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Absolute (Filter) Rating

A term used to specify the size above no particle will be found in a filtrate.

AC Coarse Test Dust (ACCTD) AC Fine Test Dust (ACFTD)

see Arizona Test Dust


A process which affects the cake resistance and the rate of filtration by the age of the suspension.


A bunch of particles holding each other. The formation of an agglomerate is called agglomeration.

Air Blow

The process of forcing a flow of pressurized air through a filter medium. A reversed air blow is usually used to clean the filter or remove a filter cake. Also Blowback.

Arizona Test Dust

Standardized test dust classified from natural Arizona dust. Generally referred to as AC fine test dust (ACFTD) and AC coarse test dust (ACCTD) (AC = Air Cleaner). Its use is deprecated in favor of other standards like ISO, ASHRAE etc.


Degeneration caused by rubbing or friction. Normally results in unwanted fine particles and slow material loss of the surface.

Average Particle Size

All particle systems contain a range of particle sizes, unless specially manufactured. The distribution can be expressed on number, surface, mass or volume basis. To locate the center of a distribution several methods are used, the most important being mode, median and (arithmetic) mean. Mode - The most frequent particle size in the distribution. A distribution can have more than one mode (Bimodal, Trimodal, etc.). Median - The particle size that divides the distribution into two halves, where 50% is finer than the median and 50% coarser. Mean - The most common statistical average. The sum of all particle sizes that occur divided by the total number.


A periodic cleaning process by reversing the flow through the filter media.

Backing Cloth Backing

A strong coarse mesh placed directly over a filter medium to protect the filter or to provide better flow conditions.


A method of cleaning or regenerating a filter medium by forcing wash fluid in the reverse direction through the medium. In case of a filter bed the bed can become fluidized if the flow is high enough.

Batch Batch Mode

Leaf filters and filter presses are usually referred to as batch machines, since the filtration process is discontinuous. The filtration cycle is interrupted regularly for cake discharge.

Belt Discharge

A method for removing filter cake from a rotary drum filter.

Belt Filter

An endless belt or band of woven wire mesh used to continuously filter solids out of liquids.

Belt Press

Belt presses generally use two continuous tightened filter bands. The feed is introduced at the open end and then transported through the progressively approaching belts causing any liquid to be squeezed out of the solids.

Bicomponent Fibre

Fibre made from two different polymers. Those can be extruded into a single filament. A core with a mantle of different material is the most common configuration, although other are possible.


A distribution with two modes. see Average Particle Size


When using cake filtration bleeding can occur during the initial phase of filtration when the cake hasn't formed yet. This causes particles to occur in the filtrate that usually are filtered out. This bleeding will stop once the cake has formed.


Two or more types of fibres used to make a yarn.


Blinding, also called plugging, is the process of particles clogging the pores of a filter medium which can't be removed by a cleaning process. This leads to an increased pressure drop and finally to a flow resistance which is too high for effective operation.

Blocking Mechanism

Description of how a particle can block a pore of a filter media. See also Complete Blocking Filtration, Standard blocking Filtration, Bridging Filtration, Intermediate Blocking Filtration.


See Air Blow.


The process of solids or gas passing through a formed filter cake, usually causing solids to appear in the filtrate. In cake washing terminology used to describe the first appearance of wash liquid on the downstream face of the filter cake.

Breakthrough Pressure

In a dewatering process, the minimum pressure needed to start the reduction in cake saturation, describing the moment when liquid start to move out of the pores.

Bridging Filtration

When cake formation begins, particles form an arch over the pores. The effect is enhanced by higher particle concentration in the feed and doesn't depend much on the size of the particles. As long as the direction of flow doesn't change, those bridges are quite stable.

Bubble Point

The differential gas pressure needed to cause the first bubble to appear from the pores of an immersed, horizontal filter media. See also Porometer.

Cake Concentration

Specifies the amount of solids in a filter cake, usually given as a percentage.

Cake Cracking

During cake deliquoring the varying stresses caused by gas-liquid interfaces lead to cracking of the cake. Cake cracking and channel formation have a negative influence on effective cake deliquoring or washing.

