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Membrane processes: Versatile technology for cutting costs and protecting the environment
BRANEPROCESSES
*
Membrane processes: Versatie technology for cuttiqg costs and protecting the environment In this article Dr John Cross of Elga describes some of the benefits of using membranes to purify water and de-alcohollse beer. embranes are playing an increasingly important part in process technology. Used initially in specialised fields such as desalination and renal dialysis, membrane technology has grown into a multi-billion-dollar international business expanding at around 15% per annum. Water purification is a major segment of the membrane market, and Figure 1 illustrates the principal techniques used and their range of application. Other industrial areas in which membranes are employed include effluent treatment, cell harvesting, de-alcoholisation of beer, metal recovery and solvent dewatering. The dramatic growth in membrane technology has been driven by both environmental and commercial forces. Since most membrane processes take place at ambient temperatures, energy
M
LOW
MOLECULAR
REGION
0.1 "Ill
I
I
Purifying process water for biotechnology/pharmaceuticals A new generation of polishing ultraiYtration (UF) membranes, developed within the past five years, promises to revolutionise the production of apyrogenie process water for the biotechnology and pharmaceutical industries. The membranes - which have a hollow iibre configuration with an active skln on both the inside and outside of the fibres - can not only be operated at temperatures up to SOT, but can withstand steam sterilisation at 121°C.
ORGANIC MACROMOLECULES
1
ill
consumption is minimal. In addition, membranes do not require aggressive regenerant chemicals or produce hazardous waste, so they are both costeffective to use and environmentally benign. Moreover, in the food and beverage markets, membrane technology brings a further bonus because ‘cold’ processing techniques help to enhance the taste of the final product.
AND
10
EACTERIAIFINE PARTICLES
COLLOIOS
1,000
100
I
I
I
I
10
100
1000x
1 Micron
REVERSE
The double-skinned, polymeric membranes have a separation threshold of 6000 daltons, so they are particularly effective in purging feedwater of pyrogens, which typically have a molecular weight in excess of 20,000 daltons. Although the British and European Pharmacopoeias insist that ‘Water for Injections’ (WFI) must be prepared by distillation, apyrogenic water produced by ultrafiltration can be used for rinsing ampoules, making up culture media and other non-critical purposes. Figure 2 shows a water purification system providing apyrogenic process water for the manufacture of recombinant blood proteins secreted by genetically manipulated micro-organisms. In the first part of the system, raw water is deionised before passing into the apyrogenic section, which incorporates a polishing ultrafilter featuring steamsterilisable UF membranes. Feedwater entering the ultrafilter splits equally into permeate and concentrate streams; the permeate is
10.000 I 10 Micron
F&urn 1. Ap~lim.tim nanges of membrane pmx.%.z~ [D.I. indicates deionisatfon, E.D. indicates eketmdialgsis) .
1
OSMOSIS
ULTRAFILTRATION
I
HlCROFlLTRATlON
388
September/October
1992
Flltratlbn& Separation
BRANEPROCESSES
Figure 2. Elga q&em for producing deionised and apymgenic process water
delivered to user points in the production area, while the concentrate is recycled back to the recirculation tank and reused. A small bleed from the concentrate returned to an upstream holding tank - prevents any build-up of contaminants in the system. The apyrogenic loop is pasteurised daily at 80°C and steam-sterilised every six months or so, depending on the microbiological quality of the water. ‘Purified Water’ is the grade of pharmaceutical process water required for the manufacture of noninjectable products, and most satis@ criteria of chemical purity defined by the pharmacopoeias. In practice, it must have a total dissolved solids (TDS) content of less than 5 mg/l, and ideally a conductivity below 5 us/ cm. Although water of this quality can readily be produced by ion-exchange, in many cases it is preferable to use an
environment-friendly membrane process. With this in mind, Elga has recently introduced a family of multiple-pass membrane systems known as the Intercept Gemini range. These systems are baaed on the latest reverse osmosis (RO) technology, and incorporate staging (in which the water passes through a series of membranes) of both permeate and concentrate streams. As a result, Gemini units are able to produce high-purity water typically with a conductivity of 1 us/ cm - that satisfies pharmacopoeia requirements without using corrosive chemicals or producing toxic effluent.
