Technique for the periodic cleaning of a filter medium by a flow of filtrate, additional backflush liquid or air in the reverse direction of the filtrate flow, initiated after a preset period of time or when a preset pressure drop through the filter cake is reached. To attain an economic process the amount of filtrate or additional backflush liquid required for backflushing may not exceed a fraction of the filtrate obtained in the previous filtration cycle.

Backflush Filtration

Backflush filtration is employed to filter large amounts of liquids with very low to moderate solids content. In most cases backflush filtration is a clarifying filtration and it is the objective to produce clean liquids. The solids to be removed are often of small size with particle sizes x < 10 µm. The liquid used for cleaning of the filter medium by the backflush can be either filtrate or an additional, external l iquid.

Backing Cloth

A robust coarse mesh cloth placed between the filter cloth and the filter cell to prevent deformation of the filter cloth and to improve filtrate drainage.

Backing Fabric
Backing Cloth
Bacteria Retaining Test

With a bacteria-retaining test, the germ load limit of a microfiltration membrane can be established for the degerminating filtration. A known area of a porous membrane is charged with a suspension of test germs and the charged ratio of the organism in the starting solution and in the filtrate is deter mined (titer reduction). A capacity of 107 germs /cm2 is the minimum for a filter medium to be called a sterilizing filter (test germ: pseudomonas diminuta).

Bag Filter

Bag filters belong to the discontinuously working cake filters. A filter bag, hanging in a perforated support basket, is fed with a pressurized suspension and the filtrate is collected in the containing pressure tank from which it exits. When the bag is full with solids or if an upper pressure loss is reached the solids are often disposed of together with the filter bag. Generally, bag filters are employed to clean up liquids with low contamination. Parallel bag filters are often set up in large units to increase throughput or to operate continually.

Barometric Leg

Attachment at continuous vacuum filters to discharge the filtrate from the vacuum system without the need of a filtrate pump. It is a dip tube that when completely filled with filtrate and immersed deep enough into the filtrate pool equalizes with its suction pressure the vacuum applied on the filter. Additionally produced filtrate can therefore drain off freely through the lower outlet. The installation of a barometric leg requires a sufficient overhead height, e.g. at least 8 m to produce a vacuum of 0.8 bar.

Basket Centrifuge
Pendulum Centrifuge
Basket Weave
Plain Weave
Batch Process

Discontinuously operating batch wise separation.

Batch Separation
Batchwise Separation
Batchwise Separation

Discontinuous processing of a given quantity of suspension. During the batchwise separation the individual steps of the separation process, such as cake formation, cake washing, or cake demoisturing are not synchronized with each other and are independently and individually adjustable. In this manner one is flexible to meet the specific requirements of the product to be separated. A continuous manner of operation can be realized by a time-delayed parallel pattern of discontinuously working machines.


Conical part of the bowl of a decanter. The settled particles are transported by a screw, which rotates with a differential speed to the bowl upwards the cone out of the liquid pond and along the dry part of the cone to the solids discharge opening.

Beaker Centrifuge

Discontinuously working centrifuge, used mainly in the laboratory for design experiments, for preparative purposes, or for preparing small and difficult to separa-te product quantities, e.g. in biotechnology. Beaker centrifuges with a horizontally ro-tating axis feature beakers rigidly mounted to the rotor. In those with a vertical axis, the beakers are mounted vertically on a pivoting joint at the end of the rotor, and only move under the influence of the centrifugal force into a horizontal plane. The beakers can be equipped either for sedimentation or filtration. Beaker centrifuges offer only very small filter areas of a few square centimeters but can be accelerated up to 10,000 g. The warming up of bucket and product caused by the air friction at high numbers of revolutions and long centrifugation times, can be controlled in cooling centrifuges. A special design of the beaker centrifuge is the long arm centrifuge.

Belt Control

Attachment in continuous filter machines with circulating filter medium to ensure straightness. It usually consists of a sensor for registering lateral belt deflections and a deflection roller correcting the belt position. Belt controls are used in belt filters, drum filters with leaving filter belt, sieve belt presses, and press filter automats.

Belt Discharge Special method of filter cake discharge from a drum filter with the help of a ‚ leaving filter belt.

Belt Filter

Continuously or quasi-continuously operating, horizontal vacuum filter. Belt fil-ters are offered with filter areas ranging from 1 m2 up to over 100 m2. They occupy large floor spaces and are relatively expensive compared to other continuously working vacuum filters. They are especially suited for easy filtering products that require an intensive cake washing, since the washing medium can be applied on the filter cake from the top as a pool. The field of applications of belt filters is extremely broad and reaches across many industries from chemicals to mineral processing.

