Tools & Microfiber2 min read

RO Rejection and Recovery: Two Percentages That Measure Different Things

Calculate RO rejection and recovery separately, understand their different denominators, and compare systems under relevant conditions.

Updated · NANOLAB Knowledge

Reverse-osmosis rejection describes the reduction in a specified solute concentration. Recovery describes the share of feedwater collected as permeate. A system can have a high rejection percentage without a high recovery percentage, because the denominators are different.

Calculate each percentage separately

For an illustrative concentration comparison, feedwater at 400 units and permeate at 20 units gives rejection of (400 − 20) ÷ 400 × 100 = 95 percent. The units and measurement method must match on both sides.

For an illustrative flow comparison, 40 litres of permeate from 100 litres of feedwater gives 40 percent recovery. That leaves 60 litres in the concentrate stream, ignoring other losses for this simplified example. The 95 percent rejection does not mean 95 litres became product water.

Compare under representative conditions

Pressure, temperature, feedwater composition and operating configuration can affect performance. Record those conditions and use the supplier's procedure for evaluating or normalizing results. A casual comparison across different seasons may be misleading.

When using conductivity as a practical surrogate for dissolved salts, preserve that description. Do not relabel a calculated conductivity rejection as a laboratory measurement of every contaminant.

Use the result for the right decision

Rejection helps assess the selected water-quality property. Recovery helps assess water use and system operation. Both should be considered with permeate production rate, maintenance needs and downstream requirements.

Do not alter valves or operating limits simply to maximize a percentage. The equipment supplier should determine a suitable operating configuration for the feedwater and system. For a detailing business, the meaningful outcome is enough acceptable rinse water delivered reliably, with understood water consumption and operating cost—not the highest isolated number on a sales sheet.

Related guidance

Technical references: DuPont: RO and NF principles and terminology.

For product-specific application instructions, consult NANOLAB technical documentation or ask about your surface and intended use.