Cake Deliquoring

Reduction of the among of liquid in a filter cake, usually by sucking or blowing air through the cake or by compressing the cake.

Cake Filter

A filter forming a cake of particles that grows with filtration time.

Cake Filtration

Used with high concentration feeds to form a cake on the medium. Filtration will then be done by the cake.

Cake Growth Rate

The average rate of cake formation during filtration.

Cake Moisture Content

The ratio of mass of liquid in the cake to the total cake mass.

Cake Thickness

The thickness of a forming cake during cake filtration.

Cake Washing

Post-treatment process that washes soluble impurities out of a filter cake by forcing a clean wash fluid through the cake.


Surface treatment of filter cloths by passing the cloth between heated, pressurised rollers. Can be used for polymer cloths as well as metal meshes.

Candle Filter

A cylindrical filter element where flow direction is from outside to inside. Cake will form on the outside and can be removed by various means.

Capillary Pressure

The differential pressure across the interface of two immiscible phases occupying the void spaces in a filter.

Capture Mechanism

Describing the way how particles are removed from the feed by the filter. There are seven methods known: Diffusion, Sedimentation, Straining, Interception, Inertial Impaction, Electrostatic Interaction and Hydrodynamic Interaction.

Clogging Point

The clogging point usually describes the point at which the filter must be replaced because the particles trapped in the filter cause a pressure drop that will be seen as excessive.

Further links to the topic:
GKD: Filter Plates
Wikipedia: Filter cake

Cloth Tracking

An electric or pneumatic system to keep the cloth of a belt filter in a central position.

Coagulation / Flocculation

A process of contact and adhesion whereby the particles of a dispersion form larger-size clusters. Also known as agglomeration, aggregation, flocculation or coalescence.

Complete Blocking Filtration

A blocking mechanism where each particle seals off the pore it reaches on the surface of the filter. Flow rate will drop until no further flow can occur. Low/medium concentration in the feed favour this blocking mechanism.

Composite Filter Media

Filter media fabricated from more than one material and/or layer.

Computational Fluid Dynamics (CFD)

Usually computer based calculations to solve problems in fluid flow over complex geometries.


The mass of one phase in a unit volume of a mixture. There are several ways of expressing a concentration: Mass fraction (mass of solids to total mass), Mass ratio (mass of solids to mass of liquid), Voids ratio (volume of voids to volume of solids), Volume fraction (volume of solids to total volume), Volume ratio (volume of solids to volume of liquid). Units like ppm, ppb should be avoided due to ambiguity.

Constant Pressure Filtration

Cake filtration that uses a constant total pressure. Since cake formation leads to increasing pressure drop across the cake, flow rate will decrease over filtration time.

Constant Rate Filtration Constant Flow Filtration

Cake filtration that uses a constant flow of feed towards the medium. Cake formation will lead to increased pressure needed for a constant flow.

Contact Angle

The angle formed between a droplet and a solid surface. Hydrophilic surfaces lead to near zero contact angles, hydrophobic surfaces show angles in excess of 90°.

Coulter Counter

A particle counter by Beckman Coulter. It counts the particles in an electrolyte and measures their volume by measuring the change in conductivity of the electrolyte by the displacement of conductive volume inside a capillary where the particles pass through.

Crossflow Filter Crossflow Filtration

Passing a solution along the surface of a filter media (usually a membrane) to increase the concentration of solids in the liquid. Shear stresses caused by the motion of the fluid prevent cake formation on the filter media.

Cut point

A test that determines the cut point of a filter medium. Particles of known size, quantity and distribution are forced through a filter and the particles passing through are measured.

Darcy (Unit)

Unit of permeability. 1 m² = 1.013x1012 Darcy.

Dead Area

Places in and around a filter medium where no filtering occurs.

Dead Void Volume

Volume of voids not occupied by a flowing fluid.


See Tex System

Deep Bed Filter Deep Bed Filtration

A filter unit that usually uses a relatively thick filter media. Particles passing though this filter media are captured inside by different Capture Mechanisms. Once pressure drop gets too high, the filter media is cleaned by Backwashing.


A physical capture mechanism in filtration. By means of brownian motion particles move towards a fiber and will be captured by adhesion at contact.

