Lab Water Purification: RO vs Deionized vs Distilled Water Compared

Lab Water Purification: RO vs Deionized vs Distilled Water Compared

Selecting the right water purification technology for your laboratory is essential for reliable, reproducible results. The three most common laboratory water purification methods are reverse osmosis (RO), deionization (DI), and distillation, each with distinct advantages, limitations, and appropriate applications. Understanding these differences helps you match water quality to analytical requirements.

Reverse Osmosis (RO) Water

• Mechanism: Pressure-driven transport through a semipermeable membrane that rejects 95-99% of dissolved solids

• Removes: Dissolved salts, organics (>200 Da), bacteria, particles, pyrogens

• Does not remove well: Dissolved gases (CO2), small organic molecules, some pesticides

• Typical quality: 1-10 uS/cm conductivity; suitable for Type III/IV laboratory water

• Cost: Low operating cost ($0.01-0.05/L); moderate capital cost

• Maintenance: Membrane replacement every 1-3 years; prefilter changes quarterly

Deionized (DI) Water

• Mechanism: Ion exchange resins swap dissolved cations for H+ and anions for OH-

• Removes: All dissolved ions to very high purity (18.2 MO.cm theoretical maximum)

• Does not remove: Organics, particles, bacteria, pyrogens, dissolved gases

• Typical quality: 0.05-1 uS/cm; suitable for Type I-III laboratory water (when polished)

• Cost: Moderate (resin replacement or regeneration); low capital for cartridge systems

• Maintenance: Resin replacement when conductivity rises; more frequent with high-TDS feed

Distilled Water

• Mechanism: Evaporation and condensation separates water from non-volatile impurities

• Removes: Dissolved salts, particles, bacteria, pyrogens, most organics

• Does not remove well: Volatile organic compounds, dissolved gases that co-distill

• Typical quality: 1-5 uS/cm; suitable for Type III laboratory water

• Cost: High energy cost ($0.10-0.50/L); high capital for stills

• Maintenance: Regular descaling of the boiling chamber; replacement of heating elements

ASTM/ISO Water Quality Types

• Type I (Ultrapure): 18.2 MO.cm resistivity, <5 ppb TOC. Used for HPLC, trace analysis, cell culture. Typically produced by RO + DI + UV + UF

• Type II: >1 MO.cm, <50 ppb TOC. Used for general laboratory, buffer preparation, spectrophotometry. RO + DI or double distillation

• Type III: >0.05 MO.cm, <200 ppb TOC. Used for glassware rinsing, autoclave feed, general washing. Single-pass RO or single distillation

Tech Inc. provides RO systems and water quality monitoring equipment for research laboratories, ensuring consistent water quality for membrane research and analytical applications.

Frequently Asked Questions

Which water type should I use for membrane testing?

For RO membrane testing, use Type II or Type III water as the base for preparing synthetic feed solutions. For preparing calibration standards and analytical samples, use Type I water. Always prepare NaCl test solutions gravimetrically with analytical-grade reagents.

Can I use RO water instead of DI water?

It depends on the application. RO water still contains 1-5% of feed water TDS, so it is not suitable for applications requiring very low ionic content. However, RO + DI combined systems are the most cost-effective way to produce high-purity water.