Deep bed filter use sand of a defined grain size as a filter layer. Sand filters are preferably employed in water purification to treat large volume flow rates. Generally, sand filters work discontinuously and have to be regenerated after reaching the dirt capacity of the filter layer. However, there are continuous sand filter variations where the sand layer is continuously recirculated and regenerated (Dyna-Sand Filter).

A simple pre-separation tank for wastewater treatment, in which denser sands are first removed by gravitational sedimentation to reduce the burden on the downstream clarythickener, i. e. to prevent blocking of the rake. The design of these tanks is either circular or rectangular.

Special design of a weave where the weft threads and warp threads cross at right angles in an exactly determined manner. The resulting surface is smooth and without any texture, either on one side (i. e. single satin weave) or on both sides (i. e. double satin weave). The highs and lows of the wefts and warps appear slightly on the surface and are positioned in a diagonal direction without touching each other.

Normalized characteristic of the liquid content in a porous bulk. The saturation degree S is the ratio of liquid volume in the bulk V_L over the entire void volume V_V

If all pores in the bulk were filled with liquid, then S = 1. If an entirely dry bulk is present, then S = 0. The saturation degree of different bulks made of different substances can be compared with each other as long as the porosity of different bulks is similar. This is not possible with the mass related residual moisture.

presents the mean particle size corresponding to the specific surface S_V of the entire particle collective. If one would divide the volume of the investigated substance into spheres of uniform size, so that the sum of their surfaces would be as large as the total surface of the collective, then these spheres would have a Sauter mean diameter d_3,2:

Projecting the results from a laboratory apparatus or from a pilot plant to full-scale size equipment.

Special type of cake removal at drum filters and disc filters. The filter cake is either cut off in the removal section with the scraper working as a knife, or discharged by a compressed air blowback and the scraper serves as a deflector plate. In any case, the scraper must keep a certain clearance to the filter medium to prevent damage.

features a cylindrical sieve section added behind the solids cone to de-water a granular solid by filtration; e. g. fine coal after it has been pre-concentrated out of a diluted suspension by sedimentation. These special designs of a decanter centrifuge are built with drum diameters of approx. 0.2÷1.8 m, operate with C-values of approx. 300÷6,000, and their suspension throughput can range from 1÷80 m.t./hr. A broad particle size spectrum of about 2÷10,000 μm can be separated due to the sedimentation prior to the filtration.

Discharge or loading device for transferring pasty solids in or out of the process room of a separation apparatus. Discharge screws are central discharge organs of decanter centrifuges, where they circulate with differential speed per minute to the main number of revolutions of the centrifuge drum. They are also used in peeler centrifuges, if the solids cannot be demoistured sufficiently and thus remain sticky and a discharge chute would get clogged. Screws have a basic body on which the screw blades are welded on as spirals. The screw blades display a gap or respectively a screw pitch. They can be equipped with a single or double spiral, left or right handed. The screw pitch can be constant or varies over its length. Screws with a diminishing screw pitch are employed at the worm extruder.

can influence the pressure level, leaking unintentionally through a by-pass, or deliberately through a valve. The quality of the vacuum in filters can be adjusted by the latter in vacuum filtration. Secondary air flow can increase at the shrinkage crack formation point in filter cakes, so that the vacuum collapses and the filtration process discontinues.

Flow towards the shrinkage dip tube in hydrocyclones that evolves behind the primary vortex’s flow reversal at the throttled apex nozzle. Fine particles in the secondary vortex, i. e. smaller than the cut size, are carried out with the cyclone overflow.

A completely saturated bulk of particles formed during a settling process in a gravitational or centrifugal field. The bulk must display a greater specific density against the surrounding liquid.

Settling process of particles in a suspension in a gravitational or centrifugal field, if the solids have a higher specific density than the surrounding liquid.

decanter, screen bowl decanter centrifuge, tubular centrifuge, disc stack separator

Clearly defined region below the clear liquid zone, in which the entire particle collective settles at the same velocity. It develops readily in hindered sedimentation, which can occur once a critical suspension concentration is surpassed.

