Electrodialysis and Electrodialysis Reversal AWWA MANUAL M38 First Edition© FOUNDED 1881 American Water Works Association Copyright (C) 1999 American Water Works Association All Rights Reserved Contents Preface, v Acknowledgments, vii Chapter 1 Introduction . 1 Basic Water Chemistry Concepts, 1 Operating Principles of ED and EDR, 3 Development of ED and EDR Systems, 5 Applications, 10 Chapter 2 Design . 13 Components of ED and EDR Design, 13 Staging, 20 Limiting Parameters, 22 Water Recovery, 25 Pretreatment, 26 Operating Principles for Design, 29 Posttreatment, 31 Concentrate Disposal, 32 References, 35 Chapter 3 Equipment and Installation . 37 Equipment Subsystems, 37 Installation, 41 Costing, 42 References, 44 Chapter 4 Operation and Maintenance . 45 Operation Procedures, 45 Maintenance Requirements, 47 Safety, 52 Abbreviations, 55 Additional© Sources of Information, 57 Index, 59 iii Copyright (C) 1999 American Water Works Association All Rights Reserved AW WA M A N UA L M38 Chapter 1 Introduction Electrodialysis (ED) is an electrically driven membrane process used to demineralize brackish water. Brackish waters lie under approximately two thirds of the United States, and inland rivers, such as the Rio Grande and the lower reaches of the Colorado, also contain high levels of salinity. Water is classified as brackish when mineral 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) and seawater more than 30,000 mg/L TDS. ED and electrodialysis reversal (EDR) reduce TDS in brackish source water by electrically removing contaminants that exceed acceptable levels for drinking and process water. An overview of membrane process applications based on the molecular weights of contaminants appears in Figure 1-1. The ED and EDR processes are competitive with reverse osmosis (RO) in treating brackish waters. Typical ED systems include chemical feed systems for antiscalant and perhaps acid addition, a cartridge filter for prefiltration, the ED unit, and equipment for aeration, disinfection, and stabilization. EDR systems can often operate without fouling and scaling chemical 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 and EDR 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 water chemistry concepts is provided here. Ionic Solutions An ion is a charged atom, molecule, or radical, the migration of which affects the transport of electricity through an electrolyte solution. For example, common table salt is a typical ionic compound. The chemical name for this crystal is sodium chloride, and the chemical symbol is NaCl. The crystal consists of two types of 1 Copyright (C) 1999 American Water Works Association All Rights Reserved 2 ELECTRODIALYSIS 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. Figure 1-1 Membrane processes overview charged atoms, sodium and chloride, that are held together by electrically attractive forces. If a crystal of salt is dissolved in water, water molecules will orient themselves around the charged atoms and nullify the attractive force between them. This is known as the solution and dissociation (dissolving) of a salt in water. When this occurs, two electrically charged particles are formed, one with a positive charge (sodium, represented as Na+) and one with a negative charge (chloride, represented ©– as 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 that accepts an electron during the dissociation process will have a net charge of –1. An atom that gives up an electron during the process will have a net charge of +1. These resulting charged particles are ions. The positively charged ions are called cations, and the negatively charged ions are called anions. These two types of ions are completely dissociated and mobile in water. In the same manner as salts, minerals and acids may also dissociate into ions in solution. Some common ions that may be found in natural water are shown in Table 1-1. Some of the ions listed in Table 1-1 have more than one positive or negative Copyright (C) 1999 American Water Works Association All Rights Reserved INTRODUCTION 3 Table 1-1 Common ions found in natural waters Cations Anions Sodium (Na+) Chloride (Cl–) +2 – Calcium (Ca ) Bicarbonate (HCO3 ) +2 –2 Magnesium (Mg ) Sulfate (SO4 ) + – Potassium (K )Nitrate (NO3 ) charge associated with them (e.g., calcium has a charge of +2). In these cases, the ion has accepted or given up more than a single electron during the dissociation process. Electrical Conductivity The 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 and immersed in pure water, no electric current passes between the electrodes because no ions exist in the solution to transport the current. In an ionic solution, however, the dissociated ions transport the electric charge between the two electrodes. The ability of a solution to carry an electric charge is known as conductivity and is measured in either micromhos per centimetre (µmho/cm) or microsiemens per centimetre (µS/cm). Conductivity is affected by the concentration of ions, the ionic composition, and the temperature 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 move through the solution more quickly. • Raising the temperature increases ion mobility, resulting in an increase in conductivity. OPERATING PRINCIPLES OF ED AND EDR __________________ Electrodialysis is an electrochemical separation process in which ions are transferred through ion exchange membranes by means of a DC voltage. This process can be understood more clearly by referring to Figure 1-2, which shows a tank filled with an NaCl 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 produce – two 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 ion exchange 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 conductive membranes that are water impermeable and allow only negatively charged ions to pass through Copyright (C) 1999 American Water Works Association All Rights Reserved 4 ELECTRODIALYSIS Source: Ionics Inc. Figure 1-2 Sodium chloride solution under the influence of a DC potential Source: Ionics Inc. Figure 1-3 Ion exchange membranes in an NaCl solution (DC circuit open) • cation transfer membranes (C in Figure 1-3), which are electrically conductive membranes that are water impermeable and allow only positively charged ions to pass through Varieties of these basic types of membranes exist that are selective to ions that are either monovalent (having a charge magnitude of 1) or divalent (having a charge magnitude of 2). Other types can be formulated to enhance the passage rates of selected© ions. For example, membranes exist that show an affinity for nitrate passage over other anions. In Figure 1-3 there is no DC potential applied to the electrodes and no movement of ions. Figure 1-4 shows what occurs when DC potential is applied across the electrodes. The figure shows six compartments separated by ion exchange membranes. The membranes influence ion behavior as follows: 1. Compartments 1 and 6 — Compartments 1 and 6 contain metal electrodes where reduction and oxidation occur. 2. Compartment 2 — Cl– ions pass through the anion membrane (A) into compartment 3, while Na+ ions move through the cation membrane (C) into compartment 1. Copyright (C) 1999 American Water Works Association All Rights Reserved INTRODUCTION 5 Source: Ionics Inc. Figure 1-4 DC potential applied across electrodes for an NaCl solution with ion exchange membrane 3. Compartment 3 — The Na+ ions cannot move through the anion membrane and remain in compartment 3. The Cl– ions cannot pass through the cation membrane and also remain in compartment 3. 4. Compartment 4 — The Cl– ions pass through the anion membrane into compartment 5, while Na+ ions pass through the cation membrane into compartment 3. 5. Compartment 5 — The Na+ ions cannot pass through the anion membrane and remain in compartment