| Depth filtration | Fluid passes through a thick, porous medium that captures particles throughout its depth. | Particles suspended in liquids; performance depends on media and particle characteristics. | Cartridge, bag, or granular-media filter | Water treatment, process liquids, and prefiltration | Holds contaminants within the media; replacement or cleaning is needed when pressure drop rises. |
| Surface filtration | A screen, fabric, or membrane retains particles primarily at its surface. | Particles larger than the effective openings of the filter medium | Screen, strainer, or pleated cartridge filter | Liquid prefiltration, equipment protection, and coarse solids removal | Retained solids can form a surface cake, increasing pressure drop and requiring cleaning or replacement. |
| Fibrous mechanical filtration | Airborne particles are captured by a combination of interception, impaction, and diffusion as air passes through fibers. | Dust, pollen, and other airborne particles across a range of sizes | Panel, pleated, or bag air filter | Ventilation systems, air handlers, and industrial dust control | Efficiency and pressure drop vary with filter class, face velocity, loading, and installation. |
| High-efficiency particulate filtration | A dense fibrous medium captures fine airborne particles through several mechanical collection mechanisms. | Fine airborne particles, including particles near the most penetrating particle size | HEPA filter unit | Cleanrooms, healthcare settings, and critical air-cleaning applications | Under the U.S. HEPA definition, a filter must remove at least 99.97% of particles 0.3 micrometers in size when tested under specified conditions; fit and sealing matter. |
| Electrostatic precipitation | Particles receive an electrical charge and are attracted to oppositely charged collection plates. | Particulate matter carried in gas streams | Electrostatic precipitator | Large-scale industrial exhaust and combustion-gas cleaning | Collection performance depends on particle properties, gas conditions, and equipment operation; collected material must be removed from the plates. |
| Inertial separation | Changes in gas-flow direction cause particles with sufficient inertia to separate from the flow. | Relatively coarse dust and droplets | Cyclone separator or inertial separator | Dust collection and pre-separation ahead of finer filters | Separation depends on particle size, density, airflow, and separator geometry; fine particles may pass through. |
| Adsorption | Contaminant molecules adhere to the internal surface of a porous adsorbent. | Selected gases, odors, and vapors | Activated-carbon filter unit | Odor control, vapor treatment, and selected air or water purification duties | Does not generally remove ordinary dust; media selection and replacement depend on the contaminant and loading. |
| Membrane separation | A selective membrane separates components according to size and, depending on the process, other properties. | Suspended particles, microorganisms, dissolved macromolecules, or dissolved salts, depending on membrane type | Microfiltration, ultrafiltration, nanofiltration, or reverse-osmosis unit | Water and process-liquid treatment | Membrane processes differ in separation capability; fouling, feed quality, and operating pressure affect performance. |