Belt Filter with Reversing Vacuum Trays

Vacuum belt filter with periodically interrupted vacuum, in order to either advance the filter medium a step, or to retract the filtrate suction trays, that are rol-ler-mounted below the filter medium, by one step. The ability to cut the vacuum elimi-nates in these filters the need for a dragging seal and a circulating carrier belt. However, they consume more energy due to the periodic cell ventilation.

Belt Press
Double Belt Press
Bench-Scale Apparatus
Laboratory Apparatus

Bentonites are clay minerals (hydrous silicate of Mg and Al; main constituent montmorillonite) with a high capacity for swelling and adsorption. Bentonites are employed among other uses in the clarification of beverages as wel

Beta Value

The Beta value is a frequently used term for the filter cloth resistance. It can be established on the basis of permeation experiments with a particle-free liquid, or directly from a filter experiment via the Darcy Equation.


Type of crossing of weft and warp threads in a weave. One classifies three basic bindings: linen or plain weave, twill weave and satin weave.

Blaine Number

Blaine Number measures according to the Blaine procedure the volume- or mass- based surface of a granular substance. The principle of the Blaine measurement is based on the gas permeation of a specified tablet manufactured of the solid to be investigated, and the evaluation of pressure-dependent gas volume flow according to the Carman & Kozeny-equation. The density of the solids, the porosity of the briquette, the pressure applied, the volumetric flow rate of the gas, and its viscosity all have to be known. It is important to recognize that with this measurement only the outer, wetted surface of the particles is taken into account, and that the numerical value obtained has to be understood less as an absolute value but more for comparing different kinds of solids. In mineral processing, for example, the Blaine Number correlates with the filterability of a suspension. Thus a Blaine number of 2000 cm–1 characterizes a product principally well suited for vacuum filtration.


The progressive clogging of the pores in a filter medium with particles from the suspension. The free cross-sectional flow area decreases through particles penetrating the pore structure of the filter medium, and consequ


Clogging of the ‚ pores of a filter medium by particles that penetrate out of the suspension into its structure.

Blocking Filtration
Sieve Filtration
BoCross Filter

Crossflow filter for dynamic filtration and dynamic sieve filtration by the BOKELA company. It is employed for thickening, washing, diafiltration and classifying (dynamic sieve filtration) of suspended micro- to nano sized particles. The modular structured machine comprises a sequence of disc-shaped filter chambers, with stirring elements inside on a common central shaft that rotates to prevent cake formation by generating a crossflow filtration across discs. The sequence of filter chambers form a completely closed hermetical system. The slurry flows meander-like from one chamber to the next one and becomes accordingly more concentrated since filtrate is discharged from each chamber. After the last chamber, the concentrate (retentate) is extracted with an automated drain valve. With the BoCross Dynamic Filter highly concentrated and/or highly viscous feed materials can be thickened and washed up to concentrations of 65 Vol-% in one step. When used for dynamic sieve filtration the BoCross Micro Sieve Filter enables sieving i.e. removing of coarse particles from very viscous and concentrated disperse systems and suspensions with a sharp sieve cut and small amount of retentate, that means the product loss (part of fines and/or liquid remaining with the coarse partic-les) is low.

Body-Feed Filtration

Special type of cake filtration utilizing a filter aid that is admixed to the suspension to be processed to improve its filterability. Generally, they consist of fibrous or granular substances, which open up the pore structure of the developing filter cake and thus increase its permeability. It can also be a coarse fraction of the solids already in the suspension called a supporting grain. Body-feed filtration is an alternative to precoat filtration; it is employed in vacuum as well as in overpressure filters and press filters.


Portable laboratory pressure filter apparatus of the BOKELA company for investigating vacuum, overpressure, and press type filtrations. lt is based on the principle of the discontinuously working monoplate fitter, has a filter area of 20 cm2 and can be pressurized up to 11 bar. All process steps of the cake fittration as weil as of the cake formation, cake washing, cake demoisturing can be investigated. An electronic measurement data registrating and processing system records the filtration data and issues them in the form of an experimental test protocol. The apparatus corresponds to the VDl-guideline No. 2762: "Mechanical solid-liquid separation by cake filtration. Determination of filter cake resistance."

