Yes, in the big scheme of things its a rounding error against industrial uses but that still doesn't mean I'm going to leave my hose bib running 24/7.
That's actually a very optimistic figure IME:
I recently started using a consumer under-sink RO system from GE Appliances at my off-grid cabin, the kind you find at Home Depot.
If you use the included pressurized accumulator tank, the stated efficiency is ~10%.
Since I'm off-grid, the reject line is run to a 5-gallon water cooler bottle, making product:reject water ratio comparisons trivial. Even 10% is somewhat optimistic.
The efficiency is about double if you disable the pressurized accumulator, that way there's no pressure on the other side of the membrane (except when stopped). It's not a simple fixed ratio since the efficiency decreases as the accumulator fills and the pressure difference across the membrane approaches zero. It's actually pretty insane how wasteful the system is as-delivered if you use the accumulator, towards 100% full it's mostly just rejecting water for an hour+.
Without the accumulator, but using the included reject water restriction orifice, it's more like 20-25%. I've ended up adding an adjustable restrictor valve on the reject water line to keep it closer to the 3:1 you described, which shouldn't damage the membrane AIUI.
I guess it's just safe defaults they ship in an unsophisticated system connecting to potentially high inlet pressures. This combined with a luxurious pressurized accumulator tank, makes it spectacularly wasteful of perfectly good water. Most wouldn't even realize the waste volumes having the reject line plumbed into the sink drain.
I've been making huge hexagonal concrete pavers with the reject water...
Anyone with an RO system living in a water-scarce region should install one! It'll save you many gallons a day, and has no downsides, unlike most other water reduction devices (like weak-flushing toilets or barely-misting shower heads).
The "pressurized accumulator" I'm referring to is just a water storage tank in the form of a bladder inside an air-pressurized steel vessel. A smaller version of the tank you find next to practically any well-pump to conserve pump cycling/wear and tear/smooth pressure spikes... Its participation in this system post-membrane is wasting water, not conserving it.
It's included from the factory just so nobody has to wait for filtered water to pass the membrane at time of use, instead experiencing an instant powerful jet of water out of the filtered water faucet.
Installed with a standard hydraulic shut off valve (ASO) you won't see any issues but you won't get much in the way of improved storage pressure. You will just see improved product/waste ratio as the pump negates the back pressure from the tank. The downsides are the periodic thumping noise and the added complexity of the extra tubings and fittings.
Installed without an ASO valve the permeate pumps will cause some TDS creep and bleed high TDS product water into the storage tank each time the system stops and starts. This will mix with the low TDS product water and raise the average up somewhat. The amount will vary on usage behavior but it's a noticeable TDS bump for most users.
Edit to add: Apparently I'm easily confused. I thought water softening was happening by reverse osmosis, but it's a different mechanism.
Googling for reverse osmosis minerals will bring you to a large number of probably fabricated claims about water pH and health that ultimately align on taste preference.
> “Remineralization is typically to enhance the flavor of the water, because reverse osmosis reduces the level of minerals in the water significantly, and some consumers prefer the taste of the water when there is enhanced mineral content,” explains Rick Andrews, of the National Sanitation Foundation’s Global Water Program. - https://www.thespruce.com/best-reverse-osmosis-systems-45868...
https://www.homedepot.com/p/ISPRING-Premium-10-in-Universal-...
https://www.homedepot.com/p/Express-Water-Alkaline-Water-Fil...
https://thewatergeeks.com/how-to-remineralize-ro-water/
etc
If you don't do this and have particularly hard water the membranes foul pretty rapidly.
In the case of undersink systems pumping out two gallons or so a day from standard municipal water, it's probably less important.
I only know this because I've been playing with osmosis systems for a few years and have built one that does around 150 gallons a day from undersink system parts with 2 membranes, some filters and pumps for my garden (otherwise my water is simply too hard to grow all the things I want to grow).
I think the RO filter is still a small cartridge, the thing that looks like a white bbq propane tank is just a water reservoir since the filtering is very slow.