Purifying beverage water and de-alcoholising beer The performance of three generations of reverse osmosis membranes is summarised in Table 1. Current RO
An Elga Intercept lJE CurLt@l&a#%m) unit.
Flltration
& Separation
September/October
1992
membranes not only have a better overall performance than their predecessors, but a sharper selectivity; whereas early cellulosic membranes had a molecular weight cutoff of about 500 daltons, thin-film composite membranes have a cutoff of approximately 100 daltons. The combination of high rejection rates and finely tuned selectivity has enabled the later membranes to be used both for puri&ing beverage water and for de-alcoholising beer. For example, a leading soft drinks manufacturer was using a traditional combination of chemical dosing, precipitation and filtration to reduce the TDS and alkalinity of the mains water. However, during a hot, dry summer, a white floe began to appear in bottles of tonic water and other carbonated products within 24 hours of bottling. Despite exhaustive changes in the chemical dosing programme, the floe continued to form. The contaminants were identified as a mixture of harmless - although undesirable polysaccharides and polypeptides derived from algal growth on local surface water. Following successful field trials with a reverse osmosis unit - which completely eliminated the floe from the products - a large RO system was installed, capable of providing 250 m3/day of purified beverage water. Figures 3 and 4 demonstrate how reverse osmosis can be used to dealcoholise beer. In the first stage of the process, the RO plant provides purified water (permeate) which is fed into the diafiltration tank; the concentrate is discharged to drain (Figure 3). In the second stage (Figure 4), full 389
BRANEPROCESSES
*
Substance
Removal efficiency, % Spiral-wrap cellulosic
Suspended solids Colloids Bacteria Viruses Pyrogens
(Above)
A reverse omnosis plant for remming
Table 1 (right).
Comparison of pqtbnnance
strength beer forms the RO feedstock, but this time the concentrate is kept and fed back to the beer tank The highly selective membranes retain the large molecules responsible for the characteristic colour, taste and aroma of beer, while allowing alcohol and water to permeate through. The alcoholic permeate is discharged to drain, and water losses are made up by continually pumping the diafiltration water into the beer tank Because reverse osmosis takes place at ambient temperatures, the lowalcohol beer is not tainted by ‘cooked’ flavours. Moreover, RO plant is reliable, simple to use and requires minimal operator involvement. In addition, reverse osmosis scores over traditional alcohol-removal technology in having substantially lower capital and operating costs; a typical RO system consumes 68% less energy per barrel than distillation. Thus membrane processes offer many advantages over conventional technology. However, although membranes have been widely adopted by advance nations worldwide, it seems that there is still an untapped market potential in the UK. The popularity of membrane technology is likely to be boosted by increasingly stringent environmental legislation governing effluent q quality and processing methods. For&rt&
irlforrnatin,
Molecular weight cutoff, daltons
alcohol from beer
of RO membranes.
TDS rejection rate, inorganic ions Notes: (i) Polyvalent
(ii) Bicarbonate
Hollow fibre asymmetric
Spiral-wrap composite
100
100
100
99.5
> 99.5
> 99.5
500
250
100
85-95
90-95
96-98
ions n$ected mom q@kimtly than monovalent ions. andfluoride rejected mom e-t&j at high PH.
Silica rejection rate
82-90
88-90
96-98
L
RO system forpmdming low-alcohol beer, using mmme osnwsis to produce Nguw 3 [above). di@Xtmkim water (F indicates feed, C indicates comenhate and P indicates permeate).
