Rejection rate is the number that tells you whether your RO membrane is actually working. It measures what percentage of a contaminant in the feed water is blocked by the membrane and kept out of the product water. A new, well-maintained membrane rejects 95–99.7% of dissolved solids depending on the contaminant and membrane type. A fouled or damaged membrane rejects less — and the drop in rejection rate is often the first measurable sign that something is wrong.
The Rejection Rate Formula
Rejection rate for any contaminant (or for TDS overall) is calculated from two measurements: the concentration in the feed water and the concentration in the permeate.
Rejection (%) = (1 − Cpermeate / Cfeed) × 100
Example: Feed water TDS = 800 ppm, permeate TDS = 12 ppm
Rejection = (1 − 12/800) × 100 = 98.5%
In the field, TDS rejection is measured with a TDS meter or conductivity meter on the permeate and feed lines — the fastest and most practical field measurement for ongoing monitoring. Specific contaminant rejection (for nitrates, arsenic, PFAS, etc.) requires laboratory analysis of both feed and permeate samples.
Rejection Rates by Contaminant
Different contaminants are rejected at different rates by the same membrane. The key variables are molecular size (larger molecules are more completely rejected by size exclusion) and ionic charge (divalent ions are rejected more completely than monovalent ions).
| Contaminant | Typical Rejection (New Membrane) | Primary Rejection Mechanism | Notes |
|---|---|---|---|
| NaCl (sodium chloride) | 99.0–99.7% | Charge repulsion + size | Standard test parameter for membrane certification |
| Calcium (Ca²⁺) | 96–99% | Size + divalent charge | Divalent ions rejected more completely than monovalent |
| Magnesium (Mg²⁺) | 96–99% | Size + divalent charge | Same as calcium |
| Sulfate (SO₄²⁻) | 97–99% | Size + divalent charge | High rejection — divalent anion |
| Nitrate (NO₃⁻) | 85–95% | Charge repulsion (monovalent) | Lowest rejection of common ions — monovalent anion |
| Fluoride (F⁻) | 90–96% | Charge repulsion + size | pH-dependent; higher pH improves rejection |
| Arsenic As(V) | 92–96% | Charge repulsion + size | As(III) — only 40–70%; oxidize to As(V) before RO |
| Arsenic As(III) | 40–70% | Size only (uncharged at neutral pH) | Pre-oxidation to As(V) is required for effective removal |
| Lead (Pb²⁺) | 95–99% | Size + divalent charge | Effective across typical pH range |
| Chromium-6 (Cr(VI)) | 92–97% | Charge repulsion | Exists as CrO₄²⁻ or HCrO₄⁻ at typical water pH |
| PFAS (PFOA, PFOS) | 90–99% | Size exclusion | Large molecules; high rejection by size regardless of charge |
| Bacteria | >99.9% | Size exclusion | Cells physically too large to pass membrane pores |
| Viruses | 99–99.9% | Size exclusion | Some smaller viruses may pass; UV post-treatment for drinking water |
| CO₂ (dissolved carbon dioxide) | <10% | None — passes freely | Small uncharged molecule; can lower permeate pH |
| H₂S | <20% | Low | Dissolved gas; degassing equipment required for effective removal |
Why the Monovalent/Divalent Distinction Matters
The polyamide active layer carries a slight negative charge that repels dissolved anions. Divalent anions (SO₄²⁻, CO₃²⁻) carry twice the charge of monovalent anions (Cl⁻, NO₃⁻, F⁻) and are repelled more strongly. This is why:
- Sulfate rejection (97–99%) is higher than nitrate rejection (85–95%) on the same membrane
- Nitrate is the most challenging common contaminant for RO to remove — its monovalent charge and moderate molecular size give it the lowest rejection of the regulated anions
- High-nitrate applications may require higher-rejection membranes (BW30HR) or a second RO pass to achieve sufficient removal
Observed Rejection vs. True Rejection
The rejection rate measured from feed and permeate samples in the field is called observed rejection. It differs slightly from true rejection because concentration polarization — a layer of elevated-concentration water that builds up at the membrane surface — makes the effective feed concentration at the membrane surface higher than the bulk feed concentration measured upstream.