BoHiBar Drum Filter

The BoHiBar Drum Filter (formerly named as Oyster Filter) is a special variant of the BOKELA BoHiBar Filtration technology especially developed for the filtration, washing and demoisturing of suspensions in the chemical. pharmaceutical and food industry. The BoHiBar Drum Filter is mounted in a vertically opening pressure vessel (optional: shell-like opening) which provides for a good accessibility of the filter. A special feature of this innovative pressure drum filter are the individually exchangeable drum filter cells (optional). The BoHiBar Drum Filter can be designed as a continuously working pressure filter or steam pressure filter.

BoHiBar Filter

Development by the BOKELA company. Continuously working pressure filter system with disc filters or drum filters enclosed in a pressure vessel according to the principle of the hyperbaric filter. For separation tasks with cake washing usually a drum filter is applied. A BoHiBar Filter can also be designed as a steam pressure filter. A special variant of BoHiBar Filter Series is the BoHiBar Drum Filter design for smaller sized filters which are enclosed in a shell-like opening clamshell pressure vessel. BoHiBar Filters are applied in the extractive, chemical and food industry for fine and difficult to filter products, to achieve high throughput rates in continuous operation, to meet high quality standards with respect to purity and dryness of the solids, to prevent contamination of solids and/or filtrate and to enable filtration with high temperature and high process pressure. For filtration of hot suspensions and volatile liquids BoHiBar Filtration can be performed as counterpressure filtration with high temperature up to T = 200 °C, high vessel pressure up to p1= 15 bar and pressure difference ΔpFiltration = p1 – p2 with a pressure p2 above the boiling point Tb, pb of the liquid (p1 > p2 > pb). This enables draining off the filtrate with a high pressure p2 > pb > patm. Thus, boiling of the liquid is prevented as well as crystallisation of undesired solids which could contaminate the filter cake and/or lead to clogging of the filter cloth and scaling of the filtrate pipe system. It also allows direct processing of hot filtrates in hot downstream processes without intermediate cooling and re-heating.

Bond Curve

Special interpretation of the capillary curve, frequently applied in centrifugation to characterize the product moisture that can be attained when the forces are at equilibrium. The plot of a Bond curve shows the moistur that can be attained when the forces are at equilibrium. The plot of a Bond curve shows the moisture, expressed as the saturation degree or the residual moisture, against the Bond number, which is the ratio of the respective forces expelling and retaining the pore liquid. A Bond curve falls into four characteristic sections. The first describes the conditions before reaching the capillary entry pressure, the second the region where the capillary rise still exists, the third is the so-called Bond plateau, and the last refers to the demoisturing of interstitial liquid.

Bond Number

A non-dimensional characteristic number in centrifugation describing the ratio of the forces expelling and retaining the pore liquid, respectively. The Bond number is displayed together with the residual moisture in the Bond curve. Different Bond numbers Bo can be defined for the coarse capillary system and for the region of the interstitial liquid.

Bo1 is valid for the region of the coarse capillary liquid:

dh  hydraulic pore diameter, h cake thickness, ρL  liquid density, g earth acceleration, C  C-value,  γ L surface tension and δ  wetting angle. In the region of the interstitial liquid the Bond2 number is valid:

Bond Plateau

Special section of the Bond curve where the freely flowing capillary liquid in coarse pores has been completely removed from the bulk whereas interstitial liquid, adhesive liquid, and inner liquid still are remaining. Depending on the surface roughness of granular solids, the Bond plateau is flat for smooth particles, and it declines for rougher particles towards a lower residual moisture with increasing Bond numbers.


A narrowing in a flow conduit that controls the throughput of a separation machine. Such constrictions often result from undersized filtrate pipes.

Boundary Layer

In fluid dynamics a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface. It is a phenomenon of viscous forces and is related to the ‚ Rey-nolds Number.

BoVac Disc Filter

A high performance type of disc filter of the BOKELA company. These continuously working vacuum filters are especially suited for large throughput performance, obtained by optimizing design and process know-how. Contrary to standard disc filters the BoVac Disc filter can rotate even at 6 rpm due to the excellent hydraulic characteristics leading to a significantly increased throughput performance. The BoVac Disc belongs to the BOKELA rotary vacuum filter family consisting of disc filters, drum filters and pan filters which are of an innovative design and represent a new generation of rotary filters.

Breathing Filter Cartridge

Special filter cartridge on the basis of a stack of flat sheets. Filtration takes place through the gaps between the sheets from the outside to the inside and particles are held back at the outer surface of the filter stack. The filter stack is held together by the force of an internal spring. To remove the particles the stack can be widened du-ring back flushing by increasing the internal pressure against the spring force and the cartridge is regenerated very effectively.

Bridge Block

Insert into the control disc of rotary disc filters, rotary drum filters and rotary pan filters for adaptation of the control zones i.e. cake formation zone and cake demoisturing zone in the control head.

