| Reverse Osmosis (RO) | Pressure-driven membrane separation through a semipermeable membrane, usually supported by sediment and carbon prefilters. | Typically reduces dissolved salts, many heavy metals, fluoride, nitrate, and numerous other dissolved contaminants. Actual performance depends on membrane condition and feed-water chemistry. | Good with pretreatment
Hardness can cause scale and shorten membrane life. A softener, antiscalant, or suitable pretreatment may be required. | Excellent
Commonly used where TDS is high, including brackish-water applications when the system is properly designed. | Good, but not standalone disinfection
The membrane can reduce many microorganisms, but post-treatment disinfection and sanitary design are recommended. | Moderate after prefiltration
Suspended solids can foul the membrane quickly without sediment filtration. | Point-of-use systems commonly recover about 20–50%; larger or optimized systems may achieve higher recovery. Reject water is produced. | Electric pump power is often used, especially for low-pressure supplies or high-TDS feed water. | Prefilter replacement is commonly required every 6–12 months; the RO membrane often lasts about 2–5 years depending on feed water and maintenance. | Drinking water, food service, laboratories, desalination, and locations with high TDS or multiple dissolved contaminants. | Produces concentrate water, needs adequate pressure, and may remove beneficial minerals. Feed water should be tested before selection. | Very High
The most versatile option for dissolved-contaminant reduction when correctly sized and maintained. |
| Ultrafiltration (UF) | Low-pressure membrane filtration with pores smaller than conventional microfiltration but larger than RO membranes. | Reduces suspended solids, colloids, bacteria, and many cysts. It generally does not remove dissolved salts, most dissolved metals, or most small dissolved organic chemicals. | Fair
Hardness passes through and may cause scaling in downstream equipment, although the membrane itself is less affected than an RO membrane in some applications. | Poor for TDS reduction
Most dissolved salts remain in the treated water. | Good for many bacteria and cysts
Virus reduction varies by membrane rating and system validation. It should not be assumed to provide complete disinfection. | Good after coarse pretreatment
Effective against many particles, but heavy sediment loads require prefiltration. | Often produces little or no continuous reject water; recovery depends on flushing and system design. | Usually low energy; many household systems operate using existing mains pressure without a booster pump. | Periodic backwashing or flushing may be needed. Prefilters are commonly replaced every 6–12 months. | Municipal water with low-to-moderate TDS, rural drinking-water treatment, and applications prioritizing mineral retention. | Does not solve high TDS, salinity, hardness, nitrate, or fluoride problems. | High for low-TDS feed water
Efficient where particulate and microbial control are more important than dissolved-contaminant removal. |
| Ultraviolet (UV) | Ultraviolet-C light damages the DNA or RNA of microorganisms and prevents replication. | Inactivates many bacteria, viruses, and protozoa when the delivered UV dose is adequate. It does not remove dissolved chemicals, salts, metals, or particles. | Good
Hardness does not directly reduce microbial inactivation, but scale on the lamp sleeve can reduce UV transmission. | Poor for TDS
Leaves dissolved salts and other non-microbial contaminants unchanged. | Excellent when properly designed
Requires suitable UV dose, flow control, clear water, and reliable electricity. It does not provide a residual disinfectant. | Poor without pretreatment
Turbidity and color can shield microorganisms from UV light. | Normally produces no reject stream and can provide near-continuous flow at the rated capacity. | Requires continuous electricity during operation. | The lamp is commonly replaced about annually, while the quartz sleeve should be inspected and cleaned to control scale. | Microbial control for treated municipal water, private wells, and as a final barrier after filtration. | Does not physically remove contaminants; treated water can be recontaminated after leaving the chamber if storage and plumbing are unsanitary. | High as a disinfection stage
Best combined with sediment filtration and, where needed, RO or activated carbon. |
| Distillation | Boils water and condenses the vapor, leaving many nonvolatile impurities behind. | Can significantly reduce salts, many heavy metals, microorganisms, and numerous nonvolatile contaminants. Volatile compounds may require activated-carbon treatment. | Excellent
Hardness minerals remain primarily in the boiling chamber and must be removed during cleaning. | Excellent
Highly effective for dissolved solids when the unit is operated correctly. | Excellent
Boiling provides strong microbial destruction; safe storage is still necessary after treatment. | Good after settling or filtration
Suspended matter is left behind but can foul the boiler and require cleaning. | Low water recovery is typical, and production is relatively slow compared with pressure-driven systems. | High electricity or heat demand because water must be vaporized and condensed. | Regular descaling and cleaning are required. Carbon polishing may be needed for volatile organic compounds and taste control. | Small-volume laboratory, medical, technical, and household applications requiring very low mineral content. | Slow output, higher operating cost, heat generation, and substantial energy use make it less practical for large continuous demand. | High for very low-mineral water
