Filtration

Filtration is a physical process that uses a filter medium with a complex internal structure to separate solid matter from a fluid. Only the fluid can pass through this medium; the solid particles that are too large to get through are called oversize, and the fluid that successfully passes is the filtrate. These oversize particles can build up into a filter cake on the medium’s surface, or they can clog the filter’s internal lattice, a problem known as blinding that stops fluid flow. The largest particle size that can get through a given filter is its effective pore size. The separation is never perfect: the solid residue will retain some fluid, and the filtrate will contain fine particles, depending on factors like pore size, filter thickness, and biological activity.

Filtration happens naturally and in human-made systems, appearing in biological, geological, and industrial contexts. In everyday language, people often use the verb “strain”—for instance, using a colander to drain cooking water from pasta. Oil filtration is a specific industrial method that purifies oil by removing impurities that degrade its quality. These contaminants come from machinery wear, environmental factors, or improper handling during oil changes, and the goal is to improve oil performance and extend machinery life.

The term also covers biological and physical systems that do more than separate solids from fluids: they remove chemical species and organisms through entrainment, phagocytosis, adsorption, and absorption. Examples include slow sand filters and trickling filters. It is also a general term for the process used by filter feeders—organisms like the microscopic *Vorticella*, the basking shark, and baleen whales—that filter small food particles from their environment.

**Physical processes** Filtration separates particles and fluid in a suspension, where the fluid may be a liquid, gas, or supercritical fluid. Depending on the application, either one or both components can be isolated. As a physical operation, it can separate materials of different chemical compositions by using a solvent that dissolves one component but not the other; the dissolved component passes through the filter, while the other is retained. Filtration is widely used in chemical engineering and can be combined with other unit operations, such as in a biofilter, which is a combined filter and biological digestion device.

Filtration differs from sieving, which separates particles at a single perforated layer (a sieve) based on size. In sieving, particles too large for the holes are retained. In filtration, a multilayer lattice traps particles that cannot follow the tortuous channels. Oversize particles can form a cake on top or block the lattice (blinding). Commercially, the term “filter” is applied to membranes where the separation lattice is so thin that the surface is the main zone of particle separation, even though these products might be called sieves. Filtration also differs from adsorption, which relies on surface charge; devices with activated charcoal or ion-exchange resin are commercially called filters, but filtration is not their main mechanical function. Similarly, magnetic removal of contaminants from fluids (like lubrication oil, coolants, and fuel oils) uses no filter medium, though devices called “magnetic filters” are sold based on their use, not their mode of operation.

In biological filters, oversize particles are trapped and ingested, and the resulting metabolites may be released. For example, renal filtration in animals (including humans) removes waste from the blood. In water and sewage treatment, undesirable constituents are removed by adsorption into a biological film grown on or in the filter medium, as in slow sand filtration.

**Methods** Filters can be used to remove unwanted liquid from a solid residue, clean unwanted solids from a liquid, or simply separate solid from liquid. Many different filtration methods exist, all aiming to separate substances through some form of interaction between the filter and the material to be removed. The substance that passes through must be a fluid—a liquid or gas. Methods vary depending on whether the targeted material is dissolved in the fluid or suspended as a solid.

Laboratory filtration techniques include hot, cold, and vacuum filtration, chosen based on the desired outcome—such as removing impurities from a mixture or isolating solids. Hot filtration is used to separate solids from a hot solution, preventing crystal formation in the filter funnel and other apparatus. The apparatus and solution are heated to avoid a rapid temperature drop, which would cause solids to crystallize in the funnel and hinder the process.