In principle it can be done, of course, but it wouldn't be economical. The abrasive sludge that accumulates in the bottom of the machine is full of large and small bits of various different materials, which complicates filtering. Even if you only cut a single material, you end up with significant amounts of abraded metal from the machine parts (slats, fittings, clamps, etc.). You would also have to spend a significant amount of energy evaporating and drying the sludge.

In any case, the main problem is that the abrasive garnet particles become decidedly less abrasive once they've been smashed into a piece of steel at 75000psi. Think of it like one of those tumblers you use to smooth and polish stones to make them pretty.

EDIT: As someone else mentioned, it's also a QC issue. Even with brand new abrasive, you sometimes get the tiniest bit of something the wrong size or weight and the solenoid feeder gets jammed and has to be take apart. Recycled sand and who knows what else would be a nightmare. Waterjets are enough work to keep running as it stands ;-)

Source: ~7 years of using an Omax waterjet cutter.

Probably because it is full of bits of whatever is being cut and can't easily be filtered after a run.

Depending on the material being cut there are might be ways. If the weight densities of the abrasive and the material differ enough we can try simple density separation. If the abrasive isn't magnetic while the material is, we can use magnets to do the same. In both cases, the big question would be how much impurities in the abrasive can the machine tolerate. If machine is somewhat tolerant, and impurities do not threaten its mechanisms, these simple methods can be economical for small businesses working with specific materials.

I studied a bit the question, in the waterjet cutting industry nobody reuses it. Basically it becomes rounder after use. A bit like the difference between river sand and desert sand.

Among other things, it gets used up, it may become too fine to cut the kerf at that IPM, it may also have impurities, etc.

Depending on the abrasive, it's theoretically reusable, but that would be very hard to quality control (even assuming you separate it effectively from the material).

This is one reason waterjets often get used only when other ways of cutting are not effective.

Also note: for things like wood, you can probably water jet them without abrasive.

That struck me as well. The abrasive cost looks to be the killer there.

Abrasive cost and maintenance (because high pressure water lines are quite needy) are the biggest expenses.

Lets say you only cut steel. Then the steel particles could be filtered out with magnets. I suppose its much more common to want to work with steel than glass or ceramics.

Nobody I've heard of recycles garnet. It is considered part of the cost of operation.

If you only cut steel you should be buying a plasma cutter.

This 100%. Water jet is great when you need to cut things that don't cut well any other way (tiles, glass,stainless steel, hardened tool steel). Otherwise, plasma, laser, router, mill, etc are all nicer and easier to work with.

Yeah, I'd love to have a water jet for making tools, but I've learned to live with the fact that these kind of things are better owned by someone else.

idk, plasma leaves a lot of slag and depending on the application requires substantial finishing work. mill is fine but has a limited work area (like this waterjet), and at least in my shop, cutter lifetimes aren't great. Thats probably because i don't have the best processes. Also holding down sheets well enough is a substantial hassle unless you're set up for a particular run. Metal routers are really noisy and are arguably more of a hassle than mills.

but I totally agree with your last point, its less convenient, but doing small to medium runs though a local water jet shop is pretty cost effective. Its mildly pricy but they will source standard material for you, and you pick up clean parts a day or two later. even ignoring maintenance - dealing with the up front cost, the consumable cost, and renting enough space to keep the thing is a pretty serious commitment. I have free access to a 4x8 cnc plasma gantry and I still send out quite a bit of work to the water jet place.