the logarithmic retention coefficient because the detection limit of coliphages was 1-10 PFU/ml. 2.8. Correction o f concentration polarization Retention of solutes can be evaluated in two different manners [8]. One is observed retention I ( C p / Q ) and the other is intrinsic retention I ( C p / C m ) where C m is the concentration at the membrane surface. In this paper, we used only observed retention which ignores concentration polarization unless otherwise mention. This is because concentration polarization of colloidal materials such as viruses is not so well understood and flow conditions were not clear for some of the modules used.
3. Results
3.1. PEG retention tests The results of retention of polyethylene glycols are shown in Table 2. Microfiltration membranes
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
24
Table 2 Retention of polyethylene glycols (%) Name
Nominal cutoff
IRIS-3065 Nuclepore AI-AO VSWP Anopore A1-AO Nuclepore CP10-1010 TS- 1000 UK-200 IRIS-3065 Cefilt USY-5 IRIS-3065 NTU-OZK IRIS-3038 IRIS-3026 NTU-3030 NTU-2020 Cefilt UP-20 UK- 10 USY- 1 NTR-7410 NTR-7250 NTR-729HF NTR-759HR
100 nm 50 nm 37 nm 25 nm 20 nm 16 nm 15 nm 10 nm 1 000 000MW 200 000MW 200000MW 50 000MW 50 000MW 40 000MW 30000MW 20000MW 20 000MW 20 000MW 20 000MW 20 000MW 20 000MW 10 000MW 10 000MW 20% Desalin. 65% Desalin. 93% Desalin. 99% Desalin.
R m ( × 1012 m - l) 0.05 7.3 9.1 14 91 100 4.0 0.39 0.88 0.057 3.1 0.48 4.5 0.41 3.0 1.4 3.8 6.7 3.8 11 16 68 94 140
PEG 20 000
PEG 4 000
PEG 600
0
0
0
4.8 0 50 44 0 0
0 0 27 23 0 0
0 0
14 0 19 48 21 95 75 61 88 0 49 0 74 > 99 > 99 > 99
1 0 11 0 5.5 63 26 17 49 0 36 0 62 98 98 > 99
Flux a ( × 10 5 m / s ) 13
0.17 0.13 0.18 0.81 28
0 0
0 0 6 0 2 14 0 0 4.4 0 2 0 24 99.2 99.8 99.9
1.6 8 2.9 2.5 0.37 0.52 0.66 0.72 2.9 1.1 2 2.0 0.89 0.46 0.22
a Volume flux during measurement of retention of PEGs.
gave almost zero retention, with the exception of the A1-AO 16 nm membrane and the Nuclepore 15 nm membrane. Ultrafiltration membranes gave lower retention values compared to the nominal cutoff size probably due to concentration polarization effect. For example, intrinsic retention after correction for concentration polarization for the IRIS-3038 membrane with nominal molecular weight cutoff size of 20 000 was calculated to be 88% retention for the PEG 20000, while observed retention was only 21%. The retention of polyethylene glycols by nanofiltration membranes was almost 100% with the exception of the NTR-7410 membrane. 3.2. Salt retention tests
The results of salt retention tests are shown in Table 3. The observed retention data for the chloride ion was lower than the nominal desalination value
because of the low pressure operation. In reverse osmosis it is understood that low pressure operation decreases retention because salt flux is relatively constant compared to volume flux. The nominal value is obtained for 1.5 MPa operation, while our testing method, which is common for the virus retention tests, adopted 0.5 MPa operation for nanofiltration membranes.
Table 3 Retention of salts by nanofiltration membranes (%) Name
NTR-7410 NTR-7250 NTR-729HF NTR-759HR
Type of anions
Flux a
PO4-P C1- NO~- NO 3 SO4:
(×10-Sm/s)
30 99 99 99
6.2 0.83 0.52 0.36
0 58 84 97
0 37 70 92
0 32 76 95
48 98 96 99
a Volume flux during measurement of retention of salts.
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
of Q~
Number of pieces tested Membrane Nominal Cutoff '~ 1 2 Name T I I 1RIS-3065 100nm 3 [~ ~ I Nuclepore 50nm 2 i ~I A1-AO 37nm 1 • VSWP 25nm 1 • Anopore 20rim 3 A1-AO 16nm 1 Nuclepore 15nm 6 CP10-1010 10rim 1 ¥~"i'F66 ............. ] ' ; ~ 6 6 ~ " ' 2 -
UK-200 IRIS-3065 Cefilt USY-5 IRIS-3065. NTU-OZK IRIS-3038. IRIS-3026" NTU-3030* NTU-2020* Cefilt UP-20 UK-10 SY-1
3
4
6
5
I
I
I I I I ..........................................................................................................................
........... ~
200,000MW 3 200,000MW 1 50,000MW 1 50,000MW 1 I 40,000MW 13 30,000MW 1 20,000MW 12 20,000MW 5 I 20,000MW 4 I-•-I 20,000MW 3 20,000MW 1 20,000MW 1 10,000MW 3 1 0 ~ _ . . . . 1 . .........................................................0......................................................................................
IW-1
i~:~i~i5 .......... ~ i ~ s a i ' i n .
NTR-7250 NTR-729HF NTR-759HR
25
~
•
65%Desalin. 3 93%Desalin. 2 99%Desalin. 1
I~ •
I
i
I
,
I
I
i //_2
1
2
3
4
5
6
7
ND
Fig. 1. Logarithmic retention coefficient of Q/3. * The Retention of Q/3 by IRIS-3065, 3038, 3026, NTU-3030, 2020 has been published in
[4].
