>Typical reverse osmosis filters produce one unit of clean water for every three units of intake
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...
Your "pressurized accumulator" sounds like a "permeate pump", which are readily available, not too expensive (~50 USD), and easily installed. It took me longer to move the filter assembly out from under my sink than to splice the device into my RO system!
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.
Seems like they misunderstood you but also gave you good advice. Using a post-RO pump removed the pressure from the RO output side. I used this setup when I had a salt-water reef aquarium and needed perfect water. The pump made a large difference in the ratio.
There can be downsides to the permeate pumps, but not many.
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.
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...