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Membrane integrity monitoring in large-scale systems
News & Views M e m b r a n e bubble p o i n t
Membrane integrity monitoring in large-scale systems Membrane systems are being increasingly used in the w a t e r i n d u s t r y to p r o v i d e a p h y s i c a l b a r r i e r to b a c t e r i a a n d o t h e r p a t h o g e n s s u c h a s Giardia and Cryptosporidiurru T h e u s e o f m e m b r a n e s a s a d e f e n c e a g a i n s t t h e s e p r o t o z o a n s is i m p o r t a n t , a s t h e o o c y s t s of t h e s e p a t h o g e n s a r e r e s i s t a n t to c h l o r i n a t i o n u s e d in w a t e r t r e a t m e n t p r o c e s s e s . Previous tests have demonstrated that these organisms are very durable.Cryptosporidium viability is n o t a f f e c t e d b y a n 18 h r e x p o s u r e to s o d i u m h y p o c h l o r i t e a t d o s e s u p t o 3 0 , 0 0 0 p p m . In a d d i t i o n , concentrations of sodium hypochlorite that are needed to e l i m i n a t e t h e o o c y s t , u p to 7 0 0 , 0 0 0 p p m , o b v i o u s l y c a n n o t b e u s e d for m u n i c i p a l w a t e r t r e a t m e n t . To e l i m i n a t e t h e r i s k of d r i n k i n g w a t e r contamination, particle counters and turbidity m o n i t o r s a r e two m e t h o d s t h a t a r e often e m p l o y e d to m e a s u r e t h e q u a l i t y of r a w a n d t r e a t e d w a t e r s . However, t u r b i d i t y a n d p a r t i c l e m o n i t o r i n g m e t h o d s a r e n o t c a p a b l e of m e a s u r i n g w a t e r q u a l i t y c h a n g e s a t t h e levels r e q u i r e d to e n s u r e r e m o v a l of p a t h o g e n s s u c h a s Giardia and Cryptosporidiurru A d d e d to t h i s , is t h e e x p e n s e a n d d i f f i c u l t y in d e t e c t i n g t h e s e p r o t o z o a through analytical means, and the as yet unresolved s e a r c h for a s u i t a b l e s u r r o g a t e . G i v e n t h i s , h o w t h e n is it p o s s i b l e to s a y t h a t p o t e n t i a l l y h a r m f u l o r g a n i s m s a r e b e i n g r e m o v e d w h e n t h e r e is n o m e a s u r a b l e c h a n g e i n f i l t e r e d w a t e r quality?. O n e a n s w e r is t h e u s e of t e c h n i q u e s t h a t m e a s u r e m e m b r a n e i n t e g r i t y d i r e c t l y , i n d e p e n d e n t of w a t e r quality p a r a m e t e r s . T h i s r e p r e s e n t s a s h i f t in t h i n k i n g f r o m w a t e r q u a l i t y monitoring to process control. Particle counters, although a useful quality monitor, a r e n o t a q u a n t i t a t i v e m e a s u r e of t h e i n t e g r i t y of t h e membrane barrier, and are therefore not an integrity monitoring technique. Two q u a n t i t a t i v e i n t e g r i t y m e a s u r e m e n t t e c h n i q u e s a r e t h e p r e s s u r e d e c a y (PDT) a n d diffusive a i r flow (DAF) t e s t s . T h e s e t e c h n i q u e s a r e a p p l i e d b y M e m c o r in a n i n t e g r i t y c o n t r o l p r o c e s s k n o w n a s M e m s u r e . T h e PDT w a s first a p p l i e d a s a n i n t e g r i t y m o n i t o r i n g tool for c o n t i n u o u s m i c r o f i l t r a t i o n s y s t e m s in t h e e a r l y 1980's. W h e r e a s t h e D A F t e s t w a s i n t r o d u c e d a s a n o p t i o n for f u r t h e r i m p r o v e m e n t s in s e n s i t i v i t y in 1991 - 1992. A c c o r d i n g to M e m c o r , v a r i a t i o n s in t h e t e s t s h a v e b e e n u s e d i n t h e p h a r m a c e u t i c a l i n d u s t r y to v a l i d a t e t h e i n t e g r i t y of c a r t r i d g e filter u n i t s for m a n y y e a r s . However, t h e t e s t s h a v e n o t b e e n a p p l i e d to monitoring large-scale continuous membrane systems until recently.
