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Glossary of Terms

Some key words and phrases that are generally associated with filtration.

Absolute
Is used in reference to the micron rating of cartridge filters, indicating that all particles larger than a specified size will be trapped within the filter and will not pass through.
Absolute Rating
Particle size in micrometers removed at a given efficiency under a manufacturer's defined test condition. Implied is 100%, but more often defined as 98.67%, 99%, 99.9% and 99.99%, according to manufacturer.
Absorb/Absorption
The process by which a liquid penetrates the solid structure of the absorbent's fibres, which then swell in size to accommodate the liquid.
The process of a fluid being taken into the pores of a solid.
Absorption
The process of taking up a substance into the physical structure of a liquid or solid by physical or chemical action, but without any chemical reaction taking place.
ACFM
Means 'actual cubic feet per minute'. In air and gas streams ACFM is the actual flow. When corrected for pressure and temperature, it can be correlated to SCFM (standard cubic feet per minute).
Acid rain
When rainfall occurs below the normal pH range, usually caused by contact of raindrops with atmospheric pollutants such as nitric & sulfuric oxides and carbon monoxide.
Activated carbon
Granulated activated carbon used to remove tastes, odor, chlorine, chloramines, and some organics from water.
Activated clay
Adsorbent clay that removes color, odor, free fatty acids, etc. from oils and tallows.
Admix
Medium that is added directly into the batch tank of pre-coat to make a permeable filter cake.
Adsorb
The act of selectively attracting and holding a gas, vapor, liquid, chemical or colloid onto the surface of a solid.
To collect and hold a fluid on the surface of a solid
Adsorption
The process by which molecules, colloids or particles adhere to the surfaces by physical action but without any chemical reaction.
Aeration
The process of adding air to the water supply for the purpose of oxidizing or mixing.
Aerosols
Liquid droplets or solid particles dispersed in air or gases, of fine enough particle size (0.1 to 100 microns) to remain so dispersed for a period of time.
Agglomerate
The process of bringing together smaller particles into a larger mass.
Alkalinity
Capacity for neutralizing acid, usually due to presence of bicarbonate or carbonate ions. Hydroxide, borate, silicate, and phosphate ions may contribute to alkalinity in treated waters.
Angstrom
A unit of length equaling 10-10 meters, 10-4 microns, 10-8 centimeters, and 4x10-9 inches.
Anion
Negatively charged ion in a solution.
Antimicrobial
An additive or chemical that inhibits and kills the growth of micro-organisms on contact.
Aquifer
Natural, underground porous formation where mineral-bearing water flows or is stored. Source of well water.
Asbestos
A fibrous silicate material, mainly comprising of calcium magnesium silicate; a noncombustible, nonconducting, and chemical resistant material; a known lung carcinogen.
ASME code
Used in relation to filter vessels. Stands for 'American Society of Mechanical Engineers, Boiler, and Pressure Vessels'.
ASTM
Stands for 'American Society for Testing and Materials'.
ASTM Test Procedure F795-88
Generally a single pass test in water at 2.5 gpm per 10 inch length to rate filter media.
Atmosphere
A unit measurement of pressure. The pressure at sea level is 14.7 psi (1 atm = 14.7 psi).
Backwash
Reversal of water or solution's flow through a filtration system. Backwash process is commonly used in sand and multi-media filters.
Bacteria
Microscopic single-celled organisms reproducing by fission or by spores. Typically round, rod-like spiral shaped bodies, often aggregated into colonies or mobile by means of flagella. Commonly found in soil, water, organic matter, and in the bodies of plants and animals. Often symbiotic in man, but sometimes pathogenic.
Bactericide
Agent capable of destroying bacteria.
Bacteriostat
Substances that inhibit bacterial growth and metabolism but does not necessarily kill the cell.
Baffle
A plate or deflector to provide flow distribution in a filter housing.
Bar
Designation of pressure units.
Beta Ratio
The ratio of the number of particles of a given size and larger upstream of a filter to the number of particles of the same size and larger downstream.
Binders
In reference to cartridge filters, chemicals used to hold or bind short fibres together in a filter.
Blind spots
Any place on a filter where liquid cannot flow through due to blinding or plugging.
Blinding
In depth and surface filtration, a buildup of particulates on or within the filter, preventing fluid flow through the filter at normal pressures.
Blowdown
In reference to boiler & cooling tower technology, the purge from the system of a small portion of water concentrated with contaminants in order to maintain the level of dissolved and suspended solids in the system below a certain maximum level.
