Comparison of Brackish Water and Seawater Desalination: A Comprehensive Analysis from Source Water Characteristics to Process Technologies

Although brackish water desalination and seawater desalination share the same fundamental objective—producing fresh water from saline sources—their differences extend far beyond feedwater salinity. This fundamental distinction, analogous to the difference between climbing a hill and summiting Mount Everest, shapes the complete technological framework, encompassing process selection, engineering implementation, economic costs, and environmental impact.

I. Fundamental Differences in Source Water: The Determining Factor for Technological Pathways

All variations stem from the inherent properties of the source water.

Brackish Water

• Salinity Range: 1,000–10,000 mg/L

• Primary Sources: High-salinity groundwater, coastal aquifers impacted by seawater intrusion, inland saline lakes, high-salinity estuarine water.

• Composition Profile: Dominated by sodium chloride, but frequently contains elevated concentrations of calcium, magnesium, sulfate, and silica. May also contain specific ions such as boron, fluoride, and arsenic.

• Contaminant Profile: Levels of organic matter, microorganisms, and turbidity are generally lower than seawater but highly site-specific.

Seawater

• Salinity Range: 30,000–45,000 mg/L

• Composition Stability: Relatively uniform composition, with sodium chloride comprising approximately 77%.

• System Complexity: Contains over 80 elements, is highly corrosive, and is rich in microorganisms, algae, and colloidal material.

• Fouling Potential: Significantly higher risk of biological fouling and colloidal fouling.

II. Comparison of Technological Pathways: Solutions for Different Challenges

1. Core Technology Selection

• Brackish Water Desalination: Reverse Osmosis (RO) is the dominant technology (>90% market share), operating at pressures of 10–30 bar. Nanofiltration (NF) is employed for specific applications.

• Seawater Desalination: Features a dual-technology landscape of RO and Thermal processes.

  • Seawater Reverse Osmosis (SWRO): Operates at high pressures (55–82 bar) and requires Energy Recovery Devices (ERDs).

  • Thermal Processes: Primarily Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED), which are thermal energy-intensive.

2. Complexity of Pretreatment Systems

• Brackish Water Pretreatment: Targets scaling and fouling. May include softening, silica removal, chemical conditioning (e.g., antiscalants), and cartridge filtration.

• Seawater Pretreatment: A complex system designed to address biofouling, colloidal fouling, scaling, and corrosion. Typically includes chlorination/disinfection, coagulation/flocculation, media filtration, and ultimately Ultrafiltration (UF).

3. Energy Consumption and Economic Comparison

• Specific Energy Consumption:

  • Brackish Water RO: 0.5–2.5 kWh/m³

  • Seawater RO: 2.5–4 kWh/m³

  • Thermal Desalination: 10–18 kWh/m³

• Levelized Cost of Water (LCOW):

  • Brackish Water Desalination: $0.3–$0.7 /m³

  • Seawater Desalination: $0.5–$1.5 /m³

III. Geographic Distribution of Application Scenarios

Brackish Water Desalination Applications

• Geographic Distribution: Arid and semi-arid inland regions such as the Southwestern USA, inland Australia, Northwestern China (Xinjiang, Inner Mongolia), and the African Sahel.

• Application Profile: Provides water for inland cities, mining operations, and agricultural areas. Scalable from community-level to municipal-scale plants.

• Representative Projects: Groundwater desalination projects in Xinjiang and Inner Mongolia, China.

Seawater Desalination Applications

• Geographic Distribution: Middle East, Mediterranean coast, coastal Australia, and water-stressed coastal cities in China.

• Application Profile: Secures water supply for major coastal cities and industrial hubs. Projects are typically large-scale.

• Representative Projects: Large-scale SWRO plants in Saudi Arabia, UAE, Israel, Singapore, and coastal cities in China like Tianjin and Qingdao.

IV. Brine Management: Divergent Environmental Challenges

Brackish Water Brine Management

• Disposal Challenges: Limited disposal options inland, lacking the dilution capacity of the ocean.

• Management Methods: Deep well injection, evaporation ponds, forced evaporation/crystallization (Zero Liquid Discharge), discharge into saline lakes or deserts.

• Environmental Risks: Potential for groundwater contamination and soil salinization.

Seawater Brine Management

• Primary Method: Marine discharge via diffusers.

• Environmental Concerns: Localized increases in salinity, thermal pollution (from thermal processes), and residual chemical discharge.

• Regulatory Requirements: Strict marine environmental impact assessments and optimized outfall design.

V. Technology Trends and Innovation Directions

1. Energy Efficiency Improvements

• Development of novel membrane materials (high-flux, fouling-resistant).

• Optimization of high-efficiency Energy Recovery Devices (ERDs).

• Integration with renewable energy sources (direct PV/wind power coupling).

2. Resource Recovery and Circular Economy

• Extraction of lithium, bromine, magnesium, and potassium from brackish water brine.

• Recovery of bromine, magnesium, uranium, and other elements from seawater brine.

• Development of brine valorization technologies.

3. Intelligent and Modular Development

• AI-driven process optimization and predictive maintenance.

• Modular, mobile/containerized desalination units.

• Application of digital twin technology for system optimization.

VI. Future Outlook: Technological Convergence Amid Divergent Development Paths

Brackish water and seawater desalination will continue to evolve along distinct yet occasionally converging paths:

• Brackish Water Desalination will advance towards greater refinement, lower energy consumption, and resource recovery, with expanding applications in inland water-scarce regions.

• Seawater Desalination will focus on large-scale, high-efficiency applications and deeper integration with renewable energy.

Simultaneously, convergence is anticipated in areas such as membrane materials, intelligent process management, and resource recovery, collectively driving desalination technologies towards greater efficiency and sustainability.

Conclusion: Brackish water desalination and seawater desalination represent distinct technological pathways adapted to different source water characteristics. The former provides a refined solution for inland saline/brackish water, while the latter constitutes a large-scale engineering response to coastal water scarcity. A thorough understanding of their fundamental differences is essential not only for technology selection but also for formulating global water resource management strategies. As technology progresses, these two pathways will continue to evolve within a sustainable development framework, offering increasingly diverse and robust solutions to global water challenges.