Filtration is a fundamental separation process used to remove solid particles from a fluid (liquid or gas). This separation is accomplished by passing the mixture through a barrier, known as a filter medium, that selectively allows the fluid to pass while retaining the suspended matter. The retained material is called the residue, and the purified fluid that passes through is the filtrate. The effectiveness and application of filtration depend heavily on the specific mechanism utilized, leading to three distinct categories designed to tackle different types of contaminants.
Mechanical Filtration
Mechanical filtration operates purely on the principle of physical size exclusion, essentially acting as a microscopic sieve or strainer. The filter medium contains pores of a specific size, trapping particles larger than the openings while allowing the fluid and smaller dissolved substances to flow through. This method is highly effective for removing suspended solids, such as sand, silt, debris, and larger organic matter, which contribute to the turbidity of a liquid.
Filter media can range from simple mesh screens to complex, multi-layered materials like sand beds or fibrous cartridges. For instance, a HEPA (High-Efficiency Particulate Air) filter uses a dense mat of fibers to capture airborne particles as small as 0.3 micrometers. The effectiveness is often described by the filter’s pore size, which can be classified as either nominal, representing a general size rating, or absolute, meaning nearly all particles above that size are guaranteed to be stopped.
In liquid systems, a cartridge filter might use pleated cotton or polyester to remove sediment, while a multi-media filter employs layers of different granular materials, such as quartz sand and anthracite. These layers trap solids within the entire depth of the medium, not just on the surface, extending the filter’s lifespan. The primary limitation of this process is that it only addresses suspended materials; it does not change the chemical composition of the fluid or remove substances that are dissolved at the molecular level.
Chemical Filtration
Chemical filtration targets dissolved substances that are far too small to be caught by the physical mesh of mechanical filters. This process relies on a direct interaction between the contaminant molecules and the filter medium, changing the fluid’s chemical makeup. The two main mechanisms employed are adsorption and ion exchange.
Adsorption is the more common method, where molecules of the contaminant are attracted to and stick onto the surface of the filter material. This is distinctly different from absorption, where a substance is soaked up or taken into the bulk of a material. The most widespread example is activated carbon, which is manufactured to possess an incredibly high internal surface area (a single gram can exceed 3,000 square meters).
Activated carbon’s porous structure makes it highly effective at removing organic compounds, volatile organic chemicals (VOCs), chlorine, and compounds causing tastes and odors. In addition to physical adsorption, some contaminants, like chlorine, are removed through a catalytic reduction reaction. This reaction involves a transfer of electrons from the carbon surface to the chlorine molecule, converting the residual disinfectant into a harmless chloride ion.
Another chemical method is ion exchange, where specialized resins swap undesirable dissolved ions, such as calcium and magnesium (which cause water hardness), for less problematic ions, typically sodium.
Biological Filtration
Biological filtration is a natural process that utilizes living microorganisms to neutralize harmful dissolved compounds, including the hardest contaminants to remove. This method is frequently the final step in a comprehensive purification system, particularly in wastewater treatment and closed aquatic environments like aquariums. The process relies on cultivating dense colonies of beneficial bacteria on a porous surface.
These bacteria consume or metabolize toxic substances, converting them into less harmful forms. The core application of this process is the nitrogen cycle, which manages the nitrogen-containing waste products generated by living organisms. The cycle begins with ammonia (NH3), a highly toxic compound produced by organic waste.
The first group of nitrifying bacteria, such as Nitrosomonas, oxidizes the ammonia, converting it into nitrite (NO2-), which is still toxic. Immediately following this step, a second group of bacteria, including Nitrobacter, converts the nitrite into nitrate (NO3-). Nitrate is significantly less toxic and can be managed through water changes or uptake by plants.
The filter material (often ceramic media or plastic bio-balls) does not physically trap contaminants, but instead serves as a high surface area habitat for the bacterial colonies. Maintaining these bacterial populations requires a constant supply of oxygen to perform the chemical conversion, a process called nitrification. These three filtration types—mechanical, chemical, and biological—often work in sequence to ensure the highest level of purification for water and air.