| Seawater Reverse Osmosis (SWRO) | Pressure-driven solution-diffusion through a dense, semipermeable membrane. Water passes through while dissolved salts are retained. | Approximately 55–80 bar for seawater, depending on salinity, temperature, recovery, and membrane design. | Typically 99.5% or higher for total dissolved salts; boron removal is lower and may require a second pass or pH adjustment. | Typically about 2.5–4.5 kWh per cubic metre for the complete modern seawater desalination process, excluding unusual site conditions. | Municipal drinking-water supply, industrial process water, water reuse, and decentralized coastal desalination. | High salt rejection, modular equipment, mature operation, and comparatively low energy use among thermal and membrane desalination options. | Sensitive to fouling, scaling, oil, suspended solids, and biofouling. Requires effective intake screening, clarification or flotation when needed, cartridge filtration, and usually ultrafiltration or other advanced pretreatment. |
| Nanofiltration (NF) | Pressure-driven transport through a membrane with very small pores. Separation combines size exclusion and charge-based ion rejection. | Approximately 5–25 bar, depending on feed quality and target removal. | High removal of multivalent ions, hardness, sulfate, color, and many organic compounds; lower rejection of monovalent salts than reverse osmosis. | Often about 0.5–2.5 kWh per cubic metre, depending on feedwater and system configuration. | Seawater pretreatment, sulfate reduction, hardness reduction, partial desalination of brackish water, and protection of downstream reverse osmosis systems. | Lower pressure than SWRO, effective hardness and sulfate control, and useful reduction of some organic contaminants. | Usually does not produce drinking water from seawater in a single pass because sodium chloride rejection is insufficient. Membrane fouling and scaling control remain necessary. |
| Electrodialysis (ED) | An electric field drives dissolved ions through alternating cation-exchange and anion-exchange membranes. | Low hydraulic pressure; the main driving force is direct electrical voltage across the membrane stack. | Commonly suitable for partial desalination, with salt removal often around 50–90% per pass depending on configuration and feed concentration. | Approximately 0.5–2.5 kWh per cubic metre for suitable brackish-water applications; energy rises as feed salinity increases. | Brackish-water desalination, industrial water recovery, and selective removal of ionic contaminants. | Energy use is related mainly to the amount of salt removed; useful for lower-salinity feeds and can provide selective ion separation. | Generally less economical for high-salinity seawater. Does not remove uncharged dissolved substances, and the process requires control of scaling, fouling, and electrode reactions. |
| Electrodialysis Reversal (EDR) | Uses the same ion-selective membrane principle as ED while periodically reversing electrical polarity and ion flow. | Low hydraulic pressure with alternating electrical polarity, commonly reversed several times per hour. | Often about 50–90% per pass for appropriate brackish-water feeds, depending on recovery and operating conditions. | Typically around 0.5–2.5 kWh per cubic metre for brackish-water treatment. | Municipal brackish-water treatment, industrial process water, cooling-water makeup, and water reuse. | Polarity reversal helps reduce buildup of scale and foulants, allowing more tolerant operation with some difficult feeds. | Best suited to brackish water rather than open-ocean seawater. Pretreatment is still required for suspended solids, oil, biological matter, and excessive hardness. |
| Forward Osmosis (FO) | Water moves across a semipermeable membrane because of an osmotic-pressure difference created by a concentrated draw solution. | Osmotic driving force rather than high hydraulic pressure; external energy is required to regenerate or separate the draw solution. | High rejection of many dissolved salts and particulates, but actual product-water quality depends strongly on reverse solute flux and draw-solution recovery. | There is no single standard value; total energy depends mainly on draw-solution regeneration and can be significant. | Concentrating difficult industrial streams, wastewater treatment, emergency water production, and hybrid desalination systems. | Low hydraulic pressure, potentially lower fouling tendency than some pressure-driven processes, and suitability for high-osmotic-pressure feeds. | Draw solute can diffuse back into the feed, and extracting it from the diluted draw solution adds complexity. Large-scale seawater desalination remains mainly application-specific and hybrid. |
| Membrane Distillation (MD) | Water vapor passes through hydrophobic microporous membranes, while liquid water and nonvolatile salts are retained. | Low hydraulic pressure; driven by a vapor-pressure difference created by a temperature gradient. | Generally greater than 99% salt rejection when the membrane remains unwetted. | Electrical demand varies widely; thermal energy is also required. Waste heat or solar heat can improve overall efficiency. | High-salinity brines, zero-liquid-discharge systems, concentrated seawater, and desalination integrated with low-grade waste heat. | Can treat feeds near or above the osmotic-pressure limit of RO, offers very high salt rejection, and can use low-temperature heat sources. | Lower flux than RO in many systems, temperature polarization, membrane wetting, scaling, and potential heat loss. Stable hydrophobic membranes and careful pretreatment are essential. |
| Ultrafiltration (UF) | Pressure-driven size exclusion through porous membranes that retain suspended solids, colloids, bacteria, and many macromolecules. | Approximately 0.5–5 bar, depending on module design and operating mode. | Negligible removal of dissolved salts; it is a pretreatment technology rather than a standalone seawater desalination process. | Typically about 0.05–0.3 kWh per cubic metre, depending on flux, backwashing, and feedwater quality. | Seawater pretreatment before SWRO, surface-water clarification, wastewater reuse, and removal of turbidity and microorganisms. | Consistent particle and microorganism removal, compact footprint, and effective protection of downstream RO membranes. | Does not remove sodium chloride or other dissolved ions. Requires backwashing, periodic cleaning, and control of organic fouling and biofouling. |
| Microfiltration (MF) | Low-pressure size exclusion through relatively large pores that remove suspended particles and many microorganisms. | Approximately 0.1–3 bar, depending on membrane type and filtration mode. | Negligible removal of dissolved salts; used for clarification and pretreatment rather than desalination. | Typically about 0.03–0.2 kWh per cubic metre, depending on filtration conditions and cleaning requirements. | Seawater intake pretreatment, removal of suspended solids, wastewater polishing, and protection of downstream NF or RO membranes. | Low pressure, simple operation, and effective reduction of turbidity and larger particles. | Limited removal of dissolved organics, viruses, and salts. Performance can decline rapidly when exposed to high organic loads, algae, oil, or fine colloids. |