For practical field monitoring, observed rejection (from TDS meter readings) is sufficient to track membrane condition over time. For precise engineering calculations, membrane manufacturers use corrected rejection values that account for concentration polarization effects. The distinction matters in system design; for field monitoring, it doesn’t.
How to Measure Rejection Rate in the Field
Field TDS rejection measurement requires two TDS meters (or one meter and two samples) and three measurements:
- Measure feed water TDS at the inlet to the RO membrane (after pre-filters, before the high-pressure pump or membrane inlet)
- Measure permeate TDS at the product water outlet
- Calculate: Rejection (%) = (1 − permeate TDS / feed TDS) × 100
What to expect on a functioning system:
- New membrane, municipal water: 98.5–99.5% rejection
- Membrane at 2–3 years, well-maintained: 97.5–99.0%
- Membrane at 4–5 years, well-maintained: 96–98%
- Fouled or damaged membrane: below 95% (investigate immediately)
What a Declining Rejection Rate Tells You
Rejection rate decline is a diagnostic signal. Different failure modes produce different patterns:
| Symptom | Likely Cause | Investigation |
|---|---|---|
| Gradual rejection decline over months | Membrane aging — normal, irreversible degradation of polyamide active layer | Compare against historical trend; plan membrane replacement if below 95% |
| Sudden sharp rejection drop | Membrane damage — chlorine/oxidant exposure, physical breach, o-ring failure allowing bypass | Check for chlorine excursion (carbon pre-filter failure), inspect o-rings and element connections |
| Rejection declines with permeate flow increase | O-ring failure or bypass — untreated feed water bypassing the membrane | Replace o-rings and interstage connectors; verify element seating |
| Rejection declines with permeate flow decline | Membrane fouling (biofouling, scaling) — diffuse transport through fouled membrane | CIP cleaning (acid for scale, caustic/biocide for biofouling); assess pre-treatment |
| Rejection low on one pressure vessel, normal on others | Individual element failure or bypass in that vessel | Element-by-element testing with port sampling |
Normalized Salt Rejection: The Right Way to Trend Performance
Raw rejection rate numbers are affected by operating conditions — temperature, feed pressure, and feed TDS all influence the measured value even when the membrane itself hasn’t changed. A cold morning can produce an apparent rejection rate drop that isn’t a membrane problem at all.
Normalized salt rejection (NSR) corrects for operating condition variations and expresses rejection as it would be at standardized baseline conditions. This allows meaningful comparison of rejection measurements taken weeks or months apart under different conditions. Normalized performance trending is the standard approach in commercial and industrial RO system maintenance programs — it separates real membrane degradation from operating condition noise.
For systems without automated data logging, take rejection measurements consistently: same time of day, after the system has run for at least 30 minutes from a warm start, and record feed pressure, temperature, and flow rates alongside the TDS readings. Even without formal normalization, consistent measurement conditions make trending useful.
Rejection Rate vs. Recovery Rate: The Distinction
Rejection rate and recovery rate are the two fundamental RO performance parameters, and they measure different things:
- Rejection rate: How well the membrane removes contaminants from the water that passes through it. A membrane property.
- Recovery rate: What fraction of feed water becomes product water. A system hydraulic parameter.
You can have 99% rejection at 50% recovery, or 99% rejection at 80% recovery. You can also have 90% rejection (degraded membrane) at 75% recovery. They’re independent variables. Both must be monitored. See RO Recovery Rate Explained for the full treatment of recovery.
Monitoring RO system performance? AMPAC USA provides commissioning baseline data (feed TDS, permeate TDS, flow rates, operating pressure) with every system shipment so you have a documented starting point for long-term performance trending. Contact us for technical support on any AMPAC USA system.
Related: How Reverse Osmosis Works | RO Membrane Types | RO Recovery Rate Explained | RO Maintenance Guide | RO System Calculators