Mass unit, named after the physicist Dalton. It is defined as mass of a hypothetical atom with the unified atomic weight of 1. As mass unit the decimal multiples are applicable, for example 1kiloDalton (kDa) instead of 1000 Dalton. At the ultrafiltration the cut-off is referred to the molecular mass instead to the particle size. This „molecular weight cut-off“ (MWCO) is quoted in the unit „Dalton“.

Vibration Damper

One “Darcy” is the unit for measuring the permeability of a cube with side length of 1 cm of a porous body under a pressure differential of 1 bar and a liquid viscosity of 1 mPas. Dimension of the unit „Darcy“ is [cm2]

Darcy’s Law

Fundamental equation for describing the laminar permeation of a porous layer with a Newtonian, incompressible fluid. The flow velocity v (empty pipe velocity) caused by the driving pressure difference Δp, through a bulk material with the thickness hc and the specific cake resistance rc with a liquid of the ‚ viscosity ηL amounts to:

Rm denotes therewith the filter medium resistance. If one applies Darcy’s law to a compressible fluid, such as air or to a two-phase flow, the equation has to be adjusted according to the respective circumstances.


Removal of the fine grain fraction of a particle size distribution in a suspension, for example with a hydrocyclone. Purpose of the defining is either an improvement of the product’s filtrability as it will be coarser then, or the liberation of the product from contaminants, which dominantly are in the fine grain region.

Dead End Filtration

Discontinuous process variation of microfiltration or ultrafiltration. With the dead end filtration the filtration process through a microporous membrane is continued until a given upper value of the pressure loss is reached, due to depositing of retained suspension contents. The filtration process has to be interrupted and the membrane has to be regenerated, e.g. through a periodic flow reversal. At very large volume flows, as in water treatment plants, the dead end filtration represents, in contrast to the crossflow filtration with its necessarily large pump performances, the more feasible variant in terms of energy consumption.

Dead Flux

The flow of fine particles, smaller than the cut size, discharged with the coarse underflow in a hydrocyclone.

Debye Length

Term of the DLVO theory for the description of the characteristic penetration depth ID of an electrical field, caused by surface charge on a suspended particle, into the surrounding electrolyte:

ε = dielectric constant, k = Boltzmann-constant, T = absolute temperature, e = ele-mentary electronic charge, n = density of the charge carrier, z = valence of the char-ge carriers


Continuously working solid bowl centrifuge in which a suspension is separated through sedimentation of the solids in the centrifugal field. The settled solids are removed out of the process room, which is conical tapered, by means of a conveyor screw at one end of the cylindrical drum. Hereby it turns with a differential number of revolutions compared with the main number of revolutions of the solid bowl drum. The exceeding clear liquid drains off over a usually adjustable weir at the opposite end of the solid bowl drum. Decanter centrifuges are offered in extremely different varieties, which conform according to the respective separation tasks. Thus, decanter centrifuges exist for the pre-thickening of sludges, for the maximum demoisturing of settled sludge, for the separation of granular products, for classifying and for sorting. Furthermore decanters in a special design are also suited for the separation of three-phase mixtures, consisting of two liquids, incapable of mixing with each other (i.e. water and oil), and solid particles. Decanters are build with drum diameters of up to 2 m and are operated with a number of revolutions of up to 10,000 rpm. Large numbers of revolution per minute are applied with small rotor diameters for reasons of material strength. For the improvement of the sedimentation of difficult to separate substances the particles, to be separated in decanters, are often agglomerated as a preparative measure by means of polymeric flocculants. Decanters are utilized in different industrial sectors and thus separate different products, such as granular PVC and compressible sewage sludge.

Decanter Centrifuge
Deep Bed Filter

Filter type used for deep bed filtration as for instance sand filters. Typically, the filter medium of a deep bed filter is a relatively thick bed of granules or fibres which is regenerated by a backflush of filtrate after a preset period of time or when a preset pressure drop through the filter cake is reached.

Deep Bed Filtration

Filtration process at which the particles to be separated accumulate inside a coarse-pored layer. Deep bed filtration should only be realized in the region of extremely low suspension concentrations to prevent an early blockage of the filter surface. They typically serve in the purification of minutely polluted liquids. Water and beverage filtration present large fields of application.

