Sediment Filtration: Micron Ratings and Particle Capture

At a glance

Sediment filtration is mechanical: the cartridge physically blocks particles larger than its rating. Common ratings are 50, 20, 10, 5, 1, and 0.5 micron. Sediment is almost always the first stage in a multi-technology train because it protects every downstream component from being clogged by particulates. It does nothing about chemicals, dissolved minerals, bacteria, or viruses smaller than the rating.

What a micron is

One micron (micrometre) is one millionth of a metre, or one thousandth of a millimetre. For reference, a human hair is about 70 microns in diameter. Visible silt and rust are typically in the 20 to 100 micron range. Bacteria range from about 0.2 to 5 microns. Viruses are 0.02 to 0.3 microns. A sediment filter rated 5 microns will capture most visible particulates and many bacteria-sized particles, but no viruses. A 1-micron filter approaches the lower end of bacterial capture; a 0.5-micron absolute filter can capture cysts (Cryptosporidium, Giardia).

Common micron ratings and what each captures

Rating	Captures	Common use	Will not capture
50 micron	Coarse sand, large rust flakes, scale fragments	Spin-down pre-filter on a well	Anything finer than visible silt
20 micron	Fine sand, smaller rust, light silt	Pre-filter on city water with frequent main breaks	Cysts, fine clay
10 micron	Most visible sediment, fine silt, clay	Polishing on city water; mid-stage on well	Bacteria, viruses, dissolved iron
5 micron	Most particulate matter; UV pretreatment minimum per EPA	Pre-filter for UV, RO, or carbon block	Bacteria smaller than 5 micron, viruses
1 micron	Some bacteria-sized particles; pretreatment for cyst-rated systems	Polishing in well-water trains	Most bacteria, all viruses, dissolved chemicals
0.5 micron (absolute)	Cysts (Cryptosporidium, Giardia)	Cyst-rated POU and POE filters	Bacteria smaller than 0.5 micron, viruses, all dissolved chemicals

Nominal vs absolute ratings

Sediment cartridge ratings come in two flavours, and the difference matters.

A nominal rating means the cartridge captures a high percentage (typically 85 percent) of particles at or above the rated size. A nominal 1-micron filter will let some particles between 1 and (perhaps) 5 microns pass through. Nominal ratings are unregulated; manufacturers use their own internal test methods.

An absolute rating means the cartridge captures essentially all particles at or above the rated size, typically verified by NSF/ANSI 42 Class testing. An absolute 1-micron filter is a stricter specification than a nominal 1-micron filter. For cyst removal under NSF/ANSI 53, the rating must be absolute. For UV pretreatment, EPA's UV Disinfection Guidance Manual specifies a 5-micron absolute filter, because nominal ratings cannot guarantee the water clarity UV requires.

NSF/ANSI 42 Particulate Reduction Classes

NSF/ANSI 42 covers aesthetic effects, including particulate reduction. The standard defines six classes by particle size:

Class I: 0.5 to less than 1 micron
Class II: 1 to less than 5 micron
Class III: 5 to less than 15 micron
Class IV: 15 to less than 30 micron
Class V: 30 to less than 50 micron
Class VI: 50 micron and larger

A cartridge certified to NSF/ANSI 42 Class I has been independently tested to capture particulates between 0.5 and 1 micron. Without certification, "1 micron" on a label is the manufacturer's claim and may use any internal test method.

Multi-stage configurations

Most well-water installations use two or three sediment stages. The standard configuration runs from coarse to fine, with each stage protecting the next.

Two-stage configuration: 50 micron spin-down (cleanable) + 5 micron depth (replaceable). The spin-down cartridge captures large rust and sand and can be flushed without removal. The 5-micron depth cartridge polishes finer silt and protects downstream carbon, RO membranes, or UV.

Three-stage configuration: 50 micron + 20 micron + 5 micron. Used in wells with high turbidity or visible iron. Each stage extends the life of the next; replacing a clogged 50-micron cartridge is far cheaper than replacing a clogged carbon block.

