Advanced RO Rejection Rate Calculator

Formula Used

RO rejection rate (%) = ((Feed concentration - Permeate concentration) / Feed concentration) × 100.

Salt passage (%) = (Permeate concentration / Feed concentration) × 100.

Recovery (%) = (Permeate flow / Feed flow) × 100.

Concentration factor = Feed flow / Concentrate flow.

Mass balance closure (%) = ((Permeate load + Concentrate load) / Feed load) × 100.

How to Use This Calculator

1. Enter feed concentration from the raw or inlet sample.
2. Enter permeate concentration from the product water sample.
3. Add feed and permeate flow for recovery and mass balance.
4. Enter concentrate values if you measured them.
5. Set a target rejection value for a quick pass check.
6. Press calculate, then export the report if needed.

Example Data Table

Understanding RO Rejection Rate

Reverse osmosis membranes separate dissolved ions from water by pressure and selective diffusion. The rejection rate shows how much of the feed contamination is removed before water reaches the permeate line. A higher value usually means better membrane separation. The calculator compares feed concentration with permeate concentration. It also estimates salt passage, recovery, concentration factor, and simple mass balance. These related values help operators see whether the result is physically reasonable.

Why It Matters

RO performance changes with membrane age, pressure, temperature, fouling, flow rate, and feed chemistry. A new membrane can reject salts well, yet poor sealing or scaling can lower the observed value. Feed TDS alone is not enough. Permeate quality must be checked at the same time. Flow data adds another useful layer. If recovery is very high, dissolved solids concentrate on the reject side. That can increase scaling risk and reduce stable output.

Reading The Outputs

Rejection percentage is the main result. Salt passage is its opposite. Recovery tells what share of feed flow becomes product water. Concentration factor estimates how strongly the brine stream is concentrated. Feed mass load, permeate load, and concentrate load are shown for a basic check. Small differences can happen because field meters, flow gauges, and sampling points are not perfect. Large differences suggest bad input data or a system issue.

Best Practice

Use clean samples from steady operation. Flush the unit first if it has been idle. Enter matching units for feed and permeate concentration. Use the unit selector only for display context, because the ratio stays the same when both readings use the same unit. Record pressure, temperature, and membrane model in your site log. Compare new results with past values. A falling rejection rate may point to membrane damage, chemical attack, fouling, or an O ring leak. This tool gives a fast screening result. Final decisions should use validated instruments and the membrane maker guidance.

Operational Tips

Test at normal pressure and normal feed temperature. Avoid samples taken during startup. Label each reading with date, cartridge status, and cleaning history. When rejection drops suddenly, inspect instruments, valves, seals, and pretreatment first. This prevents replacing a good membrane too early during costly maintenance decisions later.

FAQs