A 100 micron filter is a coarse strainer, not a purifier. Its job is catching sand, grit and scale flakes before they reach the finer cartridges, the softener valve, the pump seals or a membrane. On a well with any visible sediment it is the cheapest component in the house and the one that makes everything downstream last.
What a hundred micron catches
At a hundred microns you are removing what you can see and feel: sand grains, silt clumps, rust and pipe scale, and the debris that comes with a pump start or a burst on the utility main. You are not removing fine silt, turbidity, dissolved iron, hardness, chlorine or anything biological. That sounds like a limitation and it is precisely the point. A coarse screen has enormous dirt-holding capacity relative to its size, drops almost no pressure when clean, and takes the load that would otherwise blind a five micron cartridge in a week. It is a bodyguard for the expensive equipment, not the equipment itself.
Spin-down screens versus cartridges
At this rating the common product is a spin-down filter: a clear housing with a stainless mesh screen inside and a flush valve at the bottom. Water enters tangentially so heavy particles are thrown outward and settle in the bowl, and opening the flush valve for a few seconds sends the accumulated grit to a drain or a bucket without opening anything. The screen is reusable, so consumable cost is nothing, and maintenance is a habit rather than a purchase. The alternative is a hundred micron pleated or spun cartridge in a normal housing, which is simpler to plumb but means changing a cartridge rather than turning a valve.
Where it goes in the train
It goes first, immediately after the pressure tank on a well, or right after the meter and main shutoff on a municipal supply. Everything else follows: fine sediment cartridge, then any oxidizing filter, then the softener, then point-of-use polishing. Put it ahead of the softener specifically, because grit is very hard on a softener control valve and on the resin bed. If a UV lamp is in the train, the coarse screen protects the fine cartridge that protects the lamp, and clarity is a hard requirement for UV to work. Our best sediment filter for well water guide covers building the train in order for a particular well profile.
Flushing, sizing and the failure to watch for
Flush a spin-down on a routine, and flush it after any event that stirs the well: a pump replacement, heavy rain on a shallow well, or a main break in the street. Size the unit for your peak household flow, not your average, since a screen that is fine at one shower will restrict two. The failure to watch for is a screen left unflushed until it packs solid, which shows up as pressure falling gradually across the whole house. Fit a gauge on each side and the diagnosis takes a glance instead of an afternoon. Clean the screen properly once a year with a brush, because a rinse leaves a biofilm the flush cannot shift.
Questions people ask about 100 micron filter
Is a 100 micron filter enough on its own?
Only if your water is already clean and you just want protection from grit. It passes fine silt, dissolved iron, hardness and everything biological.
Where does a 100 micron filter go?
First in the train, right after the pressure tank on a well or after the main shutoff on city water, ahead of every finer stage and the softener.
Can the screen be cleaned and reused?
Yes. Spin-down screens are stainless mesh, flushed through a valve at the bottom and brushed clean periodically, so there is nothing to replace.
Will it reduce my water pressure?
Not when clean and correctly sized. A packed screen will, which is why a gauge on each side makes the problem obvious immediately.