Cyst configuration: 5 micron pretreatment + 0.5 micron absolute cyst filter. Required where Cryptosporidium or Giardia have been detected. NSF/ANSI 53 cyst-rated cartridges must use absolute ratings; nominal ratings are not permitted.

What sediment filters cannot do

Sediment filtration is mechanical. It cannot affect anything dissolved in the water and cannot inactivate microorganisms.

It cannot remove dissolved iron (Fe2+), only particulate iron oxide (Fe2O3) that has already precipitated.
It cannot remove chlorine, chloramine, or any chemical contaminant.
It cannot remove hardness (calcium and magnesium are dissolved ions).
It cannot remove lead, arsenic, nitrate, or any other dissolved species.
It cannot inactivate bacteria smaller than its rating; viruses are universally too small for any practical sediment filter.
It cannot remove dissolved organic compounds, VOCs, or PFAS.

When to replace

Sediment cartridge life depends entirely on water turbidity. A pressure-gauge differential of 10 to 15 PSI across a single cartridge is a typical replacement trigger; some manufacturers recommend replacement at 15 to 20 PSI. Without a gauge, replace every 3 to 6 months on city water with low turbidity, and every 1 to 3 months on a well with visible sediment. A clean cartridge installed in a well housing immediately develops a brown stain from dissolved iron oxidising on contact with air; this is visual only and does not mean the cartridge is loaded.

Common questions

What is the difference between 1 micron and 5 micron filters?

A 1 micron filter captures particles down to 1 micrometre, including most bacteria-sized particles and cysts (if absolute-rated). A 5 micron filter captures particles 5 micrometres and larger, including most visible silt and rust. The 5 micron rating is the EPA UV pretreatment minimum because UV cannot penetrate water with particles larger than 5 micron suspended in it. The 1 micron is used downstream of 5 micron when finer polishing is required, especially before any cyst-rated cartridge.

Do I need an absolute or a nominal sediment filter?

For routine sediment removal, a nominal-rated cartridge is acceptable. For NSF/ANSI 53 cyst certification, EPA-compliant UV pretreatment, or any application where particle pass-through cannot be tolerated, an absolute-rated cartridge is required. Look for the NSF/ANSI 42 Class designation on the cartridge label; that designation is the verified absolute rating.

Can a sediment filter remove dissolved iron?

No. Sediment filtration captures iron only after it has oxidised from soluble ferrous (Fe2+) to insoluble ferric (Fe3+) form, typically as visible rust particles. Dissolved ferrous iron is a clear-water condition that requires oxidation followed by filtration, or an iron-specific media (birm, manganese greensand) before sediment filtration is effective. Wells with measurable iron need a dedicated iron filter, not just a sediment cartridge.

How often should sediment cartridges be replaced?

Replace based on differential pressure or visible loading, not on a calendar. A pressure-drop of 10 to 15 PSI across the cartridge is a typical replacement trigger. Without a pressure gauge, replace every 3 to 6 months on clean municipal water and every 1 to 3 months on a well with visible turbidity. Cartridges that visibly deform or collapse during use are loaded beyond their service capacity and should be replaced immediately.

Can sediment filtration remove cysts like Giardia?

Only with an absolute 1-micron or 0.5-micron rating that is NSF/ANSI 53 certified for cyst reduction. Cryptosporidium oocysts are typically 4 to 6 microns; Giardia cysts are typically 8 to 12 microns. A nominal-rated 1-micron cartridge will let some cysts pass through. NSF/ANSI 53 cyst certification requires both an absolute rating and verification by particulate-challenge testing.

Sources

Last reviewed: April 2026

Tier 2 - Standards body
NSF International. NSF/ANSI 42 Particulate Reduction Classes
Tier 1 - Federal regulator
U.S. EPA. UV Disinfection Guidance Manual - Sediment Pretreatment
Tier 1 - Federal regulator
U.S. EPA. Drinking Water Treatability Database - Particulates
Tier 1 - Federal regulator
CDC. Healthy Water - Cryptosporidium
Tier 3 - State health department
Minnesota Department of Health. Filtration Information for Private Well Owners

Related: UV-C disinfection requires sediment pretreatment. Well water guide for multi-stage configurations. Activated carbon as the next stage after sediment.