Rate at which an individual particle or a sedimentation front settles in fluid. In the laminar flow region the sedimentation velocity of individual particles can be calculated via Stokes’ law, whereas the Richardson & Zaki-equation applies for a sedimentation front.

De-mixing phenomenon in a suspension. Often particles get classified according to their particle size, like for instance when a filtration process is superimposed by an undisturbed sedimentation caused by gravitation or centrifugation. The potential for segregation is therefore especially high in horizontal filters and filtrating centrifuges. Due to this segregation a bulk forms with an increasingly finer structured layer. The capillary entry pressure in the so-called top clogging layer is considerably higher than in a homogeneously structured cake.

Special design of a disc stack separator with periodically opened sludge discharge nozzles at the periphery of a dual-conical centrifuge drum. After a critical solids pressure is produced by the settled solids a hydraulic piston slide moves through the settled solids and the discharge nozzles open for a fraction of a second. This discharge type is especially of advantage at low solids concentration (e. g. milk skimming, beverage purification), whereas the nozzle-type separator is used for higher solids concentrations.

The pressure difference in drum filters is produced by fact that the filter cells are connected with the filtrate pipes. They lead downwards along the inner wall of the drum and in the circumferential direction against the rotational direction for approx.1÷1.5m with an open end in the drum’s inner space. The filtered liquid causes in such a pipe a hydrostatic head of approx. 0.1÷0.15bar. Self-suctioning drum filters are employed for readily filtering suspensions containing fibrous particles, for which a low pressure difference is sufficient.

Continuously working, cantilever supported filter centrifuges with a conically expanding sieve drum. The driving force for the solids transport is the surface-parallel component of the centrifugal force reduced by the adhesive friction force of the sieve. The control of such machines (e. g. sliding discharge centrifuge) is difficult, due to the fact that the friction depends partially upon the residual moisture of the product, thus changing along the path of the cone. These problems can be alleviated through adequate flow guidance (directed flow screening centrifuge) or through pulsating acceleration (tumbler centrifuge).

Selective or partial permeability in a separating layer which only allows certain components of a mixture to permeate. Membranes for the ultrafiltration pass only molecules of a certain size, which correspond to their MWCO.

Control valve in a cell less drum filter. The separation control head forms the transition from the rotating to the stationary part of this filter type. It is employed for the separation of filtrate and suctioned gas (air).

quantifies the loss of coarse grain in the fines and of fine grain in coarse material, respectively. The separation sharpness, i. e. the steepness of the fractional grade efficiency curve can be defined with characteristic values from the fractional grade efficiency curve as follows:

x_25,t respectively x_75,t represent herein the particle sizes that are separated at 25% and 75% respectively in the coarse material. In analytical separations the separation sharpness is 0.8<κ<0.9, a sharp technical separation shows 0.6<κ<0.8 and in common technical separations it is lower at 0.3<κ<0.6.

Combination arrangement of separation equipment arranged in line in order to amplify or optimize certain separation effects. It can involve similar or different separation machines. A typical series connection is the combination of a thickener followed by a filter. A series connection can also be found at the dilution washing with drum filters, or for the increasing of the cut point in hydrocyclones.

Liquid overflow edge with a jagged profile. In this manner an even liquid draining can be achieved over the entire length of the weir edge, even if it is not perfectly horizontal.

Time period during which an apparatus or an apparatus element is available for normal operation. It can cause process disturbances and influences the economic efficiency.

Centrifuge based on the separation of the particle solids at an impermeable wall by sedimentation. Primarily to be mentioned here are the decanter, disc stack separator, and tubular centrifuge. The multiple of the earth‘s acceleration, the C-value, attained in these centrifuges ranges usually from a few 1,000 up to several 10,000 in extreme cases, due to the low sedimentation velocity of the extremely small particles. Settling centrifuges are working discontinuously as well as continuously.

Circular or square container in which separation by sedimentation of solid particles and liquid occurs under the influence of gravity. The specific density of the dispersed solids necessarily has to be greater than that of the continuous liquid phase. The settled solids are removed from the tank bottom in the form of a thickened sludge. Ideally, the particle-free liquid is removed at the top of the tank by means of an overflow.

