Filtration & Micron Ratings Explained

Filtration separates particles from a liquid or gas by size. You pass the material through a filter with openings of a known size: anything larger is held back, and anything smaller passes through. Getting that size right is the difference between a clear result and a blocked filter.

This guide covers what micron ratings mean, which size to use, and how to choose the right filtration setup for your process.

What a micron rating means

A micron (µm), or micrometre, is one thousandth of a millimetre. A filter's micron rating tells you roughly how large the openings in the filter medium are. The lower the number, the finer the filter.

For scale, these are approximate sizes:

Particle	Approximate size
Smallest particle you can see with the naked eye	40 µm
Human hair	50–100 µm
Pollen grains	10–100 µm
Fine dust	1–10 µm

Nominal or absolute? A nominal rating means the filter catches most particles at that size, but some get through. An absolute rating means it catches virtually all of them. For a final clarification step, check which one you are buying.

Micron size guide

Micron range	Filtration level	Typical use
150–220 µm	Coarse	First-stage removal of plant debris and large particles
70–120 µm	Medium-coarse	General separation with good flow
25–45 µm	Medium	Refining after a coarse stage
5–15 µm	Fine	Removing smaller particulates
Below 5 µm	Very fine	Final clarification, usually with cartridges or filter paper

The finer the filter, the slower the flow and the faster it clogs. That is why most setups use more than one stage.

Use stages, not one fine filter

Pushing everything straight through a fine filter blocks it quickly. Staged filtration works coarse to fine:

Coarse stage: removes the bulk of large particles.
Medium stage: refines the material.
Fine stage: final clarification.

Each stage protects the next one, so you filter faster and replace fine media less often.

Types of filtration system

System	How it works	Best for
Filter bags and mesh screens	Liquid passes through a woven or mesh bag at a set micron	Coarse to medium stages, ice-water separation, high volumes
Cartridge filters	Replaceable cartridges, often pleated, in a housing	Fine and final filtration
Plate filtration	Filter media held between stacked plates, often inline	Larger-scale and closed-loop processing
Vacuum filtration	A vacuum pump pulls liquid through a Buchner funnel and filter paper	Lab-scale batches and faster filtration of thicker liquids
Centrifuges	Spinning separates solids from liquid by density rather than size	Removing fine solids before filtration, or replacing a filtration stage

Gravity filtration works for thin liquids. Thicker or cooler liquids usually need vacuum or pressure to move through the filter at a useful rate.

How to choose a filtration system

1. Define the goal. Are you removing large debris, refining, doing a final clarification, or protecting equipment further down the line? This tells you how fine you need to go.

2. Pick the micron range. Start from the finest rating you actually need, then add coarser stages in front of it.

3. Consider the liquid. Viscous or cold liquids flow slowly through fine media. Warming the liquid where the process allows, or adding vacuum or pressure, speeds things up.

4. Match volume and flow rate. Bags and plate systems handle larger volumes. Buchner and cartridge setups suit smaller batches. Size the system for your typical batch, not your smallest.

5. Check material compatibility. Make sure filter media, housings and gaskets are compatible with any solvents and temperatures in your process. Stainless steel and PTFE suit most applications.

6. Plan for cleaning or replacement. Stainless mesh and nylon bags can be washed and reused. Cartridges and filter papers are replaced. Factor in running costs as well as the purchase price.

Filtration FAQs

Is a lower micron rating always better? No. A finer filter catches more but flows more slowly and clogs sooner. Use the finest rating your process needs, with coarser stages in front.

Why does my filter keep clogging? Usually there is no pre-filter, or the liquid is too viscous. Add a coarser stage first, or use vacuum or pressure to keep it moving.

Can I reuse filter bags? Nylon and stainless mesh bags can usually be washed and reused if they are not damaged. Replace any bag with a tear or a stretched seam.

When should I use a centrifuge instead? When solids are too fine or too plentiful to filter efficiently. A centrifuge separates them by density first, so the filter only has to finish the job.