Unicel's Glossary of Filtration Terms
Backwash: The process of cleaning the filter medium or septum by reversing the flow of water through the filter.
Basecoat: A layer of filter aid between the precoat and septum.
Blinding: Filling in and clogging of a filter medium caused by entrapment of particles from a filtered liquid. When the medium becomes "blind," an increase in differential pressure and reduction in flow results.
Bridging: Debris which arches, or bridges, the individual pleats in the filter cartridge or between two filter elements.
Cartridge Filter: A filter that utilizes a pleated, porous medium as a filtering material.
Clarification: The process by which the filter removes progressively smaller particles on each successive turn over, thus improving efficiency and extending the cycle life. Cartridge filters use the clarification process.
Clarifier (also called coagulant or flocculent): A chemical that coagulates and neutralizes suspended particles in water. There are two types of clarifiers: inorganic salts of aluminum or iron and water-soluble organic polyelectrolyte polymers.
Coagulation: The process by which very small, finely divided solid particles -- often colloidal in nature -- are agglomerated into larger particles.
Compressibility: Degree of physical change in a filter cake when it is subject to pressure, resulting in increased differential pressure and reduced flow.
D.E.: Abbreviation for diatomaceous earth. Fossil-like skeletons of microscopic water plants called diatoms.
Denier: The relationship between cross sectional area and weight. Denier is numerically equal to the weight in grams of 9000 meters of individual fiber (i.e. 1 gram equals 1 denier). Most filtration grade Reemay use 4 denier fibers. These fibers are approximately 28 microns in diameter.
Differential Pressure: (1) The difference in pressure between two given points. (2) The combined pressure caused by the debris, filter cake, precoat and septum, expressed as ("delta P"). (3) The effluent pressure minus the influent pressure.
Effluent: Fluid which has passed through a filter. Also called the filtrate.
Enzymes: An organic protein, also known as amino acids. Enzymes are non-toxic and biodegradable. Although they are not an oxidizer or clarifier, enzymes significantly reduce cartridge maintenance by breaking down oils into carbon dioxide and water.
Feed: The mixture of particles and fluid that is introduced into the filter. Terms used synonymously include "influent" and "incoming slurry."
Filter: Verb: To pass fluid containing suspended particles through a filter medium whereby the particles are separated from the fluid. Noun: A "device" for carrying out the filtration process, consisting of the filter medium and suitable hardware for constraining and supporting it in the path of the fluid.
Filter Aid: Any material (usually diatomaceous earth) that enhances the separation of solids from liquids in the filtration process.
Filter Cake: The combined layers of solids, precoat, and debris removed in the filtration process and accumulated on the surface of the filter medium.
Filter Cycle: The operating time between cleaning or backwash cycles.
Filter Medium: The permeable material such as diatomaceous earth, sand, or polyester nonwoven material used to separate suspended particles from liquid.
Filtrate: Fluid that has passed through a filter. Also called the effluent.
Filtration: The process by which particles are separated from a liquid by passing through a permeable material.
Filtration Rate: Flow in gallons per minute (GPM) through one square foot of filter medium. For residential pools, the filtration rate should be 2 GPM per square foot of D.E. filter surface area and 1 GPM per square foot of cartridge filter surface area. For most commercial pools, the filtration rate should be 1 GPM per square foot of D.E. filter surface area and .375 GPM per square foot of cartridge filter surface area.
Flocculation: The process by which small dispersed particles combine together to form larger size particles which can be removed by the filter. The result of adding an electrolyte to the water.
Hydrophilic: Water accepting. The ability to absorb water.
Hydrophobic: Water rejecting. Lacking affinity for or ability to absorb water.
Influent: Dirty or unfiltered water introduced to the filter. Also referred to as "feed" or "incoming slurry."
OEM: Original Equipment Manufacturer or brand name.
Micron Size: Expressed as micrometer (µm), a unit of measurement equal to 1/1,000,000 of a meter (.0000394"). 40 micron is considered the smallest size particle visible to the human eye. A red blood cell is 6-8 micrometers large; one grain of table salt is roughly 100 micrometers in size.
Permeability: Expressed in cubic feet per minute (CFM), the resistance to flow through a media. The lower the perm number, the greater the resistance. The permeability of a filter media is a quality control measure for cartridge and D.E. filter materials.
Polyester: Long chain synthetic polymer composed of at least 85% by weight of ester of dihydric alcohol and terephthalic acid.
Polymer: High molecular weight organic compound whose structure can be represented by repeated small units. Synthetic polymers are formed by condensation polymerization of monomers. If two or more monomers are involved, a copolymer is formed.
Porosity: (1) The degree of open area between the fibers. (2) The void area which exists in the structure.
Precoat: The layer of filter aid (usually D.E.) formed on the media surface by introducing a slurry (generally between .1 and .2 lbs. per square foot of surface area) to the medium at the beginning of the filter cycle.
Reemay: A registered trademark of Reemay, Inc. Originally developed
by DuPont, Reemay is made of continuous filament, 100% polyester fibers.
The process by which these fibers are drawn, randomly laid and thermally
bonded is known as "spunbonding."
Septum: A permeable material used to support the filter medium or precoat on D.E. filters.
Trilobal: The three sided fiber formed by drawing polyester resin through a die or spinneret. The basic shape of Reemay fibers.
Turbidity: (1) The characteristic or property of a liquid that causes it to absorb or scatter light. (2) A measurement of water cloudiness or haziness caused by micro-organisms, algae, or suspended fine particles.