Bridge Layer

Thin layer of particles generated in the first phase of cake formation producing bridging across the pores of a filter medium.


The pore openings of many filter media used in cake filtration are generally so large that a considerable share of the particles to be separated could principally tra-verse through them. Therefore, depending on the suspension concentration as well as filtration pressure, solid particle bridges have to be built across the pores of the filter medium in the first phase of the cake formation. These bridges by themselves act then as a filter medium and can retain extensively the subsequently entering solids. Bridging thereby directly influences the rate controlling filter cloth resistance.

Brownian Motion

Stochastic movement of extremely small particles in suspensions caused by im-pacts from surrounding liquid molecules. Brownian motion is especially of relevance in the particle size region below 1 µm. Thus for example in gravity sedimentation particles with below approx. 0.5 µm diameter do not settle in water due to the Browni-an motion and thermal convection but instead remain suspended.

Brutsaert Equation

Approximation for describing the relative liquid permeability prel,L of filter cakes, defined according to Wyckoff & Botset, as a function of the relative saturation degree S. The saturation degree herein refers to the region accessible for mechanical demoisturing and therefore excludes the remanent saturation Sr:

The exponent n in this exponential equation depends on the product and has to be determined by measurements. At each saturation degree S the equation descri-bes the relative portion of the total permeability of the fi lter cake that is available du-ring demoisturing for liquid fl ow in competition to the co-currently fl owing gas.

Bubble Point

Term originating from the pore size analysis of filter media. The bubble point characterizes the largest pore in a filter medium. A filter medium completely wetted with a liquid of known surface tension γL, is subjected to a gas pressure on one side which is increased in steps. As soon as the capillary pressure pc of the largest pore is exceeded, the first gas bubble breaks through the medium. From this bubble point pressure the diameter of a circular capillary of the same pressure can be calculated with the Laplace-equation, thus defi ning a pore size:

The measuring technique is simple, quick and physically defi nite and applicable in
the pore size region between 0.1 and 100 μm.

Bubbling Zone

Special facility in drum filters for regenerating and cleaning of the filter medium. A small section in the control head of the filter at the immersion point of the filter cell is designated for air blowing at low pressure from the inside of the filter cell through the filter medium. The air blow creates strong turbulences and loosens up contaminants in the pores and on the surface of the filter medium.

Buchner Funnel

Simple vacuum filter device for laboratory use, often manufactured of china or glass, with filter areas of a few cm2 up to several 100 cm2. The Buchner funnel is generally circular and has a flat filter support that is covered with filter paper as a filter medium. The suspension to be separated is poured from above into the open funnel and filtrates into the direction of gravity. Often a water jet pump below serves as a vacuum generator.

Bucket Centrifuge
Beaker Centrifuge

Discrete, disperse aggregate of particles, touching bodily and forming a porous layer. A bulk can develop due to filtration or sedimentation; one talks about a filter cake or a sediment depending on how they are formed.

Bulk Density

The mean specific density ρm of a bulk, calculated from the density of the solids ρ and the density of the fluid ρL in the voids, according to their respective volume fractions (porosity ε):

If the fluid is gaseous, the gas density can be neglected compared to the solids den-sity and the bulk density results in:

Bulk Material
Bulk Multilayer Filter

Special form of deep bed filter or packed bed filter where the active filter layer has at least two layers of different granularity arranged on top of each other. Generally, the liquid to be purified flows first through a coarser-grained layer and then a more finely-grained layer. To avoid mixing of the different filter layer materials during back rinsing or regenerating, a fine-grained material of high density (e.g. sand) is combined with a coarse-grained material of lower density (e.g. filter coke). An impor-tant application field for these filters is the water treatment.


The static buoyancy of a particle immersed in a fluid acts on the fluid volume that is displaced by the particle in opposition to the acceleration field formed by the pressu-re gradients in the fluid. The dynamic buoyancy occurs if an asymmetric flow deve-lops around a particle due to its shape or if the particle is rotating. It also acts due to the asymmetrical pressure distribution with a force component on its surface perpen-dicular to the flow direction.


A branching off of a partial flow from the main flow; e.g. for collecting samples or for a measurement. Undesired by-passes can appear through leaks in pipelines or after crack formation in filter cakes. In the latter flows of unutilized washing liquid develop during ‚ cake washing or of wasted gas during cake demoisturing. In vacuum filtration a by-pass can be employed for regulating the filtration pressu-re, where a controlled amount of secondary air is permitted into the low-pressure zone.

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