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Deionization / DI Resin Filtration: When, Why & How

Often the final stage employed in multi-stage RO/DI water systems, Deionization removes the smallest of all impurities – known as ions – from water. But rather than purifying the water with a typical filter, which physically prevents impurities from passing through it, deionization involves passing the water through a bed of beads, known as “ion exchange resin” or “DI resin,” which attracts and sticks to the ions, removing them from the water.

DI resin beads in a DI canister.

DI resin works because ions themselves have electrical charges: ions with a negative charge are called anions, and ions with a positive charge are called cations. And just like magnets, these ions are attracted to oppositely charged substances. Therefore, when we pass water through a bed of DI resin, we pass it through a blend of positively and negatively charged beads, which binds all the corresponding ions and leaves only pure water at the end:

When water passes through the DI resin bed, the charged ions within it are attracted to, and bind to, the opposingly-charged DI resin.

The DI resin bed is commonly employed in a DI canister, which traps the ions while the water passes through. In an aquarium RO/DI system, the DI stage is frequently the final stage – it produces the absolute purest water. In a home RODI system, an additional polishing filter might follow for optimal taste, or a remineralizer might be subsequently employed, which replenishes a variety beneficial minerals to the guaranteed safe water.

Why Is Deionization Needed In Addition to Other Methods?

Deionization removes the particles that are too small for other filtration methods such as SED, GAC, CCB, and even Reverse Osmosis Membranes. Each of these purify water by filtering out particles larger than a specific size, from the SED filter which removes particles larger than 5 microns down to the RO membrane, which removes particles larger than 1/10,000 of a micron. But after RO membrane filtration, the few remaining particles – the ions – are as small as water molecules themselves, and can pass through any membranes that water can. Therefore, in order to remove them from water we employ the deionization process to attract the ions and bind them, and leave only the pure water to pass through to the other side.

Tips on How To Make Great Use of DI Resin

  • Monitor Your Resin: DI Resin is highly effective, but when it’s spent it won’t remove ions anymore. To ensure that your DI resin is working properly, check it periodically – especially when you think that your beads might be nearing their expiration date. Here’s what to look for:

    • Check for color changes: When DI resin nears its working lifespan, its color will have changed to an amber/orange or even brown color.
    • Mind your Total Dissolved Solids: When DI beads start to become saturated, ions start to creep through and TDS (Total Dissolved Solids) starts to increase. With a TDS meter, you’ll know when it’s time to change the resin.
  • Filter your feed water: DI resin removes ions extremely effectively, but it works most efficiently and economically by far when the feed water is already properly filtered. Fortunately, DI filtration is most commonly found after reverse osmosis filtration, which removes 95% of impurities. This pure feed water allows you to get the most out of your DI resin. It’s also a great practice to monitor your feed water TDS – at each stage if you can. Any change means its time to change a filter.
  • Stock up in bulk to save: Keep DI resin and other filters on hand. Buy in bulk to save and maximize the convenience and economic benefit of DI filtration.

When DI resin is employed as part of the RO DI purification process, it produces water that is guaranteed safe and more than 99.9% pure – regardless of the water’s source! DI resin filtration is dependable and economical. Please contact us if you have any questions about DI filtration, our RO DI systems, or anything related to water filtration.

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Other products you may be interested in:

6 – Stage RO DI System for Home

Aquarium RO DI Systems

6 Oz DI Canister w/ DI Resin Beads

Reverse Osmosis Pre & Post Filters