Deionization, commonly called DI, is complete demineralization of a water supply. Specifically, the term deionization is used with respect to the removal of the cations and anions from the water. In some cases, the water is passed through two separate columns of cation and anion resin. These days, most deionization is accomplished with the use of mixed-bed DI resin.
DI water is similar to distilled water, and while it produces water that is pure in a ionic respect, it does not remove bacteria. Deionized water is commonly used in the following applications:
- Food Processing
- Car Wash Rinsing
- Industrial Processes
Typically, we see Mixed-Bed DI resin in tanks or cartridges (pictured above). Year ago, many two column DI systems existed and even regenerate-in-place systems were common, but with EPA regulations the caustic and acidic nature of the regenerants have all but eliminated two column automatic regenerating systems. Some critical water processes require DI water of varying grades of purity. There are basically three (3) types of DI or ultrapure water. Type 1 is used for semiconductor final cleaning, medical devices and water for infusion. Type 2 is used in may labs, cleaning and food grade products and Type 3 is used for parts cleaning and general lab cleaning. The table is listed below:
Here's the rub: You can use high purity DI resin to deionize water, but the cost is typically 30 to 50 cents a gallon. However, if you utilize reverse osmosis ahead of any DI system, the cost will typically drop to 1 to 3 cents a gallon. If DI is your primary demineralization method the cost per gallon can be as high as 50 cents a gallon, but if DI is the secondary deionization (or polishing) method and reverse osmosis is the primary demineralization method, the cost drops to as little as a penny a gallon. Yet, there are still people who use DI as the primary demineralization method and they are deeply loved by the companies who deliver DI tanks.
I recently installed a RO system followed by DI polishing at a factory who was spending over $70,000.00 a year on DI tanks. The entire system cost $31,000.00 and the yearly DI cost is now less than $5,000.00. The water quality is the same. They don't have to wait on deliveries and the cost is dramatically lower. Why would anyone do it the DI only method? Answer: They either have too much money and don't care or they just don't know any better. Now they do!
While there are many types of DI resins, we primarily deal with three:
- Standard Mixed Bed DI Resin - Typically is for water up to 16 megaohms;
- Semiconductor Grade Mixed Bed Di Resin - Can deliver up to 18.2 megaohms; and
- Color Indicating Resin - Turns from blue to brown when it is exhausted and generally does not deliver much more than 10 megaohms.
The Semiconductor grade mixed bed DI resin is the most popular because it delivers the highest quality water and has the highest capacity. The color indicating resin is popular in reefkeeping, but it is more expensive and delivers lower quality water. Some people just like to use it because with a clear filter housing, they can observe when it is exhausted. It's actually less expensive in the long run, to use the standard semiconductor-grade resin, and buy a conductivity monitor.
In conclusion, with proper pre-treatment, a DI system can consist of a cartridge or cartridges that can be easily changed at infrequent intervals instead of relying on delivery of a large, heavy DI tank.