Deep Cone Thickener

Continuously working cylindrical or conical gravitational thickener, characterized by construction heights of up to 30 m, which are capable of producing high compressi-on layers or compression pressures, and thus high underflow concentrations for thickened sludges.


Pre-separation of coarse particles from a suspension, which could upset the following separation process, done for example with a hydrocyclone or a sieve.

Delayed Cake Filtration
Crossflow Filtration

Non-homogeneous distribution of solids in a suspension or in a bulk. Demixings can show up in terms of particle size, particle shape and composition (density). The mass forces, such as gravity, are responsible for this, leading to different sedimentation velocities of the particles. Countermeasures against unwanted demixing can be a stirring apparatus or an increase of the suspension concentration. High suspension concentrations lead, however, to swarm sedimentation.


Common expression in the solid-liquid separation for the removal of liquids out of a porous solids system capable of intra-particle force transmission due to close proximity. By comparison the removal of a liquid out of a suspension, where the particles can still be moved against each other, is called thickening. Solids systems, as presented by filter cakes or sediments, can be demoistured either by gas displacement of the pore liquid or by the reduction of the void space through compression. The mechanical demoisturing is never complete as it ends at a mechanical demoisturing boundary.

Demoisturing Angle

Expression from the field of rotary filters and here especially the drum- disc- and pan filters. The demoisturing angle α2 describes the sector in which the filter cake that is formed in the filter trough, emerges out of the suspension and where it is exposed to a gas difference pressure. In special cases the filter cake can also be demoistured by a press belt or belt rollers. The demoisturing angle α2 is connected over the filter speed (number of revolutions n) with the demoisturing time t2 as follows:

Demoisturing Equilibrium

State during the mechanical demoisturing of a porous solids systems, establishing after the completion of the demoisturing kinetics. At the equilibrium, the liquid retaining forces, such as the capillary forces, compensate the liquid removing forces, for example the centrifugal forces. Any further mechanical demoisturing can be achieved only by increasing the driving potential. All this is limited by the mechanical demoisturing boundary.

Demoisturing Kinetics

Time dependent, degressive course of liquid removal from a porous solids system effected by a driving potential such as centrifugal forces, gas over-pressure, vacuum or by mechanical means via a membrane or piston. A large quantity of liquid is removed at the onset, then the liquid outflow decreases due to the in-creasing influence of the liquid retaining forces, and finally it ends at the demoisturing equilibrium.

Depth Filtration
Deep Bed Filtration

Denotes the demoisturing of an incompressible filter cake through liquid dis-placement by gas under the influence of a gas pressure difference or a hydrostatic pressure in the earth or centrifugal field.

Design Experiment

Testing on the laboratory or pilot scale for equipment scale-up. The design experi-ment has to simulate the separation process as planned in the full-size equipment, carried out with a representative product sample as realistic as possible.


Removal of the finest particle fraction from a slurry (de-fining) by a classification process i.e. hydrocyclone


Detachment of substances (adsorbates) attached on a surface (adsorbent), by su-perceding the active forces (opposite of adsorption).


A suspension in which particles exist individually and separate from each other is called a stable suspension. Destabilization is understood to be a change of the electrical charges in a suspension in such a way, that the suspended particles can be agglomerated. For this, the electrostatic repelling of the particles has to be reduced to an extent that the Van-der-Waals attraction, which is always present as a material-constant value, becomes dominant. This can be done for example by increasing of the ion concentration or by changing the pH-value. In water treat-ment, FeCl3 is often added for this purpose.


Membrane separation process for removing of low molecular side products (e.g. salts) out of a solution of highly molecular substances (e.g. albumen, starch).


Porous separator wall used for example in electrolysis between the cathode and anode cells. In the solid-liquid separation technology a diaphragm is understood to be a membrane that is not only capable of retaining solid suspension components. For instance a hydrophilic and wetted membrane can prevent the penetration of gas up to its capillary entry pressure.

Diaphragm Press
Membrane Filter Press
Diatomaceous Earth

Natural product employed as a filtering aid consisting of finely granular skeleton fractions of diatoms. Diatomaceous earth is characterized by a very large specific surface, therefore it is capable of bonding turbid substances. It is extracted by open-pit mining in up to several 100 m thick layers, processed and offered in fractions of different granularity respective to the application. Due to its inert behavior diatomaceous earth is very often employed for the filtration of beverages such as wine, beer or fruit juices.


