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Membrane Separation Technology

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Membrane Separation Processes Neha Kathayat Rugved Pathare Daksh Pratap Singh
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Membrane Separation Processes 

Neha Kathayat

Rugved PathareDaksh Pratap Singh

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Overview 

Introduction

Ultrafiltration 

Reverse Osmosis

Electrodialysis

Summary

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Overview 

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary

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Overview 

Introduction

Ultrafiltration 

Reverse Osmosis

Electrodialysis

Summary

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Overview 

Introduction

Ultrafiltration 

Reverse Osmosis

Electrodialysis

Summary

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Overview 

Introduction

Ultrafiltration 

Reverse Osmosis

Electrodialysis

Summary

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Overview 

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary

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Introduction 

Strathmann  defines a membrane as “aninterphase separating two phases and

selectively controlling the transport of materials between those phases”  

Since the 1960s a new technology using

synthetic membranes for processseparations has been rapidly developed

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Introduction 

Strathmann  defines a membrane as “aninterphase separating two phases and

selectively controlling the transport of materials between those phases”  

Since the 1960s a new technology using

synthetic membranes for processseparations has been rapidly developed 

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 Advantages offered 

 Ambient temperature operation

Relatively low capital and running costs

Modular construction

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 Advantages offered 

 Ambient temperature operation

Relatively low capital and running costs

Modular construction

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 Advantages offered 

 Ambient temperature operation

Relatively low capital and running costs

Modular construction

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General Classification 

 Most of the membrane processes arepressure driven barring a few like

electro dialysis (ED). Pressure driven process includes micro

filtration (MF), ultra-filtration (UF),reverse osmosis (OS)

Micro filtration, Ultra filtration, Reverse-Osmosis are different with respect topore-size of membrane

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General Classification 

 Most of the membrane processes arepressure driven barring a few like

electro dialysis (ED). Pressure driven process includes micro

filtration (MF), ultra-filtration (UF),reverse osmosis (OS)

Micro filtration, Ultra filtration, Reverse-Osmosis are different with respect topore-size of membrane

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General Classification 

 Most of the membrane processes arepressure driven barring a few like

electro dialysis (ED). Pressure driven process includes micro

filtration (MF), ultra-filtration (UF),reverse osmosis (OS)

Micro filtration, Ultra filtration, Reverse-Osmosis are different with respect topore-size of membrane

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Historical Perspective 

Following the end of World War II , the USGovt. became concerned about the shortageof water before the end of century.

The US Dept. set up the Office of Saline

Waters (OSW), and committed substantialfinancial resources to the development of various separation processes for waterdesalination, a significant portion of whichwas dedicated to the development of membranes for desalination, continuing thework up to 2 decades.

Result was development of RO and UF and itsno co-incidence that US is the world leaderon this front.

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Ultra filtration 

 A Pressure Driven

Membrane SeparationProcess

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Membrane Structure and Fabrication 

The thin skin on thesurface: usually 0.1 to 1 in thickness.

Permits high hydraulicpermeability

The more open poroussubstructure (typically

120 in thickness)provides goodmechanical support.

 Virtually eliminates

internal pore-fouling.

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Today UF membranes are made from

thermally and chemically stable syntheticpolymers like PVC, PAN, polyimides(PI),polysulphone(PS) ,PVDF.

In addition there are inorganic UF

membranes made from zirconium andaluminium oxides.

Plasticizers are necessary for some

membranes, if they are to be dried, toprevent collapse of the pores duringdrying.

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Today UF membranes are made from

thermally and chemically stable syntheticpolymers like PVC, PAN, polyimides(PI),polysulphone(PS) ,PVDF.

In addition there are inorganic UF

membranes made from zirconium andaluminium oxides.

Plasticizers are necessary for some

membranes, if they are to be dried, toprevent collapse of the pores duringdrying.

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Today UF membranes are made from

thermally and chemically stable syntheticpolymers like PVC, PAN, polyimides(PI),polysulphone(PS) ,PVDF.

In addition there are inorganic UF

membranes made from zirconium andaluminium oxides.

Plasticizers are necessary for some

membranes, if they are to be dried, toprevent collapse of the pores duringdrying.

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Modification 

 Additionalstrength is

provided bycasting themembrane on aspun-bonded

poly-ethylene orpolypropylenebacking.

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Pore Size Determination 

UF membranes have “diffuse cut-off” characteristics.

The retention R (in percent) may be definedas ,R = 100 ( 1- ( Cuf / CR ) )

The convention states that the molecular 

weight cut-off of the membrane is equal to the molecular weight of the globular proteins which are 90% retained by the membrane .