BOD
Biochemical Oxygen Demand – a measure of the amount of oxygen required for the biochemical degradation of organic material in a water sample.
Body feed
The pre-coat medium that is continuously added to the filter while it is on stream so as to create a permeable filter cake.
Bottled water
Water (usually treated or purified) sold in containers for drinking and domestic use.
Bridging
The act of particles forming an arch over the openings on a membrane as well in the case where filter cakes or contamination have grown to a size where they touch each other in the filter system.
Condition of a filter loading where contaminants span the open space between adjacent sections of a filter medium, thus blocking a portion of the useful filtration area.
Bubble point
The differential gas pressure that when applied to a filter element submerged in the test fluid causes the first steady emission of gas from the filter element being tested.
Pressure drop in inches of water required to expel the first steady (continuous) stream of bubbles (fizz point) from a horizontal disc of wetted filter medium or filter cartridge immersed in a liquid.
Bubble point test
A common, nondestructive method used to test the integrity of cartridge construction to compare relative porosities of filter media or to monitor product consistency as a quality control method.
Burst strength
The ability of the filter medium to withstand disruption by pressure applied in the direction of normal flow.
Bypass
Fluid flowing through a passage other than the filter medium and/or leakage around filter media seals.
Cake
The accumulation of solids on the medium of a filter.
Candle turbidimeter
A device used to measure the turbidity of water often using the light scattering method.
Carbonate hardness
The hardness in water caused by carbonates and bicarbonates of calcium and magnesium.
Carcinogenic materials
Substance or materials capable of causing cancer.
Cartridge filter
A filter device, usually disposable. For a detailed description see end of this booklet.
Cation
A positively charged ion in a solution.
Caustic soda
Sodium hydroxide (NaOH), a commonly used chemical in water treatment.
Cellulose
Vegetable origin material, in fibrous form used as a filter medium.
CFM
Cubic feet per minute.
Channeling
Tendency for contaminant to pass through a low-density area of an inconsistent filter medium or around cartridge seal points.
Chelating agent
A molecule that is soluble in water and reacts with metal ions to hold them in solution.
Chloramine
A compound consisting of chlorine and ammonia gas which retains its bactericide qualities for a longer time compared to free chlorine.
Chlorination
The addition of small amounts of free chlorine, usually 0.2 to 2.0 ppm, to control bacteria in a water supply.
Chlorine
A chemical used as bleaching, oxidizing or disinfecting agent in water purification.
Clarity
The clearness of water or liquid, measured using a variety of methods.
Clarification
Filtration of liquids containing small quantities of solids.
Coagulant
Chemical added to water to cause formation of flocs that adsorb, entrap or otherwise bring together suspended matter defined as colloidal. Used to remove turbidity, bacteria, color and other finely divided matter from water.
Coalescing
Separation of mixtures of immiscible fluids (for example water & oil) having different specific gravities. Here fine droplets collide and by becoming larger they separate out of solution more rapidly.
COD
Chemical Oxygen Demand – a measure of the oxygen required to oxidize chemicals contained in a sample.
Collapse pressure
Pressure across a filter cartridge or bag great enough to cause it to collapse.
Colloid
A substance of fine particle size, typically in the range of 0.1 to 0.001 microns, suspended in liquid or air. Such fine particles cannot normally be filtered out but are removed by reverse osmosis, distillation, or ultrafiltration process.
Colloids
Suspension of submicron particles in a continuous fluid medium that will not settle out of the medium.
Compaction
In crossflow filtration, as a result of pressure, reverse osmosis or ultrafiltration membranes may get compressed and result in a decline in flux.
Compound
Chemical bonding of separate elements or other ingredients.
Compressibility
Degree of change in volume when a filter is subjected to pressure.
Concentrate
In crossflow filtration, the portion of a feed stream which does not permeate the medium but retains and is increased in the amount of ions, organics, and other particles which are rejected by the medium.
Concentration
The amount of material contained in a unit volume of fluid. The term is also used for the process of increasing the dissolved material per unit volume.
Concentration polarization
In crossflow filtration, the formation of a more concentrated gradient of rejected material near the surface of the membrane causing either increased resistance to solvent transport, or an increase in local osmotic pressure, and possibly a change in rejection characteristics of the membrane.
Condensate
Water obtained through evaporation and subsequent condensation. Usually the water resulting from condensing steam (generated by a boiler).
Conductivity
The ability of water to transmit electricity, the property being the inverse of resisistivity.
Contact time
The length of time an adsorbent or absorbent is in contact with a liquid prior to being removed by a filter.
