Three constructions
Spun-bonded polypropylene cartridges are made by blowing molten polypropylene into a self-supporting cylinder of very fine fibres — DWT manufactures these at its Cape Town plant. The name is the trade's, not the process engineer's: spunbond and melt-blown are two different things, spunbond drawing coarser continuous filaments that mostly serve as a support layer, melt-blown making the fine short fibres that do the filtering. A cartridge sold as spun-bonded is a melt-blown one. The fibre density increases toward the core, giving depth filtration: coarse particles trap in the outer layers, fines deeper in. One material throughout, no binders, wide chemical compatibility.
String-wound cartridges wind a textile yarn (polypropylene, cotton or glass fibre) in a diamond pattern around a core. The winding creates hundreds of tapering channels — also depth filtration, but with a more open, more tortuous structure that keeps accepting solids after a fine melt-blown surface would have blinded.
Pleated cartridges fold a thin sheet of filter media into pleats, like a car air filter. Filtration happens on the surface of the sheet, and the pleating packs several times more surface area into the same housing. Because the media is a manufactured sheet, pleated cartridges achieve the tightest, most certifiable ratings — this is where absolute ratings live — and many can be rinsed and reused a few times.
Head to head
| Spun-bonded | String-wound | Pleated | |
|---|---|---|---|
| Filtration type | Depth | Depth | Surface |
| Typical ratings | 1 – 50 µm nominal | 1 – 100 µm nominal | 0.2 – 50 µm, incl. absolute |
| Dirt-holding | Good | Best for heavy/coarse solids | Lower per change, high area |
| Pressure drop when new | Low | Low – moderate | Lowest (area advantage) |
| Temperature | To ~60 °C (PP) | To ~120 °C in glass/cotton | Media-dependent |
| Washable | No | No | Often, a few cycles |
| Relative cost | Lowest | Low – moderate | Highest |
Choosing by duty
- Municipal water, general protection duty: spun-bonded, almost always. It is the economical default and the ratings on offer match the duty.
- Borehole and river water, heavy or coarse solids: string-wound. The open structure carries a much larger solids load before the differential pressure forces a change — fewer change-outs on dirty water is the whole argument.
- Hot water or aggressive chemicals: string-wound in glass fibre or cotton over a stainless core, chosen per the compatibility tables.
- Final barrier ahead of UV, RO or a validated process: pleated, absolute-rated. Surface filtration blinds quickly on dirty water, so a pleated final stage assumes decent water arriving — put a depth cartridge in front of it.
- High flow with low solids (polishing loops): pleated, because the surface area buys the lowest pressure drop per 10-inch length.
The combination is the answer
These are stages, not rivals. The standard industrial train is a coarse depth cartridge (or media filter) taking the load, then a fine depth or pleated cartridge providing the guarantee. Asking one cartridge to be both the workhorse and the barrier is how systems end up with weekly change-outs.
Rule of thumb: spun-bonded for everyday duty, string-wound when the water is genuinely dirty, pleated when the rating itself is the point. If change-out frequency is annoying you, the fix is usually a coarser stage in front — not a better cartridge in the same slot.