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Combination filter purifier
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News and Views Engineering, 33, 3, pp 416--425, July 1993 (a translation from Chemle-lngenieur-Technlk, 62, 3, pp 175-182). The ecological problems of ammonia and ammonium salts in wastewater, and the advantages and disadvantages of the various recovery methods are also discussed. Industrial wastewaters containing residual amounts of ammonia have up until now been regarded as having little relevance as far as the environment is concerned. Over the last few years, however, ecological knowledge has advanced to the state where ammonium-containing wastewater is analysed to have the potential to cause environmental damage. Methods which are available for overcoming the problem of ammonium in wastewater include recovery of the ammonium and conversion of the ammonium nitrogen into molecular nitrogen. Ammonia can be converted to a reusable form using a liquid membrane. The process uses an organic liquid membrane which is built up as a barrier between two aqueous phases. In this case it is between the wastewater and the product phases. Emulsion membranes have proven to be particularly useful for industrial purposes. These are produced by bringing an aqueous solution (the acceptor phase) into an organic phase (the membrane phase) in such a way as to form a water-in-oil emulsion. The acceptor phase usually employed in the recovery of ammonia is dilute sulphuric acid. The ammonia which is produced when the pH is raised diffuses through the organic membrane and is converted to ammonium sulphate in the aeceptor phase. This is insoluble in the membrane. The biggest problem hampering the development of this technique is the stability of the emulsion. It can be stabilized by adding a tensible, but then subsequent breaking at a later stage can be uneconomic. A compromise has to be reached. Another useful technique is reverse osmosis. Most of the experience that has been gained relating to the separation of ammonia has been with the treatment of ground infiltration water. Separation efficiencies of 90% have been achieved with feed concentrations of approximately 400 mg/litre NH4+, with a water yield of 80%. In the purification of industrial wastewater, the limlting values of reverse osmosis (the osmotic pressure and the saturation concentration of crust-forming substances) can exert a greater influence than that of ground infiltration water. Thus a limit exists to the degree of concentration of ammonium that can be achieved, due to the associated increase in osmotic pressure. As a process for separating electrolytes from aqueous solutions, electrodialysis is also useful for the recovery of ammonium ions from wastewaters. Separation and simultaneous formation of a concentrated ammoninm-salt solution can be achieved with an ion-exchange membrane in conjunction with an electrical field. The degree of desalination that is achieved by electrodialysis is determined, essentially, by the current between the cathode and anode and the available surface of the membrane. Further information from: Prof. Dipl.-Ing. Dr. Rolf Marr, Instltut fur Thermische Verfahrenstechnik und Umwekttechnlk, Technische Universltat Graz, Inffeldgasse 25, A-8010 Graz, Austria. Combination filter purifier A combination filter purifier has been developed for ultrapurification of gases at the University of Arizona, Tucson, USA. A commercial unit will be available in two years from Pall Corp. Normally filtration and purification are done separately, but this limits purity. Ordinary filters produce some gaseous contaminants through adsorption and desorption, while friction in packed-bed purifiers produces particles. The university's method avoids this by using ceramic or metal membranes bearing proprietary reactive metal sites that scavenge gaseous contaminants to the parts per trillion level. Simultaneously, the membranes filter particles as small as 0.01 micrometer. Because the membrane filter eliminates the need for a packed-bed purifier, the combination will be only 10-12% the size of a standard two-unit system. Reverse osmosis membrane developed from aromatic polyamide A reverse osmosis membrane fabricated from aromatic polyamide can be used without pretreatment to purify tap water, claims manufacturer Toyobo Co. The Osaka-based spinning firm says the material is also chlorine- and acid-resistant. Toyobo is currently producing about four tonnes of the membrane every month, but will enter full- scale production in 1994. Further information from: Toyobo Co Ltd, 2-2-8, Dojimahama, Klta-ku, Osaka 530, Japan. Tel: +81 6 348 3111. Fax: +81 6 348 3192.(c) 2 Membrane Technology No. 40
Transcript of Combination filter purifier
N e w s a n d V i e w s
Engineering, 33, 3, pp 416--425, J u l y 1993 (a t r a n s l a t i o n from Chemle-lngenieur-Technlk, 62, 3, pp 175-182). The ecological p r o b l e m s of a m m o n i a a n d a m m o n i u m sa l t s in wastewater , a n d the a d v a n t a g e s a n d d i s a d v a n t a g e s of the va r ious recovery m e t h o d s are a lso d i scus sed .
I n d u s t r i a l was t ewa te r s c o n t a i n i n g res idua l a m o u n t s of a m m o n i a have u p un t i l now b e e n rega rded as hav ing little re levance as far as the e n v i r o n m e n t is conce rned . Over the las t few years , however, ecological knowledge ha s a d v a n c e d to the s ta te where a m m o n i u m - c o n t a i n i n g was tewate r is a n a l y s e d to have the po ten t i a l to c ause e n v i r o n m e n t a l damage .
Methods which are avai lable for overcoming the p rob lem of a m m o n i u m in was tewate r i nc lude recovery of the a m m o n i u m a n d conver s ion of the a m m o n i u m n i t rogen in to mo lecu l a r n i t rogen .