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Retention Characteristics 

1. Size and shape considerations

2.  Adsorption losses

3. Charged membranes

4. Pressure effects

5. Temperature effects

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1. Size and Shape considerations 

Retention differswith linear or

sphericalstructure of molecules.

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2. Adsorption losses 

The polymer which makes the UFaffects the retention if it adsorbs the

species on the membrane surface. Retention of membranes are often

measured in stirred cells.

 A mass balance on cell, integrated overtime t .

R = 100 * (ln( Cf / Co ) )/ln( Vo / Vf )) 

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3. Charged membrane 

Charged UF membrane reject low conc.Of salts, because the fixed chargedgroup on membrane skin reject ionicsolutes via repulsion of co ions.

Obviously, divalent, trivalent ions arerejected better than monovalent ions.

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4. Effect of pressure  

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5. Effect of temperature 

It has been found experimentally for alarge no.of membranes systems and

feed streams that the permeation rateis inversely proportional to fluidviscosity.

 Viscosity of water decreases by 2.5%

for every C rise, researchers refer to3% rule that flux increases 3% per 1 C as rule of thumb.

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Evaluation of Mass Transfer 

Co-efficient  To evaluate m.t.c, laminar parabolic velocity profileis assumed to be established at the channelentrance

Leveque’s solution gives,  Sh = 1.62 ( Re Sc dh / L)0.33

 

(For 100< (Re Sc dh /L) <5000) Generally, K = .816 (γ D2 /L )0.33

γ = the fluid shear rate at the membrane surface.= 6U/b for rectangular slits.= 8U/d for circular tubes.

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Evaluation of Mass Transfer 

Co-efficient  To evaluate m.t.c, laminar parabolic velocity profileis assumed to be established at the channelentrance

Leveque’s solution gives,  Sh = 1.62 ( Re Sc dh / L)0.33

 

(For 100< (Re Sc dh /L) <5000) Generally, K = .816 (γ D2 /L )0.33

γ = the fluid shear rate at the membrane surface.= 6U/b for rectangular slits.= 8U/d for circular tubes.

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Evaluation of Mass Transfer 

Co-efficient  To evaluate m.t.c, laminar parabolic velocity profileis assumed to be established at the channelentrance

Leveque’s solution gives,  Sh = 1.62 ( Re Sc dh / L)0.33

 

(For 100< (Re Sc dh /L) <5000) Generally, K = .816 (γ D2 /L )0.33

γ = the fluid shear rate at the membrane surface.= 6U/b for rectangular slits.= 8U/d for circular tubes.

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UF Plant Design 

Mode of operation

The arrangement of membrane moduleand their mode of operation can affectthe economics as much as the moduledesign.

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 Applications 

Semi conductor industry

Reclamation of waste lubricating oil

Decontamination of crude oil

Waste treatment

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 Applications 

Semi conductor industry

Reclamation of waste lubricating oil

Decontamination of crude oil

Waste treatment

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 Applications 

Semi conductor industry

Reclamation of waste lubricating oil

Decontamination of crude oil

Waste treatment

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 Applications 

Semi conductor industry

Reclamation of waste lubricating oil

Decontamination of crude oil

Waste treatment

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Summarizing U.F 

Twenty-five years of invention.

Reliable and economic.

Eliminated severe pollution problems. Recovery of by-products adds to the profit.

Grafting eliminates fouling problems.

Stringent environmental controls may

necessitate UF. Bio-reactors using UF, have tremendous

potential for continuous enzyme reactors.

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Reverse Osmosis 

 A Pressure Driven

Membrane SeparationProcess

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Osmosis 

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Reverse Osmosis 

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R.O membranes 

High resolution electron microscopycannot resolve the extensive pore in the

separating layer of the R.O. membranes Therefore it is generally considered that

they do not contain pores and that they

operate mainly by “solution diffusion” mechanism

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Solution diffusion mechanism 

Introduced by SOLTANEIH and GILL.