Number of pieces Membrane Nominal Cutoff Name
~ 1 I
of
(I) 2 I
3 I
4 I
T4 5 I
6 I
7 I
, ND
//--1
IRIS-3065 Nuclepore AI-AO VSWP Anopore AI-AO Nuclepore
100nm 1 • 50nm 2 37nm 1 25nm 1 20nm 3 16nm 1 15nm 6 ...C.~IO:.IOlO ................... !.9_n_m___! .......................................................................................................................... TS- 1000 1,000,000MW 0 UK-200 200,000MW 0 IRIS-3065 200,000MW 0 Cefilt 50,000MW 1 USY-5 50,000MW 1 I A IRIS-3065 40,000MW 4 , v NTU-OZK 30,000MW 1 IRIS-3038 20,000MW 4 I IRIS-3026 20,000MW 0 NTU-3030 20,000MW I NTU-2020 20,000MW 0 Cefilt 20,000MW 1 UP-20 20,000MW 1 UK-10 10,000MW 2 NTR-7410 NTR-7250 NTR-729HF NTR-759HR
20%Desalin. 65%Desalin. 93%Desalin. 99%Desalin.
1 1 3 2
• • • • •
i
• •
I
I •
i
I
I
i
i
J
1
2
3
4
5
6
Fig. 2. Logarithmic retention coefficient of T4.
i//_2 7
ND
26
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
3.3. Virus retention tests 3.3.1. Q[3
The results of retention of Q/3 are shown in Fig. 1. Microfiltration membranes with larger nominal pore size than 20 nm gave very low retention, while the membranes with smaller nominal pore size than 16 nm gave very high retention. In the case of ultrafiltration membranes, Q/3 can penetrate all types of membranes and all pieces of membranes. The nanofiltration membranes also gave incomplete retention results. Nominal cutoff size had no significant effect on retention of Q/3 in the case of ultrafiltration membranes and nanofiltration membranes. The typical logarithmic retention value by ultrafiltration membranes and nanofiltration membranes ranged from 3 to 6, which corresponds to retentions of 99.9 to 99.9999%. 3.3.2. T4
T4 is a larger virus than Q/3. The results of virus retention tests in Fig. 2 show generally higher retentions compared to the cases of Q/3. Several types of membrane retained T4 completely. However in the case of the IRIS-3065 membranes, NTU-3030 membranes and NTR-729HF membranes T4 was retained only at the same level as was observed for Q/3. The same level retention can be explained by abnormal pores which is discussed later.
4. Discussion 4.1. Factors affecting virus retention
The adsorption of viruses on the membrane surface is discussed here. Q/3 is negatively charged in water because its isoelectric point is 5.3. Because we used no strongly positive charged membranes, the electrostatic force may not be a major factor to promote adsorption. Even if we assume that 90% of the viruses are retained by adsorption mainly caused by hydrophobic interactions, the retention increases by as much as only 1 in the logarithmic retention by adsorption. The adsorption never decreases retention. Consequently, the leakage of viruses through ultrafiltration membranes and nanofiltration membranes cannot be explained by the adsorption. Only the size sieve will be a dominant factor in virus retention in a
very high retention range though other factors such as operating pressure and flow conditions in the bulk solution also affects retention of viruses. The results in Fig. 1 show that the retention by microfiltration membranes is closely related to their pore sizes. This suggests that the results are less affected by adsorption. If it is true that the separation mechanism is mainly size sieving, the discussion using coliphage can be generalized to the fate of human viruses. 4.2. Cause o f leakage o f viruses by ultrafiltration membranes and nanofiltration membranes
We examined many types of ultrafiltration membranes and nanofiltration membranes in this study. The results indicated that the virus leakage is not an exceptional phenomenon but an often observed typical phenomenon. We did experiments with many pieces of membrane of the same type. For example, 13 pieces were tested for retention of Q/3, and 4 pieces for retention of T4 in the case of IRIS-3065 membranes. The results seemed to be scattered but they never showed complete retention. IRIS-3038 membranes always show very high retention for T4. NTR-729HF membranes always gave lower retentions compared to other types of nanofiltration membranes though the retention of salts by NTR-729HF was higher than those by NTR-7410 or NTR-7250. We can consequently conclude the retention phenomena of viruses by membranes are not random phenomena but dependent on the types of membranes. Virus leakage was also observed in the case of holder type membranes. This leakage cannot be explained by the damage during installation of the membranes to the modules because these membranes are ready-packed membranes. In a previous paper [4], we had already come to the conclusion that some ultrafiltration membranes must have abnormally large pores. The reason is that log normal pore size distributions determined by polyethylene glycol retention data predicts higher retention for 23 nm particles than for experimental retention of Q/3 as shown in Fig. 3. Most of the Q/3 detected in the permeate did not pass through pores included in the log-normal pore size distribution. They passed through abnormally large pores. The number of abnormal pores was calculated to be of
27
T. Urase et a l . / Journal of Membrane Science 115 (1996) 21-29
Q)
8
0 k~ u~ 0 D
7
Calculated cutoff curve (IRIS-3038) d0--4.5nm o--0.1
6
0 4J ~J
0
~4 r6
0
5
m
4 3 2
-
Calculated cutoff cu (NTU-3030) /
5
-
dO--4.5.m
[
I
°2/i-/
i 0 1
/
,
2
5 Solute
,
I
10
I
20 25
diameter
[nm]
Fig. 3. Calculated cutoff curves by a pore model in which log normal pore size distribution was assumed, dO is mean pore diameter and ~r is standard deviation in log normal distribution. The parameters of the distribution were obtained by PEG rejection tests. This figure was rewritten by using the model in [4].
the order of l0 9 of the total number of pores in the case of the IRIS-3038 membrane. The abnormal pores have a vital consequence in discussing virus retention though they have no significant effect on normal retention properties such as retention of polyethylene glycols. In this paper, the results using another type of coliphage T4 helped us to estimate the size of abnormal pores. If retention of T4 is complete by a membrane which leaked Q/3, the diameter of abnormal pores lies between 23 and 80 nm. In the case of the NTU-3030 membrane the diameter of the abnormal pores would be larger than 80 nm because the retention of T4 is as high as the retention of Q/3 by the NTU-3030. In the case of IRIS-3038 membrane the diameter of the abnormal pores would be in the range between 23 to 80 nm because the retention of T4 by the IRIS-3038 is very high.
retention of PEG 20000 as general retention property of membranes. The figure shows no close correlation between virus retention and the general retch-
ND~ -c )CPI0-1010 " iNuc i epore 10rim
o
QAI -AO 16nm
5
io~_i0
i IRIS-3026
o
l
l~R-759HR
1
3
Cefilt
20,000
NTR-729HF
ONTU-OZK 2
P VSWP 25nm
~ C e f i l t 50,000 l ' - ~ U P - 2 0 , USY-1, USY-B, TS-1000 N u c l e p o r e 50rim, IRIS-3065 100rim
0~
4.3. The correlation of retention of PEG and Q fl The retention of Q/3 was plotted against the retention of PEG 20000 in Fig. 4. We can regard
7
iI t i ~ 50
I
L
tlll t i ~,k
90 95 Retention of PEG#20,000
l 1001%]
Fig. 4. Correlation between the retention of PEG and retention ol
Q/3.