Special processing technique in ultrafiltration where for example salts are separated from higher molecular solutions by adding new solvents commensurate with the flow rate of permeate, thus effecting a washing process.

Differential Speed

Difference in revolutions generated with gears and drives by two co-axially linked running rotors. In several centrifuge types the solids are discharged from the process chamber with the aid of a conveyer screw that is rotating with a differential speed in the direction of the centrifuge drum. Examples are the decanter and the worm screen centrifuge.

Diffuse Double Layer
Electic Double Layer

The spontaneously occurring mixing of substances, especially gases and liquids, that are directly in contact, and the equalization of their concentration differences in solutions. This is caused by the thermo-kinetic movement, whereby molecularly one substance penetrates into the other, i.e. they diffuse.


Denotes the behaviour of a Non-Newtonian Fluid to show increasing viscosity with increasing shear forces (shear thickening).

Dilution Washing

Process for the washing of a filter cake by re-suspending in wash liquid and subsequent filtration. Dilution washing can be realized for example with inseries operating drum filters. By re-suspending a filter cake with a dry mass mS, with a liquid load w1 (w1=mL,1/mS) and a contaminant content X1 (based on dry mass: mC,1/mS) in an amount mW of wash liquid and subsequent filtration to a new li-quid load of w2 (w2=mL,2/mS), the amount X2 of contaminant (mC,2/mS) in the new filter cake is:


Dipole Force
Hydrogen Bridge
Directed Flow Screen Centrifuge

Continuous screen centrifuge with a conically widening rotor where the de-scending force component of the centrifugal forces acting on the particles convey them into the direction of the rotor outlet. Ring shaped assembled elements divide the glide path into guiding channels for better control as well as for lengthening of the path. A jagged edge on these channels continuously mixes the particles. Direc-ted flow screen centrifuges are applied for suspensions of higher concentrations and coarser granularity (x < 500 µm).

Disc Filter

Continuously working rotary vacuum filter or pressure filter (see Hi-Bar Filtration and hyperbaric filter) for relatively easy to filter suspensions and larger amounts of suspension flows. Up to twelve radially sectioned filter discs are arranged on a horizontal filter shaft and filter on both sides. Each of the sectional filter cells (filter segments) is covered with a tightly stretched filter bag and connected at the cell foot with a filtrate pipe to the control head, which controls the applied vacuum. The rotating discs dip into a filter trough filled with suspension, where the filter cake is formed. The filter trough can be designed as common trough with agitator whereas for disc filters with large disc diameter the slurry is homogenised by the rotating filter discs and the trough is designed as single trough or joint-single trough without agitator. As soon as the cake emerges from the suspension, it is demoistured and dried with air. Finally, the cake is blown off by compressed air blowback and discharged over a deflection plate. Alternatively, a basic scraper discharge is used. Disc filters with standard design rotate at 0.5 ÷ 2.5 rpm whereas modern high performance disc filters of the BoVac Disc Filter type can rotate at much higher filter speed of up to 6 rpm with significantly increased throughput performance and complete cake discharge even with very thin filter cakes of 4–5 mm. Disc filters can have up to several 100 m2 filter area in a single machine and have a very large filter area to floor area ratio. High performance disc filters with large disc diameter like the BoVac Disc Filter have a filter area to floor area ratio of up to 7.5 m²/m².

Disc Package

Assembly of separating elements in disc stack separators. The circular-conical metal-sheet discs are stacked with bumper rods for a gap widths of approx. 1 mm. In this manner the effective sedimentation area in the centrifuge can be increased significantly. For separating immiscible fluids the disc package is equipped additionally with vertical bore holes, which are called rising channels.

Disc Stack Separator

Continuously working sedimentation centrifuge where the available separation area is drastically increased through the installation of a disc package into the centrifuge drum. In combination with extremely high C-values up to 15,000, some disc stack separators can realize an equivalent clarifying area of several 100,000 m . The discharge of the settled sludge through nozzles at the outer circumference of the dual-conical designed drum is either continuously (nozzle separator) or periodically (self-cleaning separator). Disc stack separators can be employed for solid-liquid, liquid-liquid, or solid-liquid-liquid separation, as well as for extraction.

Discharge Chute

Simple duct or pipe, through which the separated solid leaves the separation equip-ment under the influence of gravity. For solid products that tend to adherence in the course of time a discharge screw is recommended because they can clog over time.