It states that the membrane is non

porous, and that solvent and solutescan only be transported across themembranes by first dissolving in, and

subsequently diffusing through, themembrane

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HAASE and BELFORT model 

The solvent flux through the membraneis proportional to pressure gradient:

For the solute it is found that:

)(11 P  K  J 

222 C  K  J 

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Thus, solvent (water) flow occurs onlywhen | ΔP| > |ΔΠ|, solute flow isindependent of | ΔP|

Hence, increasing the operatingpressure increases the effectiveseparation

Typically for brackish water (1.5-2kg/m3 salts), |ΔΠ| = 0.1- 0.7MPa and| ΔP| = 3-8 MPa 

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Thus, solvent (water) flow occurs onlywhen | ΔP| > |ΔΠ|, solute flow isindependent of | ΔP|

Hence, increasing the operatingpressure increases the effectiveseparation

Typically for brackish water (1.5-2kg/m3 salts), |ΔΠ| = 0.1- 0.7MPa and| ΔP| = 3-8 MPa 

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Thus, solvent (water) flow occurs onlywhen | ΔP| > |ΔΠ|, solute flow isindependent of | ΔP|

Hence, increasing the operatingpressure increases the effectiveseparation

Typically for brackish water (1.5-2kg/m3 salts), |ΔΠ| = 0.1- 0.7MPa and| ΔP| = 3-8 MPa 

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Salt rejection 

Rejection of ions at R.O membranesdepends on valence

The rejection of organic moleculesdepends on molecular weights

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Membrane modules 

There are currently four genericconfigurations for membranes inindustrial use :

1. Tubular modules2. Hollow fiber modules3. Flat plate modules

4. Spiral wound modules.

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Tubular module 

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Hollow fiber modules 

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Flat- sheet module  

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Spiral wound modules 

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Plant configuration 

Batch Recirculation

Feed and Bleed configuration

Continuous Single-pass

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Plant configuration 

Batch Recirculation

Feed and Bleed configuration

Continuous Single-pass

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Plant configuration 

Batch Recirculation

Feed and Bleed configuration

Continuous Single-pass

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Batch Recirculation 

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Feed and Bleed Configuration 

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Single pass configuration 

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 Applications 

In some applications the product is theretentate, and the objective is toconcentrate or purify the retained

species and in others the product ispermeate.

Whereas, in some both retentate and

filtrate are important. For example, if avaluable product or by-product is apollutant in a waste stream, recoveryand use of the product will often pay

for pollution abatement.

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R.O. in water treatment 

R.O. is a well-established large scaleindustrial process for the desalination of 

brackish water More than 1000 units in operation, each

capable of producing 105 m3 /day of 

drinking water

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Membranes employed 

The two basic membranes employedin in the commercial R.O. systems are

1. The Thin Film Composite (TFC)membranes

2. Cellulose acetate blend (CAB)

membranes

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Membranes employed 

The two basic membranes employedin in the commercial R.O. systems are

1. The Thin Film Composite (TFC)membranes

2. Cellulose acetate blend (CAB)

membranes

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Membranes employed 

The two basic membranes employedin in the commercial R.O. systems are

1. The Thin Film Composite (TFC)membranes

2. Cellulose acetate blend (CAB)

membranes

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Case Study: R.O at MRL 

Objective is to reuse sewage water asprocess water for MRL

Spiral wound module employed.

MRL uses advanced composite membrane(ACM) which is in TFC form.

It is made by first casting the porous

polysulfone film on fabric support. The thin(0.05- 0.2 μ) polyamide membrane is thenformed on the film surface by polymerizationof an aromatic diamine and cycloaliphatictricarbonyl chloride

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 Advantages of ACM 

Excellent biological stability

Excellent chemical stability except

toward Cl. Resistant to membrane compaction

Longer life as compared to CAB

membranes.

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 Advantages of ACM 

Excellent biological stability

Excellent chemical stability except

toward Cl. Resistant to membrane compaction

Longer life as compared to CAB

membranes.

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 Advantages of ACM 

Excellent biological stability

Excellent chemical stability except

toward Cl. Resistant to membrane compaction

Longer life as compared to CAB

membranes.

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 Advantages of ACM 

Excellent biological stability

Excellent chemical stability except

toward Cl. Resistant to membrane compaction

Longer life as compared to CAB

membranes.

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Thin Film Composites 

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Process block diagram 

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Pretreatment 

R.O almost always requirespretreatment to control fouling

Pretreatment scheme1.  Addition of HCl to control pH

2.  Addition of SHMP to avoid calciumsulfate scale

3. Micron cartridge filter to removeparticles greater than 10 μ size

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Pretreatment 

R.O almost always requirespretreatment to control fouling

Pretreatment scheme1.  Addition of HCl to control pH

2.  Addition of SHMP to avoid calciumsulfate scale

3. Micron cartridge filter to removeparticles greater than 10 μ size

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Pretreatment 

R.O almost always requirespretreatment to control fouling

Pretreatment scheme1.  Addition of HCl to control pH

2.  Addition of SHMP to avoid calciumsulfate scale

3. Micron cartridge filter to removeparticles greater than 10 μ size

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Pretreatment 

R.O almost always requirespretreatment to control fouling

Pretreatment scheme1.  Addition of HCl to control pH

2.  Addition of SHMP to avoid calciumsulfate scale

3. Micron cartridge filter to removeparticles greater than 10 μ size

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Permeate water characteristics 