Rate of settling of solid particles in a liquid under the influence of a gravitational or a centrifugal force. The description of the settling velocity of single particles in the laminar flow region is based on Stokes’ law while in the sedimentation in concentrated suspensions (swarm sedimentation), the settling velocity of the mutually hindering particles can be described with the Richardson & Zaki equation.

Non Newtonian flow. The viscosity increases with increasing shear stress.

Flow behavior of Non-Newtonian fluids. The viscosity decreases with an increased shear stress.

A deep bed filter employed for the purification of liquids, e. g. beverages, with the outer appearance of a filter press. Instead of forming a cake in the filter chambers, the diluted suspension permeates under pressure the filter sheets, on which contaminants separate. These filter sheets have to be replaced once their absorption capability is exhausted, as generally they can not be regenerated.

Cracking of filter cakes occurs during demoisturing of fine grained and compressible filter cakes when the pore liquid is getting displaced by gas. Capillary tensile stress can induce shrinkage of the filter cake by relocation of particles which effect the formation of cracks at locations where interparticle stresses can not be compensated by the particle matrix of the cake. Cracks in the filter cake make cake washing and cake demoisturing inefficient since wash liquor and demoisturing air passes mainly through the cracks. This leads to reduced throughput, higher residual moisture of the filter cake and drastic increase of operation cost which in a worst case leads to a collapse oft the filtration process.

Crossflow-filter press that look like a filter press; the plates, however, are divided in so-called concentrate and permeate plates with a porous separation membrane in between. The flow channel spirals from the outer edge of the plates into the center. Along this path an initially diluted suspension is thickened.

Shut down process of a separation machine from an operational state to stand still.

Operational phase of a separation machine. One differentiates between a normal turning off and an emergency off, where the equipment has to be turned off immediately due to a disturbance.

Surface filtration for the purification of liquids contaminated with particle which does not necessarily have to produce a real filter cake; rather flow is often interrupted at an early stage for a backflush, after attaining a pre-set pressure loss. A special variant of the sieve filtration is the dynamic sieve filtration with the BoCross MicroScreen Filter.

Innovation of the drum filter cell design by the BOKELA company for small to medium sized drum filters. Individual, exchangeable filter cells on a BOKELA vacuum BoVac filter or BoHiBar Drum Filter which are fixed on the drum collar with butterfly screws allowing a very fast and easy exchange. The filter cloth is made as filter bag comparable to disc filters which enables to hold a complete set of filter cells pre-equipped with filter bags on stock. Thus, re-clothing can be performed in a very short time of some 30 minutes.

Candle shaped sieve filter for the cleaning of liquids with low particle contamination. They are typical by-pass filters, because the changing of the filter elements is not possible without flow interruption. A possible solution to this shortcoming are double filters or automatic filters.

Sedimentation behavior of particles in a suspension with very low solids concentration. The particles settle independently without influencing each other. According to Stokes’ law, their settling velocity depends on the density difference between solids and liquids, the liquid viscosity, a characteristic particle diameter, and gravitational or centrifugal acceleration, respectively. Single particle sedimentation is especially desired in particle size analysis, for which actual production samples often have to be diluted.

Determination of the separation ability of a filter medium under realistic conditions where a filter sample passes once. The test filter is fed with a nearly constant particle concentration until a specified maximum pressure difference is reached. A particle-measuring device monitors both particle number and size in front and behind the filter.

Special design of a filter trough for disc filters with large disc diameters. Contrary to the common trough design each filter disc runs in a trough which is a completely separated unit and the slurry is homogenised by the rotating filter discs so that no agitator is required. The single trough design is applicable for the filtration of slurries with high to medium solids concentration like Al-hydrate slurries with little danger of particle classification.

Special design of a peeler centrifuge in which the centrate is not freely ejected into the centrifuge housing, but instead is collected in a rotary siphon cup and utilized to produce a vacuum behind the filter medium. By sucking the filtrate out of the siphon cup a hight difference between filter medium and liquid surface is produced and thus a hydrostatic suction pressure. At least as important as the additional filtration pressure is the regeneration of filter medium and heel by refilling some filtrate back to the siphon cup for a backflush after each batch.