DIN Standards

Standards defined by DIN (German Institute for standardization)

Dirt Holding Capacity

The amount of contaminant, measure in grams, a filter is able to hold before reaching a maximum allowed pressure drop. This term primarily aims at depth filters.


The material leaving a filter, e.g. filtrate or solid cake.

Doctor Knife Doctor Blade

An adjustable device for removing of filter cake, e.g. from a rotary drum filter. (See Knife Discharge)


The side the filtrate flows from the filter, or any point further down the line.

Drainage Surface Drainage Grid

The support of the filter medium, that also provides unhindered flow.

Driving Force

The force necessary to provide a flow through the filter medium. Usually overpressure from above or a vacuum from below the filter, but can also be exotic like concentration gradients.

Effective Area

The area exposed to flow and usable for filtration.


A chemical, often a liquid, that is conductible by ionisation.


A suspension of two immiscible liquids, e.g. water in oil. To keep it stable, often a emulsifying agent (surfactant) is needed, otherwise the two liquids will separate over time.


The material that needs filtering and is therefor supplied to the filter.

Fibre Fiber

Monofilament or filament spun used as basic material in the textile industry.


A single, continuous strand which can be thousands of meters without break, in contrast to staple fibres like cotton.


Equipment for the purpose of performing a filtration. Consists of the filter medium and the filter holder. Sometimes the filter medium is loosely referred to as filter.

Filter Bed

A layer of particles, granular media or fibre through which filtration takes place.

Filter Cake

A filter cake is formed by the piling of particles on the filter medium during filtration.

Filter Medium

Any permeable material upon or within which solid particles are deposited. Plural is filter media. Filter media can be woven, non-woven, membranes or packed beds (e.g. sand).

Filter Medium Resistance

The resistance of the filter medium alone. The resistance can change during filtration. If the filter media is repeatedly used the resistance will stabilize to a constant value after a few cycles.
A fabric construction produced by enmeshing monofilament spirals linked together by a series of straight monofilaments. Filter belts or transport belts using this construction don't need a mechanical seam, overcoming a traditional weak link in belts.


The liquid that passed through a filter from which particles have been removed.


The process of removing solids from a liquid.

Filtration Efficiency

A quantitative measurement of the effectiveness of a filter. Usually given as a percentage of particles of a certain size retained by the filter.

Flex Fatigue

A failure process for fibres where a repeated cyclic load leads to breaking. This process is influenced by the load and the chemical and physical environment.

Flow Decay

Reduction of the flow rate by plugging or clogging of the filter medium.

Flow Rate

The rate at which a material passes through a system. Usually either mass based or volume based. Examples of units: kg/s, kg/min, t/h (mass based) or l/s, l/min, m³/h (volume based)

Flow Resistance

The resistance to fluid flow presented by e.g. the filter medium. The flow resistance is the reciprocal of the permeability. The unit of the resistance is m².


Used in membrane filtration. The rate of flow of mass or volume per unit cross-section normal to the direction of flow. For mass flow the unit is kg/m²/h or l/m²/h for volume flow. Other typical units of measurement are gallons per square foot per day (GFD or GSFD) or litres per square meter per hour (l/m2/h).

Frequency Size Distribution

A representation of the distribution of sizes in a sample. The amount of e.g. particles within a given size interval is plotted over size. If the plot includes sizes from minimum to maximum of present sizes, the plot is called Frequency Size Distribution.

Galvanic Corrosion

Galvanic corrosion occurs when two connected different metals are subject to an electrolyte. Due to the electrochemical reaction, the less noble metal will then corrode.

Gauge Pressure

Pressure reading from a gauge. To get the absolute pressure, the ambient pressure must be added to the gauge pressure.


Geometric material designer & material property preDictor A software to virtually generate filter media and predict their properties by using CFD (Computational Fluid Dynamics) to determine e.g. pressure drop or pore size.

Geometric Pore size

A method of calculating the maximum pore size of a woven mesh by using some basic parameters as input. The formulae are based on the AVIF projects A224 and A251 realized at the University of Stuttgart.

Glass Beads

Small spherical glass beads, used for filter testing.

