Setting RO recovery: big systems vs. small systems
Recovery is the single most important number a tech sets on an RO. It decides how much water you make, how hard you push the membranes, and whether the system scales. On an 8-inch system a projection has already chosen it for you. On a 4-inch system you often have to set it yourself — safely, without one. Here's how to do both.
Feed = Permeate + Concentrate · Recovery = Permeate ÷ Feed · recover more water → concentrate gets saltier
Video overview
6:24
Narrated overview of this module. The article below has the full procedure and the numbers.
01
What recovery actually is
Every gallon of feed water leaves the membrane one of two ways: as permeate (the clean product that pushed through) or as concentrate (the reject that swept the rejected salts out to drain). Recovery is just the fraction you kept as product.
The definition
Recovery % = ( Permeate flow ÷ Feed flow ) × 100
And because Feed = Permeate + Concentrate, you can get it straight off the two flow meters most systems already have: Recovery = Permeate ÷ (Permeate + Concentrate). A system making 6 gpm permeate and 4 gpm reject is running 6 ÷ 10 = 60% recovery.
Don't confuse recovery with rejection. Recovery is about how much water you make. Rejection is about how clean it is (the percent of salts the membrane blocks). This article is about recovery — the quantity dial.
02
Why recovery is the master dial — and why more isn't free
Here's the catch: the membrane rejects salts, but that rejected salt doesn't disappear — it stays in the shrinking concentrate stream. The more water you recover, the less water is left to carry the same salt load, so the concentrate gets more concentrated. That multiplier is the concentration factor, and it's why recovery drives scaling:
Concentration factor
CF ≈ 1 ÷ ( 1 − Recovery )
RecoveryConcentrate isWhat that means
50%≈ 2×Reject runs about twice feed TDS — lots of margin from scaling
75%≈ 4×Common brackish design point — needs a projection & antiscalant confidence
80%≈ 5×Getting aggressive — sparingly-soluble salts are near their limit
90%≈ 10×Only with a solid projection, the right antiscalant, and forgiving water
So recovery is a tug-of-war, and the tech's job is to land it in the right spot:
Recovery too low
Wasteful. You send usable water to drain, need a bigger feed supply, and run up water and pumping cost. Safe from scaling — but you're throwing water away.
Recovery too high
Dangerous. Concentrate supersaturates and scales the tail elements; crossflow drops so fouling and concentration polarization set in; osmotic pressure climbs; and permeate quality falls as tail-end salt passage rises.
The ceiling is set by the water, not by what you wish. The most limiting sparingly-soluble salt in the concentrate (CaCO₃, CaSO₄, BaSO₄, SrSO₄, or silica) caps how high you can safely recover. Antiscalant raises that ceiling — it doesn't remove it.
03
Two systems, two jobs
Whether a tech confirms a recovery or chooses one comes down to whether we have a projection — and that usually tracks system size.
8-inch systems recovery is designed
We have an analysis + software projection
The recovery is already chosenThe projection set it against the water chemistry, element count, and antiscalant — your job is to hit it, not invent it
Commission to the numbersSet recovery to the design value, then verify actual permeate/concentrate flows, feed & concentrate pressures, and permeate TDS match the projection
Dose antiscalant to the projected ppmThe projection's recovery only holds if the antiscalant it assumed is actually being fed (see the antiscalant procedure)
Deviation = investigateFlows or pressures off the projection means something's wrong — don't just re-throttle to force the flow
The projection is the source of truth — the tech reproduces it
4-inch systems recovery is a safety setting
Often no analysis, no projection — not practical
Set conservative first, earn higher laterWith unknown water, recovery is a safety choice, not an efficiency one — start low and let monitoring justify pushing up
Let element limits be your guardrailsManufacturer datasheet gives minimum concentrate flow and max recovery per element — those keep you safe with no model
Fewer elements = lower single-pass recoveryOne or two 40-inch elements physically can't make high recovery safely; use concentrate recycle to hold crossflow if you need more system recovery
At least get the basics if the water looks hardA quick hardness / TDS / pH read (and antiscalant) buys margin on well water you don't trust
No projection means default to caution and watch the trends
A good conservative default on unknown brackish water is around 50% recovery (roughly 1:1 permeate-to-reject). It keeps the concentrate near 2× feed and leaves plenty of crossflow. Drop lower on hard or high-TDS water; only climb above it once you have a test, an antiscalant, and clean monitoring trends behind you.
04
How to actually set recovery in the field
Recovery is set with valves and read off flow meters. Two valves do the work:
Throttle the concentrate valve closed → more permeate, higher recovery, less reject. Close too far and you starve crossflow.
Read both flow meters. Note permeate gpm and concentrate gpm at normal operating pressure. Recovery = permeate ÷ (permeate + concentrate). No meters? Time a bucket fill on each line.
Adjust the concentrate throttle valve toward target. Closing it raises recovery (more permeate, less reject); opening it lowers recovery. Move in small steps and let flows settle between moves.Small, deliberate turns — recovery reacts fast near the top.
Respect the element guardrails. Never throttle below the datasheet's minimum concentrate flow, and don't exceed max recovery per element. These are the guardrails that keep an un-projected small system safe.Rough datasheet figures: ~15–18% max recovery per 40-inch element; check the specific model.
Use recycle if crossflow is too low. On short 4-inch arrays, if you need more system recovery than the elements can sweep, route part of the concentrate back to the pump suction to hold crossflow while sending less to drain.
Confirm quality and pressure. Check permeate TDS/conductivity and the feed-to-concentrate pressure drop. Recovery is only "set" once flow, quality, and pressure all sit in a sensible place together.Log the final flows, recovery, pressures, and permeate TDS as the baseline.
GuardrailWhy it matters
Minimum concentrate flowper element / vessel, from datasheet
keeps crossflow sweeping salts off the membrane
Max recovery per element~15–18% per 40-inch element
stops any single element from over-concentrating
Max permeate flow / fluxper element rating
over-flux fouls and shortens membrane life
Feed / operating pressure limitssystem & element max
protects hardware; high recovery pushes pressure up
05
Recovery calculator
Enter what the two meters read and get recovery, the concentration factor, and a quick orientation on where that sits for un-projected water:
Recovery & concentration factor
Straight from the permeate and concentrate flow meters.
Recovery
60 %
Concentrate is
2.5 × feed
Feed flow
10 gpm
—
Orientation only, for un-projected water. On 8-inch systems the projection sets the real target and ceiling — always defer to it.
The one thing to remember
On a big system, match the projection. On a small one with no projection, recovery is a safety setting — start conservative and let the trends earn you more.
Axis Water Technologies · 855-443-2947 · info@axiswater.com · axiswater.com · 3462 W Loop 289, Lubbock, TX 79407Axis technician training library · content as of October 1, 2026