28
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
tion property. The general retention property is determined by the main pore size distribution while virus retention is determined by abnormal pores. No close relation between retention of PEG and retention of virus suggests that abnormal pores are not included in the main pore size distribution. The PEG retention data also supported the view that the cause of leakage by ultrafiltration membranes and nanofiltration membranes is abnormal pores which are not included in the main size distribution.
4.4. bzorganic ultrafiltration membranes Characteristics of abnormal pores of inorganic membranes are discussed here. Inorganic membranes are said to have a narrow pore size distribution compared to the organic membranes. Our results show that ceramic membranes with nominal molecular weight cutoff size of 20 000 and 50 000 retained T4 completely. However, the retention of Q/3 by these membranes are not different from those of organic ultrafiltration membranes. The diameter of abnormal pores of inorganic ceramic ultrafiltration membranes is small even though these membranes also have abnormal pores.
4.5. Hollow fiber membranes In the case of flat sheet membranes, seal materials in the module can disturb the membrane surface. Consequently, there is a possibility that seal materials give damage or cause abnormal pores in the membranes, while in the case of hollow fiber membranes, seal materials do not directly touch the membranes. However our results indicate that the module configuration would not be the major factor affecting virus retention. The result of the CP10-1010 membrane which is a hollow fiber type retained viruses completely while the result of the NTU-OZK which is also used in hollow fiber configuration gave low retention as was observed by using other organic ultrafiltration membranes.
retentions by these membranes are higher than those by ultrafiltration membranes and nanofiltration membranes. These membranes are really microfiltration membranes because the retention of the PEGs by these membranes are not high compared to the retention by ultrafiltration membranes and nanofiltration membranes. The high retention by these microfiltration membranes may be due to differences in membrane pore structure. Ultrafiltration membranes and nanofiltration membranes are membranes with skin layers. Slight defects in the skin layer may cause the presence of abnormal pores and consequently cause virus leakage. On the other hand, microfiltration membranes have a definite thickness for the separation layer. This thickness may prevent formation of defective pores.
4.7. Correlation of filtration resistance and ~irus retention A smaller filtration resistance is preferable for low pressure operation or energy saving in water and wastewater treatment. We plotted virus retention against filtration resistance in Fig. 5. Retentions of viruses by nanofiltration membranes are not higher than those by ultrafiltration membranes though nanofiltration membranes have very high filtration resistance. Nanofiltration membranes are suitable for removing salts but not for removing viruses. The retention of virus by ultrafiltration membranes are in
ND-
CPI0-10101
7
~
IRIS-3026 U-2020 IRIS-3065 40,000
6 CO. 5 O
IRIS-305~---
I~
,e, 3-TRTS-3065 ~-F
A1-AO 16 am, Nuclepore 15 nm and CPl0-1010 membranes gave very high retention of Q/3. The
i0 I0
•UP-20 •Cefilt
|I &
......
u~.;~uu •
i0I II Resistance
NTR-729HF
•NTU-OZK
'
4.6. Microfiltration membranes
~ NTR~759HR
1~R-7410
/ NTU-3030 20,000
O. l~J.m \TS-100~
~
• Ai-AO,16nm
UKT'10NTR_7250 ~•
IRIS-3065
4-~oo,ol o 2
Nuclep°reTl5nm~
'
i1012
T
• VSWP
Nuclepore, 50nm
I 1013 10I 14 of m e m b r a n e [ m - ~
Fig. 5. Correlation between filtration resistance and retention of Q/3.
T. Urase et al. / Journal of Membrane Science 115 (1~96) 21-29
the range from 2 - 6 in the logarithmic retention coefficient. High resistance membranes were not necessarily high retention membranes in terms of virus retention. Nuclepore 15 nm membranes and A1-AO 16 nm membrane have high retention for virus but also high filtration resistance. CPI0-1010 membrane gave high retention with relatively low filtration resistance. The example of CP10-1010 shows the possibility that we can develop high virus retention membranes with low filtration resistance.
5. Conclusion
We measured virus retention by 27 types of membranes including microfiltration membranes, ultrafiltration membranes and nanofiltration membranes by employing coliphage Q/3 and T4 as model viruses. No tested pieces of ultrafiltration membranes and nanofiltration membranes were complete barriers for the Q/3 virus (23 nm in diameter) though the retention was very high such as in the range of 9999.9999~,~ :. The analysis of the retention of different size materials confirmed our assumption that the leakage of viruses is caused by abnormally large pores which are not included in the main pore size distribution. The diameter of the abnormal pores was estimated from the difference in the retention of different coliphages. Inorganic ceramic ultrafiltration membranes have abnormal pores but the diameter of the abnormal pores is small. Module configuration
29
may not be a major factor affecting virus retention. Some types of microfiltration membranes have shown higher retention than ultrafiltration membranes and nanofiltration membranes. This suggests the possibility that we can develop high virus retention membranes with low filtration resistance.
