Reverse Osmosis Membrane Pore Size: What 0.0001 Micron Means (and What It Does Not)

On the product page for the Hydrion RO600 stage 2 membrane, the pore size is listed as 0.0001 micron. Most sediment cartridges, including the stage 1 cartridge in the same system, are rated in whole microns, such as 1 micron or 5 micron, so the jump of four decimal places is what makes the membrane's number hard to picture. It helps to put 0.0001 micron on a scale next to things you already have a feel for, and then to be honest about what the figure is actually describing: a reverse osmosis membrane's working layer is generally described as effectively solid rather than full of countable holes, so "pore size" here is a comparison rating rather than a measured gap someone found with a ruler.

• Human hair: roughly 20 to 200 microns across, something you can see and feel between your fingers.
• Bacterium: roughly 1 to 10 microns long, already too small to see without a microscope.
• Virus: roughly 0.02 to 0.3 microns across, smaller again than most bacteria.
• Stage 2 membrane rating: 0.0001 micron, below even the virus range, and the scale at which the membrane's rejection layer is described as working.

What "0.0001 Micron" on a Spec Sheet Is Actually Describing

The figure sits on the spec table for the Hydrion RO600 stage 2 membrane, under "pore size." Nearby cartridges in the same category, including ordinary sediment and carbon filters, are usually rated at 1 micron or 5 micron, which is already small enough to catch grit and cloudiness but nowhere near this membrane's number. That gap is the reason the figure confuses people who are used to reading filter specs in whole microns: it is not a slightly finer version of the same rating, it is several orders of magnitude finer. The rest of this article puts that number on a scale the reader already recognizes, then explains honestly what a reverse osmosis membrane is doing at that scale and what the rating does and does not prove about the water it produces.

Putting 0.0001 Micron on a Size Ladder

From a Strand of Hair to a Virus to the Membrane's Rating

A single human hair runs roughly 20 to 200 microns across (source: Micrometre – Wikipedia), which is why it is one of the few things on this list you can judge with your own eyes. A typical bacterium measures roughly 1 to 10 microns long (source: Micrometre – Wikipedia), already well past the point where unaided vision is any use. Many spherical viruses run smaller again, roughly 0.02 to 0.3 microns across (source: Virus – Wikipedia). Each step down that ladder is roughly an order of magnitude, and the membrane's stated 0.0001 micron sits below the virus range, at the scale its rejection layer is described as working. That placement is why reverse osmosis is the stage people reach for when a sediment cartridge alone will not do the job: the rating describes a different kind of barrier, not just a stricter version of the same one.

Why RO Membranes Aren't Really Sieves With Holes

The Solution-Diffusion Model, in Plain Terms

Membrane filtration grades are commonly compared using rough size references: general membrane pores spanning roughly 0.1 to 5,000 nanometres, microfiltration around 50 nanometres and larger, ultrafiltration around 3 nanometres and larger, and nanofiltration around 1 nanometre and larger, with reverse osmosis rejecting particles above roughly 0.2 nanometres, or about 0.0002 micron, in the same range as the membrane's own 0.0001 micron rating (source: Reverse osmosis – Wikipedia). Those numbers are useful for comparing categories, but a reverse osmosis membrane is usually described differently from the stages above it on that list: its working layer is generally treated as nonporous, rejecting dissolved material through solubility and diffusion rather than through discrete holes a particle either fits through or does not (source: Reverse osmosis – Wikipedia). In the accepted model, water molecules dissolve into the dense polymer layer on one side and diffuse across it to the other, the way a gas can pass through a rubber sheet without the sheet having a visible hole in it, rather than flowing through open channels the way water moves through a coffee filter. Read that way, "0.0001 micron" is a useful rejection rating for comparing filter stages against each other, not a hole that was measured under a microscope and counted.

What the Rating Does and Doesn't Prove

The stage 2 membrane's product page states that it reduces dissolved solids, heavy metals, fluoride and PFAS, and that it sends rejected water to drain at a 2:1 ratio while producing up to roughly 600 gallons, about 2,270 litres, of filtered water a day. Those are the manufacturer's stated figures for this product, not results from an independent lab test, and the honest way to read a spec sheet like this is to notice that distinction rather than assume a number implies a certification the page does not actually claim. Because rejection in reverse osmosis depends on more than size alone, including how soluble a dissolved substance is and how readily it diffuses through the membrane, the same mechanism that holds back contaminants also holds back most dissolved minerals, which is one reason the RO600 line pairs this membrane with a mineral-adding stage afterward rather than leaving the water with nothing dissolved in it at all; how thoroughly reverse osmosis strips minerals, and why remineralization exists, is worth reading on its own if that trade-off matters to you.

Where the Membrane Fits With the Stages Before It

The coarser stage 1 sediment-and-carbon cartridge handles the mechanical grit and chlorine taste first, so the much finer stage 2 membrane is not the system's first line of defense against the things a basic cartridge already catches; it is the stage that takes over once the water is already relatively clean. A membrane working at this scale also means some of what enters it is diverted to drain rather than through to the tap, which is the practical trade-off behind the RO600's stated 2:1 ratio between purified water and drain water. What each stage in a 3, 5 or 7-stage reverse osmosis system actually does covers that full sequence in more depth than fits here, and the complete explanation of how reverse osmosis works end to end is the better starting point if you want the whole process rather than just this one number.

When the Figure Matters for Replacing the Membrane

None of this changes how you maintain the membrane day to day, but it does explain why the product page ties its replacement interval to performance rather than a fixed date alone. Hydrion's product page states a replacement interval of around every 24 months or after about 6,800 to 7,600 litres, sooner if the display's TDS reading climbs well above its usual level or the RO indicator turns red. A rising TDS number after months of a stable reading is the practical sign that the membrane's rejection performance is slipping, even though you will never see the 0.0001 micron figure itself change; what a TDS reading on the display is actually measuring explains why that number, not the pore-size spec, is the one worth watching between replacements. For the full schedule across all three RO600 filters together, the complete replacement timeline for the PPC, membrane and TC stages lays it out stage by stage. If your own membrane is approaching that point, the stage 2 membrane's product page lists the fitting and interval next to the complete Hydrion Purity RO600 system it belongs to.