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B o t h t h e PDT a n d D A F t e s t s r e l y o n t h e p r i n c i p l e of t h e m e m b r a n e " b u b b l e p o i n t " to m o n i t o r t h e i n t e g r i t y of t h e s y s t e m . The n o m i n a l p o r e size for t h e M e m c o r microfiltration m e m b r a n e is 0 . 2 p m . Liquid in p o r e s of t h e s e d i m e n s i o n s a r e s u b j e c t to significant c a p i l l a r y forces a n d a relatively h i g h p r e s s u r e of a i r is r e q u i r e d to o v e r c o m e t h e s e forces a n d to p u s h liquid o u t of t h e p o r e s . C a p i l l a r y t h e o r y i n d i c a t e s t h a t t h e air p r e s s u r e r e q u i r e d to force liquid from t h e p o r e s is i n v e r s e l y p r o p o r t i o n a l to t h e p o r e diameter. T h u s , t h e l a r g e r t h e p o r e t h e lower t h e p r e s s u r e r e q u i r e d to d i s p l a c e t h e liquid. If a i r is a p p l i e d to o n e s i d e of t h e m e m b r a n e a n d t h e p r e s s u r e slowly i n c r e a s e d , t h e n a i r will b r e a k t h r o u g h first a t t h e p o r e w i t h t h e l a r g e s t d i a m e t e r . T h i s p r e s s u r e is k n o w n a s t h e b u b b l e p o i n t . T h e b u b b l e p o i n t of t h e M e m c o + m i c r o f i l t r a t i o n m e m b r a n e is in t h e r a n g e of 2 0 0 to 2 5 0 k P a . At a i r p r e s s u r e s b e l o w t h e b u b b l e p o i n t t h e r e will b e n o a i r flow t h r o u g h a n i n t e g r a l m e m b r a n e o t h e r t h a n a s m a l l flow d u e to d i f f u s i o n t h r o u g h l i q u i d in t h e m e m b r a n e wall. In c o n t r a s t t h e a i r flows t h r o u g h d e f e c t s s u c h a s a l e a k t h r o u g h a n ' O ' r i n g s e a l or a b r o k e n fibre, is c o m p a r a t i v e l y large. T h i s f e a t u r e of m e m b r a n e s is e x p l o i t e d b y b o t h t h e PDT a n d DAF s y s t e m s in o r d e r to a c c u r a t e l y c h a r a c t e r i s e t h e i n t e g r i t y of t h e C M F s y s t e m .
PDT a n d DAF t e s t i n g P r e s s u r e d e c a y t e s t i n g is p e r f o r m e d b y a p p l y i n g a i r a t a p r e s s u r e b e l o w t h e b u b b l e p o i n t to o n e s i d e of t h e m e m b r a n e , i s o l a t i n g , t h e n m e a s u r i n g t h e d e c l i n e in p r e s s u r e over t i m e . T h e D A F t e s t is f u n d a m e n t a l l y s i m i l a r to t h e PDT in t h a t t h e y b o t h m e a s u r e a i r flow t h r o u g h d e f e c t s during the integrity test. However, rather than measure air leakage as a pressure decay rate, the D A F t e s t d o e s t h i s b y f l l i n g t h e s h e l l of t h e m o d u l e w i t h l i q u i d a n d a l l o w i n g t h e a i r l e a k a g e to d i s p l a c e l i q u i d f r o m t h e s h e l l . T h e f i o w r a t e of d i s p l a c e d l i q u i d is t h e n a d i r e c t m e a s u r e of t h e m e m b r a n e i n t e g r i t y . W i t h r e g a r d s to t h e M e m s u r e s y s t e m f r o m M e m c o r , t h e m a i n a d v a n t a g e s of t h e D A F t e s t o v e r t h e PDT a r e t h a t i t t e n d s to b e l e s s s e n s i t i v e to a n y e x t e r n a l a i r l e a k s , is e a s i e r to m e a s u r e a c c u r a t e l y t h a n p r e s s u r e d e c a y , a n d d i f f u s i o n of a i r t h r o u g h l i q u i d in t h e m e m b r a n e w a l l c o n t r i b u t e s to a l e s s e r e x t e n t . T h e c o m b i n e d effect is t h a t t h e D A F t e s t t e n d s to b e m o r e s e n s i t i v e to c h a n g e s in i n t e g r i t y t h a n t h e PDT t e s t . T h e t r a d e - o f f for t h e i n c r e a s e d s e n s i t i v i t y is t h a t , a l t h o u g h r e a d i l y a u t o m a t e d , t h e DAF t e s t r e q u i r e s s o m e a d d i t i o n a l p i p e w o r k a n d f i t t i n g