Contaminant
Any substance or impurity in water including gases, minerals and organic materials in dissolved or suspended particles form.
Undesirable insoluble solid or gelatinous particles present in a fluid.
Crossflow membrane filtration
A separation of components of a fluid by semi-permeable membranes by application of pressure and parallel flow to the membrane surface; processes include reverse osmosis, ultrafiltration, nanofiltration and microfiltration.
Crypto
Short for Cryptosporidium, a parasite found in water. If present in drinking water, causes diarrhea, cramps, and in severe cases even death.
Cycle time/length, filter life
The length of time a filter is 'on-stream' before cleaning or replacement is needed.
The duration, measured in time or volume, that a filter can operate effectively between replacement and/or cleaning
DE
Abbreviation for Diatomaceous Earth. A material insoluble in water consists of fossilized skeletons of minute prehistoric aquatic plants.
Decarbonation
Process of removing CO2 from water.
Degasification
Process of removing dissolved gases from water.
Deionization
Process utilizing ion exchange resins, which remove ionized salts from water to obtain soft water. The process typically removes salts only and not any organics, virus or bacteria.
Delta P
Term donating the pressure drop across a filter.
Demineralization
The process of removing minerals from water, usually by deionization, reverse osmosis, or distillation.
Density
Mass per unit volume of a substance under specified conditions of temperature and pressure.
Depth filter
It employs a medium with a significant amount of thickness providing 'filtering in depth'. The path through the filter is much longer and random, providing greater possibility of both direct interception and dirt retention. Larger particles tend to be trapped in the surface layers and the finer particles trapped by succeeding layers. Generally the overall performance and dirt holding capacity of a depth filter is better than that of a surface filter.
DI
Abbreviation for deionization.
Differential pressure (pressure drop)
The difference in pressure between the upstream and downstream sides of a filter.
Difference in pressure between two points in a system. In filters, this is usually measured between the inlet and outlet of the filter housing, and is a determining factor of filter service life.
Dirt holding capacity
It signifies the relative service life of a filter and is the weight of a specified contaminant, which must be added under test conditions to produce a given differential pressure.
The weight of a contaminant fed to the filter during a test to reach a predefined terminal pressure drop.
Disinfectant
A fluid or gas used to disinfect filters, water systems, pipelines, and vessels etc.
Disinfection
The process of killing or rendering harmless pathogenic organisms in a water supply by means of heat, chemicals or UV light, etc.
Disposable
A filter or any other such part of a system, which is intended to be discarded and replaced after each service cycle.
Dissolved solids
These are the residual materials remaining after evaporating the water or solution to a dry state.
Distillate
Is the water formed by condensing vapors.
Distillation
Is the process of condensing steam from boiling water on a cool surface. This process removes all contaminants that do not vaporize with the water.
Doctor blade or knife
A hard blade or knife that cuts or scrapes off the cake from the surface of a filter.
Double open end (DOE)
A filter cartridge configuration such that both ends are open and require housings with knife edge sealing devices.
Effective area
The total area of the medium exposed to flow in a filter element.
Efficiency
The ability, expressed as a percent, of a filter to remove a specified contaminant at a given contaminant concentration from a fluid stream under test conditions.
Effluent/Filtrate
The output stream exiting a treatment system.
Electrodialysis
Dialysis that is conducted with the aid of an electromotive force applied to electrodes adjacent to both sides of the semi-permeable membrane. Charged ions are drawn through the membranes, leaving lower concentrations of ions in the feed water.
Element
Any structural part of the filter on which the membrane or media is supported.
Emulsion
A suspension of small liquid droplets within a second liquid that will not mix.
End cap
A closed cover for the end of a cartridge filter, or housing.
Endotoxin
A heat resistant pyrogen, found in the cell walls of viable and nonviable bacteria. They are more stable than bacterial cells and are not destroyed at normal bacterial disinfecting temperatures (such as in autoclaving).
EDU
Abbreviation for Endotoxin Units, a measurement for pyrogen level.
EPA
Abbreviation for Environmental Protection Agency. It is a USA agency that establishes environmental standards, including drinking water standards.
EPCB
Extended Pass Carbon Block – This filter is made from a porous carbon block and provides microfiltration (0.5 microns) for cyst removal as well as some chemicals.
Evaporation
The process where water passes from a liquid to a vapor state.
Exhaustion
A term commonly used in water softening by ion exchange. It is the point where the resins can no longer exchange ions that the process was designed for.
Extractables
Inorganic or organic elements or compounds in the filter medium that have leached into the filtrate. Usually reported by weight or percent.
