Filter Bag Micron Rating: How to Choose 5, 25 or 100 Micron
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Short answer. For most sediment and process-water jobs, start with a 25 micron filter bag. Step down to 5 micron when you are protecting something finer downstream (a cartridge filter, a UV unit, an RO membrane) or need visibly clear liquid. Use 100 to 200 micron for coarse debris, rust flakes and hops. Going finer than you need is the most common mistake: the bag blocks sooner, pressure drop climbs and you change bags more often. Felt bag ratings are nominal, so a 5 micron bag does not stop every 5 micron particle. Browse our filter bags and stainless steel bag filter housings.
What does a micron rating mean on a filter bag?
One micron (µm) is one thousandth of a millimeter. The rating on a filter bag is the particle size the bag is designed to capture. The table below puts common sizes in perspective, using figures from The Engineering ToolBox.
| Particle | Typical size |
|---|---|
| Smallest particle the eye can see | about 40 µm |
| Human hair | 40 to 300 µm |
| Fine sand | about 125 µm |
| Silt | 4 to 37 µm |
| Yeast cells | 1 to 50 µm |
| Clay | 0.5 to 4 µm |
| Bacteria | 0.3 to 60 µm |
So a 100 micron bag catches sand and visible grit but lets most silt through. A 25 micron bag removes the particles that make water look cloudy. A 5 micron bag takes out most silt and fine haze. A 1 micron bag is polishing territory and loads up quickly.
Nominal vs absolute: why a 5 micron bag is not a 5 micron wall
Most felt filter bags, including the polypropylene needle-felt bags we sell, carry a nominal rating. A nominal rating means the media removes a stated percentage by weight of particles above that size under test conditions, for example 90% of 10 micron particles. An absolute rating is defined by the largest hard particle that can pass through the media in a laboratory test, and is close to complete retention at that size. The Water Network has a longer explanation, and notes that nominal test methods vary between manufacturers, so two "5 micron" bags from different sources may not perform the same.
Felt is a depth media: particles are trapped through the thickness of the fabric, not only on the surface. As the bag loads with dirt it actually filters finer, while the pressure drop across it rises. That is normal behavior, not a fault.
Which micron rating for which job?
These are starting points, not guarantees. Your solids load, flow rate and how clean the liquid must be all move the answer. When in doubt, start one step coarser than you think and measure the result.
| Liquid or task | Starting rating | Notes |
|---|---|---|
| Coarse debris, rust flakes, weld scale, pipe scale after maintenance | 100 to 200 µm | Protects pumps, nozzles and finer filters downstream |
| General process water, cooling water, rinse water | 25 to 50 µm | Removes visible cloudiness and grit |
| Well or bore water with sediment | 25 to 50 µm, then 5 µm if still hazy | Two stages last far longer than one fine bag |
| Pre-filter before a cartridge filter, UV lamp or RO membrane | 5 to 10 µm | Check what the downstream equipment maker specifies |
| Paints, coatings, inks, resins | 25 µm down to 1 µm depending on finish | Viscous liquids need more filter area; ask us |
| Hops and large trub in brewing | 100 to 200 µm | Coarse first; yeast removal needs a finer second stage |
| Yeast haze in beer, wine, cider, kombucha | Ask us with details | Yeast is 1 to 50 µm; fine bags can block fast on a heavy yeast load |
Two mistakes that cost the most
- Going too fine, too soon. A 1 micron bag on dirty well water will block in a fraction of the time a 25 micron bag lasts, and may still pass haze because the dirt load forces liquid through the weakest part of the bag. Use a coarse stage first.
- Treating the rating as the only variable. Flow rate, solids load, viscosity and temperature decide how long a bag lasts and how clean the result is. The same 25 micron bag behaves very differently on clean rinse water and on a slurry.
Single stage, two stages, or multiple bags?
Two housings in series (coarse bag first, fine bag second) is the standard answer when you need a fine result from a dirty feed. Multi-bag housings are different: a housing with 2, 3 or 4 bags runs the bags in parallel to handle more flow, so all bags in it carry the same rating. For a line that needs both high flow and two ratings, you use two housings. If your flow or vessel requirements fall outside our standard sizes, see custom bag and multi-cartridge filter housings.
When should you change the filter bag?
Watch the pressure difference between the housing inlet and outlet. A common rule of thumb in the industry is to change the bag when the differential pressure reaches about 15 psi (roughly 1 bar); Rosedale Products gives this figure and notes it varies with the micron rating, viscosity and your system. Fit a pressure gauge on each side of the housing so you can see it. Felt bags are normally single use; a blocked felt bag does not come back to its rated performance after washing.
Bag size is not micron rating
A filter bag has two specifications. The size (#1, #2, #3, #4) is the physical dimension of the bag and must match the housing. The micron rating is the pore size of the fabric. Our size #1 and #2 housings and size #3 and #4 housings each take their own bag size; ring style is confirmed on the order. We stock the #2 polypropylene felt bag in 5, 25 and 100 micron nominal, with other sizes and ratings on request. For the housing material, see 304 vs 316 stainless steel.
FAQ
Is a lower micron rating always better?
No. A finer bag blocks faster and raises pressure drop. Choose the coarsest rating that gives the result you need, and add a second stage if one bag cannot do it.
Is 5 micron nominal the same as 5 micron absolute?
No. Nominal means a high percentage of particles above 5 micron are removed. Absolute means essentially all are. Our felt bags are nominal.
Will a 1 micron filter bag remove bacteria?
No. Bacteria range from about 0.3 micron upward, and a nominal felt bag is not a sterilizing filter. Bacteria removal needs membrane filtration or another treatment step.
Can I reuse a felt filter bag?
Felt bags are designed as single-use consumables. Rinsing removes some surface dirt but does not restore the depth of the media.
What micron rating for pre-filtering before RO?
Check the membrane manufacturer's requirement. Many systems specify a 5 micron pre-filter; a coarser bag ahead of it extends the life of the 5 micron stage.
Not sure which rating to start with? Tell us the liquid, the solids you want to remove and your flow rate and we will suggest a bag and housing combination.