Important Takeaways
Ion exchange replaces ions
Cation (H⁺) swaps positive ions; anion (OH⁻) swaps negative ions; H⁺ + OH⁻ → H₂O.
RO + DI is the sweet spot
RO removes 95–99% of load; DI polishes to ultra‑low conductivity and lowers cost per gallon.
Choose two‑bed vs. mixed‑bed
Two‑bed for bulk demineralization; mixed‑bed for the highest resistivity at point‑of‑use.
Monitor to time changeout
Rising conductivity (or TDS) after DI signals media exhaustion.
How Does Deionization Work?
Deionization (DI) uses ion exchange resins to replace dissolved ions with hydrogen (H⁺) and hydroxide (OH⁻). The released H⁺ and OH⁻ combine to form H₂O, driving conductivity down to very low levels.
- Cation resin (H⁺ form): Exchanges positive ions like Ca²⁺, Mg²⁺, Na⁺, Fe²⁺/Fe³⁺.
- Anion resin (OH⁻ form): Exchanges negative ions like Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻, silicate.
- Configurations: Two‑bed handles bulk removal; a mixed‑bed polisher achieves the lowest conductivity. Pairing RO → DI lowers cost per gallon.
Cation vs. Anion DI Resins
DI relies on two resin types working together: a strong acid cation resin (H⁺ form) removes positive ions (Ca²⁺, Mg²⁺, Na⁺), and a strong base anion resin (OH⁻ form) removes negative ions (Cl⁻, NO₃⁻, SO₄²⁻, HCO₃⁻, silicate). The H⁺ and OH⁻ released combine to form H₂O, driving conductivity very low.
Deionization Diagram
NaCl path: cation replaces Na⁺ with H⁺ (H⁺ + Cl⁻) → anion replaces Cl⁻ with OH⁻ → H⁺ + OH⁻ → H₂O.
- Type I vs. Type II anion: Type I minimizes silica leakage for ultra‑low conductivity; Type II offers higher capacity but slightly more silica leakage.
- Two‑bed vs. mixed‑bed: Two‑bed handles bulk demineralization; a mixed‑bed polisher at point‑of‑use reaches the highest resistivity.
- Best practice: Feed DI with RO to extend resin life 10–20× and stabilize water quality.
What Is Deionized Water Used For?
Common applications that benefit from very low conductivity and low silica water, including labs, aquariums, spot‑free rinsing, and industrial/boiler makeup.
Compare Water Deionization Options
When to Use DI Systems (vs. RO or Softening)
Quick guide: Softening prevents scale (hardness only), RO removes most dissolved solids, and DI polishes remaining ions to ultra‑low conductivity. Most systems pair RO → DI for the best cost per gallon.
Water Softening
Stops hardness scaling (Ca²⁺/Mg²⁺ → Na⁺/K⁺). TDS remains.
- Use when: Scale on fixtures/appliances; protect RO membranes.
- Delivers: Scale‑free water; no change to TDS/conductivity.
- Pair with: RO (pre‑treat) for better membrane life.
Reverse Osmosis (RO)
Removes 95–99% of dissolved solids and lowers DI cost per gallon.
- Use when: You need big TDS reductions and better taste.
- Delivers: 95–99% TDS removal; strong overall purity.
- Pair with: DI to reach ultra‑low conductivity.
Deionization (DI)
Exchanges remaining ions to achieve very low conductivity for sensitive uses.
- Use when: Labs, aquariums, spot‑free rinse, low silica needs.
- Delivers: Very low conductivity/near‑zero TDS water.
- Pair with: RO to extend resin life 10–20×.
Featured Deionizing Resin & Systems
Cartridges and resin for point‑of‑use polishing, plus complete systems.
Ultra High Purity DI Cartridge
Mixed‑bed cartridge for polishing RO permeate to ultra‑low conductivity.
Demineralizing Filter Cartridge
Compact DI cartridge for small housings; easy retrofit and reliable polish.
Deionizing Water Filter
- ✓ Compact DI system
- ✓ Polishes RO permeate
- ✓ Easy serviceability
- ✓ Flexible configurations
Configured DI filtration for point‑of‑use or small process lines. Great for polishing RO water where cartridge systems are preferred.
Industrial Demineralizing DI Systems
- ✓ Two‑bed + MB polish
- ✓ Low silica options
- ✓ Service exchange ready
- ✓ Custom engineering
High‑capacity two‑bed + mixed‑bed polishers with on‑site or service regeneration. Sized for flow, load, and silica targets.
Learn More About Deionization
Want the full science and sizing guidance? Read our in‑depth guide covering mechanism, EBCT, silica, and monitoring best practices.
Frequently Asked Questions
Get answers to common questions about deionization resin and DI water
What is DI resin?
An ion exchange media that replaces dissolved ions with H⁺ and OH⁻, which combine to form H₂O for very low conductivity water.
Do I need RO before DI?
For most users, yes. RO removes the bulk of dissolved solids so DI lasts longer and costs less per gallon.
Two‑bed or mixed‑bed?
Use two‑bed for bulk demineralization and a mixed‑bed polisher for the highest resistivity at point‑of‑use.
Does DI remove silica?
Yes. Strong base anion resin reduces silica; choosing Type I anion resin minimizes silica leakage. A mixed‑bed polisher achieves very low silica, especially when DI follows RO.
How long does DI resin last?
It depends on feed water quality and load. When DI follows RO, resin life is typically 10–20× longer. Monitor product resistivity or TDS to determine actual changeout intervals, and use a calculator like ours to estimate resin life and total gallons: Deionized Water Filter Calculator.
How do I know when to replace DI?
Watch resistivity or TDS after DI; rising conductivity (or TDS) indicates exhaustion. Color‑change resin helps but always confirm with a meter.