References [1] A. Ketratanakul and S. Ohgaki, Indigenous coliphages associated with suspended solids in activated sludge process, Water Sci. Technol., 21(3) (1989) 73-78. [2] C.A. Sorber, Virus rejection by reverse osmosismltrafiltration processes, Water Res., 6 (1972) 1377-1388. [3] K, Nishimura. K. Kawamura and Y, Magara, Coliphage re.iection under ultra membrane filtration ~)f activated sludge suspension, J, Jpn. Soc. Water Environ., 17(3) (1994) 187- 196 (in Japanese). [4] T, Urase, K. Yamamoto and S. Ohgaki, Effect of pure size distribution ot ultrafiltration membranes on virus rejection in crossflow conditions, Water Sci. Technol.. 3t)(9) ~ 1994) 199208. [5] R. Hull, F. Brown and C. Payne, Virology I)irectory and Dictionary of animal, bacterial anti plant viruses, Macmillan, 1989. [6] T. Urase. K. Yamamoto and S. Ohgaki, Evaluation of virus removal in membrane separation processes by using coliphage Q/3. Water Sci. Technol., 28(7)(1994)t)-lS. [7] T. Tsurumi, T. Sato, N. Osawa, H. Hitaka, T. Hirasaki. K. Yamaguchi, 5. Hamamoto, S. Manabe, T. Yamasaki and N. Yamamoto, Slructure and filtration performances ~f impr{wed cuprammonium regenerated cellulose hollow fiber flw xirus removal, Polym. J.. 22(12)(1990) 1085-. I l/)0. [8] S. Nakao and S. Kimura, Analysis of ~,olutes rejection in ultrafiltrati,m, J. Chem. Eng. Jpn.. 14(1) 11981 ) 32 37.
Journal of MembraneScience 115 (1996) 21-29
Effect of pore structure of membranes and module configuration on virus retention Taro Urase, Kazuo Yamamoto, Shinichiro Ohgaki Department of Urban Engineering, University of Tolt3'o, 7-3-1 Hongo, Bunk~'o-kn, Tokyo 113, Japan
Received 31 July 1995:accepted 3 October 1995
Abstract We measured virus retention by many types of membranes including microfiltration membranes, ultrafiltration men> branes and nanofiltration membranes. We succeeded in evaluating quantitatively virus retention in a very high retention range by employing coliphage Q/3 and T4 as model viruses. Q/3, which is the smaller virus in this study, penetrated all tested pieces of ultrafiltration membranes and nanofiltration membranes, though the retention was very high such as in the range of 99-99.9999%. The analysis of the polyethylene glycol retention data has shown that the leakage of viruses is caused by abnormally larger pores which are not included in the main pore size distribution. The diameter of the abnormal pores was estimated from the results of retention of different coliphages. The leakage of Q/3 was also observed in the case of inorganic ceramic ultrafiltration membranes, but T4, which is larger than Q/3, cannot penetrate them. Some types of microfiltration membrane have shown higher retention than ultrafiltration membranes and nanofiltration membranes. This suggests the possibility that we can develop high virus retention membranes with low filtration resistance. Kevwords: Pore size distribution:Virus retention:Water treatment:Membrane structure: Ultrafiltrationmembrane
1. Introduction Membrane processes are widely used in water and wastewater treatment including wastewater reuse systems. Chlorination has been a most popular technique for quality control of treated water in terms of microbiological safety. However, chlorination has several disadvantages in by-products and in controlling dosage in small scale plants. Moreover, viruses are generally more resistant to chlorine than bacteria. Virus retention by membrane processes without disinfection is attractive when we apply them to drinking water purification and wastewater reclamation. Water treated by membrane processes is thought to contain no suspended solids and consequently no pathogenic bacteria. Pathogenic viruses would also
be removed by the membrane processes because some of the viruses are adsorbed onto suspended solids in the environment [1]. When we want to discuss retention of viruses which are not adsorbed onto suspended solids, we must examine the virus retention performance of membrane processes experimentally. On the basis of nominal cutoff size of membranes, viruses should be completely retained by ultrafiltration membranes or nanofiltration membranes. But some reports have shown incomplete retention of viruses by these membranes [2,3]. Our earlier research also indicated that ultrafiltration membranes cannot be complete barriers for viruses because they have abnormally larger pores although the number of the abnormal pores is very small [4].
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0376-7388(95)00269-3
22
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
In this study, we investigated virus retention by many types of membranes ranging from microfiltration to nanofiltration membranes. Anopore membranes, Nuclepore membranes and inorganic membranes, which are said to have narrow pore size distribution, were also tested. Various module configurations were also tested such as flat sheet membranes, tubular membranes, monolith membranes, disk holder membranes and hollow fiber membranes.
branes appear last in the table. We selected many types of membranes such as track etched membranes, anodic oxidation membranes, sol-gel membranes as well as phase inversion membranes and composite membranes which are the popular structure of ultrafiltration membranes and nanofiltration membranes. The module configuration was different according to the shape of the membrane used. All membranes were tested in crossflow conditions except for the holder type membranes.
2. Experimental
2.2. Viruses
2.1. Membranes
In order to discuss quantitatively the virus retention even at very high level of retention, we seeded coliphages to the water. Coliphages are viruses which can attack Escherichia coli. We used two types of coliphages; one is Q/3 which is 23 nm in diameter and the other is T4 which is 80 nm in diameter [5].
The membranes used in this study are listed in Table I. The order in the table corresponds to the nominal cutoff size of the membranes. Microfiltration membranes appear first and nanofiltration mere-
Table 1 List of the membranes used in this study Membrane Name
Nominal cutoff
Manufacturer
Material
Preparation
Module
IRIS-3065 Nuclepore A1-AO VSWP Anopore A1-AO Nuclepore CPI 0-1010 TS- 1000 UK-200 IRIS-3065 Cefilt USY-5 IRIS-3065 NTU-OZK IR1S-3038 IRIS-3026 NTU-3030 NTU-2020 Cefilt UP-20 UK- 10 USY- 1 NTR-7410 NTR-7250 NTR-729HF NTR-759HR
100 nm 50 nm 37 nm 25 nm 20 nm 16 nm 15 nm 10 nm 1 000 000MW 200 000MW 200 000MW 50 000MW 50 000MW 40 000MW 30 000MW 20 000MW 20 000MW 20 000MW 20 000MW 20 000MW 20 000MW 10 000MW l 0 000MW 20% Desalin. 65% Desalin. 93% Desalin. 99% Desalin.
