An inline sediment filter is a cartridge housing plumbed straight into the main water line so every drop passes through a filter element before it reaches the rest of the house. It catches sand, silt, rust flakes and scale grit that would otherwise score valve seats, block aerators, and shorten the life of everything downstream of it.
Where it goes on the line
On a well, the housing sits after the pressure tank and before anything else you own: softener, iron filter, UV lamp, water heater. That order matters because a softener resin bed and a UV quartz sleeve are both ruined by grit, and both cost far more to fix than a cartridge costs to replace. On city water it goes right after the main shutoff, ahead of the branch that feeds the house. Leave a shutoff valve on each side of the housing plus a union or a bypass loop, so a cartridge change is a two minute job rather than a shut-the-house-down job. Give yourself at least a cartridge length of clear space beneath the sump, or you will not be able to drop the bowl without unbolting the whole assembly.
What it catches, and what sails straight through
A sediment cartridge is a mechanical strainer. It removes particles: sand, silt, pipe scale, rust that has already oxidized into a solid flake. It does nothing at all to anything dissolved. Hardness passes through. Dissolved clear-water iron passes through, then oxidizes downstream and stains the tub anyway. Bacteria pass through. Chlorine taste, sulfur smell and nitrates all pass through. This is the single most common misunderstanding we hear from homeowners who installed a filter and expected their water problem to go away. The filter is a bodyguard for the equipment behind it, and a fix for visible grit, not a treatment for water chemistry. Test first so you know which of your problems is particulate and which is not.
Watch pressure, not the calendar
Cartridge life depends entirely on how dirty your water is, so a fixed replacement schedule is guesswork. The honest method is a pair of pressure gauges, one before the housing and one after. Note the difference on the day you fit a fresh cartridge, then watch it. As the element loads up, the gap widens and flow at the far fixtures gets weak. When the drop roughly doubles, change it. If you would rather not fit gauges, the low-tech version works: pull the sump on a set day each month, look at the cartridge, and let what you see set your own interval. A cartridge left in too long can channel or collapse and dump its captured load downstream in one go.
Sizing the housing and staging the filtration
Two things decide the housing: the flow your house needs, and how much dirt the water carries. A slim standard housing is cheap and fits anywhere, but it has little surface area, so on dirty water it blinds fast and it chokes flow when several fixtures run at once. A big-body housing takes a fatter cartridge with far more surface, holds more dirt before it clogs, and passes more flow at a given pressure loss. On genuinely silty well water, staging beats a single fine cartridge: a coarse screen or a spin-down catches the sand first, then a finer cartridge polishes what is left, and the expensive fine element lasts several times longer. Filter brands publish their housings in flow-rated capacity steps, which is what you match against your peak demand.
Questions people ask about inline sediment filter
Will an inline sediment filter soften my water?
No. It removes suspended particles only. Hardness is dissolved calcium and magnesium, which passes straight through the cartridge and still needs a softener.
Should the sediment filter go before or after the softener?
Before. Grit that reaches a softener grinds the control valve seals and fouls the resin bed, so the cheap cartridge should always take the hit first.
How often does the cartridge need changing?
There is no fixed interval, because it depends on how much sediment your water carries. Change it when pressure drop across the housing has roughly doubled.
Can I run an inline filter without a bypass?
You can, but you should not. Without a bypass or a pair of isolation valves, every cartridge change means shutting off water to the whole house.