Special type of continuously working screen centrifuges. Sliding discharge centrifuges belong to the so-called self-transporting centrifuges. In these the solids are moved to the discharge solely by the surface parallel component of the centrifugal force surpassing the sum of adhesive and sliding friction force along the conically widening screen basket. Sliding discharge centrifuges have a problematic operation characteristic as the sliding behavior of solids depends among other factors on the properties of solids and liquid, the moisture degree of the bulk, and the roughness of the screen wall. Sliding discharge centrifuges are generally employed in the separation of highly viscous materials, such as molasses in the sugar industry. Other designs try to control the transport process by built-in devices such as at the directed flow screen centrifuge, or by periodically changing the centrifugal forces, such as in the tumbler centrifuge. The sliding process can be influenced especially well if the surface parallel force alone is not big enough for the transport and the friction is surpassed by an axial oscillation, adjustable in amplitude and frequency, of the screen basket like in the vibratory centrifuge.

Sand filter from the field of water treatment. Slow bulk layer filters are based on the principle of deep bed filtration and utilize filter layers with more than 1 m thickness. The filter velocity is about 0.05 up to 0.1 m/hr and filter areas reach up to 10,000 m². Often a biological reduction process for the organic substance to be separated is combined with the mechanical separation because of the low filtration process.

Highly concentrated suspension whose particles are so close that mechanical forces can be transferred between each other, but which are on the other hand still free-flowing.

Auxiliary device for improving the filter cake discharge in disc filters and drum filters. It is essential for a good cake discharge that pressure builds up in the filter cell behind the filter medium as fast as possible. The inlet cross section at the control head opens only very slowly for compressed air especially at low numbers of revolutions. Therefore, a quick-acting valve is located in the compressed air feed pipe that is only opened if the full filtrate pipe cross section is exposed.

Measure for the amount of solids in a solids-liquid mixture. For a mass-related representation the solids mass ms is referred to the total mass of the solid-liquidsystem m_s + m_L and quoted in [mass %]. Depending on whether the solids or the liquid present the continuous phase, this quantity is also described as dry substance content DS or solids mass concentration c_m:

Division by the respective densities leads to the term of solids volume concentration c_v:

A further relation for the description of the solids content in suspension presents the suspension density.

Every solid-liquid separation apparatus has discharges for respectively the cleared liquid phase and the more or less moist solid phase. In the case of sludge-like solids products, the solids discharge could be a valve or a pump, a discharge screw for very moist solids, and, in the case of powdery material, a conveyor chute.

Term for the description of the settling procedures in gravity thickeners when swarm sedimentation occurs. The solids flow S is defined as the product out of the settling velocity w and solids volume concentration c_v and possesses the dimension [m³/m2h]. The total solids flow S_tot in a gravity thickener consists of the solids flow due to the swarm sedimentation S_sed (settling velocity w_sed, concentration c_v) and the solids flow due to sludge removal (outlet) from the underflow in the thickener S_u:

The sludge outlet velocity V_u is defined as the quotient of the sludge volume stream in the underflow V_u and the thickener cross sectional area A. As concentration and settling velocity both change in opposite directions over the height of a thickener, a critical minimum of the solids flow develops at a certain point. This critical solids flow is the basis for the thickener design. It has to be determined by experiment.

Amount of solids, passing through the pores of the filter cloth at the very first moment of forming a filter cake before a bridge layer is formed on it which safely retain the following solids. Material sizes, such as pore width of the filter cloth in relation to the size of the particles to be separated, but also the adjustable parameters, such as the suspension concentration or the pressure difference significantly influence the amount of the solids lost.

Amount of particle solids in a suspension that can be processed per time unit by a separation apparatus. The throughput is additionally referred to the filter area as the specific solids throughput. For example, the specific throughput of a rotary filter is quoted in [kg/m²h].

Homogenous mixture of different substances in which the mutual diffusion and division reaches down to the level of molecules, atoms or ions ( true solutions).

