RO Recovery & Rejection Rate Calculator

What is Reverse Osmosis Membrane Performance?

Reverse osmosis (RO) forces pressurized water through a semi-permeable membrane that blocks dissolved salts and contaminants. Two metrics define membrane performance: recovery rate (what fraction of feed water becomes clean permeate) and salt rejection rate (what percentage of dissolved solids the membrane removes). Together, these determine how much clean water a system produces and how much concentrated brine it discharges.

Mathematical Foundation

System Recovery Rate

R = \frac{Q_p}{Q_f} \times 100\%

R

= Recovery rate as a percentage

Q_p

= Permeate (clean water) flow rate in GPM

Q_f

= Feed water flow rate in GPM

Salt Rejection Rate

SR = \frac{C_f - C_p}{C_f} \times 100\%

SR

= Salt rejection percentage

C_f

= Feed water TDS concentration in ppm

C_p

= Permeate TDS concentration in ppm

Brine Discharge Flow

Q_b = Q_f - Q_p

Q_b

= Brine (reject) flow rate in GPM

Q_f

= Feed water flow rate

Q_p

= Permeate flow rate

Laws & Principles

Conservation of Mass: Permeate flow can never exceed feed flow. Every gallon entering the system exits as either permeate or brine: Q_f = Q_p + Q_b.
The Recovery-Quality Tradeoff: Pushing recovery above 75-85% concentrates salts on the membrane surface, increasing the risk of scaling and reducing salt rejection. Most municipal RO systems target 75% recovery as a balance between water yield and membrane longevity.
Membrane Fouling Rule: As recovery increases, the concentration of rejected salts in the brine stream rises. When brine concentration exceeds the solubility limit of calcium carbonate or silica, scale forms on the membrane and rejection drops.

Step-by-Step Example Walkthrough

" A municipal water plant feeds 10 GPM of brackish water (500 ppm TDS) through an RO system producing 7 GPM of permeate at 25 ppm TDS. "

1. Calculate brine flow: 10 - 7 = 3 GPM of reject water.
2. Calculate recovery: (7 / 10) x 100 = 70% system recovery.
3. Calculate salt rejection: (500 - 25) / 500 x 100 = 95% rejection.

Final Result: The system recovers 70% of feed water as clean permeate while rejecting 95% of dissolved salts. The remaining 3 GPM of brine at roughly 1,600 ppm TDS requires disposal or further treatment.

Industry Application Scenarios

Seawater Desalination Plant

Feed: 35,000 ppm seawater at 1,000 m³/day.
Target: 45% recovery (450 m³/day permeate).
Rejection: 99.5% salt rejection produces permeate at ~175 ppm.
Challenge: The remaining 550 m³/day of brine at ~63,000 ppm requires ocean outfall or evaporation ponds.

Brackish Well Water Treatment

Feed: 2,000 ppm brackish groundwater at 50 GPM.
Target: 80% recovery (40 GPM permeate).
Rejection: 97% rejection gives permeate at 60 ppm — well within drinking water standards.
Brine: 10 GPM reject at ~10,000 ppm sent to deep injection well.

Water Source	Feed TDS (ppm)	Typical Recovery	Salt Rejection
Municipal Tap	200-500	85-90%	95-98%
Brackish Well	1,000-5,000	75-85%	96-99%
Seawater	30,000-45,000	40-50%	99.3-99.7%
Industrial Wastewater	5,000-15,000	60-75%	95-98%

Water Source

Feed TDS (ppm)

Typical Recovery

Salt Rejection

Municipal Tap

200-500

85-90%

95-98%

Brackish Well

1,000-5,000

75-85%

96-99%

Seawater

30,000-45,000

40-50%

99.3-99.7%

Industrial Wastewater

5,000-15,000

60-75%

95-98%

Operational Guidelines

Do This

✓Monitor rejection rate trends over time. A gradual decline in salt rejection (e.g., from 98% to 93% over months) is an early warning sign of membrane degradation or fouling that requires cleaning or replacement.
✓Use antiscalant when recovery exceeds 75%. High recovery concentrates calcium and silica in the brine stream. Chemical antiscalant dosing prevents mineral scale from forming on the membrane surface.

Avoid This

✗Don't push recovery above manufacturer limits. Every membrane has a maximum rated recovery (typically 15% per element, 75-85% per system). Exceeding it causes irreversible fouling and voids the warranty.
✗Don't confuse TDS with specific contaminants. A 99% salt rejection rate does not mean 99% removal of every contaminant. Small uncharged molecules like boron may pass through at 60-80% rejection even with excellent overall TDS removal.

Frequently Asked Questions

What is a good recovery rate for an RO system?

For brackish water, 75-85% recovery is standard. For seawater desalination, 40-50% is typical because the higher osmotic pressure of seawater limits how much water can be pushed through the membrane. Home under-sink RO units often run at only 20-25% recovery, sending most of the water to drain.

Why does salt rejection decrease over time?

Membrane degradation from oxidant exposure (chlorine), biological fouling, and mineral scaling all create micro-pathways that allow dissolved salts to bypass the rejection layer. Regular cleaning cycles (CIP) and proper pretreatment extend membrane life and maintain rejection above 95%.

What happens to the brine discharge?

Brine disposal depends on the application. Coastal plants use ocean outfall diffusers. Inland facilities may use deep well injection, evaporation ponds, or zero-liquid-discharge (ZLD) crystallizers. Brine disposal is often the most expensive and regulated part of an RO system.

Can I increase recovery by adding a second RO pass?

Yes. A second pass takes the brine from the first pass and runs it through additional membranes at higher pressure. This can push total system recovery to 90-95%. However, the second pass requires higher energy input and the concentrated brine increases scaling risk, so antiscalant dosing and careful pH control are essential.

RO Recovery & Rejection Rate

System Flow Rates

Water Quality (TDS)

Brine / Reject Flow