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Water Quality Grades
"Pure water" is not one grade but a whole spectrum. What different processes require can differ a thousandfold, and treatment cost climbs steeply with it. Below are the specifications, the differences and when to choose each of the three common grades.
Three grades
Each step up the ladder adds another stage. The fewer impurities left in the water, the harder those that remain are to remove.
RO Water · Reverse Osmosis
Pressure is applied to the raw water, driving it through a membrane fine enough to hold back most dissolved salts, organics and microbes. It is the common starting point for every higher grade.
DI Water · Deionized
Ion-exchange resin or a CEDI module follows the RO stage, removing as much of the remaining cations and anions as possible. Resistivity approaches the theoretical value, but organics, particles and ultratrace metals are not yet tightly controlled.
Ultrapure Water
On top of DI, organic carbon, ultratrace metals, boron and nanoscale particles are brought under control. Resistivity reaches the theoretical limit of 18.2 MΩ·cm at 25 °C.
Full comparison
On a single parameter the three grades can differ by six orders of magnitude. Scroll the table sideways to see every column.
| Grade | RO Water | DI Water | UPW Ultrapure |
|---|---|---|---|
| Process | RO membrane | RO + ion exchange resin (or CEDI) | RO + CEDI/MDI + UV + MBP + Final UF |
| Resistivity | approx. 0.05M~1.0MΩ·cm | 1.0M~18.2MΩ·cm | 18.2 MΩ·cm (25°C) |
| Conductivity | ≤ 20 μS/cm | 1.0~0.055 μS/cm | 0.055 μS/cm (25°C) |
| TDS | < 5–20 ppm | < 1 ppm | Virtually zero |
| TOC | 100~1000 ppb | 20~100 ppb | < 2 ppb |
| SiO₂ | < 2500 ppb | < 50 ppb | < 2 ppb |
| Boron | < 500 ppb | < 10 ppb | < 1 ppb < 100 ppt for some processes, < 5 ppt for advanced nodes |
| Particle | No specific requirement | Typically < 1 μm | < 2 counts/mL at ≥ 50 nm Down to 20 nm for advanced processes |
| Bacteria | 10²–10⁴ CFU/mL | < 100 CFU/mL | < 1 CFU/L |
| Endotoxin | Not specified | Specific limits in biotech processes | Extremely low |
| Dissolved O₂ | Near saturation | Near saturation | Down to < 10 ppb |
| Metal ions | ~95–99% removal | ppb level | ppt level Down to < 1 ppt on request |
| Cost | ★ | ★★ | ★★★★★ |
| Maintenance | Low | Medium | High |
| Applications | Boiler make-up, general process, medical | Laboratory, PCB, plating, pharmaceutical | Semiconductor, wafer fab, LCD, optoelectronics |
Typical ranges — actual specification is configured to source water and process demand
What the numbers mean
The fewer ions in the water, the poorer it conducts and the higher its resistivity — which makes resistivity the most direct overall measure of water purity. For ultrapure water the theoretical limit at 25 °C is 18.2 MΩ·cm.
Note: resistivity cannot see organics, particles or weakly ionised speciesThe reciprocal of resistivity — two ways of stating the same measurement. 18.2 MΩ·cm corresponds to 0.055 μS/cm. Industrial water is usually quoted in conductivity, high-purity water in resistivity.
The total burden of organic contamination. Organics are weakly polar, very small and barely ionised.
→ How 185 nm photolysis worksSilica exists largely in weakly ionised form, so resistivity barely registers it — yet it forms deposits on wafers and boiler surfaces that are very hard to remove.
Boron barely dissociates in neutral water, which makes it one of the hardest elements to remove; ordinary RO and resin struggle to remove it. It is also a semiconductor dopant, so even trace amounts can shift device behaviour — hence the ppt-level requirement at advanced nodes.
At advanced nodes nanoscale particles have a profound effect in every respect, and different processes set different particle limits.
→ Nanoparticle removalThere is little nourishment in pure water, but bacteria may still colonise pipe walls as biofilm; dead legs and long stagnant sections are the high-risk areas. Control comes from piping design and sanitisation capability, not from filtration alone.
Fragments of bacterial cell wall left behind after the organism dies. They are heat-stable, so sterilisation does not remove them. For injectables and biotech processes, endotoxin is a control point in its own right, separate from bacteria count.
Water picks up oxygen the moment it meets air — around 8.5 ppm at ambient conditions. Oxygen left in the water oxidises the metal in a microcircuit and causes defects; in power plants it corrodes turbine blades.
→ Why dissolved gases must goUltratrace metals alter the electrical behaviour of the device, so semiconductor processes must specify them at ppt level. Achieving that depends not only on process configuration and resin selection but on how much the material itself leaches.
→ Piping & materialsHow to choose
Cost does not rise linearly with purity. Going from RO to DI is roughly an order of magnitude of investment; DI to UPW is another — and a UPW system keeps costing in maintenance, consumables and monitoring.
Over-treating wastes capital and energy; under-specifying shows up directly in yield. The right approach is to work backwards from what the process can actually tolerate — not to start by deciding which system to buy.
Typical mapping — confirm against your own process specification
Not sure which grade you need?
We will come back with the grade and treatment train your conditions actually call for — not a blanket recommendation for the highest specification.
