Every application needs a different grade of water – and getting it wrong shows up in your results, your equipment, or your compliance file. This hub sets out what Type I, II and III water actually mean, how Purite’s purification technologies produce them, and answers the questions our engineers hear most.
Why water grade is a variable you can’t ignore
Water dissolves almost everything it touches, which is exactly what makes it useful – and exactly why it’s rarely pure on its own. Mains and borehole water carry dissolved salts, organic matter, dissolved gases, particulates and mircoorganims, all of which change with geography, season and local treatment. In a lab, a hospital or a boiler house, that variability becomes a variable in your results, your equipment lifespan or your compliance record.
Rather than treating “purified water” as one thing, most standards – and Purite’s own systems – split it into grades matched to what the water is actually for. Feed water for a boiler has a very different specification to water for LC-MS. Getting the grade right is usually cheaper than over-purifying, and always cheaper than a failed batch, a fouled membrane, or a contaminated sample.
Water grades: Type I, Type II and Type III
Purite laboratory systems are built to deliver water anywhere from 1 MΩ.cm up to 18.2 MΩ.cm resistivity, covering the full Type I – III range used across ASTM D1193, ISO 3696 and CLSI classifications. Here’s what each grade means in practice.
Type I – Ultrapure water
Also called: reagent-grade water, UPW
| Resistivity | 18.2 MΩ.cm |
|---|---|
| TOC | < 10 ppb |
| Bacteria | < 10 CFU/ml |
| Endotoxins | Non-detect |
Water at the theoretical limit of purity – effectively H2O and a balanced trace of H+ and OH- ions, with organics, particulates, bacteria and dissolved gases removed to the lowest practical levels. It’s produced by polishing pre-treated or RO-fed water through mixed-bed deionisation, UV oxidation and final-stage filtration.
Typical uses
- HPLC & LC-MS mobile phase
- GC-MS & trace analysis
- ICP-MS / GFAAS
- PCR & molecular biology
- Mammalian cell culture
Clinical analysers
Type II – Pure / analytical grade
Also called: RO/DI water, general lab grade
| Resistivity | 1 – 15 MΩ.cm |
|---|---|
| TOC | < 50 ppb |
| Bacteria | < 100 CFU/ml |
| Particulates | 0.2 µm filtered |
A step down from ultrapure, Type II water still has the majority of ions, organics and microorganisms removed, typically via reverse osmosis followed by ion exchange or electrodeionisation (EDI). It’s the workhorse grade for day-to-day laboratory chemistry, where ultra-trace purity isn’t the limiting factor.
Typical uses
- General reagent make-up
- Buffer & media preparation
- Autoclave & glassware wash feed
- General wet chemistry
- Feed to a Type I polisher
- Routine QC testing
Type III – RO / feed grade
Also called: pre-treated water, process feed water
| Resistivity | 0.05 – 1 MΩ.cm |
|---|---|
| Conductivity | < 20 µS/cm |
| Turbidity | Low, RO-treated |
| Role | Feed / general purpose |
Typical uses
- Glassware pre-rinse
Feed to Type I/II systems - Boiler & humidifier feed
- Autoclave & steriliser feed
- General washdown
- Non-critical process water
How each grade gets made: the technology glossary
No single technology produces every grade of water. Purite systems combine several of the methods below, in sequence, to reach the required spec cost-effectively.
Reverse osmosis (RO)
Reverse osmosis (RO)
Deionisation (DI)
Deionisation (DI)
Electrodeionisatoin (EDI)
Electrodeionisatoin (EDI)
Ion exchange (IX)
Ion exchange (IX)
Ultraviolet treatment (UV)
Ultraviolet treatment (UV)
Activated carbon filtration
Activated carbon filtration
Particulate & sub-micron filtration
Particulate & sub-micron filtration
Distillation
Distillation
Frequently asked questions
Grouped by the questions we’re actually asked – by lab managers, estate teams, and procurement.
Choosing a water grade
Start from the technique, not the budget. Trace analysis, mass spectrometry and cell culture need Type I ultrapure water because even parts-per-billion contamination affects results. General reagent prep, media-making and glassware washing are usually well served by Type II. Feed, boiler and washdown applications rarely need better than Type III. If you’re unsure, it’s more cost-effective to size correctly than over-purify everything to Type I.
