clarification filters for liquids – principles, classification, selection · 2016-02-24 ·...

1. Introduction

Lowly concentrated particles notexceeding 10 μm in size take center stagein the following considerations. These par-ticles should be completely removed or,otherwise, their concentration should belimited. The selection of adequate clarify-ing filters is a hard job for everyone who isnot a specialist in solid-liquid separation -and sometimes even for the experts. Cost-related decisions based on maximumequipment efficiency might help at a laterphase of the planning process when only afew candidates are left. For early planningphases, however, tools are needed thatcompare the targets and feedstock specifi-cations of the individual case with the typ-ical performance and capabilities of theavailable filters thus contributing to a pre-selection of the most appropriate filtersand technologies.

Such a comparison is not easy toaccomplish. The huge number of variousfilters is confusing. Moreover, the designa-tion of filters frequently does not indicatethe principles of their operation. The con-fusion might have been caused by the his-toric development of filtration in isolatedmarkets and applications and by a tenden-cy to claim new filter concepts even whenwell-known principles have been slightlymodified or combined. There are manyapparently similar filters that apply ratherdifferent concepts and - on the other hand- filters that look very different on a firstglance applying similar mechanisms.

14 F & S International Edition No. 13/2013

Clarification filters for liquids –principles, classification, selectionR. Berndt*

To select an adequate clarifying filter in an early planning phase, the process targets and feedstock specifications onthe one hand are compared with the typical performance and capabilities of available filters on the other hand. Lowlyconcentrated particles not exceeding 10 µm in size take center stage. They should be completely removed or,otherwise, their concentration should be limited. Clarification filters can be classified referring to the way in whichparticles are distributed during filtration and subsequently removed or disposed of. A first selective question asks ifsolid contaminants can be disposed of with the filter media or not. Further decisive criteria refer to the size of thesmallest particles to be removed, the concentration of solids in the feedstock and the desired liquid recovery and/or thefinal concentration of solids. Additional criteria further reduce the number of appropriate solutions. Combinations offilters with filters or with other solid-liquid separation equipment bridge the gap between the performancecharacteristics of single filters and the requirements of the application.

* Rolf Berndt (VDI)Prof. Dr.-Ing. habil.RBFM ConsultingAm Bieberbach 43, 63128 Dietzenbach/GermanyPhone: +49 (0) 6074-26347Fax: +49 (0) 6074-483580e-mail: [email protected]

Fig. 1: Filters and filtration technologies for clarification of liquids

Fig. 2: Methods of shear-induced fouling-layer control /1/

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Obviously there is a need for an easy clas-sification of clarification filters by onlyfew principles and for the application ofthis classification for a systematic surveyand selection guide referring to typicalapplications and performance parameters.

2. Classification of clarification filters

Industrial clarifying filtration of liquidsand contamination control of small parti-cles < 10 μm in size at low solids concen-tration take centre stage in this paper.Consequently strainers and screens areexcluded as well as filters that make use ofa filter cake spontaneously created by thesolids to be removed (cake filtration).Chemical manipulation of the feedstock(e. g. flocculation, coagulation) shall beexcluded, too. With these exclusions, clar-ifying filtration is understood as an opera-tion meant to remove particles that form apractically non-permeable fouling layer.Such a fouling layer would result in rapidflux decay and/or an increase of the differ-ential pressure.

Two primary countermeasures areapplied for control or total removal offouling layers (Figure 1): Either the foul-ing layer is removed or controlled byhydrodynamic means (Group H) or thefouling layer is disposed of in/with the dis-posable filter media (Group D). Two fur-ther, supporting strategies may avoidimpermeable deposits and help to maintainthe filtrate flux: Preferably two-dimen-sional distribution of the particles on largelow-cost filter areas (a) or distribution in three dimensions of a depth filter media (d).Group H filters are for instance- Membrane filter modules/systems apply-

ing either permanent or periodic cross-flow for fouling layer control (typicallyequipped with UF membranes or MFmembranes; Figure 2, Figure 3)

- UF or MF membrane filter modules thatare cleaned by periodic backwash

- Backwash filter elements (e.g. cartridgesequipped with MF membranes)

- Deep bed filters (granular beds, e.g. sandfilters, multimedia filters) cleaned byperiodic backwash.

Crossflow in membrane filters/modules is generated by three different meansincluding - Recirculation of a bigger part of the feed-

stock through flow-through membranemodules (Figure 2, right, Q1)

- Shear-gap filters with rotating membraneelements and/or agitator disks (Q2)

- Shear-gap filters with vibrating mem-brane elements (Q3).

Crossflow (permanent or periodical)may be combined with backwashing and

other periodic shear-induced membranecleaning procedures (Figure 2, right).

