A micron filter is rated by the size of particle it is built to stop, and the number on the label is only half the story: a nominal rating is a rough average and an absolute rating is a tested guarantee, and the same figure can mean very different things between the two. Getting this right decides whether your filter protects the house or just strangles the flow.
Nominal versus absolute ratings
A nominal rating says the cartridge captures most particles at or above that size, with no promise about the rest. It is the cheaper construction, usually a spun or wound polypropylene cartridge, and it is perfectly good as a bulk sediment trap. An absolute rating means the cartridge has been tested to hold back essentially everything at that size, which requires a pleated or melt-blown structure with a controlled pore distribution. When a claim matters, for instance holding back cysts or protecting a reverse osmosis membrane, only an absolute rating is meaningful. Two cartridges carrying the same number can behave completely differently.
Coarse to fine, in stages
Sediment loading is why single-cartridge installs fail early. A fine cartridge fitted straight onto a dirty supply plugs solid in weeks, and the symptom is falling pressure at every fixture. The fix is staging: a coarse cartridge first to take out sand, grit and rust flakes, then a finer one behind it to catch the silt and fines that got past. The coarse stage is cheap and takes the punishment; the fine stage lasts far longer because it only sees what the first one missed. On a well with heavy sediment, a backwashing sediment tank ahead of both saves a great deal of cartridge changing.
Flow rate and pressure drop
Every cartridge has a rated flow, and running past it does two things: pressure falls at the fixtures, and particles that would have been captured at a slower speed are pushed through the media instead. The remedy is surface area, not a coarser rating. A twenty inch big blue housing carries far more media than a standard ten inch one and will hold the same rating at a much higher flow, which is why whole house installs use the larger format. Pleated cartridges also offer more area than spun ones for the same size. Fit a pressure gauge before and after the housing and change the cartridge when the difference climbs.
What a micron rating cannot do
Micron ratings only address particles. Anything dissolved in the water passes straight through regardless of how fine the cartridge is: hardness, iron in its dissolved ferrous form, nitrate, fluoride, sodium, chlorine and most chemical contaminants. Bacteria are small enough that a sediment cartridge is not a barrier either, and a loaded cartridge can become a place for them to grow. So a micron filter is a mechanical stage in a larger train, protecting the equipment behind it, not a treatment system on its own. Our whole house water filter guide sets out what belongs in front of it and behind it.
Questions people ask about micron filter
Is a lower micron rating always better?
No. A finer cartridge clogs faster and costs more pressure. Choose the coarsest rating that protects whatever sits downstream, and stage a finer one behind it only if you actually need it.
How do I know when to change a micron filter?
By pressure drop, not by the calendar. A gauge either side of the housing tells you when the cartridge is loading up; falling pressure at the fixtures is the same signal noticed later.
Will a micron filter make cloudy water clear?
It depends on the cause. Suspended silt and rust are filterable, but cloudiness from air bubbles or from dissolved iron oxidizing after the tap is not, and needs a different fix.
Does a sediment cartridge remove bacteria?
No. Bacteria pass through ordinary sediment ratings, and a loaded cartridge can support growth. Disinfection is a separate stage, typically ultraviolet or chlorination.