Glossary of Filtration Terms



Absolute: A degree of filtration that guarantees 100% removal of suspended solids over a specified size

Absorb: The process by which a liquid penetrates the solid structure of the absorbent's fibers or particles, which then swell in size to accommodate the liquid.

Activated carbon: Carbon activated by high temperature forming an absorptive media which removes dissolved color, odor and taste from liquids or gases.

Adsorb: The act of selectively attracting and holding a gas, vapor, liquid or colloid onto the surface of a solid.



Backwash: A reverse flow of liquid to remove solids from the filter. This backwash can be from an outside source, raw water or other fluids, from the filtrate or compressed air or even an inert gas ( for safety reasons.)

Baffle: A plate or deflector to provide flow distribution in a filter. Primary functions are to prevent erosion of pre-coat and settling of body feed in the filter tank.

Blowdown: The use of air or an inert gas pressure to displace a liquid out of a filter cake. Continued blowdown is used to dry the cake in situations.

BOD: Biochemical Oxygen Demand: A measure of the amount of oxygen required by bacteria for the biochemical degradation of organic material in a water sample.

Body feed: The pre-coat media that is continuously added to the filter while it is on stream. Its purpose is to create a permeable filter cake.


Cellulose: A fibrous material of vegetable origin used as a filter media.

CFM: Cubic feet per minute.

Clarity: The clearness of a liquid as measured by a variety of methods.

Cloth: A type of woven filter spectrum made from natural or synthetic yarns.

Coalescing: The separation of mixtures of immiscible fluids, (oil and water) with different specific gravities, can be accomplished due to the fact that small droplets of each pass through the coalescing media to enlarge, thus become larger droplets, then separate out of solution.

Crossflow membrane filtration: A separation of the components of a fluid by semipermeable membranes through the application of pressure and tangential flow to the membrane surface. This includes the processes of R/O, UF, Nanofiltration and Microfiltration.


D.E. The commonly used abbreviation for Diatomaceous Earth. Diatomaceous Earth is the fossilized skeletons of minute, prehistoric aquatic plants. Insoluble in water. 

Delta P: A commonly used symbol denoting the pressure drop across a filter.

Differential pressure: The difference in pressure between the upstream and downstream sides of a filter or of a filter cake. It can also be the difference in pressure between two points in a system or of a component in such a system.

Dissolved solids: Any solid material that will dissolve in the liquid that is being filtered such as sugar in water. Filters cannot remove these "solids". Also the residual material remaining after filtration and evaporation.


Efficiency: The ability, expressed as a percent, of a filter to remove a specified artificial contaminant at a given contaminant concentration under specified test conditions.


Feed: The mixture of solids and liquids that enters the filter. Synonyms: prefilt influent and incoming slurry.

Filter aid: Any material that assists in the separation of solids from liquids. Usually used on difficult filter applications.

Filter cake: The accumulated particles on a filter surface, usually from a slurry mixture (made up of a pre-coat media, such as D.E.).

Filter system: The combination of a filter and associated hardware required for the filtration process.

Filtration: The process by which solid particles are separated from a liquid by passing the liquid through a permeable material. Also, the physical or mechanical process of separating insoluble particulate matter from a fluid, such as air or liquid, by passing the fluid through a filter medium that will not let the particulates through.

Filtration rate: The volume of liquid that passes through a given area in a specified time. Usually expressed as gallons per square foot per minute (or hour).

Flux: In crossflow filtration, the membrane throughput, usually expressed in volume per unit time per area, i.e., gallons per day per square meter.


Gauge: Thickness of steel used to manufacture filter vessels. The lower the gauge, the thicker the steel.


Head: An end closure for the filter case or bowl which contains one or more ports. Also the measurement of pressure in a column of liquid expressed in feet of liquid x0.433=pound per square inch.

Housing: A ported closure which directs the flow of fluid through the filter element.

Hydrophilic: Water accepting.

Hydrophobic: Water rejecting.


Influent: The fluid entering the filter.

In-line filter: A filter assembly in which the inlet, outlet and filter element axis are in a stright line.



Manifold: A pipe or assembly into which the filter elements are connected to from one common discharge for the filtrate.

Media: The material that performs the actual separation of solids from liquids. Sometimes erroneously used to mean septum.

Membrane: Media through which a liquid is passed; usually associated with an extremely fine or tight type of filtration. Highly engineered polymer film containing controlled distribution of pores. Used as the separation mechanism in R/O, Electrodialysis, UF, Nanofiltration and Microfiltration.

Mesh: Number of strands in a linear inch of woven filter fabric, usually wire. It is also used as a septum

Micron or Micrometer (um): The National Bureau of Standards decreed that the term "micron", a millionth of an inch, now is officially "micrometer" (um), and nanometer (nm) is to be used instead of millimicron (mu).






Screen: A term commonly used for septum, Also a wire mesh screen used to screen out large sized particles that would clog a filter cartridge. Usually installed on the suction side of a pump.

Separator: A spiral-wound membrane cartridge or element in crossflow membrane systems. Modular and replaceable.

Sparger: A device to introduce compressed air or gas into a liquid to agitate it or to dissolve the air or gas in this liquid. Spargers are made of porous ceramic or stainless in various grades (porosities) to provide a specific sized "bubble".



Viscosity: That property of fluids by which they offer ressistance to flow. Measured in poise, kinematic viscosity, centistokes, SUS, seconds saybolt, degree Engler and degree Barbey, Garner-Holt, etc.





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