Many buyers only focus on how much pure water an RO system can produce per hour when purchasing reverse osmosis equipment. However, they ignore two other critical indicators: recovery rate and salt rejection rate. Permeate flow, salt rejection rate and recovery rate together determine the actual performance of the whole RO system. Missing any one of them may lead to bad purchasing decisions.
1. Permeate Flow: System Output Capacity
Permeate flow refers to the volume of qualified pure water produced by the RO system under design conditions, usually marked as m³/h or T/H. For example, a 50m³/h single-stage RO system means it can produce 50 tons of pure water per hour under standard conditions. Important note: Permeate flow is calibrated at 25°C standard temperature. The actual output will drop when the raw water temperature goes down. Many suppliers overrate permeate flow, and the equipment cannot reach the advertised flow at low water temperature.
2. Salt Rejection Rate: Purification Capacity
Salt rejection rate shows the ability of RO membranes to remove dissolved salts from raw water, which decides the final water quality. Formula: Salt Rejection Rate = (Raw Water TDS - Permeate TDS) ÷ Raw Water TDS × 100% A standard single-stage RO system achieves ≥97% salt rejection. Double-stage RO can reach above 99%. Higher salt rejection means lower TDS in product water and purer water quality. If you have strict water quality requirements, checking only permeate flow will risk unqualified outlet water.
3. Recovery Rate: Water Utilization Efficiency
Recovery rate is the ratio of pure water output to total raw water intake, reflecting how efficiently you use water resources. Formula: Recovery Rate = Permeate Flow ÷ Feed Flow × 100% For an RO system with 50 tons per hour permeate flow, if feed water is 75 tons and concentrate discharge is 25 tons, its recovery rate is 66.7%. Higher recovery rate is NOT always better. Excessively high recovery will raise salt concentration on the membrane surface rapidly, causing scaling and fouling, shortening membrane service life and increasing maintenance cost. Normally, single-stage RO is designed at 50%~75% recovery rate, adjusted based on raw water hardness and SDI.
4. The Relationship Between The Three Indicators
These three values interact and restrict each other instead of working independently:
- With fixed permeate flow, higher recovery leads to higher salt concentration in concentrate, which requires anti-fouling RO membranes.
- For high TDS raw water, you may need to reduce single-element recovery to guarantee stable salt rejection.
- Water temperature and operating pressure affect both permeate flow and salt rejection. Reliable manufacturers will calculate all three indicators according to your raw water test report, instead of only quoting a permeate flow number.
5. Tips For RO System Selection
✅ Permeate flow: Confirm it is rated at 25°C, distinguish nominal flow and actual flow under low temperature.
✅ Salt rejection rate: Match it with your water quality requirement; single-stage and double-stage RO have obvious difference.
✅ Recovery rate: Design according to raw water hardness and scaling risk, do not blindly pursue high recovery.
When selecting RO equipment, do not judge only by permeate flow. Check recovery rate, salt rejection rate and permeate flow together. Only in this way can your system run stably for a long time, maintain stable water quality and control operation cost.
Q1: What is the ideal recovery rate for a single-stage RO system?
A1: The standard design range is 50%–75%. It must be adjusted according to raw water hardness, SDI and scaling tendency. Higher recovery will increase fouling risk.
Q2: Why does permeate flow drop when raw water temperature decreases?
A2: Water viscosity rises at lower temperature, slowing water penetration through RO membranes. All nominal permeate flow is calibrated at 25°C.
Q3: Can I get 99% salt rejection with a single-stage RO?
A3: Normally no. A standard single-stage RO reaches around 97% salt rejection. For 99% or higher salt rejection, double-stage RO is recommended.
Q4: Is it good to set the recovery rate as high as possible?
A4: No. Excessively high recovery causes concentrated salt on the membrane surface, triggering scaling, membrane fouling and early membrane replacement.
Q5: Which parameter should I prioritize when comparing RO suppliers?
A5: You need to check all three together: permeate flow, salt rejection and recovery rate. A supplier quoting only permeate flow may hide unreasonable design or overrated data.