A m m o n i a c a n be conver ted to a r e u s a b l e form u s i n g a l iquid m e m b r a n e . The process u s e s a n organic l iquid m e m b r a n e wh ich is bu i l t u p as a ba r r i e r be tween two a q u e o u s phases . In this case it is be tween the was tewa te r a n d the p roduc t phases . E m u l s i o n m e m b r a n e s have p roven to be pa r t i cu la r ly use fu l for i n d u s t r i a l pu rposes . These are p r o d u c e d by b r i n g i n g a n a q u e o u s so lu t ion (the acceptor phase) in to a n organic p h a s e (the m e m b r a n e phase) in s u c h a way as to form a wa te r - in -o i l emuls ion . The acceptor p h a s e u s u a l l y employed in the recovery of a m m o n i a is d i lu te s u l p h u r i c acid. The a m m o n i a which is p r o d u c e d w h e n the pH is ra i sed di f fuses t h r o u g h the organic m e m b r a n e a n d is conver ted to a m m o n i u m s u l p h a t e in the aeceptor phase . This is i n so lub le in the m e m b r a n e .
The biggest p rob lem h a m p e r i n g the deve lopment of th i s t e c h n i q u e is the s tab i l i ty of the emuls ion . It c a n be s tabi l ized b y a d d i n g a tensible , b u t t h e n s u b s e q u e n t b r e a k i n g at a la ter s tage c a n be uneconomic . A c o m p r o m i s e h a s to be reached.
Ano the r use fu l t e c h n i q u e is reverse osmosis . Most of the exper ience tha t h a s b e e n ga ined re la t ing to the s epa ra t i on of a m m o n i a ha s b e e n wi th the t r e a t m e n t of g r o u n d inf i l t ra t ion water. Sepa ra t i on efficiencies of 90% have b e e n ach ieved with feed c o n c e n t r a t i o n s of approx ima te ly 400 m g / l i t r e NH4+, wi th a water yield of 80%. In the pur i f i ca t ion of i n d u s t r i a l wastewater , the l iml t ing va lues of reverse osmos i s (the osmotic p r e s s u r e a n d the s a t u r a t i o n c o n c e n t r a t i o n of c r u s t - f o r m i n g s u b s t a n c e s ) c a n exert a greater in f luence t h a n t ha t of g r o u n d inf i l t ra t ion water. T h u s a l imit exis ts to the degree of c o n c e n t r a t i o n of a m m o n i u m t h a t c a n be achieved, due to the assoc ia ted inc rease in osmot ic p ressure .
As a p rocess for s epa ra t i ng electrolytes from a q u e o u s so lu t ions , e lec t rodia lys is is also use fu l for the recovery of a m m o n i u m ions from was tewaters . Sepa ra t i on a n d s i m u l t a n e o u s fo rma t ion of a c o n c e n t r a t e d a m m o n i n m - s a l t so lu t ion can be achieved wi th a n i on -exchange m e m b r a n e in c o n j u n c t i o n wi th a n electr ical field. The degree of d e s a l i n a t i o n t ha t is achieved by electrodialysis is de t e rmined , essent ia l ly , by the c u r r e n t be tween the ca thode a n d a n o d e a n d the avai lable su r face of the m e m b r a n e .
Further information from: Prof. Dipl.-Ing. Dr. Rolf Marr, Instltut f u r Thermische Verfahrenstechnik und Umwekttechnlk, Technische Universltat Graz, Inffeldgasse 25, A-8010 Graz, Austria.
Combinat ion f i l ter purifier A c o m b i n a t i o n filter purif ier h a s b e e n developed for u l t r apur i f i ca t ion of gases at the Univers i ty of Arizona, Tucson , USA. A commerc ia l u n i t will be avai lable in two years from Pall Corp.
Normal ly f i l t ra t ion a n d pur i f i ca t ion are done separate ly , b u t th is l imits pur i ty . O r d i n a r y filters p roduce some ga se ous c o n t a m i n a n t s t h r o u g h adso rp t ion a n d desorpt ion , while fr ict ion in packed -bed pur i f iers p r o d u c e s part icles .
The un ive r s i ty ' s m e t h o d avoids th is by u s i n g ceramic or me ta l m e m b r a n e s be a r i ng propr ie ta ry react ive me ta l s i tes tha t scavenge gaseous c o n t a m i n a n t s to the pa r t s per tr i l l ion level. S i mu l t a ne ous l y , the m e m b r a n e s filter par t ic les as smal l as 0.01 micrometer . Because the m e m b r a n e filter e l imina t e s the need for a p a c k e d - b e d purifier, the c o m b i n a t i o n will be only 10-12% the size of a s t a n d a r d two-un i t sys tem.
Reverse o s m o s i s m e m b r a n e deve loped from aromat ic po lyamide A reverse osmos i s m e m b r a n e fabr ica ted from aromat ic po lyamide c a n be u s e d wi thou t p r e t r e a t m e n t to purify tap water, c la ims m a n u f a c t u r e r Toyobo Co. The O s a k a - b a s e d s p i n n i n g firm says the ma te r i a l is also chlor ine- a n d ac id - res i s t an t .
Toyobo is c u r r e n t l y p r o d u c i n g a b o u t four t o n n e s of the m e m b r a n e every m o n t h , b u t will en te r full- scale p r o d u c t i o n in 1994.
Further information from: Toyobo Co Ltd, 2-2-8, Dojimahama, Klta-ku, Osaka 530, Japan. Tel: +81 6 348 3111. Fax: +81 6 348 3192.(c)
2 M e m b r a n e Techno logy No. 40
