What micron sediment filter do I need?

It depends on what you are protecting and how dirty your water is. For visible grit and sand on a well, start with a coarse spin-down filter at 50 to 100 microns, then add a 5-micron cartridge to catch finer particles before a softener or carbon filter. If you are feeding a reverse osmosis system, the membrane manufacturer usually calls for a 5-micron pre-filter, and a 1-micron filter is worth adding when you want to catch cysts like Giardia and Cryptosporidium. Going straight to 1 micron on dirty water just clogs the filter fast and drops your pressure.

What is the difference between nominal and absolute micron ratings?

A nominal rating means the filter captures roughly 85 to 90 percent of particles at the stated size, so a nominal 5-micron filter lets some 5-micron particles through. An absolute rating means it captures around 99.9 percent at that size under defined test conditions, which is the stricter measurement. String-wound and melt-blown filters are usually rated nominally. Pleated filters are more often given an absolute rating. When a particle size really matters, like a 1-micron absolute filter for cyst reduction, the word absolute is the one to look for on the spec sheet.

Is a lower micron number always better?

No. A lower micron number means a tighter filter, which catches smaller particles but also clogs faster and restricts flow more. If your water has a lot of coarse sediment, a 1-micron filter will plug within days and you will be changing cartridges constantly. The right approach is staging: a coarse filter first to take out the big stuff, then a finer filter to polish. That spreads the load and makes the fine filter last.

Do sediment filters remove chemicals or bacteria?

No. A sediment filter is mechanical filtration only. It removes suspended solids like sand, silt, rust flakes, and clay by physical straining. It does not remove dissolved chemicals like lead, nitrates, PFAS, or chlorine, and it does not disinfect water or kill bacteria. A 1-micron absolute filter can physically reduce some larger pathogens like Giardia and Cryptosporidium cysts, but for dissolved contaminants you need carbon, reverse osmosis, or another treatment, and for bacteria you need UV or disinfection.

How often do you change a sediment filter?

There is no fixed schedule because it depends entirely on how much sediment your water carries. Some well owners change a fine cartridge monthly, some city users go six months or longer. The honest answer is to watch your pressure. A clogged sediment filter drops the flow at your taps, so a pressure gauge before and after the filter housing tells you when it is loaded. A spin-down filter is different, you flush it with a valve instead of replacing it, which is part of why it works well as the first stage.

Sediment Filters and Micron Ratings: Spin-Down vs. Cartridge, and What Micron Size to Pick

Sediment is the contaminant nobody searches for and almost everybody has. Sand, silt, rust flakes, clay, scale chips off old pipe. It is the grit at the bottom of a glass left to sit, the cloudiness after the utility flushes a hydrant, the orange flecks in well water after a heavy rain.

A sediment filter is the cheapest, simplest piece of water treatment you can buy, and it is the one that makes everything downstream work. A softener resin bed, a carbon block, a reverse osmosis membrane, a UV lamp, all of them get wrecked or choked by sediment that should have been caught upstream. So before you buy any of those, you buy this. The only real question is which type and what micron size, and that is where most people guess wrong.

Sediment Is an Aesthetic Problem, Not a Health One

Worth being clear up front: sediment itself is rarely a health hazard. The particles are usually inert minerals, sand, and oxidized metal. The EPA covers cloudiness and visible particles under its Secondary Drinking Water Standards, which are non-enforceable guidelines for nuisance issues like color, taste, staining, and turbidity. These are aesthetic standards, not health-based limits.

That matters for how you think about a sediment filter. You are not buying it to make unsafe water safe. You are buying it to clear cloudy water, stop grit from wearing out faucet valves and appliances, and protect the more expensive filtration that handles the contaminants that do affect health. If your real concern is rust-colored water, the cause matters before the cure, and our pages on brown water from the tap and brown well water causes help you tell sediment apart from dissolved iron, which a sediment filter will not catch.