For most labs, healthcare and precision industrial applications, no. Mains water is treated to drinking-water standards, not laboratory or process standards – it still carries dissolved minerals, chlorine, organics and microorganisms that vary by region and season. That variability is exactly what purification is designed to remove, so results, reagents and equipment stay consistent regardless of what’s coming out of the mains that week.
They’re two ways of measuring the same property – how easily water conducts an electric current, which depends on dissolved ionic content. Conductivity (µS/cm) rises as ionic content rises; resistivity (MΩ.cm) is its inverse and rises as water gets purer. Ultrapure water’s theoretical maximum is 18.2 MΩ.cm, equivalent to 0.055 µS/cm – the point at which virtually no ions remain to carry a current.
Not automatically – it means lower risk of contamination affecting sensitive techniques. For applications that aren’t ion- or organic-sensitive, Type I water offers no measurable benefit over Type II, just added cost. It’s also worth noting that ultrapure water is chemically “hungry”: it actively tries to reabsorb contaminants from air, storage vessels and tubing, so it needs to be used promptly and stored correctly to stay at spec.
Systems & technology
Reverse osmosis forces water through a membrane under pressure, rejecting a broad range of contaminants – salts, bacteria, viruses, organics – in one physical process. Deionsiation uses ion-exchange resin to target dissolved ions specifically, reaching much higher resistivity than RO alone. Most systems use RO as a first-pass, high-capacity stage, then DI or EDI to polish the water further.
Conventional DI resin needs periodic regeneration or exchange once it’s exhausted – that means downtime, chemical handling, or a cylinder exchange service. EDI produces high-purity water continuously using an electric field, without consumable resin changes, which suit sites with high or constant demand. Resin-based DI still has a place where upfront cost or simplicity matters more than continuous throughput.
If your feed water is hard – high in dissolved calcium and magnesium – a softener upstream of RO or DI protects membranes and resin from scaling, extending their service life. It’s pre-treatment, not purification in itself: softened water is still far from lab or process grade, just gentler on the equipment that purifies it further.
No – UV disinfection deactivates microorganisms and, at a different wavelength, breaks down trace organics, but it doesn’t remove dissolved ions, particulates or the resulting cell debris from the water. It’s typically used alongside RO, DI and particulate filtration, not instead of them.
Healthcare & dialysis
Dialysis water has its own strict standards (such as ISO 13959/23500) covering chemical contaminants, bacteria and endotoxins, because the patient’s blood is in direct contact with treated water across the dialyser membrane. Systems typically combine RO, DI and ultrafiltration or endotoxin-retentive filtration to meet these limits reliably and continuously, with ongoing monitoring built in.
Both are healthcare-grade, but they serve different processes. Sterile services water feeds washer-disinfectors, autoclaves and decontamination equipment for reusable medical devices, where mineral and organic content affects instrument cleanliness and equipment scaling. Dialysis water is patient-contact water with tighter chemical and microbiological limits, because it interacts directly with the patient’s bloodstream.
Industrial, boilers & data centres
Untreated feed water leaves behind dissolved minerals as scale on heat-exchange surfaces, reducing efficiency and raising energy use, while dissolved oxygen and other gases drive corrosion. Softening, RO or deionisation – matched to the boiler’s operating pressure – keeps scale and corrosion in check and extends plant life.
It depends on the cooling architecture, but commonly includes pre-treatment and filtration of the source water, softening or RO to control scale in cooling loops, UV for biological control (particularly in systems prone to legionella risk, like cooling towers), and increasingly rainwater harvesting or recycling to reduce both water usage and energy overhead.
Operating & buying water systems
In-house purification has a higher upfront cost by typically lower running costs over time, especially at moderate-to-high usage, and gives on-demand water with real-time quality monitoring rather than a fixed shelf specification. Bottle water avoids the capital outlay but adds ongoing procurement, storage, expiry tracking and single-use plastic waste – and purity can start declining the moment a bottle’s seal is broken.
It’s a low-capital way to get purified water without owning a full purification system: you use a purchased cylinder until the resin is exhausted, then exchange if for a fresh one against a purchase order or voucher. It suits lower-volume or intermittent demand, sites without the space or budget for fixed plant, or as backup capacity alongside an existing system.