pH = 7.5

TDS = 400 ppm

Total Hardness as CaCO3= 100ppm

Free ammonia = 0.1 ppm

Nitrates = 1ppm

Silica = 10 ppm

BOD =2ppm COD= 5ppm

total Phosphates = 0.1 ppm

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 ACM characteristics 

Membrane configuration = spiral wound

Material = polyamide

Supplier = dupont

Dimension= 8” dia x 40” long 

Rated operating pressure = 200-350 psig

Temperature range = 0-45oC

pH range = 4-11 Membrane SA = 37.2 m2

Cl tolerance = 0.25 ppm (pH>8) ; 0.1ppm(pH<8)

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Permeate can be used as 

Process water

Landscaping, gardening

Ground water discharge for controllingintrusion of sea water into the groundwater table

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Summarizing R.O 

Can be widely used as captive wastewater recycle plants in industries

Used for the production of drinkingwater in European countries. Largestsuch plant produces 140000 m3 / daywater for north Paris

In temperate climates nanofiltration ismore economical on this regard. 

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Electrodialysis 

 An electrically driven

membrane separationprocess.

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Flow-diagram for ED 

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The membranes 

Thin films of polymeric chains containingelectrically charged functional sites.

 Anion-exchange membrane (e.g. withquartenary ammonium groups)

Cation-exchange membranes (i.e. withsulfonate groups)

 Various methods of producing

Close to 98% efficiency

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ED Stack 

1: Polypropylene end plate 2: Electrode 3: Electrode chamber 4: spacer-sealing PVC 5: Spacer fabric 6: Screws 7: Steel frame 8: Inlet anode cell 9: Inlet concentrate cell 10: cation exchange membrane

11: AAM 12: Inlet diluate cell 13: Inlet cathode chamber

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Process configurations 

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Typical process configuration 

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Typical Applications

Demineralization

Concentration of electrolytes

Ion-replacement reactions

Metathesis reactions

Separation of electrolysis products

Fractionation of electrolytes

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Electrodialysis reversal (EDR)

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Cation-neutral ED

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Limiting current density 

Transference number is the fraction of current carried by an ion

t+

for cations and t-

for anions ts

- for anion through solution (say .5)

tm- for anion through anion exchange

membrane (say 1.0)

So at membrane there is a depletionlayer set up 

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Contd… 

 At a certain current theconc. becomes 0

This is called limiting

current density Beyond this H+ and OH- 

is transported acrossthe membrane

Loss of efficiency andppt of salts due to pHchanges

Ilim= limiting currentdensity

D= diffusion coefficient

F= Faraday’s const. 

l=Equivalent filmthickness

)(

lim

 smi t t l 

 DF 

 zC 

i

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Typical process configuration

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Typical Applications 

Demineralization

Concentration of electrolytes

Ion-replacement reactions Metathesis reactions

Separation of electrolysis products

Fractionation of electrolytes

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Electrodialysis reversal (EDR) 

Deals with fouling of the membrane andscale formation

Reversal of polarityseveral times anhour along withalternation of conc.

and demin stream Useful with Ni, Zn,

Fe salts etc

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Variations of ED 

Cation exchangeand neutralmembrane

Elimination of anionmembrane

Greater flexibility inselecting flow rates

and feed and o/pconc

Electrosorption

Neutral inner layerbetween a cation ex

membrane andanion ex membrane

During normaloperation the

neutral gets loadedand in reverse getsunloaded 

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Compact water purification   electrodialysis units are connected directly to themain water supply system through a flexible hose.

The pressure of 1 –3 atm is quite sufficient,because their operating pressure is within 0.3 –0.4atm. Thus, in comparison with RO there is no need of additional pump of high pressure; 

as a result there is another main advantage:consumers have the opportunity to regulate themselves the taste of water by means of a simpleturn of the tap;

residual chlorine in water does not influence thedesalination process, therefore there is no need of cartridges before treatment;

simplicity and slight adjustment of the process,low power consumption and low cost of units  promises a great advantage in the future;

when using these units there is no need of 

bottle water purchase. 

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Seawater desalination with ED 

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Comparison of RO and ED  


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