electrodialysis and electrodialysis reversal m38 · introduction© electrodialysis (ed) is an...

©1881

American Water Works Association

Electrodialysis andElectrodialysis Reversal

AWWA MANUAL M38

First Edition

FOUNDED

Copyright (C) 1999 American Water Works Association All Rights Reserved

©Contents

Preface, v

Acknowledgments, vii

Chapter 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Basic Water Chemistry Concepts, 1Operating Principles of ED and EDR, 3Development of ED and EDR Systems, 5Applications, 10

Chapter 2 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Components of ED and EDR Design, 13Staging, 20Limiting Parameters, 22Water Recovery, 25Pretreatment, 26Operating Principles for Design, 29Posttreatment, 31Concentrate Disposal, 32References, 35

Chapter 3 Equipment and Installation . . . . . . . . . . . . . . . . . . . . 37

Equipment Subsystems, 37Installation, 41Costing, 42References, 44

Chapter 4 Operation and Maintenance . . . . . . . . . . . . . . . . . . . . 45

Operation Procedures, 45Maintenance Requirements, 47Safety, 52

Abbreviations, 55

Additional Sources of Information, 57

Index, 59

iii


©Introduction

Electrodialysis (ED) is an electrically driven membrane process used todemineralize brackish water. Brackish waters lie under approximately two thirds ofthe United States, and inland rivers, such as the Rio Grande and the lower reaches ofthe Colorado, also contain high levels of salinity. Water is classified as brackish whenmineral content ranges between that of fresh drinking water and that of seawater.Brackish water contains more than 500 mg/L of total dissolved solids (TDS) andseawater more than 30,000 mg/L TDS.

ED and electrodialysis reversal (EDR) reduce TDS in brackish source water byelectrically removing contaminants that exceed acceptable levels for drinking andprocess water. An overview of membrane process applications based on the molecularweights of contaminants appears in Figure 1-1. The ED and EDR processes arecompetitive with reverse osmosis (RO) in treating brackish waters. Typical EDsystems include chemical feed systems for antiscalant and perhaps acid addition, acartridge filter for prefiltration, the ED unit, and equipment for aeration, disinfection,and stabilization. EDR systems can often operate without fouling and scalingchemical feed, and they can treat high-fouling sources more efficiently than RO.However, it is important to remember that the types of membranes used in ED andEDR systems do not provide a barrier to remove microorganisms as do RO,nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) membranes.

BASIC WATER CHEMISTRY CONCEPTS ______________________A basic understanding of salts and water is necessary to understand the design,operation, and maintenance of a water demineralization system. A review of waterchemistry concepts is provided here.

Ionic SolutionsAn ion is a charged atom, molecule, or radical, the migration of which affects thetransport of electricity through an electrolyte solution. For example, common tablesalt is a typical ionic compound. The chemical name for this crystal is sodiumchloride, and the chemical symbol is NaCl. The crystal consists of two types of

AWWA MANUAL M38

Chapter 1

1


©charged atoms, sodium and chloride, that are held together by electrically attractiveforces.

If a crystal of salt is dissolved in water, water molecules will orient themselvesaround the charged atoms and nullify the attractive force between them. This isknown as the solution and dissociation (dissolving) of a salt in water. When thisoccurs, two electrically charged particles are formed, one with a positive charge(sodium, represented as Na+) and one with a negative charge (chloride, representedas Cl–).

The subatomic particle responsible for the electrical charge is called an electron.An electron, by convention, has an assigned charge of negative one (–1). An atom thataccepts an electron during the dissociation process will have a net charge of –1. Anatom that gives up an electron during the process will have a net charge of +1. Theseresulting charged particles are ions. The positively charged ions are called cations,and the negatively charged ions are called anions. These two types of ions arecompletely dissociated and mobile in water.

