When discussing design solutions with peers and customers, we often encounter questions about the proper setting of RO system recovery rates. The core issue is whether the set recovery rate exceeds the system’s upper limit (the limiting recovery rate).
General Conclusion of RO System Recovery
Regarding the recovery rate of reverse osmosis systems, values derived from membrane element performance and experience can generally be expressed as follows:
Single-pass RO system, small system (total number of membrane elements < 6): Recovery rate set to 50–75%
Single-pass RO system, medium to large system: Recovery rate set to 75%
Two-pass RO system: Recovery rate set to 85%+
Conclusive statements are always easier to remember, but conclusions without limiting conditions can often be misleading.
Parameters Related to RO Recovery Rate
Recovery Rate Calculation Formula
R = Fp / Ff × 100%
Where:
Ff = Feed flow rate to the RO membrane (system) (m³/h)
Fp = Permeate flow rate from the RO membrane (system) (m³/h)
R = Recovery rate of the RO membrane (system)
Concentration Factor Calculation Formula
CF = 1 / (1 – R)
Where:
CF = Concentration factor
R = Recovery rate of the RO membrane (system)
Concentration Polarization Factor
β (single-stage RO) ≤ 1.2
β (two-stage RO) ≤ 1.4
Note: For traditional RO membranes, the upper limit of recovery rate for a single membrane element is 15%, and the limiting recovery rate upper limit is 15% * 1.2 = 18%. In RO software design systems, the former is generally used as the design standard (exceeding it will trigger a warning).
appropriately.
| Number of Membrane Elements in Series (pcs) | 1 | 2 | 3 | 4 | 5 | 6 | 8 | 12 | 18 |
|---|---|---|---|---|---|---|---|---|---|
| Maximum System Recovery Rate (%) | 18 | 32 | 42 | 50 | 55 | 59 | 68 | 80 | 90 |
For small RO systems with fewer than 6 total membrane elements, the above maximum recovery rate limit can be relaxed.
Factors Influencing RO System Recovery Rate
In principle, the factors influencing RO system recovery rate can be divided into four categories: sparingly soluble salt scaling, concentration polarization, concentrate flow at the tail element, and membrane flux equalization. The corresponding influencing factors are as follows:
- Sparingly soluble salt scaling:Raw water quality, recovery ratio
- Concentration polarization:Salt concentration buildup on the concentrate side of the membrane element surface
- Tail element concentrate flow:Sufficient concentrate flow at the tail element to carry away high-salinity concentrate, reducing scaling risk on the tail concentrate side
- Membrane flux:Balanced membrane flux helps ensure uniform working efficiency of front and rear membrane elements, preventing excessively high recovery rate or membrane flux for a single element.
The combined effects of 1, 2, and 3 occur on the concentrate side surface of the tail element (the last membrane element). Here, the risk of sparingly soluble salt scaling is highest. The corresponding solubility product (Ksp) of the sparingly soluble salt is equal to the product of the raw water concentration factor and the square of the concentration polarization factor (for AB-type sparingly soluble salts; for A₂B and AB₂ type, it is the cube). Taking a system recovery rate of 75% with calcium carbonate as an example:
Concentration factor CF = 1 / (1 – 0.75) = 4
β = 1.2
CF × β = 4.8
The Ksp is (4.8)² = 23.04 times the Ksp of the raw water (feed water).
Considering that 2 can be addressed by short-term low-pressure flushing, and 3 and 4 can be appropriately solved by reasonably arranging membrane elements, the factor that often determines the RO system recovery rate in conventional systems is the scaling problem of sparingly soluble salts. The upper limit of its influence is the combined effect of 1, 2, and 3, while the lower limit is the effect of the system’s concentration ratio alone. Using the same example above, the upper limit is over 23.04 times, and the lower limit is 16 times.
Therefore, when we assume that the system’s membrane element arrangement is ideal, the most important factor affecting the recovery ratio is actually the raw water quality, with calcium salt and silicate deposition being the primary concerns. Calcium scaling is mainly divided into calcium carbonate and calcium sulfate scaling. Calcium sulfate scaling generally plays a decisive role only when the sulfate ion concentration is much higher than the alkalinity (commonly seen in mining and industrial wastewater). Otherwise, calcium carbonate scaling is usually dominant. Silicate deposition is greatly influenced by temperature and antiscalants and generally does not limit the system’s recovery rate (except in silica-rich water sources).
Below is a table I previously derived through pure mathematical calculation (LSI and Ksp) showing the upper limit of recovery rate caused by sparingly soluble salt scaling for different raw water qualities. (This table considers the influence of concentration polarization; the actual upper limit of recovery rate will be slightly higher than the table values.)
