Electrodeionization (EDI) is a purification process that is electrically driven and features a combination of ion exchange resin and ion-selective membranes. EDI, which is normally coupled with reverse osmosis, provides a useful alternative to other purification methods. It provides laboratory reagent water at high volumes without the need for deionization cartridges. This approach avoids the decrease in product water quality associated with cartridges as they become exhausted as well as the associated cartridge replacement costs.

How does electrodeionization work?

EDI has evolved from electrodialysis (ED). The principle of ED is that water is purified in a cell containing two types of ion selective membranes - cation-permeable and anion-permeable - placed between a pair of electrodes. When a direct electric potential is applied across the cell, the cations in the water are drawn towards the negatively charged cathode and the anions are drawn towards the positively charged anode. The cations can pass through the cation-permeable membrane, but not through the anionic one. Likewise, the anions can pass through the anion-permeable membrane, but not through the cationic one. The net result is the movement of ions between chambers and the water in one section can become deionized while that in another section becomes concentrated.

In practice, ED can only be used economically to produce water of relatively high conductivity (200 µS/cm or more) because of the prohibitively high electrical voltages required to drive ions through water of increasingly high purity.

This problem is overcome in EDI technology by filling the spaces between the membranes with ion exchange resins. The resins provide a conductive flow path for the migration of ions, enabling deionization to be virtually complete and resulting in the production of high-purity water. A further benefit of EDI is that the continuous electrolysis of water occurring in the cell produces hydrogen and hydroxyl ions. These ions maintain the resins in a highly regenerated state, thereby avoiding the need for chemical reactivation. The resins used in EDI systems can either be separate chambers of anion or cation beads, layers of each type within a single chamber or an intimate mixture of cation and anion beads.

Some EDI systems incorporate mixed resin beds in a plurality of narrow cells. This is particularly effective in large-scale plants for pharmaceutical and other applications. Veolia Water Systems, ELGA's parent company, is the leading supplier of a wide range of CDI technologies which address these larger scale applications.

ELGA's ADEPT (Advanced Deionization by Electrical Purification Technology) process utilizes separate beds of cation and anion resins as well as a bed of intimately mixed resins. The separate beds of cation and anion resins are housed in wide cells that provide a flow path for the ions in transit. This offers advantages in the flexibility of design and mechanical simplicity on a laboratory scale. The relatively high volume of resin in the cells provides a buffer against changes in feedwater quality. The quality of water produced is then further enhanced by a mixed resin bed.

The multiple-pass process in which feedwater pre-purified by reverse osmosis flows through a cation exchange bed, an anion exchange bed and a bed of mixed resin is analogous to many large scale high purity water purification systems.

Typically, the product water has a resistivity of 10-18 MΩ-cm (at 25°C) and a total organic carbon content below 20 ppb. Bacterial levels are minimized because the chemical and electrical conditions within the system inhibit the growth of micro-organisms.

EDI very effectively complements reverse osmosis. RO is a pressure-driven process in which the water is stripped of its contaminants as it passes through the membrane. It does not however remove all the ionic species and cannot remove dissolved species such as carbon dioxide. EDI can remove carbon dioxide as well as other weakly ionizable species, such as silica, by ionizing them and moving them through the membrane.

 What are the drawbacks of distillation?

Distillation only produces purified water slowly. It is not an on-demand process. Due to this, a quantity of water must be distilled and stored for later use. This storage of the distillate can be problematic if the container in which the water is stored is not made of an inert material. Ions or plasticizers will leach out of the container and re-contaminate the water. In addition, bacteria grow very well in water that has been standing for some time.

To maintain sterility, sterile storage bottles are used and the collected water autoclaved, but once the bottle is opened it is exposed to bacteria and contamination begins. In hard water areas stills require frequent acid cleaning, due to scale build-up, unless the feedwater is pre-treated by softening or reverse osmosis.

Why does Singapore need NEWater? As a small island , we are dependent on our neighbour, Malaysia, for water. However, if hostilities were to occur, and have occurred in the past, we might find ourselves lacking the most basic and important resource neccessary to survive. Thus, Singapore has embarked on this project to gradually decrease our reliance on Malaysis for water and one day, we might be able to depend on ourselve alone for this natural resource.

Therefore, how does NEWater work? As of every water purification system, there are certain steps involved.

The second barrier is the first stage of the NEWater production process known as Microfiltration (MF). In this process, the treated used water is passed through membranes to filter out and retained on the membrane surface suspended solids, colloidal particles, disease-causing bacteria, some viruses and protozoan cysts. The filtered water that goes through the membrane contains only dissolved salts and organic molecules.

The third barrier or the second stage of the NEWater production process is known as Reverse Osmosis (RO). In RO, a semi-permeable membrane is used. The semi-permeable membrane has very small pores which only allow very small molecules like water molecules to pass through. Consequently, undesirable contaminants such as bacteria, viruses, heavy metals, nitrate, chloride, sulphate, disinfection by-products, aromatic hydrocarbons, pesticides etc, cannot pass through the membrane. Hence, NEWater is RO water and is free from viruses and bacteria and contains very low levels of salts and organic matters.

At this stage, the water is already of a high grade water quality. The fourth barrier or third stage of the NEWater production process really acts as a further safety back-up to the RO. In this stage, ultraviolet or UV disinfection is used to ensure that all organisms are inactivated and the purity of the product water guaranteed.

With the addition of some alkaline chemicals to restore the acid-alkali or pH balance, the NEWater is now ready to be piped off to its wide range of applications.