Grade Efficiency

Filter media show different filter effectiveness at different particle sizes. This variation in efficiency is called grade efficiency. Plotting the filtration efficiency over particle sizes gives the 'grade efficiency curve'.


An alternative word for particle. Usually one that is clearly visible to the naked eye.


The fraction of a filter cake remaining on the filter medium after discharge. Sometimes called residual heel.


HEPA means High Efficiency Particulate Air or High Efficiency Particulate Arrestance. HEPA filters are usually throwaway media for air filtration. HEPA filter are classified in DIN EN 779 and EN 1822-1:1998.


Particles consisting of a range of sizes. Same as polydisperse.

Horizontal Belt Filter

A vacuum filter with an endless cloth moving on a support belt. The belt and cloth move continuously around two rollers and across several suction boxes.


Having an affinity for water. Opposite of hydrophobic.


A particle that is repelled from water Opposite of hydrophilic


The ability of a substance to attract and hold water.


One of seven physical capture mechanisms of filtration. The movement of the center of mass of a particle would pass the side of a fiber, but the geometric dimension of the particle causes the particle to touch the fiber and stick to it due to adhesion.

Intergranular Corrosion (IGC)

A form of corrosion attacking the boundaries of grains inside a metal. Can be avoided by accurate control of the chemical composition and by performing heat treatment.

Intermediate Blocking Filtration

A blocking mechanism where a particle will block a pore at the surface of a filter medium. Later arriving particles may settle on already deposited particles. Not every particle will block a pore, but there is a probability for this to occur.

Irreducible Saturation

The value of saturation of a filter cake at which no further desaturation is possible, other than by evaporative means. Irreducible saturation is best determined experimentally, since calculations are often unreliable.

Knife Discharge

The most common method of cake discharge on rotary drum filters. The knife is places close to the medium and leaves a heel of cake on the cloth to avoid damaging contact. Knife discharge can be supported by blowback if the cake isn't thick enough to discharge by its own weight.


A flat filter element that contains a filter medium.

Leaf Filter

A filter in the shape of a thin leaf which can be mounted horizontally or vertically.


The weight of particles per unit area deposited on a filter surface.


Having an affinity for a solvent ('solvent loving') Is the solvent water the term hydrophilic is used. Opposite of lyophobic.


Lacking an affinity for a solvent ('solvent hating') Is the solvent water the term hydrophobic is used. Opposite of lyophilic.

Maximum Pore Size

Diameter of a pore represented as a capillary of circular cross-section with the same capillary pressure as the pore.

Mean Flow Pore Size

Pore size equivalent to the mean flow rating

Mean Flow Rating

Diameter of the pore that divides the volume of flow through all pores into two equal parts.

Mesh Count/ Mesh Size

Number of openings per inch or per centimeter Sometimes deprecated units like "per rheinisch" or "per frensh" are used.

Mode Modal Particle Size

see Average Particle Size


A dispersion in which particles are all of the same size. Usually used when 90% of the particles are within ±10% of the mean.


A single, continuous fibre, usually of a synthetic material.


A fabric yarn made from multiple twisted fibres, usually of a synthetic material.

Newtonian Fluid

A fluid that obeys Newton's law of viscosity. It shows a linear proportionality between shear stress and shear rate. The constant of proportionality is viscosity.

Nominal (Filter) Rating

Based on filtration tests where 90% (sometimes 95% or 98%) by mass of particles are stopped by the filter. This method is discouraged in favour of the absolute filter rating.

Non-newtonian fluid

A fluid whose viscosity changes with the shear rate. It doesn't obey Newton's law of viscosity.

Number Distribution

The relative frequency size distribution by number within a size distribution. Can also be shown as a cumulative size distribution.


Having an affinity for oils ('oil loving') Opposite to oleophobic.


Lacking an affinity for oils ('oil hating') Opposite to oleophilic.

Open Area

The pore area of a filter medium. Usually given as a percentage of the total area.


Open Field Operation And Manipulation A general purpose CFD simulation software

Particle Counter

A device to count the particles in a fluid.

Particle Size

The value of the physical dimension of a particle. Unless the particle is of spherical shape, it isn't possible to state a single value. For those particles the method of measurement must be stated.