Most membrane filtration systems arebased on crossflow version Q1. Forremoval of particles, tubular modules aremore suitable than flat membrane modules(Table 1). Monolithic ceramic multi-chan-nel elements in steel housings and hollow-fiber or capillary modules are preferred(Figure 3).

Backflushing filters are typicallyequipped with cylindrical backwash car-tridges (Figure 4) made from sinteredpowder (metal, polymer), wire cloth or

sintered wire cloth. Surface filter elementsdominate the market. Depth filter elementsprovide an interesting alternative for cer-tain applications /4/. The elements arecleaned by periodically reversed fluidflow, typically filtrate, sometimes externalfluids or gas/liquid mixtures. Specialbackflush techniques are used to achievehigh cleaning efficiency.

Deep bed filters (e.g. sand filters, multi-media filters /5, 6/) use beds from granularor crushed material which is periodicallybackwashed. The so-called “rapid” sandfilters (grains about 0.5 mm in diameter)

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Tab. 1: Membrane modules /3/

Fig. 3: Examples of crossflow membrane filter arrangements /2/ (left: hollow fibre module, right:monolithic multi-channel ceramic element)

Fig. 4: Cross sections of backwash Filter Elements /4/ (left: Surface element Pall Dia-Schumalith®, right: Rigid depth filter element Pall Supafine®)


are frequently cleaned by reversing thefluid flow (filtrate, sometimes with air) ata rate about four to ten times higher thanthe filtration flow. This is to expand andfluidize the filter bed to release trappedsolids into the wash water. Multimedia fil-ters use materials different in density andgrain size (sand, garnet, anthracite) inorder to get an appropriate grain size dis-tribution after cleaning and resettling.Group D comprises- Surface filter elements (pleated mem-

branes or flat fixed fiber sheets, non-pleated surface filter elements)

- Depth filtration elements, e.g. depth filtercartridges or sheet filter elements

- Flat depth filter media such as filtersheets in plate-and frame filters

- Granular filter aids for pre-coatingand/or body feed.

There is a smooth transition from puresurface filters to depth filters, especially inthe world of submicron particles. Denselyfolded membrane cartridges provide typi-cally 0.5 to 0.8 m2 area per 10” cartridgelength for prevalent surface filtration. Thefiltering membrane is combined with meshlayers for support and drainage, pleatedtypically in folds parallel to the axis of thecartridge and supported by a perforated

center tube for the filtrate and surroundedby a protective screen external to thepleats. Polymeric membranes such aspolyamide, polyvinylidene fluoride(PVDF) and polytetrafluoroethylene(PTFE) are applied. Exchangeable car-tridges are mounted in segmented pressurehousings. Disposable systems consist ofcartridges fixed in disposable housingsand are typically applied in pharmaceuti-cal and microelectronic industries (Figure5).

Membranes for surface filtration arepleated tightly. Other, thicker media suchas bonded non-woven fabrics or melt-blown fiber mats with partial or prevailingdepth filtration effects are pleated in moreopen structures or applied as non-pleated,thick sheet on a filtrate drainage core tube(Figure 6). Gradients of filter fineness areachieved by decreasing fiber diameters(preferable) or by decreasing porosity inflow direction.

Filter sheets (Figure 7) are basicallyproduced in the same manner as paper.Though they are thicker (2…6 mm) andmade from cellulose fibers with additionof kieselguhr, perlite and finest polymericfibers (fibrides). They have traditionallybeen used in plate-and-frame filters toclarify beverages or to (pseudo-)sterilize

pharmaceutical solutions /5, 6/.Assemblies from lenticular disk sheetswith polymeric cages and central filtratecollectors are called depth filter modulesand are used in filter housings similar tofilter cartridges.

Granular filter aids are inert solids witha large surface area, e.g. kieselguhr(diatomaceous earth), perlite, cellulosefibres. They are either dispersed in the sus-pension to be filtered (“body feed”) and/orused to form a temporary filter layer on acoarser filter media. Traditionally appliedin beer filtration and filtration of other fer-mentation products, body feed filtrationuses pressure filters usable for cake filtra-tion. Pre-coat filtration is frequently per-formed by rotating vacuum drum filters.

3. Comparison of clarification filters

Filtration is an interaction of a filterwith a suspension to be filtered under cer-tain conditions. Thus all characteristic fea-tures of a filter depend not only on the fil-ter itself but always - more or less - on theparameters of the suspension and theprocess as well. This is the reason for thehuge challenges connected with the“objective” characterization of filters,resulting in conclusions which are eithervague or correct only for fixed conditions.The parameters used hereafter for compar-ison are- the size of the smallest removable

particle- typical figures for the area-related filtrate

flow (filtration velocity, filtrate flux)- typical figures for the volumetric

fraction of particles in the feed- the way of solids discharge and typical

solids fraction figures in case ofpumpable concentrates.


