EDI systems use ion exchange membranes, ion exchange resins and electricity together in order to remove ions from water. Ion exchange resins in the EDI system are continuously regenerated during the operation of the system without the need for any regenerant chemicals. Electric current is applied between the cathode and anode electrodes in the unit. The electric field formed between the cathode and anode attracts positive ions towards the cathode and negative ions towards the anode. There are also membranes made of cation and anion ive resin in the EDI unit. The cation ive membrane allows the passage of only cation ions, while the anion ive membrane allows the passage of anion ions. The cation and anion ive resin filled between cationic and anionic membranes, on the other hand, reduce the electrical resistance in the product water compartment and facilitate the transfer of ions. The dense water, which is transferred to the concentrate compartment and enriched in ionic charge, is drained. EDI concentrate water is generally 10 - 20 times richer in ions than feed water. As water is purified as it moves through the product water compartment and due to the increase in electric potential, an electrochemical reaction occurs and the water molecules begin to separate into H+ and OH- ions. This process, during which acid and caustic are formed, provide continuous regeneration of cation/anion based resins. EDI unit creates its own chemical regenerants naturally. Therefore EDI systems do not require external chemicals.
As the industry's need for high quality pure water increased over time, the need for pure water production systems started to increase. As the technology improved, pure water production systems evolved as shown below.
FIRST GENERATION (IXDI ION-EXCHANGING RESIN SYSTEMS) PRETREATMENT > CATION/ANION IXDI > MIXBED IXDI
In earlier times when pure water production systems were used, the treatment technology was based on delivering water passed through cation / anion ion exchange resin columns to mixbed DI columns at the final stage. This technology was based on regeneration of cation and anion resins used in the system with acidic and basic regenerant chemicals. This technology was used successfully for years until the reverse osmosis membrane technology was developed. Compared to today's technologies, the most important disadvantage of ion-exchange resin systems was the constant consumption of acid (HCl – H2SO4) and caustic (NaOH).
SECOND GENERATION (RO + MIXBED) PRETREATMENT > REVERSE OSMOSIS > MIXBED IXDI
Reverse Osmosis (RO) system which utilize cross flow filtration technology, subsequently replaced Cation/Anion Deionization systems. Since the reverse osmosis systems used before the Mixbed DI unit provide 98-99% ion removal, RO lowers the regeneration frequency of Mixbed deionzation system which is applied after. Hence reducing the consumption of regenerant chemicals and operating cost of water treatment plant. This configuration is still successfully applied today in processes that do not require ultra-pure water (Conductivity: EC< 0,1 µS/cm (Resistivity R >10 MOhm) quality.
THIRD GENERATION (REVERSE OSMOSIS + ELECTRODEIONIZATION) PRETREATMENT > REVERSE OSMOSIS > ELECTRODEIONIZATION
Development of Electrodeionization technology has ended chemical consumption during ultra pure water production. With the development of systems with RO + EDI configuration, it has become possible to produce ultra-pure quality (conductivity EC< 0,06 µS/cm (Resistivity R >16,7 MOhm) process water at high flow rates.