Water Deionization 101

Water Deionization 101
By Mark Timmons
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Years ago, high purity water was used only in limited applications. Today, deionized (or "DI") high purity water has become an essential ingredient in hundreds of applications. Just a few include the following:
  • Medical offices
  • Laboratories
  • Pharmaceutical companies
  • Cosmetics companies
  • Large aquariums,fish tanks,and reefkeeping facilities
  • Electronics manufacturing plants
  • Food processing plants
  • Plating and anodizing
  • And even the final rinse at the local car wash!
DI or deionized water is simply water that has had most of the mineral content and dissolved ion content removed. It is just pure water. The cleanliness of the water is rated as its resistance value.  It should be noted that pure water will not conduct electricity. It's the ions in the water that conduct the electricity. Deionization is simply an ion-exchange process in which water flows through resin beds or resin beads. Cation resin exchanges hydrogen ions (H ) for positive ions, and anion resin exchanges hydroxide ions (OH-) for negative ions. Sometimes there are two column of Deionization - one for the Anions and one for the Cations, but the highest quality water is produced by what is called mixed-bed DI or "MBDI" whereby the Cation exchange and Anion exchange resins are mixed together. Separate Anion and Cation columns are now used infrequently because of the superiority and economy of reverse osmosis.  Deionized water quality is measured by conductivity or resistivity.  In the chart below, resistivity is represented above the line in ohms or megohms ands conductivity is below the line measured in microsiemens. Type III Purified Water is considered to be 1 meg ohm to 0.02 ohms or 1 to 50 microsiemens. Type II Pure Water is considered to be 1 to 10 meg ohms or 1.0 to 0.1 microsiemens. Type I Ultra-Pure Water is considered to be 10 to 18.2 meg ohms or .01 to 0.0555 microsiemens. Quite frequently, potential customers say that they just want to deionize their water and not use reverse osmosis, not understanding that if you use deionization as the primary demineralization method, the cost per gallon will be 10 to 20 times higher than if you were to use RO as the primary disinfection method and DI simply for polishing.  When that tandem is used the cost per gallon can be two to four cents a gallon, but if you rely on just DI, the cost can exceed fifty cents per gallon.
Several years ago, I tried to explain this to a plant manager, but he insisted that he just wanted DI and decided to go with another company who was only too happy to do that.  When the operating costs of the first year were calculated, the cost for DI was $74,000.00. The cost of a RO system would have been less than $15,000.00, and for their water quality needs (Type III), they did not even need DI as the RO system produced the water quality they needed.  His boss was not impressed, and sadly, he lost his job.
If you are just needing a gallon or two a day of deionized water, then maybe it's not worth it to have an RO system (although a small RO system might cost only a couple hundred dollars.  But if you are using 10, 20, 50 or even 100 gallons of water a day, then it is imperative that reverse osmosis be the primary demineralization method before DI.  That is the most cost effective method. What a high purity Type III water treatment system might look like for an application that might need 300 gallons a day (as an example): Step 1: Granular Activated Carbon Filter - Removes chlorine and chlorine and chloramine which is detrimental to RO Membranes. Step 2:  Anti-Scalant System - Prevents scale from forming on the membrane,s increasing system efficiency and longevity. Step 3:  Reverse Osmosis System -  This is the primary demineralization method which removes up to 99% of the total dissolved solids (TDS), so that the DI components only have to polish the water off and remove that 1%. Step 4:  Deionization  System- US Water Systems has a number of DI configurations, but this is one of our most popular. Also, if you need water that is Type I or Type II, we also have systems that deliver that, like THESE. Most RO - DI systems are custom engineered for our customers particular applications.  If you are unsure of what type of DI or High Purity you need, please feel free to talk to one of our Certified Water Specialists or Commercial Engineers.  We have over 200 years of combined experience and likely have a solution for you that is proven effective and economical.
July 25, 2017
Dr.Mohamed F Shaaban
June 6, 2020 at 8:49 PM
I want to get a demineralized water with EC of 0.1 microsiemens/cm .I used a strong cation exchange unit then strong Anion -weak anion exchange unit and mixed bed exchange unit. the conductivity before the mixed bed was 6 microsiemens/cm and after mixed bed reached 0.6 microsiemens/cm while I need to get 0.1 microsiemens. how can I get it. Please advice me.
Mark Timmons
June 15, 2020 at 10:59 AM
Dr. Shaaban, There are many factors that affect this. I have copied one of our DI specialists and he will reach out directly to you.