Understanding
Deionization Resins

How DI Resins Work

Deionization Resins, or more appropriately, Ion Exchange Resins, are made of Polymer Beads that are 0.5-1mm in diameter. They are either negatively charged (anions) or positively charged (cations). These are used in conjunction to remove ionic contaminants from your water source. In solution, salts separate into positively charged cations and negatively charged anions. As the water comes into contact with the resins, the salts in the water bind to the anions and cations. The cation resins release Hydrogen ions (H+) during the ion exchange process, and the anion resins release Hydroxide ions (OH-). These released ions then combine to form pure water (H20). 

How Long Should Do They Last?

Removal capacities of resins are expressed as ppm-gallons. In order to figure out how much DI water your cartridge will produce before it is exhausted, you would divide the ppm-gallons rating by the TDS of the RO water before it enters the DI stage. For example, if your DI cartridge is rated at 6000 ppm-gallons and the TDS of your RO water is 6ppm, then you should be able to process 1000 gallons of water before your DI cartridges become exhausted. (6000 ppm-gallons / 6 ppm)

Note: This is only a rough estimate of the life of your DI resins as it does not take into account the presence of gases such as ammonia and CO2 which do not register as TDS but are removed by the DI stage.

Phosphate and Silicate Dumping

It is important to understand the difference between filtration and ion exchange. As the pore sites in a carbon prefilter get filled up, the water will pass through it basically untouched. This is very different from what happens with DI cartridges. When the DI resins no longer have Hydrogen or Hydroxide ions to exchange, they release the ions with the weakest bonds in order to bind with the ions in the water that have the strongest bonds. These weakly bonded ions include phosphates and silicates. Continuing to run water through an exhausted DI cartridge results in what is referred to as phospate and silicate dumping. This can actually result in a higher TDS in the water exiting the DI stage than was initially in the RO water and may have disastrous results for your reef tank.

Why SpectraPure Resins Are The Best

There are literally thousands of different variations of ion exchange resins, all of which are configured for specific purposes. At SpectraPure, we have been custom blending resins for laboratory, industrial and aquatic applications for almost 30 years now. While other aquatics companies use generic resin blends in their DI cartridges, SpectraPure is the only company that custom blends resins specifically for the aquatics trade. We have thousands of dollars of testing equipment and test every batch of resin that we produce. Our proprietary MaxCap® and SilicaBuster™ resins are the result of these decades of research and testing.

Since pure water is a poor conductor of electricity, purity is best measured using resistivity (megohms). Water is considered to be ultrapure when the resistivity is 18.2 megohms. This is represented by the vertical axis on the above chart. The horizontal axis represents the total number of gallons processed. Standard inline TDS (Total Dissolved Solids) meters will typically read 0 ppm until they reach approximately .25 megohms, at which time they will read 1 ppm. They will not indicate the efficiency of the removal capacity of the DI resins up to that point. You will notice that 4 of the samples reach the point of exhaustion (.25 megohms) at approximately the same point but their paths to that point are quite different, resulting in entirely different removal capacities.

Points to consider when reading the above chart:
Does the water achieve a reading at or near 18.2 megohms?
As you can see in the above chart, the mixed bed resin from Competitor 1 doesn’t even achieve half of that value. It starts to drop quite radically after 40 gallons and is seriously depleted after processing just over 60 gallons of water.

How long does the curve stay at or near 18.2 megohms?
The removal capacity of the DI resins is represented by the area under the curve. Generally speaking, the longer the curve stays at or near 18.2 megohms, the greater the removal capacity of the DI resins.

How sharply does the curve drop from 18.2 megohms to the point where your meter reads 1 ppm?
This depicts how long your DI resins work at a reduced capacity before your meter shows that it is time to replace them. The steeper the drop on the curve, the more time your DI resins spend at their peak efficiency.

By analyzing and comparing the area under the curves, we can obtain a correlation between the removal capacities of the different resins.

Although both of SpectraPure’s resins stay at the maximum removal capacity for the same amount of time, the color-indicating resin drops more sharply resulting in a removal capacity of .9 that of our standard mixed bed resin. Both of SpectraPure’s resins remain at their peak removal capacity for a longer amount of time and have a higher removal capacity than any of our competitor's mixed bed or color-indicating resins.