Melt blown cartridges are the plain white sediment elements that fill most whole house housings. They are made by spraying molten polypropylene fibers onto a spinning core, which builds a matted tube with no seams, no adhesive and no separate media inside it. That construction is why they are inexpensive, why they hold a lot of dirt, and why they cannot be cleaned and reused.
Depth filtration, not a screen
A screen catches everything at one surface. A melt blown cartridge is a depth filter: the fibers are laid coarse on the outside and progressively finer towards the core, so large particles are stopped in the outer layers and finer ones work their way further in before they are trapped. The whole thickness of the wall does filtration work rather than just the face. That is what gives these cartridges their dirt holding capacity, and it is also why the micron rating on the label describes the tightest inner layers, not the surface you can see.
Choosing the rating, and staging it
A tighter cartridge is not automatically better. On a supply carrying visible sand and rust, a very fine element blinds off within weeks and starves the house of pressure, while a coarse one passes silt that then fouls a softener or a membrane downstream. The usual answer on dirty water is two housings in series, a coarse one first and a fine one second, so the cheap cartridge absorbs the bulk of the load and the fine one lasts. On municipal water with occasional main works, a single mid range cartridge is normally enough.
Melt blown against pleated and string wound
Pleated cartridges fold a sheet media into a large surface area, run at a lower pressure drop for their rating, and some can be rinsed and put back for a second life, which suits water that is intermittently dirty. String wound elements are wound yarn on a core and behave similarly to melt blown but shed fibers as they age. Melt blown wins on cost per cartridge and on holding capacity for grit, and it is the sensible default where cartridges are changed on a schedule rather than nursed. Our replacement cartridge guide covers matching sizes to housings.
Knowing when it is spent
Do not judge by color. A cartridge can look filthy and still flow perfectly, or look nearly clean and be blinded by fine silt that never showed as a stain. Pressure is the honest signal: note the pressure with a fresh cartridge in, and change when it has dropped noticeably at the same flow, using gauges either side of the housing if the supply matters. Failing that, change on a fixed calendar interval and note the date on the housing, because a cartridge left in far too long can channel and release what it was holding.
Questions people ask about melt blown filter cartridges
Can melt blown cartridges be washed and reused?
No. The dirt is held throughout the depth of the fiber wall, not on a surface, so rinsing does not recover it. These are change and discard items.
Is a finer micron rating always better?
No. Too fine for the water blinds off quickly and costs pressure. On dirty supplies use a coarse cartridge first and a finer one after it.
Melt blown or pleated?
Melt blown is cheaper and holds more grit. Pleated runs at lower pressure drop and some can be rinsed once, which suits intermittently dirty water.
How do I tell when to change it?
By pressure drop, not by how dirty it looks. Note the pressure with a new cartridge fitted and change when it has fallen noticeably at the same flow.