Mitsui Petrochem Costar Nakao Lab. Millipore Whatman Nakao Lab. Costar Toray Toso Advantech Mitsui Petrochem Nihon Gaishi Advantech Mitsui Petrochem Nitto Denko Mitsui Petrochem Mitsui Petrochem Nitto Denko Nitto Denko Nihon Gaishi Advantech Advantech Advantech Nitto Denko Nitto Denko Nitto Denko Nitto Denko
Polyvinylidene fluoride Polycarbonate Aluminum oxide Cellulose acetate Aluminum oxide Aluminum oxide Polycarbonate Polyacrylonitrile Polysulfone Polysulfone Polyvinylidene fluoride Ceramic Poly sulfone Polyvinylidene fluoride Polysulfone Polyacrylonitrile Sulfonated polysulfone Polysulfone Polyolefine Ceramic Aromatic polyamide Polysulfone Polysulfone Sulfonated polysulfone Polyvinyl alcohol Polyvinyl alcohol Aromatic polyamide
Phase inversion Track-etch Anodic oxidation Phase separation Anodic oxidation Anodic oxidation Track-etch Phase inversion Phase inversion Phase inversion Phase inversion Sol-gel Phase inversion Phase inversion Phase inversion Phase inversion Phase inversion Phase inversion Phase inversion Sol-gel Phase inversion Phase inversion Phase inversion Composite Composite Composite Composite
Flat sheet(U) Flat sheet25 Flat sheet45 Flat sheet25 Holder Flat sheet45 Flat sheet25 Hollow Fiber Flat sheet62 Flat sheet62 Flat sheet(U) Monolith Holder Flat sheet(U) Hollow Fiber Flat sheet(U) Flat sheet(U) Tubular Tubular Monolith Flat sheet62 Flat sheet62 Holder Flat sheet(R) Flat sheet(R) Flat sheet(R) Flat sheet(R)
MW: Molecular weight cutoff size (Dalton).
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29 The size and structure of Q/3 closely resembles enteric viruses which are most important in considering water reclamation. The concentration of coliphages was measured by the double layer agar method [1] with the host cell of Escherichia coil K I 2 A / A (F +) as to Q/3, and Escherichia coli B as to T4. 2.3. Experimental set-up and sampling In real environmental samples the presence of suspended solids increases the retention of viruses due to the gel-layer effect [6]. In this paper viruses were suspended in 1 m M phosphate buffer solution to avoid the gel-layer effect. W e used pure water for 1 h filtration before virus retention tests started. Virus retention tests were conducted in the crossflow conditions except for the holder type membranes. Crossflow velocity was typically 0.8 to 1.2 m / s . Applied pressure was typically controlled at 2 0 - 5 0 kPa with the exception of 98 kPa for VSWP, A1-AO, UK-10, UP-20, TS-1000, UK-200 membranes, 196 kPa for Nuclepore membranes and 500 kPa for nanofiltration membranes. The pressures were determined based on the low flux operation which is usual in water and wastewater treatment. 2.4. Poh,ethvlene glycol retention tests In order to know the retention performance of the membranes, we examined the retention of polyethylene glycols (PEGs) whose molecular weights are 20 000, 4 000 and 600. The measurements were carried out in 0.5 g / l solution under the same operating conditions as the measurement of retention of viruses. The concentrations of PEGs were measured by HPLC with a RI detector. 2.5. Sah retention tests Salt retention tests were carried out for nanofiltration membranes by using composite salt solutions of K H 2 P O 4 44 r a g / l , NaNOz 99 mg/1, KNO 3 144 mg/1, NaCl 50 m g / 1 and M g S O a • 7 H 2 0 77 m g / 1 under the same operating conditions as the measurement of retention of viruses. Salt retention was characterized by anion retention. Anion concentrations were measured by HPLC by the indirect UV method.
23
2.6. Resistance q[ membranes The filtration resistances of the membranes were calculated by pure water permeability as follows. AP R
Ill
- -
r/J,
where R m is the filtration resistance of the membrane, ,.IP the applied pressure, r 1 is viscosity and J , is volume flux. 2. Z El'aluation o f retention &tta The permeate and bulk solution were sampled 1 h after the virus retention test started. The logarithmic retention coefficient was defined by the following equation in order to evaluate quantitatively the very high retention level [7]. Cb qb = log I~.-~4
where Cb is the concentration in the bulk, Cp the concentration in the permeate and qb is the logarithmic retention coefficient. The concentration of the viruses in the seeded bulk solution was 107--10 s P F U / m l . We succeeded in measuring a retention as high as 7 of the logarithmic retention coefficient because the detection limit of coliphages was 1-10 PFU/ml. 2.8. Correction o f concentration polarization Retention of solutes can be evaluated in two different manners [8]. One is observed retention I ( C p / Q ) and the other is intrinsic retention I ( C p / C m ) where C m is the concentration at the membrane surface. In this paper, we used only observed retention which ignores concentration polarization unless otherwise mention. This is because concentration polarization of colloidal materials such as viruses is not so well understood and flow conditions were not clear for some of the modules used.
3. Results
3.1. PEG retention tests The results of retention of polyethylene glycols are shown in Table 2. Microfiltration membranes
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
24
Table 2 Retention of polyethylene glycols (%) Name
Nominal cutoff
IRIS-3065 Nuclepore AI-AO VSWP Anopore A1-AO Nuclepore CP10-1010 TS- 1000 UK-200 IRIS-3065 Cefilt USY-5 IRIS-3065 NTU-OZK IRIS-3038 IRIS-3026 NTU-3030 NTU-2020 Cefilt UP-20 UK- 10 USY- 1 NTR-7410 NTR-7250 NTR-729HF NTR-759HR
100 nm 50 nm 37 nm 25 nm 20 nm 16 nm 15 nm 10 nm 1 000 000MW 200 000MW 200000MW 50 000MW 50 000MW 40 000MW 30000MW 20000MW 20 000MW 20 000MW 20 000MW 20 000MW 20 000MW 10 000MW 10 000MW 20% Desalin. 65% Desalin. 93% Desalin. 99% Desalin.
