A string filter, more properly a string wound cartridge, is made by winding yarn around a central core in a controlled pattern. It is one of the oldest cartridge constructions still in general use, and it survives because the winding produces a genuine depth filter with high dirt holding capacity at a low price.
How the winding filters
The yarn is wound so that the passages are wider at the outside and progressively tighter towards the core, which makes the whole thickness of the cartridge do filtration work rather than just the outer face. Large particles are caught in the outer windings and finer ones penetrate further before being trapped. That depth structure is why these hold a great deal of dirt before pressure drop climbs, and why they suit water carrying a heavy and varied sediment load.
Against melt blown and pleated
Melt blown polypropylene works on the same depth principle with a matted fiber structure, holds comparable dirt, and does not shed. Pleated cartridges fold a sheet media for a large surface area, run at lower pressure drop for their rating and can sometimes be rinsed once, which suits intermittently dirty water. String wound competes on cost and on capacity for coarse material, and it is often available in materials that suit higher temperatures or chemical duties where polypropylene is unsuitable.
The drawback worth knowing
String wound cartridges can shed fibers as they age, and the shed material travels downstream. That matters little on a garden or irrigation duty and matters a great deal ahead of a reverse osmosis membrane, a UV chamber or anything with fine passages. It is also why a string cartridge left in far too long is worse than a spent melt blown one: the latter simply blinds, while the former can put material into the water it was fitted to clean.
Where to use one
Coarse pre filtration on well water carrying heavy sediment, ahead of a finer stage that will catch anything shed. Industrial and agricultural duties where cost per cartridge dominates. High temperature applications where the winding material is chosen for the duty. For drinking water pre filtration and anywhere a membrane sits downstream, prefer melt blown or pleated. Our whole house filter guide covers staging coarse and fine cartridges properly.
Questions people ask about string filter
How does a string wound cartridge filter?
By depth. The winding is looser outside and tighter towards the core, so the whole thickness traps particles rather than just the surface.
What is its main drawback?
It can shed fibers as it ages, which travel downstream. That matters greatly ahead of a membrane or a UV chamber.
How does it compare with melt blown?
Similar depth filtration and dirt capacity at similar cost, but melt blown does not shed, which is why it is usually preferred.
Where is a string filter a good choice?
Coarse pre filtration on heavy well sediment ahead of a finer stage, and duties where temperature or chemistry rules out polypropylene.