Sediment filters are consumables, and most of the money and most of the annoyance in owning one is in the replacement cycle rather than the purchase. Choosing the right cartridge type for the dirt you actually have, and sizing the housing so it lasts, is the difference between a filter you change twice a year and one you fight with every month. Here is how the common cartridges differ and how to keep a stock that fits.
Spun, pleated and string-wound
Spun polypropylene is a melt-blown depth cartridge: dirt is captured through the whole thickness of the wall, often in a graded structure that catches coarse material at the outside and finer material deeper in. It holds a lot before it blinds, which makes it the default on a well. Pleated cartridges fold a thin medium into a much larger surface area, so on lightly silted water they run for a long time at a low pressure drop, and many can be rinsed and reused a few times. String-wound cartridges are yarn wrapped over a core, tolerant of coarse grit and warm water, and they shed less than the cheapest spun cartridges do.
Reading the rating properly
A nominal rating means the cartridge captures most particles at that size, while an absolute rating means it captures essentially all of them. Two cartridges printed with the same number can therefore behave very differently, and a nominal cartridge is not adequate protection where something downstream genuinely depends on clarity, such as a UV lamp. Staging is the usual answer: a coarse cartridge takes the bulk of the load and a finer one polishes, so the fine cartridge lasts far longer than it would on its own. That is also the cheapest configuration to run, because the sacrificial coarse cartridge is the one you replace most often.
How often to replace, and how to tell
Calendar intervals are a starting point, not an answer, because the load depends entirely on your water. Gauges either side of the housing turn it into a measurement: the pressure difference across a clean cartridge is small and grows steadily as it loads, and you replace when the gap reaches the point where household pressure suffers. Absent gauges, weak flow at the furthest fixture is the signal most people notice first. Never let a cartridge run to full blockage on a well, because a badly loaded filter can channel and release accumulated dirt downstream in a single slug. Our replacement water filters guide covers scheduling across a whole system.
Housings, sizes and keeping spares
Cartridges come in a small number of standard lengths and diameters, and the large diameter housings hold several times the dirt of a standard one at the same rating, which usually pays for itself in changes avoided. Record the exact size and rating somewhere permanent when the system is installed, because working it out later from a soaked cartridge is unpleasant. Keep at least one spare on hand: sediment events arrive without warning after a main break or a heavy rain, and that is precisely when you cannot wait for delivery. Buy the housing wrench when you buy the housing, and keep the o-ring lubricated with silicone grease at every change.
Questions people ask about sediment filters
How often should I change a sediment cartridge?
It depends entirely on the sediment load, so measure rather than guess. A growing pressure difference across the housing, or weak flow at the furthest fixture, tells you the cartridge is spent.
Is a finer cartridge always better?
No. A cartridge finer than your water needs plugs quickly and costs you household pressure. Match the rating to the dirt, and stage coarse before fine if the load is heavy.
What is the difference between nominal and absolute?
Nominal means most particles at that size are caught; absolute means essentially all of them are. Absolute is the stricter claim and matters when a UV lamp or membrane depends on the water being clear.
Do I need to sanitize the housing when I change cartridges?
A rinse and a wipe is normally enough on treated municipal water. On a well, or after any bacteria issue, sanitizing the housing at each change is a sensible habit.