Particle Size Analysis

The process of measuring the size of a particle or the size distribution of many particles. Methods include microscopy (e.g. optical), electrical sensing (e.g. Coulter Counter), light diffraction, light scattering and photon correlation spectroscopy (PCS).

Particle Size Distribution

A table of numbers or a graph giving the frequency of particles of a chosen size range.

Pick Count

Used for woven filter media, giving the number of weft wires per unit length.

Pitting Corrosion

When the protective passive film of a metal is damaged and can't be regenerated faster than corrosion occurs, a pit is formed with time. Once formed, the corrosion continuous at an ever growing rate. In extreme cases this will lead to perforation of the metal.

Pitting Resistance Equivalent Number (PREN)

A number representing the resistance of a metal against pitting corrosion. The higher the number, the more resistant the metal is. The PREN can be calculated from the contents of the alloy. PREN = %Cr + 3.3 * %Mo + 16 * %N


A fold formed by doubling a sheet of material back on itself. Pleating leads to an increased filter surface within a given volume. A pleat is a single fold.

Pleat Support Pleat Spacer

Used to prevent a pleat from collapsing.


see Heterodisperse


A hole or cavity in a body

Pore Flow

A convectional transport mechanism through a porous material.

Pore Size

The size of a pore in a porous material. The size and/or size distribution can be measure by a porometer.

Pore Size – Absolute Rating

The pore size at which particles equal or greater are retained with 100% efficiency.

Pore Size – Nominal Rating

The pore size at which particles are retained with an efficiency less than 100% (typically 90 to 98%).

Pore Size Distribution

Similarly to particle size distribution, this describes the distribution of pores of specified sizes. This affects filtration performance as the ratio of pore size to particle size influences plugging and bridging. Can be measure by a porometer.

Pore Velocity

The velocity of a fluid flow through the pores of a filter cake.


A measuring device to determine the pore size distribution.


A device to measure the pore volume and hence the porosity of a filter medium. Often (incorrectly) used as term to describe a porometer.


The ratio of the void volume to the total volume of a material.


The physical unit calculated by force per unit area. SI Unit to be used is Pascal (Pa = N/m²).

Pressure Drop

The loss of pressure in a fluid which flows through an obstacle, e.g. filter medium.

Pressure Filter Pressure Filtration

A class of filters that uses pressure above the filter media as driving force for the fluid to pass the media.

Primary Particle Size

The size of an individual particle, e.g. prior to agglomeration.


The process of particles, which have been deposited on a filter medium, re-enter the flow, e.g. by sudden change in flow velocity or vibrations.

Resonant Frequency

The natural frequency of a system where the largest amplitudes occur. Resonance can be used on a filter medium to reduce cake formation and fouling.


The study of flow and deformation properties of matter, such as elasticity, plasticity and viscosity, under the influence of an applied stress. See Newtonian Fluid, Non-Newtonian Fluid


A device containing apertures of uniform shape and size arranged in a rectangular manner. In the paper industry used as synonym for transport band.

Sieve Diameter

The minimum width of the square apertures of a sieve.

Size Fraction

The fraction of a system of particles limited by two given sizes.


The process of measuring the size or size distribution.

Standard Blocking Filtration

A blocking mechanism in liquid filtration where it is assumed that particles stick to the walls of the pores, leading to decreased flow and increased pressure drop. This will either lead to total blockage of the filter medium or a breakthrough which will contaminate the filtrate. Standard blocking filtration is favoured by low concentration of particles in the feed.

Stress Corrosion Cracking (SCC)

A form of corrosion that appears in chloride or halide environments when the metal is subjected to tensile stresses. High nickel contents promote this kind of corrosion.

Tex System

Metric system of units describing the linear density of a fibre. 1 tex = weight in grams of 1000 m of fibre 1 decitex (dtex) = weight in grams of 10000 m of fibre

UHEPA Ultra High Efficiency Particulate Air ULPA Ultra low penetration air

see HEPA


Describes the approaching fluid or flow before the filter.

Wet Sieving

Sieving with the aid of a liquid. Measuring the particle distribution usually leads to bigger particles passing the filter compared to a dry sieving of the same filter medium.

Wire Mesh

A term referring to a woven medium.

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