In the same manner as salts, minerals and acids may also dissociate into ions insolution. Some common ions that may be found in natural water are shown in Table1-1. Some of the ions listed in Table 1-1 have more than one positive or negative

Figure 1-1 Membrane processes overview

Metal Ions Aqueous Salts Viruses Humic Acids Bacteria Cysts Antimony Sodium Salts Infectious Trihalomethane Salmonella Protozoa Arsenic Sulfate Salts Hepatitis Precursors Shigella Giardia Nitrate Manganese Salts Vibrio cholerae Cryptosporidium Nitrite Aluminum Salts Cyanide etc.

2 ELECTRODIALYSIS


©charge associated with them (e.g., calcium has a charge of +2). In these cases, the ionhas accepted or given up more than a single electron during the dissociation process.

Electrical ConductivityThe most important property of an ionic solution is its ability to conduct electricity.When two electrodes are connected to a direct current (DC) power supply andimmersed in pure water, no electric current passes between the electrodes because noions exist in the solution to transport the current. In an ionic solution, however, thedissociated ions transport the electric charge between the two electrodes. The abilityof a solution to carry an electric charge is known as conductivity and is measured ineither micromhos per centimetre (µmho/cm) or microsiemens per centimetre (µS/cm).Conductivity is affected by the concentration of ions, the ionic composition, and thetemperature of the solution in the following ways:

• Increasing ion concentration results in increased electric conductivity.

• Smaller ions and those with more than one electric charge tend to movethrough the solution more quickly.

• Raising the temperature increases ion mobility, resulting in an increase inconductivity.

OPERATING PRINCIPLES OF ED AND EDR __________________Electrodialysis is an electrochemical separation process in which ions are transferredthrough ion exchange membranes by means of a DC voltage. This process can beunderstood more clearly by referring to Figure 1-2, which shows a tank filled with anNaCl solution and electrodes (cathode and anode) placed at either end.

When DC potential is applied across the electrodes, the following take place:

• Cations (Na+) are attracted to the cathode, or negative electrode.

• Anions (Cl–) are attracted to the anode, or positive electrode.

• Pairs of water molecules break down (dissociate) at the cathode to producetwo hydroxyl (OH–) ions plus hydrogen gas (H2).

• Pairs of water molecules dissociate at the anode to produce four hydrogen ions(H+), one molecule of oxygen (O2), and four electrons (e–).

• Chlorine gas (Cl2) may be formed at the anode.

The movement of ions in the tank can be controlled by the addition of ionexchange membranes that form watertight compartments, as shown in Figure 1-3.The two types of ion exchange membranes used in electrodialysis are

• anion transfer membranes (A in Figure 1-3), which are electrically conductivemembranes that are water impermeable and allow only negatively chargedions to pass through

Table 1-1 Common ions found in natural waters

Cations Anions

Sodium (Na+) Chloride (Cl–)Calcium (Ca+2) Bicarbonate (HCO3–)Magnesium (Mg+2) Sulfate (SO4–2)Potassium (K+) Nitrate (NO3–)

INTRODUCTION 3


©• cation transfer membranes (C in Figure 1-3), which are electrically conductivemembranes that are water impermeable and allow only positively chargedions to pass through

Varieties of these basic types of membranes exist that are selective to ions thatare either monovalent (having a charge magnitude of 1) or divalent (having a chargemagnitude of 2). Other types can be formulated to enhance the passage rates ofselected ions. For example, membranes exist that show an affinity for nitrate passageover other anions.

In Figure 1-3 there is no DC potential applied to the electrodes and nomovement of ions. Figure 1-4 shows what occurs when DC potential is applied acrossthe electrodes. The figure shows six compartments separated by ion exchangemembranes. The membranes influence ion behavior as follows:

1. Compartments 1 and 6 — Compartments 1 and 6 contain metal electrodeswhere reduction and oxidation occur.

2. Compartment 2 — Cl– ions pass through the anion membrane (A) intocompartment 3, while Na+ ions move through the cation membrane (C)into compartment 1.

Source: Ionics Inc.