Discharge Screw

A transporting device which removes by force a separated solid out of a separation apparatus. Discharge screws are employed especially when the solids are sticky, due to insufficient demoisturing, and a discharge chute would foul up.

Discharge Scroll
Discharge Screw
Discontinuous Separation Apparatus

Employed for batchwise separation, meaning a complete processing of indivi-dual volumes of suspension.


Distribution of the dispersed phase in the continuous phase. The mixing of solids particles in a liquid for the production of a suspension is an example for this.

Disperse Phase

A phase distributed as individual elements in another, contiguous surrounding phase (continuous phase). Examples for disperse phases are solids particles in suspensions, gas bubbles in liquids, or liquid droplets in gas (mist).


A system, consisting of two or more phases, in which one phase (disperse phase) is evenly distributed in the dispersion medium (continuous phase).


Term in the particle measuring technology, characterizing the physical property that is utilized for measuring. Thus, the dispersivity characteristic can be a settling rate, a scattered light distribution, an attenuation, a voltage impulse, or similar phenomena. Through a physical relationship an equivalent diameter of particle can be determined out of such a dispersivity characteristic. An example would be the calculation of a particle diameter based on the settling velocity according to Stokes’ law.

Displacement Washing

Denotes the cleaning of filter cakes by feeding a wash liquid. Under the influence of the driving potential, which can be either vacuum, gas overpressure, hydraulic pressure, or centrifugal pressure, the wash liquid is passed through the filter cake. The resulting removal of the mother liquor occurs in two different mechanisms. For one, the main amount of the pore liquid is displaced in plug flow. The still remaining residuals of the substances to be removed are added to the flowing wash liquid by diffusion, which is considerably more time intensive. Quality criteria for displacement washing are a high wash degree and a low consumption of wash liquid, which is expressed by the wash ratio. An alternative process to the displacement washing is dilution washing, where the filter cake is re-suspended in the wash liquid and subsequently filtered again.


Named after Derjaguin and Landau (Russia, 1941) as well as Verwey and Overbeek (Holland, 1948) and used for describing the stability of suspensions from a balance of the attracting Van-der-Waals forces and repelling electrostatic forces. This theory is important for the solid-liquid separation technology as it allows to describe the state of suspension and agglomeration as well as flocculation of particles.

Double Belt Press

Continuously working press filter, in which a filter cake is pressed out between two filter belts. A pressing power in the magnitude of 2 ÷ 4 bar can be applied via press and deflection rollers, whereby the press belts are guided. Double belt presses can reach working lengths of several meters and are employed for the separation of difficult to filter and extremely compressible sludge. The feed slurries that are normally strongly flocculated have to be pre-demoistured by gravitational filtration in the straining zone so that they can be drawn between the belts. In the wastewater sector they compete mainly with the decanter and the filter press for sewage sludge separation.

Double Filter

Discontinuous, candle shaped sieve filter for the purification of liquids with minor amounts of particulate contamination. Double filters are mainly employed in the main flow with up to 100 % filter area on stand-by. This can be by a put on flow by a three-way valving. An alternative to double filters present the automatic filters.

Double-Flap Sluice Gates

Attachment employed in the solid-liquid separation for the discharge of demoistured solids from a continuous pressure filter system like the BOKELA Hi-Bar-Filter. Generally, the cylinder of double-flap sluice gates is filled first under the acting internal pressure of the filter apparatus with a closed outer gate; then the cylinder is isolated by the inner gate from the pressure chamber of the filter. It is emptied after the pressure has been released by opening the outer gate to atmospheric pressure. A special locking technique has to prevent that both gates open simultaneously with the filter vessel still under pressure.

Double Layer

Electric double layer of counter ions, surrounding an electrically charged surface of suspended solid particles.

Double Weave

Weaves with different separation properties, strongly connected together. Double weaves display usually a fine-pored upper side facing the suspension to ensure the retention of solids, and coarse underside lending stability to the media.

Drag Effect

Observed especially with decanter centrifuges: particles are discharged with the clear liquid although they should have settled according to Stokes’ law. Instead, the drag by the liquid overflowing the sediment causes a stirring up of already settled particles and carries them away.

Driving Potential

Force effect utilized for the separation of particles from of liquids, coming from vacuum, gas overpressure, or hydrostatic or centrifugal pressure, or hydraulic, mechanical capillary pressure.