Segregation of a particle collective into fractions of different particle types.

Coarse-sized, meshed insert between two microporous membranes employed in the ultrafiltration as distance spacers to provide for the unhindered flow of respectively concentrate and permeate. Spacers are found, for instance, in coil and spiral modules.

Surface area of a particle typically relating to either volume or mass of the solids. The volume related specific surface Sv is connected with the Sauter mean diameter of a particle collective d_3,2 as follows:

Filter element design used in the crossflow filtration, especially in the ultrafiltration. Spiral modules are rolled up alternating layers of microporous membranes and spacers. This creates an extremely large membrane area per module volume. The feed liquid flows coaxially through the module, starting from the front end of the roll. The permeate, after evolving from the membranes, flows on a spiral-shaped path to the center and is extracted through a core pipe.

Suspension whose particles are prevented from agglomerating by electrostatic repelling forces resulting from surface charges.

Fibers (e. g. natural fibers such as cotton) made out of short fiber pieces by twisting. In contrast to monofilaments, which are endless and smooth threads, staple fibers possess a greater roughness due to protruding fiber ends. This can increase the separation efficiency of a filter cloth woven out of it. However, it possibly complicates the cake discharge due to larger adhesion forces between cake and cloth.

Device for the discharge of bulk material from a pressure vessel with a pressure p₁. A rotor, divided into individual pockets, rotates sealed inside a horizontal cylinder with openings on the upper and bottom side. The pockets are filled from above with product and emptied downwards into the atmosphere or into a process room with a pressure p₂ < p₁. The principle of the star feeder can only be employed for mildly abrasive materials, because otherwise the sealing will deteriorate due to the abrasion.

The start-up of the operation of separation machines from a standstill to stable and stationary operational conditions at the operation point. Very often the start-up is understood to be the initial taking into operation of a new plant.

Specialy designed steam hood by the BOKELA company for performing the steam pressure filtration process on BoHiBar Filters

Supplementary attachment on continuous vacuum and pressure filters for improving the cake demoisturing through steaming.

Innovative process by the BOKELA company in form of the BoHiBar-Filters for cake demoisturing and cake washing. An externally supplied, saturated or overheated steam is used as the gaseous displacement medium instead of air. The steam condenses on the cold filter cake surface and forms a sharply defined and evenly developed condensate front which moves through the cake. Thus, the cake is heated up to the condensate temperature and a fingering with a premature gas breakthrough is prevented which can occur when air is used as gaseous displacement medium. The particular advantage of this process is that most of the filtrate is mechanically removed from the cake, followed immediately by thermal convection drying, which results in extremely low moisture levels. In addition to demoisturing a washing of the cake is also performed, due to condensation and thus a pure liquid input into the bulk. So far, the process has been implemeted on hyperbaric filters with disc filters and drum filters.

Measure supporting the cake demoisturing in the vacuum filtration mainly employed for continuous filters, such as belt filters, drum filters, or pan filters. Saturated and overheated steam is applied via a steam hood on the previously demoistured filter cake surface. While the steam is sucked through the cake, it transfers its heat into the cake and condenses there. The heating leads to a reduction of the filtrate viscosity and consequently to an acceleration of the demoisturing process. Decreased product moistures are reached within the limited demoisturing time available in continuous filters. In addition to this, the surface tension of the filtrate is decreased and the capillary pressure is slightly lowered. After discharge the heated cake can additionally lose further liquid through post-evaporation.

Separation of all viable microorganisms in the size range of 0.1÷1 μm by filtration. A separation below 0.1 μm, e. g. viruses, dissolved toxins, or pyrogenes, is performed by adequate filter beds in an adsorptive manner. The suitability of a filter medium as a sterile filter is examined by the bacteria retaining test.

Sterilization of apparatuses without dismanteling. During steam sterilization the inner surface of an appartus is exposed to overheated steam for a defined time. Before the apparatus can be sterilized it is cleaned by cleaning in place.