| Note: pH=7, Concentration unit: mmol/L | 25°C | 15°C | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No antiscalant added | Antiscalant added | No antiscalant added | Antiscalant added | |||||||||||
| Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | |||
| Low Hardness Tap Water | Na+ | 2 | 88.06% | 70.41% | 95.88% | 74.36% | 80.74% | 70.99% | 93.36% | 74.79% | ||||
| Ca2+ | 0.25 | |||||||||||||
| SO42- | 0 | |||||||||||||
| SiO2 | 10 | |||||||||||||
| TDS (mg/L) | 169.5 | |||||||||||||
| Conventional Tap Water | Na+ | 5 | 82.09% | 68.92% | 93.82% | 73.24% | 71.11% | 69.55% | 90.04% | 73.71% | ||||
| Ca2+ | 0.5 | |||||||||||||
| SO42- | 0 | |||||||||||||
| SiO2 | 15 | |||||||||||||
| TDS (mg/L) | 393.5 | |||||||||||||
| Surface Water / Mountain Spring Water | Na+ | 5 | 70.15% | 65.23% | 89.71% | 70.55% | 51.85% | 66.02% | 83.40% | 71.12% | ||||
| Ca2+ | 2 | |||||||||||||
| SO42- | 0 | |||||||||||||
| SiO2 | 25 | |||||||||||||
| TDS (mg/L) | 646.5 | |||||||||||||
| Note: pH=7, Concentration unit: mmol/L | 25°C | 15°C | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No antiscalant added | Antiscalant added | No antiscalant added | Antiscalant added | |||||||||||
| Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | Y (CaSO₄) | Y (SiO₂) | Y (CaCO₃) | |||
| Surface Water / Mountain Spring Water | Na+ | 5 | 70.15% | 65.23% | 89.71% | 70.55% | 51.85% | 66.02% | 83.40% | 71.12% | ||||
| Cl⁻ | 5 | |||||||||||||
| Ca2+ | 2 | |||||||||||||
| HCO₃⁻ | 4 | |||||||||||||
| SO42- | 0 | |||||||||||||
| SiO2 | 25 | |||||||||||||
| TDS (mg/L) | 646.5 | |||||||||||||
| Conventional reclaimed water | Na+ | 10 | 68.98% | 82.09% | 88.69% | 93.82% | 71.97% | 71.11% | 89.32% | 90.04% | ||||
| Cl⁻ | 10 | |||||||||||||
| Ca2+ | 5 | |||||||||||||
| HCO₃⁻ | ||||||||||||||
| SO42- | 5 | |||||||||||||
| SiO2 | 15 | |||||||||||||
| TDS (mg/L) | 1290 | |||||||||||||
| High-salinity reclaimed water | Na+ | 20 | 69.13% | 70.15% | 64.78% | 87.09% | 89.71% | 70.23% | 71.97% | 51.35% | 65.59% | 87.76% | 83.40% | 70.80% |
| Cl⁻ | 20 | |||||||||||||
| Ca2+ | 6 | |||||||||||||
| HCO₃⁻ | 2 | |||||||||||||
| SO42- | 5 | |||||||||||||
| SiO2 | 25 | |||||||||||||
| TDS (mg/L) | 2057 | |||||||||||||
From the table above, we can see that in most cases, calcium carbonate scaling is the main factor limiting system recovery rate. Only when the temperature is low and the silicic acid content is high does silicate precipitation become the determining factor, and it is greatly influenced by the effectiveness of antiscalants. Therefore, without using a softener in the pretreatment stage, it is very difficult for the system’s recovery rate to reach 75% (even with antiscalant addition). When a softener is properly configured (Ca²⁺ < 0.03 mmol/L), the scaling risk of calcium salts decreases sharply, and the upper limit of the system’s recovery rate often exceeds 75%.
Conclusions and Recommendations of RO System Recovery Rate (for Single-stage RO Systems)
Small RO Systems
Number of membrane elements < 6, recovery rate significantly limited by system configuration:
Good raw water quality (e.g., tap water): Recommended recovery rate 60–66.7% (concentration factor 2.5–3). Add softening/antiscalant if necessary.
Poor raw water quality (e.g., reclaimed water, brackish water): Recommended recovery rate 50–60%. If a softening/antiscalant device is installed, recovery rate can be increased to 60–66.7%.
Small to Medium RO Systems
6 < number of membrane elements < 20, recovery rate somewhat limited by system configuration:
Good raw water quality (e.g., tap water): Recommended recovery rate 66.7–75% (concentration factor 3–4). Add softening/antiscalant if necessary.
Poor raw water quality (e.g., reclaimed water, brackish water): Recommended recovery rate 60–75%. Install a softening/antiscalant device.
Medium and Larger RO Systems
Number of membrane elements > 20, recovery rate hardly limited by system configuration, unless space is constrained:
Good raw water quality (e.g., tap water): Recommended recovery rate 75% (concentration factor 4). Install a softening/antiscalant device.
Poor raw water quality (e.g., reclaimed water, brackish water): Recommended recovery rate 60–75%. Install a softening/antiscalant device.
Note: The feed water quality for two-pass RO systems is good, and the risk of sparingly soluble salt scaling is low. The upper limit of recovery rate for a single membrane element is 30%. Theoretically, three membrane elements connected in series can achieve a recovery rate exceeding 80%.
Final Remarks: The theoretical calculations and empirical conclusions above will deviate to some extent from design software such as WAVE and ROSA. Precise conclusions should be based on software calculations under detailed raw water quality conditions. The conclusions in this article are for simple reference only. For medium and larger RO systems, please use software design or verification (please refer to previous articles for software usage instructions and download links).
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