How to Calculate the Capacity of a Deionization System (SDI)
The ion exchange capacity of a resin is calculated according to the total dissolved solids in the influent.

The capacity of a deionization system (SDI) can be measured in several ways. It can be calculated using milliequivalents (MEQ) per liter, per cubic foot of resin or in grains. Generally speaking, 1 MEQ is equal to 1 mg of total dissolved solids (TDS) in CaCO3. In terms of grains, they represent 1 mg of TDS = 0.0584 grains.

Estimating the treatment capacity of a deionization system allows you to calculate the quantity of resin required to meet your water treatment needs in terms of both flow rate and quality. It is important to note that the treatment capacity of a deionization system is intimately linked to the quality of the water to be treated. The higher the total dissolved solids, the more quickly the resin is exhausted.

Calculate the treatment capacity of a deionization system

To begin, convert the number of milligrams of total dissolved solids (TDS) present in the water to grains per gallon (GPG). Once the conversion is complete you can then use the following data to estimate the treatment capacity of a cubic foot of resin

  • New mixed bed resin: 10,000 grains/ft3
  • Regenerated mixed bed resin: 9000 grains/ft³

To put things in perspective, let's imagine that the inlet water has a concentration of 25 grains per gallon. With a 20 cubic foot capacity tank of new resin, you could treat about 8000 gallons of water. Here is the calculation:

  • 10,000grains|ft3 x 20ft3 / 25gpg = 8000 gallons of water

Example of the City of Montreal

Let's use the example of the city of Montreal to better highlight the importance of properly calculating SDI capacity. Montreal water has a total dissolved solids (CaCO3) concentration of 15 grains/gallon. With this kind of concentration, a 520 PG cylinder with a capacity of 3.6 cu. ft. of resin would have a treatment capacity of approximately 2400 gallons of water. If this cylinder was used for a flow rate of 5gpm, which is very low, the resin would have to be changed every 8 hours

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Procedure for determining MCFM flow capacity - “Filterability Test”
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