RO system for producing low-akohal &nm 4 (below). alcoholtie beer (F indicates feed, C &dim&s con-, Cdimtes level controls).
beer, using reverse osmosis to deP indicates permeate and LX
contact:
Elga Ltd High Wet, Lane End High W@cmnbe Bucks. HP14 3JH, UK. Tel: + 44 CO)494 881393 Fax: + 44 (0) 494 881007. JW
September/October
1992
Filtration6 Separation
*
Membrane processes: Versatie technology for cuttiqg costs and protecting the environment In this article Dr John Cross of Elga describes some of the benefits of using membranes to purify water and de-alcohollse beer. embranes are playing an increasingly important part in process technology. Used initially in specialised fields such as desalination and renal dialysis, membrane technology has grown into a multi-billion-dollar international business expanding at around 15% per annum. Water purification is a major segment of the membrane market, and Figure 1 illustrates the principal techniques used and their range of application. Other industrial areas in which membranes are employed include effluent treatment, cell harvesting, de-alcoholisation of beer, metal recovery and solvent dewatering. The dramatic growth in membrane technology has been driven by both environmental and commercial forces. Since most membrane processes take place at ambient temperatures, energy
M
LOW
MOLECULAR
REGION
0.1 "Ill
I
I
Purifying process water for biotechnology/pharmaceuticals A new generation of polishing ultraiYtration (UF) membranes, developed within the past five years, promises to revolutionise the production of apyrogenie process water for the biotechnology and pharmaceutical industries. The membranes - which have a hollow iibre configuration with an active skln on both the inside and outside of the fibres - can not only be operated at temperatures up to SOT, but can withstand steam sterilisation at 121°C.
ORGANIC MACROMOLECULES
1
ill
consumption is minimal. In addition, membranes do not require aggressive regenerant chemicals or produce hazardous waste, so they are both costeffective to use and environmentally benign. Moreover, in the food and beverage markets, membrane technology brings a further bonus because ‘cold’ processing techniques help to enhance the taste of the final product.
AND
10
EACTERIAIFINE PARTICLES
COLLOIOS
1,000
100
I
I
I
I
10
100
1000x
1 Micron
REVERSE
The double-skinned, polymeric membranes have a separation threshold of 6000 daltons, so they are particularly effective in purging feedwater of pyrogens, which typically have a molecular weight in excess of 20,000 daltons. Although the British and European Pharmacopoeias insist that ‘Water for Injections’ (WFI) must be prepared by distillation, apyrogenic water produced by ultrafiltration can be used for rinsing ampoules, making up culture media and other non-critical purposes. Figure 2 shows a water purification system providing apyrogenic process water for the manufacture of recombinant blood proteins secreted by genetically manipulated micro-organisms. In the first part of the system, raw water is deionised before passing into the apyrogenic section, which incorporates a polishing ultrafilter featuring steamsterilisable UF membranes. Feedwater entering the ultrafilter splits equally into permeate and concentrate streams; the permeate is
10.000 I 10 Micron
F&urn 1. Ap~lim.tim nanges of membrane pmx.%.z~ [D.I. indicates deionisatfon, E.D. indicates eketmdialgsis) .
1
OSMOSIS
ULTRAFILTRATION
I
HlCROFlLTRATlON
388
September/October
1992
Flltratlbn& Separation
BRANEPROCESSES
Figure 2. Elga q&em for producing deionised and apymgenic process water
delivered to user points in the production area, while the concentrate is recycled back to the recirculation tank and reused. A small bleed from the concentrate returned to an upstream holding tank - prevents any build-up of contaminants in the system. The apyrogenic loop is pasteurised daily at 80°C and steam-sterilised every six months or so, depending on the microbiological quality of the water. ‘Purified Water’ is the grade of pharmaceutical process water required for the manufacture of noninjectable products, and most satis@ criteria of chemical purity defined by the pharmacopoeias. In practice, it must have a total dissolved solids (TDS) content of less than 5 mg/l, and ideally a conductivity below 5 us/ cm. Although water of this quality can readily be produced by ion-exchange, in many cases it is preferable to use an
environment-friendly membrane process. With this in mind, Elga has recently introduced a family of multiple-pass membrane systems known as the Intercept Gemini range. These systems are baaed on the latest reverse osmosis (RO) technology, and incorporate staging (in which the water passes through a series of membranes) of both permeate and concentrate streams. As a result, Gemini units are able to produce high-purity water typically with a conductivity of 1 us/ cm - that satisfies pharmacopoeia requirements without using corrosive chemicals or producing toxic effluent.
