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All You Need to Know About Deionization DI 101

by Mark Timmons April 20, 2022 0 Comments

Before I start this blog, I want to make sure that the reader understands the Point-of-View from which it is being written.

What it is not: This is not a scholarly white paper for the seasoned water treatment professional. God knows there are plenty of those out there. This blog is not for that purpose. What it is: This is an educational piece designed to provide the end-user of Deionized Water (also called DI water) with an overall understanding of what deionized water is and how to best decide what is the best method to provide it for their particular application.

What is DI Water?

DI water systems typically consist of beds (tanks or cartridges) comprised of cation and anion resins. DI systems produce various endpoint water qualities at various flow rates, depending on the configuration of the system. Therefore, each application must be considered independently.

Years ago, high purity water was used only in limited applications. Today, deionized 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
  • Aquariums, fish tanks ,and reefkeeping
  • Electronics manufacturing plants
  • Food processing plants
  • Humidification (humidors, libraries, etc.)
  • Plating and anodizing
  • And even the final rinse at the local car wash!

Here are some of the most common types or grades of DI water:

Not unlike any other water treatment application or technology, deionization or DI, as it is called for short, has its inherent limitations. These limitations are based upon the following:

  • The water quality of the water supply being treated;
  • The quality of water needed as the end product; and
  • The volume of that water required for the application or process.

The vast majority of dissolved impurities in modern water supplies are ions such as calcium, sodium, chlorides, and iron. Through using a deionized water system for laboratory-grade water, these ions can be removed through a technique called ion exchange. Positively charged ions (cations) and negatively charged ions (anions) are exchanged for hydrogen (H+) and hydroxyl (OH-) ions, respectively, due to the resin's greater affinity for other ions. The ion exchange process occurs on the binding sites of the resin beads. Once depleted of exchange capacity, the resin bed is regenerated with concentrated acid and caustic, which strips away accumulated ions through physical displacement, leaving hydrogen or hydroxyl ions in their place.

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.

Deionized water quality is measured by conductivity or resistivity. In the chart below, resistivity is represented above the line in ohms or megohms and conductivity is below the line measured in microsiemens.

For low volume/low quality commercial and industrial applications requiring 50K-Ohm to 2 Meg Ohm resistivity water (Type II or Type III DI Water), cartridge systems can prove extremely cost effective. DI cartridges have a finite capacity, based on cartridge size, to perform the ion exchange before becoming exhausted and requiring replacement. These are the simplest of systems, as they are flow-through systems requiring little more than a line in and line out for installation, along with a chlorine reduction pre-filter (carbon).

For higher volume and higher purity DI water applications (like laboratory-grade water), it is typically recommended to pretreat the water via reverse osmosis (RO). In doing so, the reverse osmosis unit will perform the bulk of the total dissolved solid (TDS) reduction, allowing the DI cartridges to polish the low TDS water. The resulting benefits are higher water purity (2 to 17+ Meg Ohm) and longer DI cartridge runtimes, resulting in dramatically lower consumable costs.

For ultra-high purity critical applications, which require up to 18.2 Meg Ohm resistivity water (Type I DI Water), additional system components will be required. These can include recirculation loops, pumps, UV sterilization, TOC reduction, along with additional final filtration and a more highly sophisticated monitoring system. Such a system requires a combination of pretreatment, reverse osmosis and DI to meet the water specification. These systems can be self-contained in a bench-top system or floor-standing, based on the application and volume.

  • Type III Purified Water is considered to be 1 meg ohm to 0.02 ohms or 1 to 50 microsiemens. Type III DI water is used for parts cleaning and general lab cleaning.
  • Type II Pure Water is considered to be 1 to 10 meg ohms or 1.0 to 0.1 microsiemens. Type II DI water is used in may labs, cleaning and food grade products.
  • Type I Ultra-Pure Water is considered to be 10 to 18.2 meg ohms or .01 to 0.0555 microsiemens. Type I DI water is used for semiconductor final cleaning, medical devices and water for infusion.