Particularly useful where electricity is reliable and water demand is limited. |
| Activated Carbon Filtration | Adsorption onto porous carbon media, often combined with sediment filtration. | Can reduce chlorine, many taste-and-odor compounds, and selected organic chemicals. Performance varies greatly with carbon type, contact time, and contaminant concentration. | Poor for hardness
Calcium and magnesium generally pass through. | Poor for TDS
Does not substantially reduce dissolved salts. | Limited
It is not a dependable standalone disinfection method. Bacteria can grow on poorly maintained carbon filters. | Fair after sediment filtration
Carbon beds can clog when exposed to high turbidity. | Usually provides continuous flow with little or no reject water. | Generally requires no electricity unless paired with a pump or automatic control valve. | Cartridges or media require regular replacement based on capacity and water quality; exhausted carbon can release trapped contaminants. | Chlorine and taste reduction, RO pretreatment, and polishing of otherwise treated municipal water. | Not suitable as the only treatment for salinity, nitrate, hardness, or microbiologically unsafe water. | High as a pretreatment or polishing stage
Usually part of a multi-stage system rather than a complete purifier. |
| Electrodeionization (EDI) | Uses ion-exchange membranes, resin, and an electric field to continuously remove ionized impurities from pretreated water. | Produces very low conductivity water by removing ions. It is not intended to remove particles, microorganisms, or non-ionized organic compounds without upstream treatment. | Good after RO
Hardness must be largely removed beforehand to prevent scaling and performance loss. | Excellent after RO pretreatment
Feed water with high ionic loading generally exceeds EDI limits without upstream desalination. | Requires upstream control
EDI is not a primary disinfection process; sanitary design and microbial control are essential. | Poor without pretreatment
Particles and colloids can foul the membranes and resin. | Usually operates as a continuous process with a small concentrate or electrode-rinse stream, depending on design. | Requires continuous electricity, but avoids routine chemical regeneration of mixed-bed resin in normal operation. | Needs high-quality RO permeate, periodic sanitization, monitoring of conductivity, and replacement of modules after long service intervals. | Laboratories, pharmaceutical water systems, electronics manufacturing, and other high-purity applications. | High capital cost, sensitive feed-water requirements, and unsuitable performance when used directly on untreated or highly variable water. | Very High for high-purity systems
Best selected as a polishing stage after effective pretreatment, especially RO. |
| Nanofiltration (NF) | Pressure-driven membrane filtration with separation between UF and RO; particularly effective for multivalent ions and selected organic compounds. | Often reduces hardness, color, larger organic molecules, and some sulfate and heavy-metal ions. Monovalent salt removal is generally lower than RO. | Good
Can substantially reduce hardness, but scaling control remains important. | Good for selected TDS components
May reduce TDS but usually does not desalinate as completely as RO. | Moderate with validated membrane performance
Should be paired with disinfection where microbial safety is required. | Moderate after pretreatment
Suspended solids and colloids can foul the membrane. | Often recovers about 50–85% in well-designed systems; the actual rate depends on feed quality and operating pressure. | Requires pump energy, generally less demanding than RO at comparable conditions but dependent on feed-water pressure. | Needs sediment and often carbon pretreatment, periodic cleaning, and monitoring for scaling and membrane fouling. | Water softening with partial desalination, color reduction, food processing, and industrial pretreatment. | May not adequately remove monovalent salts, nitrate, or very high salinity when low conductivity is required. | High for targeted treatment
Useful when hardness and selected organics must be reduced without maximum desalination. |
| Solar Distillation | Uses solar heat to evaporate water and condense the vapor in a separate collection surface. | Can reduce salts, many metals, and microorganisms. Volatile contaminants may carry over unless additional treatment is included. | Excellent
Hardness remains in the brine or basin and must be periodically removed. | Excellent in principle
Can treat saline or brackish water, but production depends strongly on sunlight and ambient conditions. | Excellent through evaporation
Collected water must be protected from contamination after condensation. | Good with proper basin management
Sediment remains behind but can reduce efficiency if it accumulates. | Low and variable output; production depends on solar radiation, collector area, temperature, and weather. | Very low operating electricity demand, although pumps or controls may require power. | Requires cleaning of the evaporator and condenser surfaces, brine removal, and inspection for biological growth. | Remote areas, emergency supply, small-scale desalination, and locations with strong solar resources. | Weather-dependent, space-intensive, slow, and generally unsuitable for consistent high-volume supply without energy storage or backup. | Moderate for off-grid use
Valuable where sunlight is abundant and water demand is modest. |