Rotating, cylindrical element of a separation apparatus that is perforated and generally made from metals. In its interior or on its outer mantel area the solids are separated from the suspension. However, the drum used in sedimentation processes is impermeable and always fed in the interior.

Drum Filter

Most widely used type of the continuously working rotary vacuum filters with many variants. Drum filters allow excellent cake washing and are built with different variants for removing the filter cake from the filter cloth what makes drum filters an appropriate filtration technology for very different products. The most common type is characterized by a hollow, horizontal filter drum, often with filter cells arranged coaxial on the outer face of the drum that is divided by longitudinal strips. The filter cells are fitted with cell inserts (drainage grids) to enhance filtrate drainage and to support the filter cloth. The filter cells are connected through filtrate pipes with the control head. The filter cloth is held in place by caulking, bolt down clamping or by a taut wire wound around the drum. An easier method of cloth fixing is achieved with FrameTrak which uses individual filter cloth panels. A different, innovative design by the BOKELA company for small to medium filter sizes uses up to 48 exchangeable, individual filter cells (single cell) with the filter cloth made as filter bag comparable to disc filters allowing very fast and easy re-clothing. The drum, rotating with approx. 0.2 ÷ 3 rpm (typically up to 2 rpm), is immersed up to half of its diameter in a filter trough, where the feed suspension is contained and homogenized by an agitator. The filter cake forms on the filter cell while it moves through the suspension. When the cell emerges with the filter cake out of the suspension the cake is washed (optionally) and demoistured and dried with air. Then the cake is removed from the drum with the help of various methods for cake discharge. Depending on the filter cake characteristics (grainy, sticky or fibrous) different discharge methods are applied: compressed air blowback, roller discharge, belt discharge, knife discharge at precoat filtration or string discharge. Drum filters can be equipped with various supplementary devices as for instance wash hoods or press belts and press rollers for mechanical compression of the cake and they can be completely encapsulated to prevent the escape of heat and/or emissions. Other variants of drum filters are for instance the interior drum filter or the cell less drum filter. Drum filters are built with sizes up to 125 m2 filter area and even more but generally filter sizes range from 1 to 80 m². Drum filters are also available as continuous rotary pressure filters (see also BoHiBar Filter, BoHiBar Drum and hyperbaric filter).

Dry Substance

Mass related definition of the dry substance content in a bulk following the separation process. The determination of the dry substance DS is simple and is performed by weighing, respectively, the moist and the dry cakes. The mass of the solids mS is then related to the total mass of the moist bulk mtot, made up of the solids mass mS and the liquids mass mL. The dry substance is quoted in weight percentage. Substances of different densities cannot be compared on the basis of their dry substance values.

Dynamic Buoyancy
Dynamic Crossflow Filter

Crossflow filter with a shear flow, adjustable independently from the pump-pressu-re. The flow is generated by a rotor-stator system, where the suspension is pro-cessed in its annulus. Rotor as well as stator or both can be designed for filtering. There are axial and coaxial dynamic crossflow filters. The BoCross Filter of the BOKELA company represents a special form of the radial dynamic crossflow filters.

Dynamic Filtration

Alternate expression for the ‚ crossflow filtration, whereby the feed suspension flows tangentially to the filter medium. A special design in dynamic filtration is the BoCross Filter by the BOKELA company.

Dynamic Sieve Filtration

Innovative process of sieve filtration by the BOKELA company based on the principle of dynamic filtration with the BoCross Filter. At dynamic sieve filtration the BoCross Filter is used for a continuous separation of coarse particles from suspensions (classifying). The fine particle fraction and the mother liquor pass the sieve medium while the substantially smaller coarse fraction is held back by the sieve and discharged as highly concentrated residual suspension.

Dynamic Viscosity

Refers to the viscousness or internal friction of a fluid. It characterizes a material property, according to which tangential forces appear, acting against a parallel dis-placement of liquid or gas layers relative to each other. The dynamic viscosity is measured in [N sec /m2] or [Pa sec]. It is the force required to flow for a thin layer of gas or liquid of unit size area with a velocity that is 1m/sec higher than that of a layer 1m away.

Dyno Filter

Former brand name for the BoCross Filter technology BoCross Dynamic and BoCross Microscreen

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