R m ( × 1012 m - l) 0.05 7.3 9.1 14 91 100 4.0 0.39 0.88 0.057 3.1 0.48 4.5 0.41 3.0 1.4 3.8 6.7 3.8 11 16 68 94 140
PEG 20 000
PEG 4 000
PEG 600
0
0
0
4.8 0 50 44 0 0
0 0 27 23 0 0
0 0
14 0 19 48 21 95 75 61 88 0 49 0 74 > 99 > 99 > 99
1 0 11 0 5.5 63 26 17 49 0 36 0 62 98 98 > 99
Flux a ( × 10 5 m / s ) 13
0.17 0.13 0.18 0.81 28
0 0
0 0 6 0 2 14 0 0 4.4 0 2 0 24 99.2 99.8 99.9
1.6 8 2.9 2.5 0.37 0.52 0.66 0.72 2.9 1.1 2 2.0 0.89 0.46 0.22
a Volume flux during measurement of retention of PEGs.
gave almost zero retention, with the exception of the A1-AO 16 nm membrane and the Nuclepore 15 nm membrane. Ultrafiltration membranes gave lower retention values compared to the nominal cutoff size probably due to concentration polarization effect. For example, intrinsic retention after correction for concentration polarization for the IRIS-3038 membrane with nominal molecular weight cutoff size of 20 000 was calculated to be 88% retention for the PEG 20000, while observed retention was only 21%. The retention of polyethylene glycols by nanofiltration membranes was almost 100% with the exception of the NTR-7410 membrane. 3.2. Salt retention tests
The results of salt retention tests are shown in Table 3. The observed retention data for the chloride ion was lower than the nominal desalination value
because of the low pressure operation. In reverse osmosis it is understood that low pressure operation decreases retention because salt flux is relatively constant compared to volume flux. The nominal value is obtained for 1.5 MPa operation, while our testing method, which is common for the virus retention tests, adopted 0.5 MPa operation for nanofiltration membranes.
Table 3 Retention of salts by nanofiltration membranes (%) Name
NTR-7410 NTR-7250 NTR-729HF NTR-759HR
Type of anions
Flux a
PO4-P C1- NO~- NO 3 SO4:
(×10-Sm/s)
30 99 99 99
6.2 0.83 0.52 0.36
0 58 84 97
0 37 70 92
0 32 76 95
48 98 96 99
a Volume flux during measurement of retention of salts.
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
of Q~
Number of pieces tested Membrane Nominal Cutoff '~ 1 2 Name T I I 1RIS-3065 100nm 3 [~ ~ I Nuclepore 50nm 2 i ~I A1-AO 37nm 1 • VSWP 25nm 1 • Anopore 20rim 3 A1-AO 16nm 1 Nuclepore 15nm 6 CP10-1010 10rim 1 ¥~"i'F66 ............. ] ' ; ~ 6 6 ~ " ' 2 -
UK-200 IRIS-3065 Cefilt USY-5 IRIS-3065. NTU-OZK IRIS-3038. IRIS-3026" NTU-3030* NTU-2020* Cefilt UP-20 UK-10 SY-1
3
4
6
5
I
I
I I I I ..........................................................................................................................
........... ~
200,000MW 3 200,000MW 1 50,000MW 1 50,000MW 1 I 40,000MW 13 30,000MW 1 20,000MW 12 20,000MW 5 I 20,000MW 4 I-•-I 20,000MW 3 20,000MW 1 20,000MW 1 10,000MW 3 1 0 ~ _ . . . . 1 . .........................................................0......................................................................................
IW-1
i~:~i~i5 .......... ~ i ~ s a i ' i n .
NTR-7250 NTR-729HF NTR-759HR
25
~
•
65%Desalin. 3 93%Desalin. 2 99%Desalin. 1
I~ •
I
i
I
,
I
I
i //_2
1
2
3
4
5
6
7
ND
Fig. 1. Logarithmic retention coefficient of Q/3. * The Retention of Q/3 by IRIS-3065, 3038, 3026, NTU-3030, 2020 has been published in
[4].
Number of pieces Membrane Nominal Cutoff Name
~ 1 I
of
(I) 2 I
3 I
4 I
T4 5 I
6 I
7 I
, ND
//--1
IRIS-3065 Nuclepore AI-AO VSWP Anopore AI-AO Nuclepore
100nm 1 • 50nm 2 37nm 1 25nm 1 20nm 3 16nm 1 15nm 6 ...C.~IO:.IOlO ................... !.9_n_m___! .......................................................................................................................... TS- 1000 1,000,000MW 0 UK-200 200,000MW 0 IRIS-3065 200,000MW 0 Cefilt 50,000MW 1 USY-5 50,000MW 1 I A IRIS-3065 40,000MW 4 , v NTU-OZK 30,000MW 1 IRIS-3038 20,000MW 4 I IRIS-3026 20,000MW 0 NTU-3030 20,000MW I NTU-2020 20,000MW 0 Cefilt 20,000MW 1 UP-20 20,000MW 1 UK-10 10,000MW 2 NTR-7410 NTR-7250 NTR-729HF NTR-759HR
20%Desalin. 65%Desalin. 93%Desalin. 99%Desalin.
1 1 3 2
• • • • •
i
• •
I
I •
i
I
I
i
i
J
1
2
3
4
5
6
Fig. 2. Logarithmic retention coefficient of T4.
i//_2 7
ND
26
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
3.3. Virus retention tests 3.3.1. Q[3
The results of retention of Q/3 are shown in Fig. 1. Microfiltration membranes with larger nominal pore size than 20 nm gave very low retention, while the membranes with smaller nominal pore size than 16 nm gave very high retention. In the case of ultrafiltration membranes, Q/3 can penetrate all types of membranes and all pieces of membranes. The nanofiltration membranes also gave incomplete retention results. Nominal cutoff size had no significant effect on retention of Q/3 in the case of ultrafiltration membranes and nanofiltration membranes. The typical logarithmic retention value by ultrafiltration membranes and nanofiltration membranes ranged from 3 to 6, which corresponds to retentions of 99.9 to 99.9999%. 3.3.2. T4
T4 is a larger virus than Q/3. The results of virus retention tests in Fig. 2 show generally higher retentions compared to the cases of Q/3. Several types of membrane retained T4 completely. However in the case of the IRIS-3065 membranes, NTU-3030 membranes and NTR-729HF membranes T4 was retained only at the same level as was observed for Q/3. The same level retention can be explained by abnormal pores which is discussed later.
