EDI systems remove ionized and ionizable species from water using electrically active media and electrical potential (DC power) to affect ion transport. By using this type of purification technology, caustic and acidic types of chemicals are not required. The water which has been treated by RO-EDI has typically exceeded customer’s high-purity standards.
Electrodeionization system is a continuous electro-chemical process of water deionization where ion specific membranes, mixed bed resin and a DC voltage across them, replace the standard hazardous acid-caustic chemical regeneration process.
In the latest technology of EDI, modules must be pretreated with RO for many reasons. However, what are the advantages and disadvantages of softening and 1-pass and 2-pass RO as pretreatment?
Many EDI systems using Pure Aqua EDI technology perform very well with 1-pass RO. It is easy to achieve 17+ Megaohm with 1-pass RO. Most USP WFI and purified water systems easily meet requirements with 1-pass RO. As a basic knowledge in the water treatment industry, softening with 1-pass RO system is less expensive and less complex than a 2-pass RO system.
Two pass RO followed by EDI gives the following advantages, and is often used for power plants or pharmaceutical applications.
- Lower conductivity = higher EDI quality
- Lower CO2 = higher silica removal
- ppb-level contaminants means infrequent EDI cleaning
- Higher rated flows for EDI
Single pass RO system can be enhanced using membrane degasser for even better EDI performance. membrane degasser can reduce the CO2 which enhances EDI performance, reduces scaling potential, and helps SiO2 removal by the EDI machine.
In summary, EDI systems have many options:
- Softener followed by RO followed by EDI modules
- Softener followed by RO followed by membrane degasser followed by EDI modules
- RO followed by caustic dosing followed by RO followed by EDI modules
It is difficult to meet the feed specification to the EDI plants without pH adjustment and/or membrane degasser. Decreasing the CO2 before the EDI will decrease the load (lower amps) and improve the product quality. Pure Aqua installed a membrane degasser on many EDI skids for CO2 removal/reduction. The membrane degasser replaced the proposed design of a conventional forced air degasser (tower). It was more cost efficient, compact and decreases installation time.