A pre filter is a sediment stage installed ahead of your main treatment equipment so that grit, rust flakes and suspended particles never reach it. It is the cheapest component in most systems and it protects the most expensive ones, which is why leaving it out or letting it clog is behind a surprising share of softener, membrane and ultraviolet failures.
What it protects, and from what
Sand, silt, rust and pipe scale do specific damage downstream. In a softener they lodge in the resin bed and create channels that let water bypass the resin, so capacity drops while the machine reports normal operation. On a reverse osmosis membrane they abrade and blind the surface, cutting output permanently. In an ultraviolet chamber they shade organisms from the lamp, so the water leaves disinfected on paper and not in fact. And in a carbon bed they occupy surface area that should be adsorbing contaminants. One inexpensive cartridge upstream prevents all four.
Choosing a micron rating
Micron rating is the size of particle the cartridge is designed to stop, and smaller is not automatically better. Too fine a rating on water carrying a lot of sediment gives you a cartridge that blinds in weeks and a house with poor pressure. The usual approach on a well is a staged one: a coarse spin-down or a large-micron cartridge first to take the bulk, then a finer cartridge to polish before sensitive equipment. Ultraviolet systems typically specify five micron immediately ahead of the chamber. Match the rating to the equipment being protected rather than choosing the finest cartridge on the shelf.
Pleated, spun and washable options
Spun polypropylene cartridges are graded so larger particles are caught on the outside and finer ones deeper in, which gives good dirt-holding capacity at low cost, and they are disposable. Pleated cartridges pack more surface area into the same housing and some are rinsable, which suits water that carries a predictable seasonal sediment load. Spin-down filters use a stainless mesh in a clear bowl that you flush from a valve at the bottom, generating no waste at all, and they are the natural first stage on a well that produces sand. Many households run a spin-down followed by a spun cartridge and change the second one far less often as a result.
Knowing when to change it
Do not judge by appearance, because a cartridge that looks filthy may still flow freely and a clean-looking one can be loaded internally. Fit pressure gauges before and after the housing and change on pressure drop, which is the only direct measure of how blocked it is. Failing that, change on a calendar and shorten the interval if you notice flow falling off. On well systems, expect a sudden change in sediment load after heavy rain, a pump service or any work on the well, and check the prefilter after any of those events. Our whole house filter guide covers where the prefilter sits relative to everything else.
Questions people ask about pre filter
Does a prefilter reduce water pressure?
A clean one causes only a small drop. A loaded one causes a large drop, which is exactly why gauges on either side are the best way to decide when to change it.
Where should the prefilter be installed?
On the main line after the meter or pressure tank and before all other treatment, with a bypass and isolation valves so you can change cartridges without shutting off the house.
Can I wash and reuse a sediment cartridge?
Only the ones designed for it, such as pleated washable cartridges and stainless spin-down screens. Rinsing a spun cartridge pushes trapped dirt deeper and shortens its life.
Do I need a prefilter on city water?
Often yes, though a coarser one. Mains work, hydrant flushing and aging pipe all release rust and scale that arrive at your house without warning.