4. Discussion 4.1. Factors affecting virus retention
The adsorption of viruses on the membrane surface is discussed here. Q/3 is negatively charged in water because its isoelectric point is 5.3. Because we used no strongly positive charged membranes, the electrostatic force may not be a major factor to promote adsorption. Even if we assume that 90% of the viruses are retained by adsorption mainly caused by hydrophobic interactions, the retention increases by as much as only 1 in the logarithmic retention by adsorption. The adsorption never decreases retention. Consequently, the leakage of viruses through ultrafiltration membranes and nanofiltration membranes cannot be explained by the adsorption. Only the size sieve will be a dominant factor in virus retention in a
very high retention range though other factors such as operating pressure and flow conditions in the bulk solution also affects retention of viruses. The results in Fig. 1 show that the retention by microfiltration membranes is closely related to their pore sizes. This suggests that the results are less affected by adsorption. If it is true that the separation mechanism is mainly size sieving, the discussion using coliphage can be generalized to the fate of human viruses. 4.2. Cause o f leakage o f viruses by ultrafiltration membranes and nanofiltration membranes
We examined many types of ultrafiltration membranes and nanofiltration membranes in this study. The results indicated that the virus leakage is not an exceptional phenomenon but an often observed typical phenomenon. We did experiments with many pieces of membrane of the same type. For example, 13 pieces were tested for retention of Q/3, and 4 pieces for retention of T4 in the case of IRIS-3065 membranes. The results seemed to be scattered but they never showed complete retention. IRIS-3038 membranes always show very high retention for T4. NTR-729HF membranes always gave lower retentions compared to other types of nanofiltration membranes though the retention of salts by NTR-729HF was higher than those by NTR-7410 or NTR-7250. We can consequently conclude the retention phenomena of viruses by membranes are not random phenomena but dependent on the types of membranes. Virus leakage was also observed in the case of holder type membranes. This leakage cannot be explained by the damage during installation of the membranes to the modules because these membranes are ready-packed membranes. In a previous paper [4], we had already come to the conclusion that some ultrafiltration membranes must have abnormally large pores. The reason is that log normal pore size distributions determined by polyethylene glycol retention data predicts higher retention for 23 nm particles than for experimental retention of Q/3 as shown in Fig. 3. Most of the Q/3 detected in the permeate did not pass through pores included in the log-normal pore size distribution. They passed through abnormally large pores. The number of abnormal pores was calculated to be of
27
T. Urase et a l . / Journal of Membrane Science 115 (1996) 21-29
Q)
8
0 k~ u~ 0 D
7
Calculated cutoff curve (IRIS-3038) d0--4.5nm o--0.1
6
0 4J ~J
0
~4 r6
0
5
m
4 3 2
-
Calculated cutoff cu (NTU-3030) /
5
-
dO--4.5.m
[
I
°2/i-/
i 0 1
/
,
2
5 Solute
,
I
10
I
20 25
diameter
[nm]
Fig. 3. Calculated cutoff curves by a pore model in which log normal pore size distribution was assumed, dO is mean pore diameter and ~r is standard deviation in log normal distribution. The parameters of the distribution were obtained by PEG rejection tests. This figure was rewritten by using the model in [4].
the order of l0 9 of the total number of pores in the case of the IRIS-3038 membrane. The abnormal pores have a vital consequence in discussing virus retention though they have no significant effect on normal retention properties such as retention of polyethylene glycols. In this paper, the results using another type of coliphage T4 helped us to estimate the size of abnormal pores. If retention of T4 is complete by a membrane which leaked Q/3, the diameter of abnormal pores lies between 23 and 80 nm. In the case of the NTU-3030 membrane the diameter of the abnormal pores would be larger than 80 nm because the retention of T4 is as high as the retention of Q/3 by the NTU-3030. In the case of IRIS-3038 membrane the diameter of the abnormal pores would be in the range between 23 to 80 nm because the retention of T4 by the IRIS-3038 is very high.
retention of PEG 20000 as general retention property of membranes. The figure shows no close correlation between virus retention and the general retch-
ND~ -c )CPI0-1010 " iNuc i epore 10rim
o
QAI -AO 16nm
5
io~_i0
i IRIS-3026
o
l
l~R-759HR
1
3
Cefilt
20,000
NTR-729HF
ONTU-OZK 2
P VSWP 25nm
~ C e f i l t 50,000 l ' - ~ U P - 2 0 , USY-1, USY-B, TS-1000 N u c l e p o r e 50rim, IRIS-3065 100rim
0~
4.3. The correlation of retention of PEG and Q fl The retention of Q/3 was plotted against the retention of PEG 20000 in Fig. 4. We can regard
7
iI t i ~ 50
I
L
tlll t i ~,k
90 95 Retention of PEG#20,000
l 1001%]
Fig. 4. Correlation between the retention of PEG and retention ol
Q/3.
28
T. Urase et al. / Journal of Membrane Science 115 (1996) 21-29
tion property. The general retention property is determined by the main pore size distribution while virus retention is determined by abnormal pores. No close relation between retention of PEG and retention of virus suggests that abnormal pores are not included in the main pore size distribution. The PEG retention data also supported the view that the cause of leakage by ultrafiltration membranes and nanofiltration membranes is abnormal pores which are not included in the main size distribution.