s i n o r d e r to m e a s u r e t h e d i s p l a c e d l i q u i d flowrate. B o t h t h e s e t e s t s a r e u s e f u l for m o n i t o r i n g t h e o v e r a l l i n t e g r i t y of a m e m b r a n e a r r a y . However, if a d r o p in i n t e g r i t y is d e t e c t e d , it is i m p o r t a n t to find o u t w h e r e t h e l e a k is o c c u r r i n g , so it c a n b e i s o l a t e d a n d r e p a i r e d . In M e m c o r s y s t e m s t h i s is d o n e u s i n g a s o n i c a n a l y s e r w h i c h is t u n e d i n t o t h e f r e q u e n c i e s of b u b b l e s p r o d u c e d d u r i n g i n t e g r i t y t e s t i n g . M o d u l e s w i t h d a m a g e d o r b r o k e n f i b r e s give significantly higher noise response than integral membrane modules.
M e m b r a n e T e c h n o l o g y No. 9 5
News & Views Potential applications
Giardia and Cryptosporidium is found. Failing this, PDT and DAF are two m e t h o d s that should come u n d e r consideration for the safe provision of drinking water, although at present no individual technique is universally accepted.
The integrity monitoring m e t h o d s described here are well suited to drinking water plants installed in remote or small communities. S u c h plants are usually small, package plants that are not staffed full-time. In these cases advance w a r n i n g of problems to allow time for response is critical. The PDT and DAF methods can monitor integrity and indicate a potential problem well before the risk of p a t h o g e n b y p a s s becomes significant. For example, if the plant is required to remove 4 log of Giardia, the PDT alarm level can be set to trigger w h e n the m e a s u r e d result drops below, say 4.5 log. This indicates to the operator that corrective action needs to be taken, either t h r o u g h isolation of modules, or their removal a n d repair. T h r o u g h o u t this time the plant will consistently achieve greater than 4 log, allowing time for a planned and contl-olled response to maintain water quality. In conclusion, it can be said that two of the most sensitive methods currently available for monitoring membrane integrity are the PDT and DAF tests. Both these tests are capable of monitoring small changes in membrane integrity, even in very large membrane arrays. Memcor report that the DAF test, for example, is capable of detecting a single broken fibre in a m e m b r a n e array containing over one million fibres. This enables operators to conh-ol m e m b r a n e integrity at levels well above 4 log removal of Giardia or Cryptosporidium, without relying on filtered water quality monitors s u c h as turbidity or particle counters. As outlined previously, a m e t h o d of c o n t i n u o u s online validation of 4-1og (99.99%) removal is a priority for the drinking water industry. At present, this m e a n s a physical r a t h e r t h a n biological technique. Perhaps this will c h a n g e if a n d w h e n a suitable surrogate for
For more information about Memsure contact:Memtec Research Ltd, Locked Mailbag No. 1, Windsor, N S W 2756, Australia. Tel: +61 45 776800;fax: +61 45 776804. E-mail: [email protected]
Commercial liquid membrane system In Membrane Technology (No. 85) almost a year ago, Tony Franken discussed whether liquid m e m b r a n e s were a viable industrial separation process or merely an academic exercise. He stated that from an industrial point of view, liquid m e m b r a n e s were an unlikely commercial option due to the lack of stability in s u c h systems. However, it seems t h a t a US c o m p a n y , C o m m o d o r e Separation Technologies (CST), disagrees with this analysis. It h a s recently i n t r o d u c e d w h a t is believed to be the first commercially available s u p p o r t e d liquid m e m b r a n e s y s t e m - - the SLiM 100. But, as Tony F r a n k e n stated in his original article, the stability of the liquid m e m b r a n e s y s t e m still
