RO permeate is the water that passes through the reverse osmosis membrane and comes out of your drinking faucet. Everything the membrane rejects leaves by the other outlet as concentrate, which goes to drain. Understanding the split between the two explains almost every question people have about how these systems behave.
Permeate, concentrate and feed
Three streams, three names. Feed is the water arriving at the membrane housing after the pre-filters. Permeate, sometimes called product water, is the portion that crossed the membrane film, leaving nearly all dissolved solids behind. Concentrate, also called reject or brine, is the remainder, carrying the rejected salts away at a higher concentration than the feed. The membrane needs that continuous sweep of concentrate across its surface, because without it the rejected minerals would pile up at the film, precipitate and scale it. That is the whole reason a reverse osmosis system has a drain line and why it can never be waste-free.
Recovery, and what it means for your bill
Recovery is the share of feed water that leaves as permeate rather than concentrate. Household systems have historically had modest recovery, which is what gives reverse osmosis its reputation for wasting water. Two things improve it: a permeate pump, which recovers energy from the concentrate stream to reduce backpressure from the storage tank, and modern tankless designs with a booster pump that run the membrane at higher pressure. Vendors publish the ratio on their product pages, so it is a specification you can compare directly when shopping, alongside the daily production rate that the same page will state.
Judging permeate quality
The everyday measure is total dissolved solids, read with a cheap handheld meter or a permanently mounted dual inline monitor. Rejection is the drop between the feed reading and the permeate reading, expressed as a proportion, and it is far more informative than the permeate number alone, because a house with very low feed solids will show a good permeate reading even from a failing membrane. Take both readings when the system is new and write them inside the cabinet door. When rejection falls off over time, the membrane is aging or has been damaged by chlorine that got past a tired carbon pre-filter.
What permeate is like to use
Because it is stripped of dissolved minerals, permeate tastes flat and makes hollow coffee, which is why remineralizing cartridges are sold as a last stage. It is excellent for anything where minerals cause trouble: humidifiers, steam irons, aquarium top-ups, car rinsing and ice that stays clear. It also has almost no buffering, so it takes on flavor from tubing and from a tank left standing, and running the faucet briefly after a few idle days is worth the habit. Keep the pre-filters fresh, since protecting the membrane is the only way to keep permeate quality where it started.
Questions people ask about ro permeate
Is permeate the same as filtered water?
It is the specific term for water that has passed through a reverse osmosis membrane, as opposed to concentrate. Carbon and sediment filtration produce filtered water but no permeate stream.
Why does my system send water to the drain?
The concentrate stream continuously sweeps rejected minerals off the membrane surface. Without it they would precipitate and scale the film, so the drain flow is protection rather than waste by design.
What is a good permeate reading?
Compare it against your feed reading rather than judging it alone, since rejection is the meaningful number. Record both when the system is new and watch for the gap narrowing over time.
Can I reuse the concentrate?
Some households collect it for plants tolerant of higher mineral content or for outdoor cleaning, but it must never be plumbed back into the feed, and it is not suitable for drinking.
Does a permeate pump improve water quality?
Its main effect is efficiency: it reduces tank backpressure so the membrane works against less resistance, which raises recovery and shortens fill time, and rejection generally improves slightly too.