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Fig. 5: Disposable filter cartridges, housingsand disposable systems /2/

Fig. 6: Cross sections of pleated and non-pleated filter cartridge media (centre), Pall Ultipleat®Profile element (left), Pall Profile® II depth filter element (right]) /2/

Fig. 7: Filter sheets, plate-and-frame sheet filter press, depth filter module /2/


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There are many different methods forrating filters in terms of removable particlesizes. (A comprehensive disquisition is farbeyond the scope of this paper.) Infor ma -tion on the “fineness” of a filter mustalways be associated with the test methodused. Generally challenge tests on the onehand and non-destructive tests on the otherhand are applied. In challenge tests, the fil-ter is evaluated by a comparison of theamount of particles of a known suspensionin the upstream and downstream flow. Thetest methods are different concerning thetype of particles (standardized test dust,spherical beads from glass or polymers,colloidal oxides or metals, polystyrenelatex, gold nanoparticles, microorganismsetc.) and the kind of test procedure (single-pass, multi-pass) on the one hand. On theother hand they vary in the way how thefilter efficiency is defined and presented.This filter efficiency may refer to the massof particles or the number of particles innarrow fractions or cumulated above a cer-tain particle size /3, 5, 6/. Figure 8 indi-cates the range of applicability of certainchallenge materials compared with typicalworking areas of separation processes.

Non-destructive tests are applied with-out use of particles and do not contaminatethe filter media. The most important onesare the bubble-point test, the pressure-hold

test and the “Forward Flow” test as anadvancement of the former. Basicallythese tests make use of the observation thatthe pressure required to force a gas by con-vection through a pore that is wetted andfilled by a liquid is inversely proportionalto the diameter of the pore.

Typical figures of the filtrate flux andthe acceptable solids feed concentration inTable 2 do not consider “exotic” condi-tions and are by far no sizing rules. Toomany features of an individual applicationmay influence these two parameters.

4. Selection of clarification filters

A first selective question asks if solidcontaminants can be disposed of with thefilter media or not. If the answer is NO,only “Group H” filters (Figure 1) areapplicable. In a second step, the capabili-ties of the remaining filters are evaluatedby means of three criteria: The size of thesmallest particles to be removed, the con-centration of solids in the feedstock andthe desired liquid recovery respectively thefinal concentration of solids. Thus a three-

Tab. 2: Typical application features of clarification filters (based on: /2, 3, 4, 5, 6, 7, 9/)


dimensional “area of applicability” foreach of the available filters is defined.Figure 9 shows the typical fields of appli-cation with respect to the smallest remov-able particle size and the solids volumefraction in the feedstock.

Additional criteria help to furtherreduce the number of appropriate solu-tions. Among those criteria are- the general targets of filtration (e.g.

process filtration vs. utility filtration,“last chance” filtration, cleaning ofprocess liquids in baths and loops)

- the desired reliability (sharpness of cut-off, up-time)

- the compatibility of filter media andother wetted parts with the fluid to be fil-tered with respect to temperature andchemistry

- the accepted level of contamination ofthe fluid by the filter (corrosion products,extractables, particle release)

- the throughput rate (on average,extremes) and the viscosity of the liquid

- the accepted volume and frequency ofmaintenance

- the general acceptance of filter types inthe target industry

- the available space- the influence on waste treatment, waste

disposal and environmentand many other features of the individ-

ual application. Tarleton and Wakemandescribe in /9/ selection methods for awider range of operations which are gen-erally applicable for the special field ofclarification filters as well.

The number of applicable filters for aspecial job can be drastically reduced byapplying the criteria above. The remainingcandidates can now be compared on a costbasis. Capital cost compared with theavailable budget is a first selective criteria,Total Cost of Ownership a second one.

The cost balance facet is decisive for theresult, especially if consequential costs areimportant. An example: If a “last chance”filter with a protective function for down-stream processes (e.g. utility filtration insemiconductor industry) fails, the conse-quential costs are often much higher thanthe potential cost savings by using acheaper, less reliable product.

5. Combination of equipment

An ideal filter should be capable in han-dling high solids concentration and lowparticle sizes in the feedstock and shouldprovide sharp, reliable cut-off as well aslow liquid content in the separated solids.Often there is no filter available that meetsthese expectations in equal measure.Crossflow membrane filtration often pro-vides a useful but not always an ideal solu-tion. Especially the moisture content of theconcentrate is high as it needs to bepumpable or at least flowing under highshear stress. To combine filters with otherfilters or with other solid/liquid separationequipment is the most important way incase of complicated applications.