Mono- to bimolecular layer of adsorbed counter ions, solidly bonded on a charged particle surface.

pressure nutsche filter equipped with a stirrer. Stirring allows the execution of numerous operations which can range from homogenisation of the suspension to re-suspending up to solids discharge. The stirred pressure nutsche filter in its most variable form develops into a filter reactor, in which further operations can be performed, such as chemical reaction, crystallization etc.

Special form of gravity filtration. The demoisturing of liquid-saturated, granular bulk material in a gravitational field is driven by the hydrostatic head of the liquid itself. This pressure has to be larger than the capillary pressure acting in the bulk. During demoisturing the liquid column gets smaller and the hydrostatic pressure decreases, respectively, until at equilibrium of the forces the capillary rise of a bulk is reached.

Equation for the description of the settling velocity v of a spherical individual particle with a diameter x and a density ρ_s in a liquid with a density ρ and the dynamic viscosity η_L under the earth’s gravitational acceleration g, if a laminar flow is present:

Particles must be able to settle unhindered by other particles present. If Stokes’ law is to be applied onto centrifugation the gravitational acceleration is to be multiplied with the C-value. It must be verified that laminar flow is still present.

A continuously rotating, drum-shaped sieve, which is loaded on the inside with for example a strongly flocculated suspension for pre-thickening under the influence of gravity.

Horizontal, pre-demoisturing zone of double belt presses in which a crossly flocculated suspension is thickened by gravitational demoisturing, so that it can be drawn subsequently into the wedge zone between the two filter belts.

Filter element for discontinuous deep bed filtration made from twisted yarn (stack fibers), that is coiled to a thick layer around a perforated core from where the filtrate flows. The coiled yarn forms the actual deep-bed filter in whose pores the contaminant particles deposit themselves.

General term for the entire class of filters that use a gas difference pressure as the driving potential, generated by the application of a vacuum behind the filter medium. Suction filters are limited in respect to the maximal pressure difference by the vapor pressure of the liquid.

Filtration where the particles to be separated are retained preferably on the surface of the filter medium in contrast to the deep bed filtration. Surface filtration can be realized as cake filtration, sieve filtration, and crossflow filtration.

Electric charge of suspended solids particles. Particles in suspensions are often negatively charged. The surface potential is partially compensated by counter ions contained in the liquid, and declines exponentially with an increase in distance from the particle’s surface. The surface potential can cause an electrostatic repulsion of particles. If one wants to agglomerate them by utilizing the Van-der-Waals forces the surface potential has to be shielded in order to bring the particles to a sufficient proximity.

Mixture of a liquid and a particle-shaped solid. The liquid is in this case the continuous phase, while the solid forms a disperse phase or discontinuous phase. At higher solids concentrations the suspension transforms into sludge, if the particles approach each other so closely that they are capable of exerting mechanical forces on each other.

The suspension density ρ_sL (s = solid, L = liquid) is defined as ratio of mass m and volume V:

c_m = solids-mass concentration , and c_V = solids-volume concentration

Settling behavior of a particle collective characterized by the fact that size, density and shape of the particles all loose their influence on the sedimentation velocity. A sharp sedimentation front appears with a clear liquid zone above. The main influencing parameter becomes the suspension concentration. Over the broad range of concentrations where swarm settling is observed some classification effects – especially in centrifugal fields – can be observed nevertheless. Only at high concentrations a totally homogeneous sedimentation of all particles takes place; this phenomenon is called zone sedimentation.

Type of leaf filter, patented in 1905 by E. Sweetland, with hanging filter leafs, arranged in a tank. Originally, the lower part of the vessel was unhinged for cleaning purposes; in present day designs the upper part of the vessel can be opened.

Cup-shaped insert in discontinuously operating beaker centrifuges with a vertically rotating axis. At rest, when the beakers are charged, they hang vertically at the end of an arm, which is attached with pivoting joints to the rotational axis. During rotation they swivel sideways into a horizontal plane. In this manner, several beakers with suspension can be simultaneously processed.

Type of membrane with uniform pore sizes across its thickness. Symmetrical membranes are mostly applied for microfiltration whereas asymmetrical membranes with an extremely fine-pored surface are applied for ultrafiltration.