This varies by system, usage and feed water quality, but most laboratory and healthcare units are placed on a scheduled maintenance contract with periodic consumable changes (pre-filters, RO membranes, UV lamps, DI cartridges) rather than a fixed universal interval. Point-of-use resistivity and TOC monitoring gives an early warning if performance drifts between services.
Glossary of water purity terms
Activated carbon
A porous carbon media that removes chlorine, chloramines, organic compounds and taste/odour from water by adsorption, and protects downstream RO membranes from chlorine damage.
CFU (colony-forming unit)
A count of viable bacteria in a water sample, expressed per millilitre (CFU/mL). Ultrapure water typically specifies <10 CFU/mL.
CLRW (Clinical Laboratory Reagent Water)
A CLSI-defined water grade for clinical and diagnostic laboratory testing, with specified limits on resistivity, TOC, bacteria and particulates.
Conductivity
A measure (µS/cm) of how well water conducts electricity, driven by dissolved ionic content. The inverse of resistivity – the purer the water, the lower its conductivity.
Demineralised water
Water that has had dissolved mineral salts removed, typically via ion exchange. Used interchangeably with “deionised water” in most industrial contexts.
Deionisation (DI)
Purification using ion-exchange resin to remove dissolved ions from water, raising its resistivity. Commonly used as a polishing stage after reverse osmosis.
Distilled water
Water purified by boiling and re-condensing the vapour, leaving non-volatile contaminants behind. Largely replaced in modern labs by RO/DI systems, which costs less to run.
Electrodeionisation (EDI)
A continuous deionisation process combining ion-exchange media with an electric field, avoiding the periodic chemical regeneration that conventional DI resin needs.
Endotoxin
A toxic compound found in the outer membrane of certain bacteria, measured in endotoxin units per millilitre (EU/mL). Must be non-detectable for water to be classed as ultrapure, and is tightly controlled in dialysis and pharmaceutical water.
Feed water
The water entering a purification system or process – anything from raw mains supply to pre-treated Type III water feeding a Type I polisher.
Ion exchange (IX)
A process that swaps unwanted dissolved ions for more benign ones using resin media – the mechanism behind both water softening (calcium/magnesium for sodium) and deionisation (ions for hydrogen/hydroxide).
Megohm-cm (MΩ.cm)
The standard unit for water resistivity. Ultrapure water’s theoretical ceiling is 18.2 MΩ.cm at 25℃ – the point at which almost no dissolved ions remain to conduct current.
Permeate
The purified water that has passed through a reverse osmosis membrane, as distinct from the “concentrate” or “reject” stream carrying the rejected contaminants.
ppb / ppt (parts per billion / trillion)
Units for very low concentrations of a contaminant – commonly used to express TOC limits in high-purity water, where even trace organics can interfere with sensitive analysis.
Resistivity
A measure of how strongly water resists an electric current, expressed in MΩ.cm. Rises as ionic content falls, making it the primary real-time indicator of water purity.
Reverse osmosis (RO)
A filtration process that forces water through a semi-permeable membrane under pressure, rejecting dissolved salts, bacteria, viruses and most organic compounds in a single physical step.
Softened water
Water treated by ion exchange to remove hardness-causing calcium and magnesium ions, replacing them with sodium. Protects downstream plumbing, boilers and membranes from scale – but is not purified water in itself.
TDS (total dissolved solids)
The combined concentration of all dissolved substances in water, usually expressed in mg/L or ppm, and closely related to conductivity.
TOC (total organic carbon)
A measure of all organic compounds dissolved in water, expressed in ppb. A key purity indicator for chromatography and mass spectrometry, where organic contamination raises background noise.
Type I / II / III water
The standard grading system (per ASTM D1193, ISO 3696 and CLSI) for laboratory water purity, running from Type III (general/feed grade) up to Type I (ultrapure, reagent grade).
UV treatment
Ultraviolet light used either to disinfect water by deactivating microorganisms, or – at a different wavelength – to break down trace organic compounds and reduce TOC.
Not sure which grade or system fits your site?
Our engineers can specify the right combination of technologies for your application – from a single benchtop unit to a custom-engineered ring.