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M e m b r a n e T e c h n o l o g y No. 9 5
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Membrane integrity monitoring in large-scale systems Membrane systems are being increasingly used in the w a t e r i n d u s t r y to p r o v i d e a p h y s i c a l b a r r i e r to b a c t e r i a a n d o t h e r p a t h o g e n s s u c h a s Giardia and Cryptosporidiurru T h e u s e o f m e m b r a n e s a s a d e f e n c e a g a i n s t t h e s e p r o t o z o a n s is i m p o r t a n t , a s t h e o o c y s t s of t h e s e p a t h o g e n s a r e r e s i s t a n t to c h l o r i n a t i o n u s e d in w a t e r t r e a t m e n t p r o c e s s e s . Previous tests have demonstrated that these organisms are very durable.Cryptosporidium viability is n o t a f f e c t e d b y a n 18 h r e x p o s u r e to s o d i u m h y p o c h l o r i t e a t d o s e s u p t o 3 0 , 0 0 0 p p m . In a d d i t i o n , concentrations of sodium hypochlorite that are needed to e l i m i n a t e t h e o o c y s t , u p to 7 0 0 , 0 0 0 p p m , o b v i o u s l y c a n n o t b e u s e d for m u n i c i p a l w a t e r t r e a t m e n t . To e l i m i n a t e t h e r i s k of d r i n k i n g w a t e r contamination, particle counters and turbidity m o n i t o r s a r e two m e t h o d s t h a t a r e often e m p l o y e d to m e a s u r e t h e q u a l i t y of r a w a n d t r e a t e d w a t e r s . However, t u r b i d i t y a n d p a r t i c l e m o n i t o r i n g m e t h o d s a r e n o t c a p a b l e of m e a s u r i n g w a t e r q u a l i t y c h a n g e s a t t h e levels r e q u i r e d to e n s u r e r e m o v a l of p a t h o g e n s s u c h a s Giardia and Cryptosporidiurru A d d e d to t h i s , is t h e e x p e n s e a n d d i f f i c u l t y in d e t e c t i n g t h e s e p r o t o z o a through analytical means, and the as yet unresolved s e a r c h for a s u i t a b l e s u r r o g a t e . G i v e n t h i s , h o w t h e n is it p o s s i b l e to s a y t h a t p o t e n t i a l l y h a r m f u l o r g a n i s m s a r e b e i n g r e m o v e d w h e n t h e r e is n o m e a s u r a b l e c h a n g e i n f i l t e r e d w a t e r quality?. O n e a n s w e r is t h e u s e of t e c h n i q u e s t h a t m e a s u r e m e m b r a n e i n t e g r i t y d i r e c t l y , i n d e p e n d e n t of w a t e r quality p a r a m e t e r s . T h i s r e p r e s e n t s a s h i f t in t h i n k i n g f r o m w a t e r q u a l i t y monitoring to process control. Particle counters, although a useful quality monitor, a r e n o t a q u a n t i t a t i v e m e a s u r e of t h e i n t e g r i t y of t h e membrane barrier, and are therefore not an integrity monitoring technique. Two q u a n t i t a t i v e i n t e g r i t y m e a s u r e m e n t t e c h n i q u e s a r e t h e p r e s s u r e d e c a y (PDT) a n d diffusive a i r flow (DAF) t e s t s . T h e s e t e c h n i q u e s a r e a p p l i e d b y M e m c o r in a n i n t e g r i t y c o n t r o l p r o c e s s k n o w n a s M e m s u r e . T h e PDT w a s first a p p l i e d a s a n i n t e g r i t y m o n i t o r i n g tool for c o n t i n u o u s m i c r o f i l t r a t i o n s y s t e m s in t h e e a r l y 1980's. W h e r e a s t h e D A F t e s t w a s i n t r o d u c e d a s a n o p t i o n for f u r t h e r i m p r o v e m e n t s in s e n s i t i v i t y in 1991 - 1992. A c c o r d i n g to M e m c o r , v a r i a t i o n s in t h e t e s t s h a v e b e e n u s e d i n t h e p h a r m a c e u t i c a l i n d u s t r y to v a l i d a t e t h e i n t e g r i t y of c a r t r i d g e filter u n i t s for m a n y y e a r s . However, t h e t e s t s h a v e n o t b e e n a p p l i e d to monitoring large-scale continuous membrane systems until recently.
