In the field of water treatment, reverse osmosis technology can effectively remove various pollutants in water, including dissolved solids, salts, organic matter, and microorganisms, ensuring that the treated water quality meets the standards for drinking or industrial use. Modern reverse osmosis systems usually have a high level of automation, are relatively simple to operate, can operate efficiently, and reduce the need for manual intervention. It is the core technology used in water treatment.
So, how many RO membrane elements should we match when designing a reverse osmosis system? Let's find out.
Why do we need to calculate the number of RO membranes?
Calculating the number of RO membranes is a key step in the design of a water treatment system for the following reasons:
- Accurate membrane calculation ensures that the system can meet specific water treatment requirements. Different application scenarios have different requirements for water flow and water quality. By determining the required number of membranes, it can be ensured that the system will not affect the water treatment capacity due to insufficient membranes during operation.
- A reasonable calculation of the number of membranes helps optimize the system's economy. If the number of membranes is too large, it will lead to unnecessary investment and operating costs and increase maintenance and replacement frequency. Insufficient membranes will lead to an inefficient system, affect water quality, and incur additional treatment costs. Therefore, accurate calculation can find the best balance between performance and cost.
- Calculating the number of membranes can also help designers consider the membrane's operating conditions, such as pressure and recovery rate, to ensure that the system operates in the best operating state, thereby extending the membrane's service life. Through scientific design, the membrane's blockage and contamination are reduced, and its operating efficiency is improved.
Key parameters for RO system design
Water production (GPD or m³/day)
- Definition: The amount of pure water that the system needs to produce per hour/day.
- Unit conversion: 1 m³/day ≈ 264.17 GPD (gallons/day).
- Design basis: Determined according to user needs or project specifications, a 10-15% margin is required to cope with peak demand.
Recovery Rate
- Definition: Ratio of water output to water intake (%).
- Typical values:
- Desalination system: 40-50% (high salinity requires low recovery rate).
- Brackish water/wastewater reuse: 70-85%.
Salt Passage
- Definition: The ratio of influent salt to produced water, reflecting the membrane desalination efficiency.
- Formula: Salt permeability = produced water TDS ÷ influent TDS
- Design goal: Usually, the salt permeability is required to be <1% (such as seawater membrane desalination rate> 99%).
Operating pressure (psi/bar)
- High-pressure membranes: RO membranes need to overcome osmotic pressure, and the pressure of seawater systems can reach 800-1200 psi (55-82 bar).
- Low-pressure membranes: Brackish water treatment is usually 150-300 psi (10-20 bar).
Flux (LMH or GFD)
- Definition: Water production per unit membrane area, reflecting the working strength of the membrane.
- Unit: LMH (liters/square meter/hour) or GFD (gallons/square foot/day).
- Conversion: 1 GFD ≈ 1.7 LMH.
- Safety range:
- Seawater membrane: 12-20 LMH.
- Brackish water membrane: 20-30 LMH.
The core steps to calculate the number of RO membranes
Determine design requirements
1. Target water output: e.g. 100 m³/day.
2. Influent water quality analysis:
- Influent TDS (total dissolved solids), temperature, type of pollutants (colloids/organics/hardness).
- Example: Seawater TDS=35,000 ppm, temperature=25°C.
Select RO membrane model
- Seawater desalination: high desalination rate membrane (such as SW30HRLE-400, single branch water production 7.2 m³/day @ 55 bar).
- Brackish water: low pressure membrane (such as BW30-400, single branch water production 28 m³/day @ 15 bar).
Calculate the total membrane required
- Basic formula: Number of membranes = (target water production ÷ water production of a single membrane) × (1 ÷ recovery rate)
- Note: The calculation results are integers, and all the larger ones are taken.
- Example:
- Target water production = 100 m³/day, single membrane water production = 7.2 m³/day, recovery rate = 45%.
- The number of membranes required: (100÷7.2) × (1÷45%) = 31.
Example calculation
- **Case**: Seawater desalination system
- **Target**: Water production = 200 m³/day, inlet TDS = 35,000 ppm, temperature = 20°C.
- **Membrane selection**: SW30HRLE-400, single-unit water production 7.2 m³/day (STC conditions: 25°C, 55 bar).
- **Calculation**:
- 1. Temperature correction: Water production drops by about 10% at 20°C (7.2 × 0.9 = 6.48 m³/day).
- 2. Number of basic membranes: (200 ÷ 6.48) × (1 ÷ 0.45) ≈ 69 units.
- 3. Add 15% redundancy: 69 × 1.15 ≈ 80 units.
- **Configuration plan**: The membrane housings (PV) are arranged in a 2:1 ratio, and each PV is equipped with 6 membranes, requiring a total of 14 PVs.
Conclusion
Calculating the number of RO membranes may seem simple, but it will change with the parameters and requirements of the entire system. When one of the parameters changes, the number of membrane elements required may change.
If you need us to design a reasonable water treatment solution for you, please feel free to contact us. We will respond to you as soon as possible.
