A string wound filter cartridge is yarn wound in a criss-cross pattern over a central core, creating a bed that is loose on the outside and tight at the middle. Water works its way inward through progressively finer gaps, so the cartridge traps sediment throughout its depth rather than only on its face.
Depth loading is the whole point
A pleated cartridge stops particles at a surface, and once that surface is covered the pressure drop climbs quickly. A wound cartridge grades from coarse to fine as water moves inward, so big particles are held near the outside and fines are caught deeper in. On a well that produces sand, silt or iron floc that gives a longer usable life at the same nominal rating and a far gentler rise in pressure drop, which is why wound cartridges are standard on dirty raw supplies.
Nominal ratings, not absolute ones
Wound cartridges are rated nominally: they capture most particles at the stated size, not all of them. That is fine for protecting a softener, a pump or a carbon stage, and it is not fine as the last barrier before an ultraviolet lamp or a reverse osmosis membrane, where an absolute-rated pleated or melt-blown cartridge belongs. A common well arrangement is a coarse wound cartridge first for bulk sediment, then a finer absolute cartridge to polish before the sensitive equipment.
Materials and temperature
Polypropylene yarn is the general-purpose choice: inert, chlorine-tolerant and suitable for cold potable lines. Cotton wound cartridges exist for specific applications but are not the right pick where chlorine is present. Check the temperature rating if the cartridge sits anywhere hot water can back up into it, and check the end cap style, since open-end, double-open-end and gasketed styles are not interchangeable between housings. Buy the style your housing takes rather than the one that looks similar.
Knowing when it is spent
Judge a wound cartridge by pressure, not by the calendar or by how it looks. Fit gauges before and after the housing, note the drop when the cartridge is new and change it when that drop has roughly doubled or when showers noticeably weaken. A cartridge caked with orange iron on a private well is telling you the real problem is untreated iron, which needs an oxidizing filter rather than an ever-shorter cartridge cycle. Our water filter replacement cartridge guide covers matching sizes and ratings across housings.
Questions people ask about string wound filter cartridge
Can a string wound cartridge be washed and reused?
No. Rinsing shifts trapped particles deeper into the winding and ruins the grading. Wound cartridges are consumables and are replaced, not cleaned.
String wound or pleated for well water?
Wound for heavy or variable sediment, because it loads through its depth and lasts longer. Pleated where you need a cleaner absolute cut before ultraviolet or a membrane.
Does a finer micron rating always filter better?
No. Too fine a rating on a silty supply blinds off in days and starves the house of pressure. Stage a coarse cartridge first, then a finer one behind it.