Absolute vs nominal micron rating — the real difference
Short answer: Nominal 5 micron means roughly 85 to 90 percent of particles are retained. Absolute 5 micron means about 99.9 percent. For a general RO plant nominal is sufficient and much cheaper. Absolute is needed where a guarantee is required — pharma, food, or where the membrane is particularly expensive.
The difference in plain terms
The pores inside a filter are not all identical — they cannot be.
- Nominal: "most pores are 5 micron, some are a little larger" — so a few particles get through
- Absolute: "no pore is larger than 5 micron" — so almost nothing gets through
| Nominal | Absolute | |
|---|---|---|
| Particles retained | 85 – 90% | 99.9% |
| Price | Lower | 3 – 5 times higher |
| Usual construction | PP spun, string wound | Pleated |
| Testing | No formal standard | Tested and certified |
| Suited to | General RO, industry | Pharma, food, sterile |
Is 85 to 90 percent enough?
For an RO plant, yes. The reasons:
- The 10 to 15 percent that pass are particles close to 5 micron, not large ones
- The RO membrane itself is extremely fine and retains them
- The real threat is coarse particles, and a nominal filter stops those
When absolute is genuinely required
1. Pharma and food industry
These involve validation and audit — you have to produce documentation of what the filter retains. A nominal filter carries no such guarantee.
2. Very expensive membranes
An 8040 or seawater membrane costs a great deal. Spending ₹1,000 to ₹2,000 on an absolute filter there is effectively insurance.
3. Final polishing
0.45 or 0.22 micron absolute retains bacteria. This is used in packaged water and laboratory work.
See our absolute pleated cartridges.
0.22 and 0.45 micron — sterile filtration
| Micron | What it retains | Where used |
|---|---|---|
| 0.45 | Most bacteria | Laboratory, testing |
| 0.22 | Effectively all bacteria | Pharma, sterile applications |
These are almost always absolute, because a nominal rating would be meaningless — 10 percent of bacteria passing is still a failure.
How to tell whether a filter is absolute
- The packaging states it clearly — "absolute" or "99.9% at X micron"
- It is usually of pleated construction
- The price is considerably higher
- A certificate or test report often accompanies it
If the packaging says only "5 micron" and nothing more, it is nominal.
A selling practice to watch for
In some places an absolute price is charged for a nominal filter. To protect yourself:
- Ask for the test report — it accompanies genuine absolute filters
- Check the packaging — the rating should be clearly printed
- Buy only from a supplier you trust
The decision, plainly
| Your application | What to use |
|---|---|
| General RO plant (250–5,000 LPH) | Nominal 5 micron PP spun |
| Dirty water | Nominal, but in two stages |
| Expensive 8040 membrane | Absolute 5 micron is worth considering |
| Pharma or food | Absolute, with test report |
| Packaged drinking water | Nominal before RO, absolute at final stage |
| Laboratory or sterile | 0.22 absolute |
The full micron selection method is in our micron guide.
Nominal or absolute — we will advise
Tell us your application and membrane type, and we will confirm whether nominal is adequate or absolute is required.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What is the difference between nominal and absolute micron?
Nominal means roughly eighty five to ninety percent of particles are retained, while absolute means about ninety nine point nine percent. In an absolute filter no pore is larger than the rated size, which is why it costs considerably more.
Does an RO plant need absolute filters?
Generally no. Nominal five micron is adequate for RO, because the few particles that pass are very small and the membrane itself retains them. That money is better spent on improving pre-treatment.
When should absolute filters be used?
In pharma and food industry where validation and audit apply, or where the membrane is particularly expensive such as an 8040 or seawater element. Sterile filtration uses zero point two two micron absolute.
How can I tell whether a filter is absolute?
The packaging states it clearly, either as absolute or as ninety nine point nine percent at a given micron, and a test report usually accompanies it. If it says only five micron with nothing further, it is nominal.
Can a 0.22 micron filter be fitted before RO?
No, it would choke immediately. It belongs at the final stage after RO, where the water is already clean. Before RO, five micron is the appropriate rating.