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B o t h t h e PDT a n d D A F t e s t s r e l y o n t h e p r i n c i p l e of t h e m e m b r a n e " b u b b l e p o i n t " to m o n i t o r t h e i n t e g r i t y of t h e s y s t e m . The n o m i n a l p o r e size for t h e M e m c o r microfiltration m e m b r a n e is 0 . 2 p m . Liquid in p o r e s of t h e s e d i m e n s i o n s a r e s u b j e c t to significant c a p i l l a r y forces a n d a relatively h i g h p r e s s u r e of a i r is r e q u i r e d to o v e r c o m e t h e s e forces a n d to p u s h liquid o u t of t h e p o r e s . C a p i l l a r y t h e o r y i n d i c a t e s t h a t t h e air p r e s s u r e r e q u i r e d to force liquid from t h e p o r e s is i n v e r s e l y p r o p o r t i o n a l to t h e p o r e diameter. T h u s , t h e l a r g e r t h e p o r e t h e lower t h e p r e s s u r e r e q u i r e d to d i s p l a c e t h e liquid. If a i r is a p p l i e d to o n e s i d e of t h e m e m b r a n e a n d t h e p r e s s u r e slowly i n c r e a s e d , t h e n a i r will b r e a k t h r o u g h first a t t h e p o r e w i t h t h e l a r g e s t d i a m e t e r . T h i s p r e s s u r e is k n o w n a s t h e b u b b l e p o i n t . T h e b u b b l e p o i n t of t h e M e m c o + m i c r o f i l t r a t i o n m e m b r a n e is in t h e r a n g e of 2 0 0 to 2 5 0 k P a . At a i r p r e s s u r e s b e l o w t h e b u b b l e p o i n t t h e r e will b e n o a i r flow t h r o u g h a n i n t e g r a l m e m b r a n e o t h e r t h a n a s m a l l flow d u e to d i f f u s i o n t h r o u g h l i q u i d in t h e m e m b r a n e wall. In c o n t r a s t t h e a i r flows t h r o u g h d e f e c t s s u c h a s a l e a k t h r o u g h a n ' O ' r i n g s e a l or a b r o k e n fibre, is c o m p a r a t i v e l y large. T h i s f e a t u r e of m e m b r a n e s is e x p l o i t e d b y b o t h t h e PDT a n d DAF s y s t e m s in o r d e r to a c c u r a t e l y c h a r a c t e r i s e t h e i n t e g r i t y of t h e C M F s y s t e m .
PDT a n d DAF t e s t i n g P r e s s u r e d e c a y t e s t i n g is p e r f o r m e d b y a p p l y i n g a i r a t a p r e s s u r e b e l o w t h e b u b b l e p o i n t to o n e s i d e of t h e m e m b r a n e , i s o l a t i n g , t h e n m e a s u r i n g t h e d e c l i n e in p r e s s u r e over t i m e . T h e D A F t e s t is f u n d a m e n t a l l y s i m i l a r to t h e PDT in t h a t t h e y b o t h m e a s u r e a i r flow t h r o u g h d e f e c t s during the integrity test. However, rather than measure air leakage as a pressure decay rate, the D A F t e s t d o e s t h i s b y f l l i n g t h e s h e l l of t h e m o d u l e w i t h l i q u i d a n d a l l o w i n g t h e a i r l e a k a g e to d i s p l a c e l i q u i d f r o m t h e s h e l l . T h e f i o w r a t e of d i s p l a c e d l i q u i d is t h e n a d i r e c t m e a s u r e of t h e m e m b r a n e i n t e g r i t y . W i t h r e g a r d s to t h e M e m s u r e s y s t e m f r o m M e m c o r , t h e m a i n a d v a n t a g e s of t h e D A F t e s t o v e r t h e PDT a r e t h a t i t t e n d s to b e l e s s s e n s i t i v e to a n y e x t e r n a l a i r l e a k s , is e a s i e r to m e a s u r e a c c u r a t e l y t h a n p r e s s u r e d e c a y , a n d d i f f u s i o n of a i r t h r o u g h l i q u i d in t h e m e m b r a n e w a l l c o n t r i b u t e s to a l e s s e r e x t e n t . T h e c o m b i n e d effect is t h a t t h e D A F t e s t t e n d s to b e m o r e s e n s i t i v e to c h a n g e s in i n t e g r i t y t h a n t h e PDT t e s t . T h e t r a d e - o f f for t h e i n c r e a s e d s e n s i t i v i t y is t h a t , a l t h o u g h r e a d i l y a u t o m a t e d , t h e DAF t e s t r e q u i r e s s o m e a d d i t i o n a l p i p e w o r k a n d f i t t i n g s i n o r d e r to m e a s u r e t h e d i s p l a c e d l i q u i d flowrate. B o t h t h e s e t e s t s a r