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.

TRUE STORY: Several years ago, I met with a plant manager, who had an Exchange-Tank Type DI System. They were spending $74,000.00 a year for these tanks and they only needed Type III Water, which we could deliver with just an RO system, without DI Tanks or Cartridges. We installed a Reverse Osmosis System and a Dual Bank DI Cartridge System as a backup. They never use the DI now as the RO water is sufficient for their needs, but if they ever need DI, the housings are there. The cost of the equipment and installation was $34,000.00 and they spend $2,000.00 to $4,000.00 a year on maintenance. What a nice payback!

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 few 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 1,000 gallons a day (as an example):

Step 1: Granular Activated Carbon Filter - Removes chlorine and chloramine which is detrimental to RO Membranes.

Step 2: Anti-Scalant System - Prevents scale from forming on the membranes, 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. Whether you need water that is Type I, Type II, or Type III, we also have systems specifically for that. Most RO - DI systems are custom engineered for our customers particular applications.

While there are many types of DI resins, we primarily deal with three:

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.

Typically, we see Mixed-Bed DI resin in tanks or cartridges. As mentioned earlier, years ago, many two column DI systems existed and even regenerate-in-place systems were common, but with EPA regulations the caustic and acid nature of the regenerants have eliminated many two column automatic regenerating systems.

Service exchange (SDI) or portable exchange (PEDI) DI tanks were popular for years, but now there are generally better options as the labor and transportation costs have skyrocketed. SDI and PEDI systems allow for the use of a service provider's DI tanks for the purpose of high purity water production. These tanks are used across a wide variety of industries for humidifier feed, washing/rinsing, lab water, aquatics/reefkeeping, chemical formulation, research, science and analytical applications, just to name a few. For these users, water quality is critical to their process. Exchange tanks are perceived by many as their only option, but the reality of the situation is that DI Cartridges are almost always lower in cost and convenience.

DI cartridges contain the same cation and anion resin that is utilized in mixed bed tanks, only in a smaller package, with less capacity and lower flow rate capabilities. Systems are configurable based on the application and are easily expandable in the future.

With exchange tanks:

  • Tanks are commonly exchanged with capacity remaining and which you are paying for, but never realizing.
  • Tanks are commonly exchanged after so many days, regardless of how much water you have used.
  • Tanks are exchanged at some point after you call, based on a quality indicator. At this point, the water quality is less than desirable. This can take days.
  • Tanks make you dependent on someone else.

With DI cartridges :

  • DI cartridge systems eliminate the need for an outside service provider to enter your facility.
  • DI cartridges are replaced on your schedule, not the service provider's.
  • DI cartridge systems allow you to keep spare cartridges onsite.
  • DI cartridge systems give you back control.

As with DI tanks, DI cartridge systems fed with reverse osmosis water allow for much greater run times before exhaustion/replacement. This would be the case for applications requiring greater than 2 megohm resistivity.

What does Your Business Need?

A day doesn't go by that we don't receive a phone call (usually multiple calls) with the following request: I need a price for a DI water system.

Simple enough, right! Well, not really. Sometimes to the frustration of the caller, we come back with a list of questions that the caller is not prepared to answer. More times than not, what the caller needs is vastly different from what he or she originally considered.

Frequently, we will ask people What Type of DI Water do you need? For example, what endpoint quality of DI water do you need?

Five Things to Consider:

  1. Remember, if high purity water is critical for your process or application, one size does not fit all!
  2. What is the supply water quality?
  3. What is the application specific required water quality?
  4. What is the demand? Gallons/Liters per minute and Gallons/Liters per day?
  5. What space do you have?

Mark Timmons, aka/The Water Doctor is a Master Water Specialist and has 50 years of experience in water treatment, including filtration, deionization and reverse osmosis. It was over 40 years ago that he designed and installed his first RO/DI High Purity water system to replace a distillation system in a hospital laboratory. That system functioned without issues for nearly 40 years. It is pictured below:

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