4.4. bzorganic ultrafiltration membranes Characteristics of abnormal pores of inorganic membranes are discussed here. Inorganic membranes are said to have a narrow pore size distribution compared to the organic membranes. Our results show that ceramic membranes with nominal molecular weight cutoff size of 20 000 and 50 000 retained T4 completely. However, the retention of Q/3 by these membranes are not different from those of organic ultrafiltration membranes. The diameter of abnormal pores of inorganic ceramic ultrafiltration membranes is small even though these membranes also have abnormal pores.
4.5. Hollow fiber membranes In the case of flat sheet membranes, seal materials in the module can disturb the membrane surface. Consequently, there is a possibility that seal materials give damage or cause abnormal pores in the membranes, while in the case of hollow fiber membranes, seal materials do not directly touch the membranes. However our results indicate that the module configuration would not be the major factor affecting virus retention. The result of the CP10-1010 membrane which is a hollow fiber type retained viruses completely while the result of the NTU-OZK which is also used in hollow fiber configuration gave low retention as was observed by using other organic ultrafiltration membranes.
retentions by these membranes are higher than those by ultrafiltration membranes and nanofiltration membranes. These membranes are really microfiltration membranes because the retention of the PEGs by these membranes are not high compared to the retention by ultrafiltration membranes and nanofiltration membranes. The high retention by these microfiltration membranes may be due to differences in membrane pore structure. Ultrafiltration membranes and nanofiltration membranes are membranes with skin layers. Slight defects in the skin layer may cause the presence of abnormal pores and consequently cause virus leakage. On the other hand, microfiltration membranes have a definite thickness for the separation layer. This thickness may prevent formation of defective pores.
4.7. Correlation of filtration resistance and ~irus retention A smaller filtration resistance is preferable for low pressure operation or energy saving in water and wastewater treatment. We plotted virus retention against filtration resistance in Fig. 5. Retentions of viruses by nanofiltration membranes are not higher than those by ultrafiltration membranes though nanofiltration membranes have very high filtration resistance. Nanofiltration membranes are suitable for removing salts but not for removing viruses. The retention of virus by ultrafiltration membranes are in
ND-
CPI0-10101
7
~
IRIS-3026 U-2020 IRIS-3065 40,000
6 CO. 5 O
IRIS-305~---
I~
,e, 3-TRTS-3065 ~-F
A1-AO 16 am, Nuclepore 15 nm and CPl0-1010 membranes gave very high retention of Q/3. The
i0 I0
•UP-20 •Cefilt
|I &
......
u~.;~uu •
i0I II Resistance
NTR-729HF
•NTU-OZK
'
4.6. Microfiltration membranes
~ NTR~759HR
1~R-7410
/ NTU-3030 20,000
O. l~J.m \TS-100~
~
• Ai-AO,16nm
UKT'10NTR_7250 ~•
IRIS-3065
4-~oo,ol o 2
Nuclep°reTl5nm~
'
i1012
T
• VSWP
Nuclepore, 50nm
I 1013 10I 14 of m e m b r a n e [ m - ~
Fig. 5. Correlation between filtration resistance and retention of Q/3.
T. Urase et al. / Journal of Membrane Science 115 (1~96) 21-29
the range from 2 - 6 in the logarithmic retention coefficient. High resistance membranes were not necessarily high retention membranes in terms of virus retention. Nuclepore 15 nm membranes and A1-AO 16 nm membrane have high retention for virus but also high filtration resistance. CPI0-1010 membrane gave high retention with relatively low filtration resistance. The example of CP10-1010 shows the possibility that we can develop high virus retention membranes with low filtration resistance.
5. Conclusion
We measured virus retention by 27 types of membranes including microfiltration membranes, ultrafiltration membranes and nanofiltration membranes by employing coliphage Q/3 and T4 as model viruses. No tested pieces of ultrafiltration membranes and nanofiltration membranes were complete barriers for the Q/3 virus (23 nm in diameter) though the retention was very high such as in the range of 9999.9999~,~ :. The analysis of the retention of different size materials confirmed our assumption that the leakage of viruses is caused by abnormally large pores which are not included in the main pore size distribution. The diameter of the abnormal pores was estimated from the difference in the retention of different coliphages. Inorganic ceramic ultrafiltration membranes have abnormal pores but the diameter of the abnormal pores is small. Module configuration
29
may not be a major factor affecting virus retention. Some types of microfiltration membranes have shown higher retention than ultrafiltration membranes and nanofiltration membranes. This suggests the possibility that we can develop high virus retention membranes with low filtration resistance.
References [1] A. Ketratanakul and S. Ohgaki, Indigenous coliphages associated with suspended solids in activated sludge process, Water Sci. Technol., 21(3) (1989) 73-78. [2] C.A. Sorber, Virus rejection by reverse osmosismltrafiltration processes, Water Res., 6 (1972) 1377-1388. [3] K, Nishimura. K. Kawamura and Y, Magara, Coliphage re.iection under ultra membrane filtration ~)f activated sludge suspension, J, Jpn. Soc. Water Environ., 17(3) (1994) 187- 196 (in Japanese). [4] T, Urase, K. Yamamoto and S. Ohgaki, Effect of pure size distribution ot ultrafiltration membranes on virus rejection in crossflow conditions, Water Sci. Technol.. 3t)(9) ~ 1994) 199208. [5] R. Hull, F. Brown and C. Payne, Virology I)irectory and Dictionary of animal, bacterial anti plant viruses, Macmillan, 1989. [6] T. Urase. K. Yamamoto and S. Ohgaki, Evaluation of virus removal in membrane separation processes by using coliphage Q/3. Water Sci. Technol., 28(7)(1994)t)-lS. [7] T. Tsurumi, T. Sato, N. Osawa, H. Hitaka, T. Hirasaki. K. Yamaguchi, 5. Hamamoto, S. Manabe, T. Yamasaki and N. Yamamoto, Slructure and filtration performances ~f impr{wed cuprammonium regenerated cellulose hollow fiber flw xirus removal, Polym. J.. 22(12)(1990) 1085-. I l/)0. [8] S. Nakao and S. Kimura, Analysis of ~,olutes rejection in ultrafiltrati,m, J. Chem. Eng. Jpn.. 14(1) 11981 ) 32 37.