DI is an expression meaning "deionization". This term comes from the fact that the resin mixture in the tanks treats the water through the principle of deionization. The term mixture is used to define the resins inside the tank, as there are several different types. To name only the major types, cationic resins capture the positive ions present in the water (cations) while anionic resins capture the negative ions (anions). Mixed resin tanks therefore have the ability to exchange both anions and cations, resulting in the production of highly ionically pure water.

Briefly, deionization works by exchanging unwanted ions with other types of ions. Generally, cations will be exchanged for hydrogen (H+) while anions are exchanged for hydroxide (OH-). These two ions combined (H+ / OH-) form H2O, which is why the water coming out of a DI system is so ionically pure.


What are the characteristics of a deionization system (DI)

 Deionization systems can be found in many places in a water treatment system. Typically, the DI section of an ultrapure water system consists of a tank or tanks filled with resin and quality indicators. The size of the tanks varies according to the flow rate. The larger the media (resin) bed, the higher the flow rate. While differences in the diameter of the tank(s) allow for adjustment of the filtration rate, the number of tanks and the quantity of resin allow for adjustment of the quality, continuity and redundancy of the treatment. Depending on the quality of the water to be treated, a greater quantity of resin allows the exchange of a greater quantity of contaminants. Moreover, the more resin there is, the longer your system can operate without needing regeneration.

The quality indicators found on deionization systems are generally used to indicate whether or not the ion exchange is done properly. Although there are several other types of indicators, the most commonly used quality indicator is a resistivity meter. Calculating the resistivity of water indirectly tells you how many ions are in the water since pure water that is free of contaminating ions does not conduct electricity.

Essentially, to indicate if ion exchange is occurring properly, the quality indicator lights up green. When the ion exchange is not adequate, the quality indicator lights up red. There are obviously other types of quality indicators that can be more precise in their indications, but they are rarer and can sometimes be useless. To make a long story short, if we keep in mind the red/green indicators, when the red light comes on, the resin is exhausted and must be regenerated or changed.

If you are curious about how the regeneration process works, check out this article!


What is the point of having multiple tanks in a DI system?

DI systems are built to provide water of a pre-established quality on a continuous basis. This is why there are often two tanks next to each other. The first tank is generally used to remove the majority of unwanted ions while the second acts as a polisher to ensure that the quality of the treatment is consistent.


Although the presence of two tanks ensures the consistency of operation and treatment of DI systems, quality indicators are the only way to know if the resin inside the tanks is exhausted or not. To ensure that processing can be done without interruption and to give operators time to perform the necessary steps, a quality indicator should always be installed between the first and second tank. This way, when the resin in the first tank runs out, the second tank continues the treatment and allows the operators to begin the process of changing the tanks.

 It is common to see a quality indicator after the second tank in the DI system. Typically, when an indicator is installed at this location, it is much more accurate than the one at the first tank and can provide more information to help monitor the process more appropriately. In other words, the purpose of a second quality indicator is to monitor the effectiveness of the entire DI system, while the first is to identify when the resin needs to be changed.


When and how to change DI tanks

As mentioned above, DI tanks must be changed when the quality indicators turn red, or exceed the limits established for the water treatment in question.

To start the replacement of the DI tanks, it is important to empty the water inside them. There are two ways to do this. The first is to cut off the main water inlet to the system and let it drain. The second is to bypass the tanks if the piping and valves allow it. Once the tanks are drained, make sure the system is depressurized by opening a bleed valve. If your system does not have a bleed valve, you can always safely and quietly unscrew any connection on the system.

 Once the water has been drained and the tanks depressurized, you can proceed by removing the main tank. Once this tank is removed, move the second tank, the one used for polishing the water, to the location of the first. This step is known as rotating the tanks. Since the resin in the polisher is in contact with far fewer contaminants, it does not exhaust as quickly, so rotating the tanks will make your system more efficient.

Once the main tank is reinstalled, install the tank with the regenerated resin as the polisher. Now that they are both installed, make sure that all connections are properly installed and before you start filling the system with water, make sure that the air in the system can be evacuated.


The abbreviation DI, meaning deionization, refers to ion exchangers used for water treatment. As you may have noticed, we have focused on the quality indicators as these are essential to the proper functioning of a deionization system as they allow the user to know when the resin needs to be changed or regenerated.

It is important to note that the terms " resin change " vs. " resin regeneration " are very different and one of them can save you a lot of money while promoting the respect of the environment.
To learn more about the principle of regeneration, visit this page:

 Anyway, if you have any questions, comments or needs of assistance related to your DI systems, do not hesitate to call us, write to us or leave us a comment below.