Water-Filtration Methods Compared: How Each Works

This reference page provides a technical comparison of primary water filtration and purification methods used in emergency, residential, and field contexts. It details the mechanical mechanisms, typical pore sizes, and the specific contaminant removal capabilities of each technology based on current industry standards and scientific data.

Comparison of Water Treatment Methods

Method	Mechanism	Pore Size (Microns)	Microbial Targets	Chemical/Mineral Removal
Boiling	Thermal Inactivation	N/A	Bacteria, Viruses, Protozoa [1]	None (may concentrate minerals)
Distillation	Phase Change	N/A	Bacteria, Viruses, Protozoa [2]	Heavy metals, nitrates, salts
Reverse Osmosis	Semi-permeable Membrane	~0.0001 [3]	Bacteria, Viruses, Protozoa	95-99% of TDS, lead, arsenic
Ultrafiltration	Membrane Barrier	0.01 to 0.1 [4]	Bacteria, Protozoa, most Viruses	Suspended solids, large organics
Microfiltration	Mechanical Barrier	0.1 to 10 [2]	Bacteria, Protozoa	Sediment, turbidity
UV Purification	DNA Disruption	N/A	Bacteria, Viruses, Protozoa [5]	None
Activated Carbon	Adsorption	0.5 to 50 (varies)	Limited (if <1 micron)	Chlorine, VOCs, taste/odor [6]
Chlorine/Iodine	Oxidation	N/A	Bacteria, Viruses	None (ineffective vs Cryptosporidium) [1]
Chlorine Dioxide	Oxidation	N/A	Bacteria, Viruses, Protozoa	None (requires 4+ hours for Crypto) [7]
SODIS (Solar)	UV-A + Thermal	N/A	Bacteria, Viruses, Protozoa [8]	None
Flocculation	Coagulation/Sediment	N/A	Partial (via settling)	Heavy metals (partial), turbidity
Ion Exchange	Resin Substitution	N/A	None	Hardness (Ca/Mg), Lead, PFAS [6]

Mechanical and Membrane Filtration

Reverse Osmosis (RO): Utilizes a high-pressure pump to force water through a semi-permeable membrane. With a pore size of approximately 0.0001 microns, it is capable of removing dissolved salts, metal ions, and the smallest known waterborne viruses. Most systems require a pre-filter to prevent membrane fouling by sediment [3].
Ultrafiltration (UF): Operates at lower pressures than RO. Its pore size (0.01 to 0.1 microns) is sufficient to physically block bacteria and most viruses, though it does not remove dissolved minerals or salts [4].
Microfiltration / Ceramic Filters: Commonly used in portable field filters. Typical absolute pore sizes range from 0.1 to 1.0 microns. These are highly effective against protozoa (Giardia, Cryptosporidium) and bacteria, but generally cannot remove viruses due to their smaller size (typically 0.02 to 0.4 microns) [2].

Chemical and UV Purification

Ultraviolet (UV) Light: Exposes water to UV-C radiation (typically 254 nm) to scramble the DNA/RNA of microorganisms, rendering them unable to reproduce. It is effective against 99.99% of pathogens but requires clear water to function; suspended particles can "shield" microbes from the light [5].
Halogen Disinfection (Chlorine/Iodine): Chemical oxidants that destroy the cellular structure of microbes. While effective against bacteria and viruses, they have low efficacy against Cryptosporidium oocysts, which have a thick outer shell resistant to halogen-based oxidation [1].
Chlorine Dioxide: A more powerful oxidant than standard chlorine. It is recognized for its ability to inactivate Cryptosporidium, though the process is time-dependent, often requiring 4 hours of contact time at specific temperatures to achieve a 99.9% reduction [7].

Thermal and Solar Methods

Boiling: The most reliable method for pathogen inactivation. Bringing water to a rolling boil for 1 minute (3 minutes at altitudes above 6,500 feet/2,000 meters) kills all waterborne bacteria, viruses, and protozoa [1].
SODIS (Solar Water Disinfection): A method using UV-A radiation and increased temperature. Clear PET plastic bottles are filled with low-turbidity water and exposed to direct sunlight for at least 6 hours. The synergy of UV radiation and heat (if water reaches 50°C/122°F) inactivates most pathogens [8].

Adsorption and Ion Exchange

Activated Carbon (GAC/Block): Works via adsorption, where contaminants stick to the vast surface area of the carbon pores. It is the primary method for removing organic chemicals, pesticides, herbicides, and chlorine. It does not remove minerals or most microbes unless specifically rated for sub-micron filtration [6].
Ion Exchange: Uses resin beads to swap unwanted ions (like lead, mercury, or calcium) for harmless ones (like sodium or potassium). This is the standard technology for water softening and is increasingly used for the removal of PFAS and nitrates [6].

Last verified: 2026-07-03