Figure 1-2 Sodium chloride solution under the inf luence of a DC potential

Source: Ionics Inc.

Figure 1-3 Ion exchange membranes in an NaCl solution (DC circuit open)

4 ELECTRODIALYSIS


© 3. Compartment 3 — The Na+ ions cannot move through the anion

membrane and remain in compartment 3. The Cl– ions cannot passthrough the cation membrane and also remain in compartment 3.

4. Compartment 4 — The Cl– ions pass through the anion membrane intocompartment 5, while Na+ ions pass through the cation membrane intocompartment 3.

5. Compartment 5 — The Na+ ions cannot pass through the anion membraneand remain in compartment 5. The Cl– ions cannot pass through thecation membrane and remain in compartment 5.

Compartments 2 and 4 are depleted of ions, whereas compartments 3 and 5 have ahigher concentration of ions. When these membranes are properly arranged, twomajor and separate streams are produced (demineralized and concentrated), as wellas two minor streams from the electrode compartments. For water treatment, severalhundred of these compartments are assembled into a membrane stack, forming theheart of an ED system.

DEVELOPMENT OF ED AND EDR SYSTEMS _________________ED selectively removes dissolved solids, based on their electrical charge, bytransferring the brackish water ions through a semipermeable ion exchangemembrane charged with an electrical potential. Figure 1-5 shows a schematic of anentire ED system. It points out that the feedwater becomes separated into thefollowing three types of water: (1) product water, which has an acceptably low TDSlevel; (2) brine, or concentrate, which is the water that receives the brackish waterions; and (3) electrode feedwater, which is the water that passes directly over theelectrodes that create the electrical potential.

EDR involves reversing the electrical charge to a membrane after a specificinterval of time. As described later, this polarity reversal helps prevent the formationof scale on the membranes. Figure 1-6 shows a schematic of an EDR system. Thesetup is very similar to an ED system except for the presence of reversal valves.

Demineralization of brackish water using ED was pioneered in the 1950s. EDhas been used successfully over the past 40 years to treat municipal and processwater supplies. ED process technology has advanced rapidly since its inceptionbecause of improved ion exchange membrane properties, better materials of

Source: Ionics Inc.

Figure 1-4 DC potential applied across electrodes for an NaCl solution with ion exchangemembrane

INTRODUCTION 5


©Legend:C Conductivity ControllerPRV Pressure-Regulating Valve

Legend:C Conductivity ControllerPRV Pressure-Regulating Valve

Source: Ionics Inc.

Figure 1-5 Electrodialysis system flow diagram

Source: Ionics Inc.

Figure 1-6 Electrodialysis reversal system flow diagram

6 ELECTRODIALYSIS


©construction, advances in technology, and the evolution of polarity reversal. Accordingto IDA Desalting Plants Inventory,* the installed worldwide capacity of ED and EDRmembrane treatment plants increased from 2 mgd (7.5 ML/d) in 1955 to more than200 mgd (750 ML/d) in 1992.

Custom-designed and prepackaged ED and EDR plants provide water atpredetermined TDS or salt-removal levels with high water recovery rates (i.e., withlow amounts of feedwater being sent to waste). Additional production can be achievedby adding process trains or by operating the units in parallel (side by side) ratherthan in series (one after the other). The desalting capacity can be increased withadditional stages of membranes in series. ED and EDR systems are capable oftreating variable source water quality while producing a consistent finished waterquality.

ED and EDR plants can be designed to remove from 50 to 99 percent of sourcewater contaminants or dissolved solids. Source water salinities of less than 100 mg/Lup to 12,000 mg/L TDS can be successfully treated to produce finished water of lessthan 10 mg/L TDS.

Batch and Continuous ElectrodialysisThe first type of commercial ED system was the batch system. In this type of EDsystem, source water is recirculated from a holding tank through the demineralizingspacers of a single membrane stack and back to the holding tank until the final purityis obtained. The production rate is dependent on the dissolved minerals concentrationin the source water and on the degree of demineralization required. The concentratestream is also recirculated to reduce wastewater volume, and continuous addition ofacid is required to prevent membrane stack scaling.

