How Does the Two-Pass SWRO + BWRO Process Achieve High Recovery and Low Energy Consumption? | Best Desalination System Design Guide| Insights by AQUAL

How Does the Two-Pass SWRO + BWRO Process Achieve High Recovery and Low Energy Consumption?

Seawater desalination systems traditionally operate at relatively low single-pass recovery rates, typically 40–50%, to ensure membrane safety, stable performance, and manageable scaling risks. However, modern desalination plants increasingly adopt the two-pass SWRO + BWRO series process, enabling overall recovery rates above 65–75% while simultaneously achieving lower specific energy consumption.

This advanced configuration has become one of the most energy-efficient and cost-effective solutions for large-scale seawater desalination projects worldwide.

This article provides a comprehensive explanation of the working principle, design logic, energy-saving mechanisms, and operational advantages of the two-pass SWRO + BWRO process.

1. What Is the Two-Pass SWRO + BWRO Process?

The two-pass SWRO + BWRO process combines:

•First Pass: Seawater Reverse Osmosis (SWRO)

•Second Pass: Brackish Water Reverse Osmosis (BWRO)

Process Flow Overview:

1.Raw seawater → Pretreatment → SWRO membrane system

2.SWRO permeate → Intermediate storage / conditioning → BWRO membrane system

3.Final product water → Remineralization → Distribution

Key Design Concept:

Instead of pushing seawater RO recovery beyond safe limits, the system splits the desalination task into two optimized stages, allowing:

•Safe operation of SWRO membranes

•Higher total system recovery

•Lower total energy consumption

2. Why Single-Pass SWRO Recovery Is Limited to 40–50%

Single-pass SWRO recovery is constrained mainly by:

•High scaling risk

•Severe concentration polarization

•Excessive osmotic pressure rise

•Rapid membrane fouling

As recovery increases:

•Brine salinity rises sharply

•Osmotic pressure increases exponentially

•Required operating pressure skyrockets

This results in:

•Dramatic energy consumption increase

•Accelerated membrane degradation

•Higher chemical cleaning frequency

Therefore, most SWRO systems are intentionally designed at 40–50% recovery for optimal stability.

3. How the Two-Pass Design Breaks the Recovery Limitation

The two-pass SWRO + BWRO configuration achieves high overall recovery through pressure-stage separation and salt concentration control.

Step 1: First-Pass SWRO (40–45% Recovery)

•Handles extremely high salinity seawater

•Operates at high pressure (55–70 bar)

•Produces low-salinity permeate (~300–800 ppm TDS)

Step 2: Second-Pass BWRO (80–90% Recovery)

•Treats low-salinity SWRO permeate

•Operates at much lower pressure (8–16 bar)

•Achieves very high recovery efficiently

Overall System Recovery Formula:

Total Recovery = 1 − (1 − SWRO Recovery) × (1 − BWRO Recovery)

Example:

•SWRO = 45%

•BWRO = 85%

Total Recovery = 1 − (0.55 × 0.15) = 91.75%

In practical engineering, total system recovery of 65–75% is easily achievable with high operational safety.

4. How the Two-Pass System Reduces Energy Consumption

4.1 Pressure Level Decoupling

High-pressure operation is limited only to the first SWRO stage, while the second BWRO stage operates under low pressure, dramatically lowering average energy demand.

4.2 Maximized Use of Energy Recovery Devices (ERD)

The SWRO concentrate contains massive hydraulic energy. Advanced ERDs (e.g., pressure exchangers) can recover 95–98% of this energy, transferring it directly to incoming seawater feed.

This reduces net SWRO pumping energy by up to 60%.

4.3 Lower Osmotic Pressure in Second Pass

Because SWRO permeate salinity is low:

•BWRO membranes face minimal osmotic resistance

•High recovery is achieved with very low operating pressure

Thus, the second pass contributes high water yield with minimal energy input.

5. Key Technical Advantages of Two-Pass SWRO + BWRO

6. Additional Functional Benefits

6.1 Superior Boron Removal

Boron is difficult to remove in single-pass SWRO. The second-pass BWRO, especially at high pH conditions, enables boron removal below 0.5 mg/L, meeting strict drinking water and irrigation standards.

6.2 Improved System Flexibility

Operators can:

•Adjust second-pass recovery

•Optimize pressure balance

•Improve seasonal operational adaptability

This enhances long-term operational reliability.

7. Typical Application Scenarios

The two-pass SWRO + BWRO system is ideal for:

•Municipal seawater desalination plants

•Industrial ultrapure water production

•Power plant boiler feed systems

•Semiconductor manufacturing

•High-end commercial water supply

8. Economic Advantages

Despite higher initial CAPEX, the system provides:

•Lower long-term OPEX

•Reduced membrane replacement cost

•Lower chemical consumption

•Extended equipment lifespan

Over the system lifecycle, the total cost of water production is significantly reduced.

Conclusion

The two-pass SWRO + BWRO configuration represents the most advanced desalination system architecture currently available. By intelligently splitting pressure stages, optimizing osmotic conditions, and maximizing energy recovery, it successfully achieves both high recovery and low energy consumption, solving the fundamental contradiction of traditional seawater desalination.

For large-scale, energy-efficient, and high-quality desalination projects, this process is now widely regarded as the best-practice engineering solution.