A 100 micron filter cartridge is a coarse strainer, not a purifier. It catches sand, grit, rust flakes and other visible particles while letting almost everything smaller pass, which is exactly what you want at the front of a system where the job is to protect the equipment behind it rather than to polish the water.
What a 100 micron rating means in practice
Micron ratings describe the size of particle a cartridge is meant to stop. At 100 microns you are in the range of coarse sand, well grit, scale chips and the flakes that come off aging galvanized pipe. Anything you can see in a glass of water is comfortably above that size, and anything you cannot see is almost certainly below it. That makes a cartridge at this rating a strainer rather than a filter in the way most homeowners mean the word. It will not affect taste, odor, color from dissolved iron, or anything chemical, and no cartridge at this rating should be sold as if it will.
Where it belongs in the system
The usual home for a coarse cartridge is the very first position after the well pressure tank or after the main shutoff on a city line. Put it there and it takes the abrasive load off everything downstream: the softener valve, the carbon tank, the membrane, the water heater dip tube and the fixture aerators. Some well systems run a spin-down screen at a similar rating ahead of the housings, which can be flushed rather than replaced. If your first cartridge in a multi-stage system is clogging within weeks, adding a coarse stage in front of it is usually cheaper than continually replacing the finer element.
Why finer is not automatically better
It is tempting to fit the finest cartridge that will physically go in the housing. On a supply carrying real sediment, that produces a filter that blinds off in days and a pressure drop the whole house feels: weak showers, appliances timing out, a pump cycling harder than it should. Staged filtration exists because the load has to be shared. Coarse first, finer second, finest last. A useful field test is to run a coarse cartridge for one cycle and look at what it captured. If it is loaded with grit, you have your answer. If it is spotless, you can safely step to a finer rating in that position.
Coarse filters and iron staining
A common misunderstanding is that a sediment cartridge will fix orange staining. It depends entirely on the form the iron is in. Iron dissolved in water is invisible and passes through any sediment cartridge, only turning orange after it meets air. Once oxidized it becomes particles, and coarse and fine cartridges will start catching it, which is why filters on some wells come out rust-colored. Penn State Extension notes that iron and manganese carry secondary, aesthetic standards with recommended maximums of 0.3 mg/L iron and 0.05 mg/L manganese, and that oxidized iron fouls softener resin. That is the real reason to deal with iron properly upstream rather than hoping a strainer will do it.
Questions people ask about 100 micron filter cartridge
Will a 100 micron cartridge improve taste?
No. At that rating it removes visible particles only. Taste and odor problems are handled by carbon, which is a different stage entirely.
How often should a coarse cartridge be changed?
By pressure drop rather than by calendar. When flow at the fixtures noticeably weakens, or the cartridge comes out heavily loaded, shorten the interval.
Can I use a washable screen instead?
Yes. Spin-down screens and stainless mesh elements at coarse ratings can be flushed or rinsed and reused, which suits wells that carry a steady grit load.
Does a coarse filter remove bacteria?
Not at all. Bacteria are far smaller than that rating. Disinfection requires ultraviolet treatment or chlorination, not a sediment cartridge.