The second type of commercially available system was the unidirectionalcontinuous-type ED. In this type of system, the membrane stack contains two stagesin series; each stage helps demineralize the water. The demineralized stream makesa single pass through the stack and exits as product water. The concentrate stream ispartially recycled to reduce wastewater volume and is injected with acid to preventscaling.

ED systems are unidirectional in the sense that cations move only toward thecathode and anions move only toward the anode. The current polarity does notreverse. (However, the direction of flow could reverse, and some commercial systemsuse this technique to deter the buildup of slime and foulants.) In unidirectional EDsystems, scale prevention is achieved either by the use of scale inhibitors for calciumsulfate (CaSO4) control and/or acids for carbonates control, or through the use ofpermselective membranes. Permselective membranes can be tailored to inhibit thepassage of divalent anions or cations, such as sulfates, calcium, and magnesium.Permselective refers to the ability of an ED membrane to discriminate betweendifferent ions to allow passage or permeation through the membrane. For example,the AST-type membranes show good permeation or high transport numbers formonovalent anions, such as Cl– or NO–2, but have low transport numbers and showvery low permeation rates for divalent or trivalent ions, such as SO4–2, PO4–2, orsimilar anions. This is achieved by specially treating the anion membrane, and theeffect can be exploited to separate various ions. Existing commercial membranes aremonovalent anion specific, monovalent cation specific, or hydrogen ion specific. Therelative specificities vary, with the monovalent anion membrane showing the greatest

INTRODUCTION 7

*Available from the International Desalting Association, Topsdale, Mass.


©specificity, for example, the ratio of chloride to sulfate ion transport numbers.Through the use of proper staging, with monovalent and divalent permselectivemembranes, the development of high calcium sulfate in the concentrate side of themembranes can be forestalled and scale formation prevented. Figures 1-7 and 1-8illustrate.

Figure 1-7 illustrates how a combination of monovalent anion selectivemembranes in a first stage, followed by a second stage containing monovalent cationpermselective membranes, can be used to concentrate solutions well past the normalcalcium sulfate solubility limits. In the first stage, no sulfate passes through themembrane, and so the concentrate is rich in calcium chloride. Rather than passingthis concentrate to stage 2, the stage 2 system concentrate is made up from fresh feedor another source. Here, the passage of calcium ions is retarded and the concentrateis rich in sodium sulfate. Neither stage ever exceeds the calcium sulfate solubilitylimits. Yet, when the two concentrate streams are combined, together they can farexceed the calcium sulfate limit. In fact, precipitation can result on mixing.

Figure 1-8 is a detail of the first stage from Figure 1-7 showing the use of astandard membrane with a monovalent anion permselective membrane. Theconcentrate stream is very low in sulfate, about equal to or slightly greater than thefeedwater, while the chloride and sodium concentrations, for example, could be manytimes higher than the feedwater. In other schemes, the concentrate can be made upfrom a separate water source that is already low in sulfate (for example, reverseosmosis permeate or ED dilute water) to increase water recovery.

Colloidal particles or slimes that are slightly electronegative may accumulate onthe anion membrane and cause membrane fouling. This problem is common to allclasses of ED systems. These fouling agents are removed by flushing with cleaningsystems.

Control of scale and fouling is critical to all membrane systems — ED, EDR, RO,UF, and others. Costs to install, operate, and maintain chemical feed systems as well

Source: Thomas D. Wolfe.

Figure 1-7 Use of monovalent permselective ED membranes for high recovery (concentrationof calcium sulfate in saturated waters)

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©as chemical storage facilities can significantly add to the costs of any membrane-based system.

Electrodialysis ReversalElectrode compartments in EDR perform differently from those in unidirectional ED.EDR systems, first developed in the 1960s, incorporate electrical polarity reversal tocontrol membrane scaling and fouling. These systems are designed to producedemineralized water continuously without continuous chemical addition duringnormal operation.