What a Micron Rating Actually Means

A micron, or micrometer, is one millionth of a meter. For scale: a human hair is roughly 70 microns across, fine beach sand is around 100 to 300 microns, and the cysts of Giardia and Cryptosporidium are in the 4 to 14 micron range. A micron rating on a sediment filter tells you the size of particle it is built to strain out.

The smaller the number, the tighter the filter and the smaller the particles it stops. A 50-micron filter catches sand and large grit but lets fine silt pass. A 5-micron filter catches most of what makes water look cloudy. A 1-micron filter catches fine clay and is tight enough to physically reduce some larger pathogens.

Here is the part the marketing skips. A tighter filter clogs faster. A 1-micron cartridge on water full of coarse sand will plug in days and choke your water pressure. That is not the filter failing, that is you using the wrong micron size for your water. Tighter is not better. Right-sized is better.

Nominal vs Absolute Ratings

This is the spec that separates a real filter from a hopeful one, and it is the detail most buyers never see.

A nominal rating means the filter removes roughly 85 to 90 percent of particles at the stated size. A nominal 5-micron filter lets a meaningful fraction of 5-micron particles slip through. It is a useful filter, but the number is a target, not a guarantee.

An absolute rating means the filter removes around 99.9 percent of particles at the stated size under defined laboratory test conditions. A 1-micron absolute filter is the standard worth looking for when particle size genuinely matters, for example when you want to reduce Giardia and Cryptosporidium cysts by physical straining.

String-wound and melt-blown filters are usually rated nominally. Pleated filters are more often given an absolute rating because their structure holds a consistent pore size. When you see a bare number with no word next to it, assume nominal. When the size has to be reliable, the word “absolute” is the one to find on the data sheet.

The relevant certification framing here is NSF/ANSI 42, which covers mechanical filtration and particulate reduction. Under that standard, particulate reduction is graded in classes by particle size range, so a product certified to NSF/ANSI 42 for a given particulate class has been tested to a defined performance, not just labeled by the manufacturer. You can verify any claim in the NSF certification database. A printed micron number with no certification behind it is a manufacturer’s word, nothing more.

The Four Types of Sediment Filter

There are really two formats, a flushable spin-down screen and a replaceable cartridge, and within cartridges there are three constructions. Each has a job.

Spin-Down (Flushable Screen)

A spin-down filter is a clear housing with a stainless steel or polyester mesh screen inside and a flush valve at the bottom. Water spins through the screen, the sediment drops to the bottom, and when it loads up you open the valve and blow the debris out the drain. Nothing to replace.

Spin-downs are coarse by design, typically 50 to 500 microns, with 100 microns being a common all-purpose choice. They are the right first stage on a well that pulls sand, grit, or scale. Because you flush them instead of buying cartridges, they cost almost nothing to run, which is why they earn the lead-off spot in a staged setup. Brands like Rusco and iSpring build the common residential models. What a spin-down will not do is clear cloudy water, the mesh is far too open for fine silt.

Pleated Cartridge

A pleated cartridge folds the filter media like an accordion, which packs a large surface area into a small cartridge. More surface area means more dirt-holding capacity and longer life before clogging. Pleated filters commonly run 1 to 50 microns and are often given an absolute rating.

Two more advantages: many pleated cartridges are washable, so you can rinse and reuse them a few times before replacing, and the steady pore size makes them well suited when you need a dependable particle cutoff. They cost more per cartridge than the alternatives. They are a strong choice for a fine polishing stage and for RO pre-filtration.

String-Wound Cartridge

A string-wound cartridge is yarn wound tightly around a central core, denser toward the inside. That graded density makes it a depth filter, it traps particles throughout its thickness, not just on the surface, so it holds a good amount of sediment for its price. String-wound cartridges are usually nominally rated and span roughly 1 to 100 microns. They are inexpensive and forgiving on water with a mixed range of particle sizes. The trade-off is that the nominal rating means looser real-world performance than an absolute-rated pleated filter at the same printed micron number.