FDA
Abbreviation for the U.S Food and Drug Administration.
A filter construction materials for filtration of foods, beverages, drugs or cosmetics must comply with regulations established by the FDA as listed in CFR Title 21.
Feed water
Incoming water supply prior to any treatment.
Filter aid
Any material that assists in the separation of solids from liquids. They are usually used in the more difficult to filter applications.
Filter cake
The accumulated particles that have collected on a filter surface and then act as a filter medium themselves to enhance the filtering characteristics of the original filter medium.
Filter medium
The permeable material that separates particles from a fluid passing through it.
Filter system
Consists of the filter and all associated hardware required for the filtration process.
Filtrate
Any liquid that has passed through the filter medium.
Filtration
Is the process by which solid particles are separated from a liquid by passing it through a filter medium.
Separation of particulate matter from a fluid by passing a fluid through a permeable medium that will trap a percentage of the particulates.
Filtration efficiency
The fraction of suspended particles retained by the filter.
Filtration rate
The volume of liquid that passes through a given filter in a specified time.
Floc
Coagulated mass of particles settling down to the bottom, which were formerly divided and suspended in a liquid.
Flocculant
Chemicals which, when added to water, cause suspended particles to coagulate into larger mass (flocs) which then settle by gravity.
Flocculation
Is the process of agglomerating particles into larger masses called flocs, which then settle down to the bottom by gravity.
Flow fatigue resistance
The ability of a filter to resist structural failure due to flexing caused by differential pressure.
Fluid compatibility
The suitability and chemical resistance of a filtration medium for service with the fluid involved.
Flux
In crossflow filtration, the unit membrane throughput, usually expressed in volume per unit time per area, e.g. liters per hour per m2.
A relationship of flow to surface area; expressed in gallons per minute per square foot.
Fouling
In crossflow filtration, the reduction of flux that is attributed to a buildup of solids on the surface of the membrane.
Frazier
A test to measure the air permeability of a filter medium. Usually expressed in CFM of air at Delta P of 1/2-inch water column.
FTU
Formazine Turbidity Units – a measure of turbidity, by a nephelometer, used to measure low turbidity water.
Fuller's Earth
A sorptive clay, used for filtration, acid removal, bleaching, decolorizing, clarifying, and refining aid. Also used for removal of surfactants from petrol, kerosene, diesel and jet fuels.
Gels
Compressible or semisolid materials that can pass through filter media at an undefined and inconsistent degree. Best removed by depth filtration.
Giardia cyst
A parasite found in water. Causes diarrhea, cramps and in severe case even death.
Glassing
A form of silica scaling at higher temperatures, usually occurring in high-pressure boilers.
GPD
Abbreviation for 'gallons per day'.
Graded density
The better quality depth cartridge filters have a lower density in the outer layers to trap coarser particles and a higher density towards the inner layers to trap the finer particles. Graded density filters have a higher dirt holding capacity and longer effective filter life than depth filters with constant density construction.
Variation in a cartridge that results in the filter medium being more dense toward the core and less dense toward the outside surface. This is useful where a wide range of particle sizes exists because it allows larger particles to be trapped towards the surface and smaller particles toward the core.
Grains per gallon (gpg)
A unit of concentration equal to 17.1 milligrams per liter (17.1ppm). Commonly used as a measure of hardness in water.
Ground water
Subsurface water confined in permeable sand layers or cavities between rock or clay.
Hardness
The concentration of calcium and magnesium salts in water. Gives rise to scale formation and reacts with soap.
Hemodialysis
The process of purifying a kidney patient's blood by means of dialysis membranes.
HEPA
High Efficiency Particulate Absolute – a filter which removes from air 99.97% or more monodisperse dioctyl phthalate (DOP) particles having a mean diameter of 0.3 microns.
High purity water
Highly purified and treated water with particular attention to microbiological reduction or elimination. Term is commonly used in the pharmaceutical industry.
Housing
An open-able chamber or container which holds the filter and directs the flow of fluid through the filter element.
Hydrogen sulfide
A toxic gas (H2S) that has a 'rotten egg' smell. It is found primarily in well water supplies or other anaerobic water sources. Contributes to corrosion in metallic pipes.
Hydrophilic
Water loving, the tendency of a surface to wet with water.
Hydrophobic
Water hating, the tendency of a surface not to wet with water.
Immiscible
Incapable of blending of mixing into a single homogenous phase.
Impingement
Direct impact of a particle or liquid upon the filter medium.
Influent
Fluid entering the infelt of a filter.
Injection
In context of water treatment, it is the introduction of a chemical into the process water for purpose of altering its chemistry.