The combination in Figure10 (left)comprises disposable cartridges or filtersheets in the first stage and membrane fil-ter cartridges in the second stage.Combinations of this kind are frequentlyused in biopharmaceutical industry, foodand beverages, semiconductor industryand similar areas where low levels of con-taminations need to be reliably controlled.

The combination on the right sideshows a typical way how to take advantageof the capabilities of group H filters (lowcut-off, high solids load accepted) com-bined with downstream treatment of theconcentrate. A dewatering filter (e.g. a fil-ter press) might be applied with or withoutfilter aids or coagulants/ flocculants.Sometimes the particle concentrate can

form a filter cake simply due to the higherconcentration of solids compared to theoriginal feedstock. Only in certain applica-tions, the filtrate of the second stage can berecycled to the first filter. There is - espe-cially in case of loop or tank treatment - arisk of accumulated contamination due tohigh levels of contaminants in Filtrate II,e.g. flocculants, extractables, other dis-solved matter). Instead of a dewatering fil-ter, other solid-liquid separation equip-ment such as centrifuges or evenevaporators or driers may be used.

6. Summary and Conclusions

1. Clarification filters used in industryvary in the way in which particles aredistributed during filtration and howthey are subsequently removed or dis-posed of. Hydrodynamic fouling layercontrol (crossflow, backflush) or use ofdisposable filter media on the one handare combined with solids distribution inthree dimensions of a depth filter mediaor (preferably) two dimensions on sur-face filters on the other hand.

2. Hydrodynamic fouling layer control isapplied in crossflow filters and back-wash/backflush filters of different kind.Disposable filter materials/ elementsinclude surface filter elements, depthfilter elements, flat media of differentkind and filter aids.

3. Most industrial clarification filters arecapable of handling low and moderatelevels of solids concentration in thefeedstock provided the particles to beremoved are not far below 1 μm in size.Only crossflow membrane filters mayhandle higher concentrations of submi-cron particles satisfactorily.

4. Selection of clarification filters is donein a process of elimination based oncomparison of filter characteristics andcapabilities with targets and circum-


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Fig. 8: Application range of challenge materials andfiltration processes (according to /8/)

Fig. 9: Clarification filters: Comparison of application ranges


stances of the filter job in mind. Important criteria are the size ofthe smallest particles to be removed, the concentration of solidsin the feedstock and the desired liquid recovery respectively thefinal concentration of solids. Additional criteria such as compat-ibility, tradition in an industry, space requirements, maintenanceissues and reliability further reduce the number of appropriatecandidates. Cost considerations can finally be applied to identi-fy the best solution.

5. Combinations of filters with filters or with other solid-liquidseparation equipment can bridge the gap between the perfor-mance characteristics of a single filter and the requirements ofan individual application.

References:/1/ R. Berndt: Chemie-Ingenieur-Technik 2007, 79 (11), 1809/2/ Pall Corporation, Long Island (N.Y.): Company publications. Provided compliments of Pall Europe Limited. Copyright Pall Europe Limites, 2011/3/ R. Berndt: Verfahren und Systeme für die Membran filtra tion (9). In: K. Luckert (publisher),Handbuch der mechanischen Fest-Flüssig-Trennung, Vulkan-Verlag, Essen 2004/4/ K. Defren, R. Berndt: Rückspülbare Tiefenfilter mit starrer Filtrationsmatrix für die effektiveFiltration im Bereich von 1μm bis 10 μm. ProcessNet, FA MFA, Frankfurt a. M., 2011/5/ R. Leibnitz: Tiefenfilter (8.7). In: K. Luckert (editor), Handbuch der mechanischen Fest-Flüssig-Trennung, Vulkan-Verlag, Essen 2004/6/ K. Sutherland: Filters and filtration handbook. Elsevier, 5th edition, Amsterdam, Boston, Heidelberg, London, New York 2008/7/ C. Eckerscham: Chemie Technik 2005, 34 (8), 40/8/ Takehito Mizuno et al.: IEEE Transactions on semiconductor manufacturing, 2009, 22 (4), 452/9/ S. Tarleton, R. Wakeman: Solid/Liquid separation: Equipment selection. Elsevier, 1th edition, Amsterdam, Boston, Heidelberg, London, New York 2007

F & S International Edition No. 13/2013 19

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Fig. 10: Typical combination of filters (left: series of disposable filters; right: group H filter combined with concentrate dewatering equipment)

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clarification filters for liquids – principles, classification, selection · 2016-02-24 ·...

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