Melt-Blown Cartridge

A melt-blown cartridge is spun polypropylene fibers, often graded so the outside is coarse and the core is fine. Like string-wound, it is a depth filter, and it is typically the cheapest cartridge on the shelf. Melt-blown filters are the default 5-micron sediment cartridge in most under-sink and whole-house housings. They are nominally rated, single-use, and a sensible everyday workhorse for general sediment. Pentek and Pentair build many of the standard sizes that fit 10-inch and 20-inch housings.

Staging: Coarse Then Fine

This is the single idea that separates a setup that works from one you fight with constantly.

Do not try to do all the filtering with one cartridge. If you put a single 5-micron filter on sandy well water, it loads up fast, your pressure sags, and you change cartridges every few weeks. Stage it instead. Put a coarse filter first to take out the big particles, then a finer filter to polish what is left. The coarse stage protects the fine stage, so the fine cartridge lasts far longer.

A typical well water sequence:

Stage 1: spin-down at 100 microns. Catches sand, grit, and scale. Flush it, never replace it.
Stage 2: cartridge at 20 to 30 microns. Knocks out the medium sediment the spin-down passes.
Stage 3: cartridge at 5 microns. Polishes the water clear and protects whatever comes next.

City water with occasional cloudiness usually needs less. A single 5-micron melt-blown cartridge in an under-sink or whole-house housing handles most municipal sediment on its own.

The point of staging is spreading the load across micron sizes. Each filter only has to remove the slice of particles between its rating and the rating of the stage before it, so each one lasts longer and your pressure stays up.

Where Sediment Filters Fit in a Treatment Train

A sediment filter is a pre-filter. It goes first, ahead of the equipment that does the real contaminant removal, because that equipment is what sediment damages.

Before a water softener. Grit scratches and fouls the resin bed and jams the control valve. A spin-down ahead of a softener is cheap insurance.

Before an iron filter. Sediment and iron are different problems with different fixes, and a sediment filter does not remove dissolved (ferrous) iron, it only catches iron that has already oxidized into visible flakes (ferric). For the full picture on the iron side, see best iron filters for well water.

Before reverse osmosis. This is the most important one. An RO membrane is delicate and expensive, and sediment tears it up. Nearly every RO system ships with a sediment pre-filter, usually 5 microns, exactly for this reason. If you want to physically reduce cysts ahead of the membrane too, a 1-micron absolute pre-filter is the upgrade. Our under-sink RO install guide shows where that pre-filter sits in the stack.

Before a whole-house carbon or UV system. Sediment blinds carbon and shades UV lamps so the light cannot reach the water. Clean the water mechanically first. Our best whole-house water filters page covers how these stages combine into a single system.

How to Pick, in One Pass

Test or at least look at your water first. Visible grit and sand point to a coarse spin-down lead-off. Cloudiness with no obvious particles points to a fine cartridge.

Then match the micron size to the job:

50 to 100 microns: first-stage spin-down for sand and grit on a well.
20 to 30 microns: a middle stage to bridge coarse and fine.
5 microns: the everyday standard for clearing cloudy water and protecting softeners, carbon, and RO. If you buy one filter, this is usually it.
1 micron absolute: when you want to physically reduce Giardia and Cryptosporidium cysts, or feed a sensitive RO membrane the cleanest possible water.

And do not over-tighten. Reach for 1 micron only when you have a reason for it, because a fine filter on dirty water just clogs and starves your pressure. Stage it, watch your pressure with a gauge across the housing, and change the cartridge when the flow drops, not on a calendar.

A sediment filter will not make bad water safe. It will not touch lead, nitrates, PFAS, chlorine, or bacteria, and it does not disinfect anything. What it does is clear the water you can see and protect the filtration that handles the contaminants you cannot. For matching those other contaminants to the right treatment, start with what water filter do I need.