Inside-out flow
Fluid flow through a filter element outward from its center. This flow direction is not normal for most filters.
Ion
An atom or molecule, which has lost or gained one or more electrons, thereby acquiring a net electric charge.
Ion Exchange
A process in which ions are preferentially adsorbed from a solution for equivalently charged ions attached to solid resin particles.
JTU
Jackson Turbidity Units – turbidity test units of measurement registered on a candle turbidimeter.
Laminar flow
Flow rate at which liquid is in a nonturbulent flow in which rapid random fluctuations are absent.
LSI
Langelier Saturation Index – a calculation that predicts calcium carbonate precipitation under a specific condition, temperature, pH, TDS, hardness, and alkalinity.
Mean filtration rating
A measurement of the average size of the pores of the filter medium.
Media (Medium)
It is the porous material that provides the actual filtration and separation of solids from liquids.
Media Migration
It is the release of the filtration media (fibers or other material of which the filter is constructed) into the filtrate.
Medium
The plural of this word is 'media'.
Membrane
A polymer film containing highly controlled distribution of pores. They serve as a barrier permitting the passage of materials only up to a certain size or character. Membranes are used as the separation device in reverse osmosis, ultrafiltration, nanofiltration, and microfiltration.
Mesh
Number of yarns, wires or monofilaments in a linear inch of woven filter fabric.
mg/l
Milligrams of matter per liter of water; it is a measurement similar to ppm (parts per million by weight).
Microfiltration
It is the filtration that removes bacteria and particles in the range of 0.1 to 3 microns in diameter.
Micron
Metric unit of measurement equivalent to 10-6 meters or one millionth of a meter (mictometer). The naked human eye can see particles of 20 microns and above. Particles of size 0.03 to 19 microns can be seen with a microscope and those of 0.001 to 0.2 microns only with an electron microscope.
Mixed-bed
An ion exchange tank consisting of both cation and anion resin mixed together. It is used to polish water already treated by two bed softening tanks or reverse osmosis. Mixed-bed gives the most complete deionization of water up to 18.3 megaohm/cm resistivity.
Molecular weight
Is the sum of the atomic weights of the constituents, which make up a molecule.
Molecule
Is the smallest possible unit of a chemical or compound, composed of one or more atoms, that still retains the properties of that substance.
Multifilament
Fibre strand that comprises of continuous lengths. A yarn of continuous filaments contains no short fibres.
Multipass process
A process or system in which fluid is circulated indefinitely through a filter medium
Nanofiltration (NF)
Crossflow membrane separation process for removing particles in the 250 to 1000 molecular weight range, selected salts and most organics. Nanofiltration is often used for water softening and require lower driving pressure than RO.
Nephelometer
Device used for measuring low turbidity water with the test results expressed in Nephelometeric Turbidity Units (NTU).
Nominal rating
It is an arbitrary micrometer value indicated by various cartridge filter manufacturers. It refers to an approximate size particle, the vast majority of which will not pass through the filter.
Noncarbonate hardness
It is the hardness caused by chlorides, sulfates, and nitrates of magnesium and calcium.
Nonwoven
A filter cloth formed by a method where the fibres are held together by a binder, or by fusing some of the fibres, or by needle punching which gives mechanical cohesion.
NTU
Nephelometeric Turbidity Units – a measurement obtained by passing a light beam through a low-turbidity water sample with a nephelometer.
On-stream
When a filtration system is being used and is producing a filtered product.
Osmosis
Is the flow of water from a less concentrated solution to a more concentrated solution through a semipermeable membrane until energy equilibrium has been achieved.
Osmotic pressure
It is the measurement of the potential energy difference between solutions on either side of a semipermeable membrane.
Oxidation
A process by which electrons are lost to an oxidizing agent in order to increase a molecule or ion in positive valence.
Ozonator
A device that generates ozone by passing a high voltage current through a chamber containing air or oxygen. Used for water disinfection.
Ozone (03)
It is an unstable, highly reactive state of oxygen. An excellent oxidizing agent and bactericide.
Particle filtration
Normally means filtration of particles in the size range of 1 to 75 microns and is typically done by cartridge filters.
Particle removal efficiency
Removal of particles as a function of size as determined by counting individual particles.
Particulate
Minute, separate pieces of matter.
Permeability
The property of a filter medium that permits a fluid to pass through under a pressure differential.
Permeable
A media which allows some material to pass through.
Permeate
That portion of the feed stream, which passes through a membrane, leaving behind a more concentrated stream.
Permeator
A hollow fibr