e u s e f u l for m o n i t o r i n g t h e o v e r a l l i n t e g r i t y of a m e m b r a n e a r r a y . However, if a d r o p in i n t e g r i t y is d e t e c t e d , it is i m p o r t a n t to find o u t w h e r e t h e l e a k is o c c u r r i n g , so it c a n b e i s o l a t e d a n d r e p a i r e d . In M e m c o r s y s t e m s t h i s is d o n e u s i n g a s o n i c a n a l y s e r w h i c h is t u n e d i n t o t h e f r e q u e n c i e s of b u b b l e s p r o d u c e d d u r i n g i n t e g r i t y t e s t i n g . M o d u l e s w i t h d a m a g e d o r b r o k e n f i b r e s give significantly higher noise response than integral membrane modules.
M e m b r a n e T e c h n o l o g y No. 9 5
News & Views Potential applications
Giardia and Cryptosporidium is found. Failing this, PDT and DAF are two m e t h o d s that should come u n d e r consideration for the safe provision of drinking water, although at present no individual technique is universally accepted.
The integrity monitoring m e t h o d s described here are well suited to drinking water plants installed in remote or small communities. S u c h plants are usually small, package plants that are not staffed full-time. In these cases advance w a r n i n g of problems to allow time for response is critical. The PDT and DAF methods can monitor integrity and indicate a potential problem well before the risk of p a t h o g e n b y p a s s becomes significant. For example, if the plant is required to remove 4 log of Giardia, the PDT alarm level can be set to trigger w h e n the m e a s u r e d result drops below, say 4.5 log. This indicates to the operator that corrective action needs to be taken, either t h r o u g h isolation of modules, or their removal a n d repair. T h r o u g h o u t this time the plant will consistently achieve greater than 4 log, allowing time for a planned and contl-olled response to maintain water quality. In conclusion, it can be said that two of the most sensitive methods currently available for monitoring membrane integrity are the PDT and DAF tests. Both these tests are capable of monitoring small changes in membrane integrity, even in very large membrane arrays. Memcor report that the DAF test, for example, is capable of detecting a single broken fibre in a m e m b r a n e array containing over one million fibres. This enables operators to conh-ol m e m b r a n e integrity at levels well above 4 log removal of Giardia or Cryptosporidium, without relying on filtered water quality monitors s u c h as turbidity or particle counters. As outlined previously, a m e t h o d of c o n t i n u o u s online validation of 4-1og (99.99%) removal is a priority for the drinking water industry. At present, this m e a n s a physical r a t h e r t h a n biological technique. Perhaps this will c h a n g e if a n d w h e n a suitable surrogate for
For more information about Memsure contact:Memtec Research Ltd, Locked Mailbag No. 1, Windsor, N S W 2756, Australia. Tel: +61 45 776800;fax: +61 45 776804. E-mail: [email protected]
Commercial liquid membrane system In Membrane Technology (No. 85) almost a year ago, Tony Franken discussed whether liquid m e m b r a n e s were a viable industrial separation process or merely an academic exercise. He stated that from an industrial point of view, liquid m e m b r a n e s were an unlikely commercial option due to the lack of stability in s u c h systems. However, it seems t h a t a US c o m p a n y , C o m m o d o r e Separation Technologies (CST), disagrees with this analysis. It h a s recently i n t r o d u c e d w h a t is believed to be the first commercially available s u p p o r t e d liquid m e m b r a n e s y s t e m - - the SLiM 100. But, as Tony F r a n k e n stated in his original article, the stability of the liquid m e m b r a n e s y s t e m still
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M e m b r a n e T e c h n o l o g y No. 9 5
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