In EDR systems, the polarity of the electrodes is reversed two to four times eachhour. When polarity is reversed, chemical reactions at the electrodes are reversed. Atthe negative electrode, reactions produce hydrogen gas and hydroxide ions. Hydroxideraises the pH of the water, causing calcium carbonate (CaCO3) precipitation. At thepositive electrode, reactions produce acid, oxygen, and some chlorine. The acid tendsto dissolve any calcium carbonate present to inhibit scaling.

Valves in the electrode streams automatically switch flows in the two types ofcompartments. Streams that were in demineralizing compartments become concen-trate streams, and concentrate streams become demineralizing streams, as shown inFigure 1-9.

Because of the corrosive nature of the anode compartments, electrodes areconstructed of an inert metal, usually platinum coated. The current-reversal processaffects the operation of a membrane system by

• detaching polarization films

• breaking up freshly precipitated scale or seeds of scale before they can causedamage

Source: Thomas D. Wolfe.

Figure 1-8 Principle of monovalent permselective electrodialysis

INTRODUCTION 9

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©• reducing slime formations on membrane surfaces

• reducing problems associated with the use of chemicals

• cleaning electrodes with acid automatically during anodic operation

APPLICATIONS ____________________________________________Both ED and EDR are electrically driven membrane processes that selectively removesoluble ionic constituents carrying electrical charges that pass through permeable ionexchange membranes. The natural electrical conductivity of water allows ED andEDR processes to be applied to a wide range of water treatment objectives.

In ED and EDR systems, the membranes are impermeable only to water and toparticles that have a particular characteristic (e.g., a certain charge), so these systemsdo not present a barrier to remove bacteria or noncharged organic contaminants. Incontrast, RO, NF, UF, and MF systems filter water through membranes designed toremove contaminants in the molecular and ionic size ranges, effectively removingGiardia cysts and enteric viruses. RO can be used in combination with ED and EDRto remove these contaminants and to further concentrate the waste stream.

Reduction of Total Dissolved SolidsReduction of TDS to meet drinking water standards is the most common applicationof ED and EDR technology. Plants treating brackish sources that contain up to10,000 mg/L TDS can reliably and economically yield product water containing lessthan 500 mg/L TDS. For example, an EDR plant in Sarasota County, Fla., treatsbrackish well water with 2,500 mg/L TDS and yields product water of less than350 mg/L TDS — an overall reduction in dissolved solids of 86 percent — with85 percent recovery. The same plant can produce product water with 500 mg/L TDSfrom source waters containing as much as 3,600 mg/L TDS at the same rate ofrecovery. This flexibility is particularly important in applications for which multipleor variable source waters are used.

Control of Inorganics and Ionized ContaminantsED and EDR also control specific inorganic constituents or ionized contaminants inwater. Common applications include the reduction of naturally occurring levels ofsodium, chloride, fluoride, or sulfate to below the US Environmental ProtectionAgency (USEPA) regulatory levels.

ED and EDR can be used to remove or reduce some of the following commonionized constituents:

• TDS

• chromium

• sodium

• mercury

• chloride

• copper

• sulfate

• uranium

• fluoride

• nitrate and nitrite

• iron

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©• selenium

• hardness

• barium

• bicarbonate

• cadmium

• strontium

Removal rates for these ionized constituents are similar to TDS removal rates.In addition to overall TDS reduction, ED and EDR systems effectively treat sourcewater problems, such as saltwater intrusion; high nitrate–nitrite and selenium levelsfrom agricultural contamination; high, naturally occurring fluoride levels; and heavymetals contamination.

A. Before polarity reversal

Source: Ionics Inc.

Figure 1-9 Reversed polarity in EDR systems

B. After polarity reversal

INTRODUCTION 11


electrodialysis and electrodialysis reversal m38 · introduction© electrodialysis (ed) is an...

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