The last step in creating RO/DI water is deionization, which is simply removing ions from water. This step is done in the single canister on the right in the photo below. Next to the canister is a digital read-out that gives the Total Dissolved Solids (TDS) found in the RO/DI water leaving the unit. The goal is to have a TDS of zero.
Over time, the resin found in this canister becomes spent and needs to be replaced. We know we need to do this when the TDS starts to rise above zero. We live in a rural area with well water, so for us we need to change this resin every 4 months or so. The resin we purchase is from Bulk Reef Supply and cost around $11 for this 1.25 lb bag. They also have some great videos on how to refill cartridges properly.
The resin looks like this inside the vacuum sealed bag. The consistency reminds me of moisturising beads found in body washes.
We have a refillable DI cartridge. We simply dump the spent resin in the trash and refill with new.
The resin should be packed in very tightly, with almost no give when pressed down. We used the entire bag of resin in this cartridge. A foam ring gets crammed into the top followed by a screw on lid.
The cartridge can then go into the DI canister of the RO/DI unit. The first few minutes of water from the RO portion of the RO/DI process will have a high amount of total dissolved solids and is known as TDS creep. This water should be diverted to help save on the DI resin. When this particular type of resin starts to go bad it turns a golden or brown color like in the photo below.
Because we make our own RO/DI water at home, procedures like this are a necessary maintenance in order to have the convenience of having RO/DI water right in our home when needed. And for $11 every few months, I’m not complaining!