Purifying beverage water and de-alcoholising beer The performance of three generations of reverse osmosis membranes is summarised in Table 1. Current RO
An Elga Intercept lJE CurLt@l&a#%m) unit.
Flltration
& Separation
September/October
1992
membranes not only have a better overall performance than their predecessors, but a sharper selectivity; whereas early cellulosic membranes had a molecular weight cutoff of about 500 daltons, thin-film composite membranes have a cutoff of approximately 100 daltons. The combination of high rejection rates and finely tuned selectivity has enabled the later membranes to be used both for puri&ing beverage water and for de-alcoholising beer. For example, a leading soft drinks manufacturer was using a traditional combination of chemical dosing, precipitation and filtration to reduce the TDS and alkalinity of the mains water. However, during a hot, dry summer, a white floe began to appear in bottles of tonic water and other carbonated products within 24 hours of bottling. Despite exhaustive changes in the chemical dosing programme, the floe continued to form. The contaminants were identified as a mixture of harmless - although undesirable polysaccharides and polypeptides derived from algal growth on local surface water. Following successful field trials with a reverse osmosis unit - which completely eliminated the floe from the products - a large RO system was installed, capable of providing 250 m3/day of purified beverage water. Figures 3 and 4 demonstrate how reverse osmosis can be used to dealcoholise beer. In the first stage of the process, the RO plant provides purified water (permeate) which is fed into the diafiltration tank; the concentrate is discharged to drain (Figure 3). In the second stage (Figure 4), full 389
BRANEPROCESSES
*
Substance
Removal efficiency, % Spiral-wrap cellulosic
Suspended solids Colloids Bacteria Viruses Pyrogens
(Above)
A reverse omnosis plant for remming
Table 1 (right).
Comparison of pqtbnnance
strength beer forms the RO feedstock, but this time the concentrate is kept and fed back to the beer tank The highly selective membranes retain the large molecules responsible for the characteristic colour, taste and aroma of beer, while allowing alcohol and water to permeate through. The alcoholic permeate is discharged to drain, and water losses are made up by continually pumping the diafiltration water into the beer tank Because reverse osmosis takes place at ambient temperatures, the lowalcohol beer is not tainted by ‘cooked’ flavours. Moreover, RO plant is reliable, simple to use and requires minimal operator involvement. In addition, reverse osmosis scores over traditional alcohol-removal technology in having substantially lower capital and operating costs; a typical RO system consumes 68% less energy per barrel than distillation. Thus membrane processes offer many advantages over conventional technology. However, although membranes have been widely adopted by advance nations worldwide, it seems that there is still an untapped market potential in the UK. The popularity of membrane technology is likely to be boosted by increasingly stringent environmental legislation governing effluent q quality and processing methods. For&rt&
irlforrnatin,
Molecular weight cutoff, daltons
alcohol from beer
of RO membranes.
TDS rejection rate, inorganic ions Notes: (i) Polyvalent
(ii) Bicarbonate
Hollow fibre asymmetric
Spiral-wrap composite
100
100
100
99.5
> 99.5
> 99.5
500
250
100
85-95
90-95
96-98
ions n$ected mom q@kimtly than monovalent ions. andfluoride rejected mom e-t&j at high PH.
Silica rejection rate
82-90
88-90
96-98
L
RO system forpmdming low-alcohol beer, using mmme osnwsis to produce Nguw 3 [above). di@Xtmkim water (F indicates feed, C indicates comenhate and P indicates permeate).
RO system for producing low-akohal &nm 4 (below). alcoholtie beer (F indicates feed, C &dim&s con-, Cdimtes level controls).
beer, using reverse osmosis to deP indicates permeate and LX
contact:
Elga Ltd High Wet, Lane End High W@cmnbe Bucks. HP14 3JH, UK. Tel: + 44 CO)494 881393 Fax: + 44 (0) 494 881007. JW
September/October
1992
Filtration6 Separation