Filtration is one of the most broadly spread operation in industrial workshops processing products is a liquid phase and on machines using a liquid as an ancillary working tool or as a source of energy.


Among other solid-liquid separation techniques, i.e. centrifugation, settling, flotation and cycloning, filtration is the most versatile one. Its possible equipments and operation principles are so different that most of problems can be solved by a filter, whatever the scope of the separation: either to recover the solid phase from a solid liquid mixture or to give the liquid a given degree of clarification or a specified suspended solid concentration.

The filtering media plays the key role in a filter. To satisfy the requirements of both the separation process and the technology, a great variety of media has been developed. Their choice is a difficult task: first all above requirements cannot always be stated or quantified what means that the user's needs are sometimes unknown in their whole. Second the ability for use in each specific condition of each existing media can hardly be determined for technical and cost reasons.

Tests have thus to be performed in laboratory conditions to simulate as faithfully as possible actual field operating conditions so as to quantify the properties of the media in relation with the function they will have to full fill.

Each professional sector then creates its own test methods and standardization is a way of simplifying and clarifying disclaim of the media characteristics.

Applications of Liquid Filtration

It is not the scope of this paper to list all current applications of filtration processes: only a book could do it and a brief reading of the "Filters and Filtration Handbook" (1) even if not application oriented, is the best base for such an overview.

Compared to other solid-liquid separation techniques, the filtration characterizes by the use of a porous solid medium having the fundamental function of letting the whole liquid flowing through with the lowest hydraulic resistance whilst partly or entirely retaining solids. Due to this retention which always takes place both on the surface and within the porous structure, but in variable proportions, the porosity decreases, the permeability decreases, the differential pressure increases and/or the flow rate decreases to a maximum/minimum acceptable value. Then the process has to be stopped and/or the filter media has to be backwashed or replaced. So fundamentally, filtration is a discontinuous process and only technological solutions/advances can make the whole operation continuous.

With the testing aspect in mind, one has to recall that filtration applications can be classified within two families:

  • The concentration and/or dehydration of the solid phase to be extracted from the liquid. Some examples are the ore extraction, the paper pulp manufacturing, the sewage sludge dehydration, minerals or coal processing, the pharmaceutical or chemical precipitate collection and so many others;
  • The clarification and/or decontamination of a liquid by a reduction of its solid phase content to the appropriate concentration. It finds universal applications: fuel and lubricating oils of all kind of engines, beverage and drinking waters, machine tool coolants, nuclear power plant liquids, micro-electronic component manufacturing chemicals, all chemical and pharmaceutical liquids and so many others as well.

In the first case of application, the properties, especially the performances, of the porous media are less critical because the separation process is mainly operated by the deposit of the particles, called a cake. On the contrary, in clarification, the suspension is continuously in intimate contact with the media and any defect of the media will impact the filtration process and the quality of the filtrate.



"The Handbook of Filter Media" (2) and several similar books give an excellent description of the broad variety of filtering media available to the market. To summarize we will adopt the following classification:

  • Granular media: made of individual rounded shape particles of various nature (silica, carbon, cellulose), they are mainly used as thick layers (from few millimeters as precoats to a thousand as depth media) to clarify the liquid and to collect solids. They are consumable or backwashable.
  • Fibers: they are used as individual particles (and could have been classified in the above category) in thin layers only (precoats) but mainly associated and bound together in a solid form: papers, nonwovens, plates, agglomerates… They are in cellulose, glass, polymers, carbon or stainless steel and a key difference is the way they are bound: polymer coating, sinterization, thermal bonding…
  • Metal plates : uniform material with holes made by perforation or "electroerosion" in all possible metals.
  • Membranes: thin (less than 1 mm) materials with pore sizes less than 10 microns and a narrow distribution, they are available in organic (cellulose nitrate, acetate or ester, polysulfone, polycarbonate, nylon, PVDF…) and inorganic (carbon, zircon, ceramic, metal, glass …) substances. Their porosity and pore shape depend on their manufacturing process (sinterization, track edging, solvent casting, stretching, …)




As well as it was not intended to survey and describe all available filtering media, all available filters are not listed but only reminded. For a complete survey ask for IFTS documentation service ( or read above mentioned literature. One can consider the following families:

  • Deep bed filters (sand, activated carbon) are closed or open tanks associated with back flushing circuits
  • Belt filters use rolls and woven wires to support synthetic or metallic woven wires and nonwovens which continuously or regularly move to an extremity of the belt where the cake is discharged
  • Candle filters are closed housings and candles are vertical "porous" cylinders of various diameters made of woven wires, sintered beads, stackdiscs, ...
  • Rotary drum filters, generally include separate quarters submitted to successively vacuum or pressure, rotating in a hemicylindrical trough containing the suspension. The cake formed can be washed and is discharged as a thin layer by a scraping knife or totally on outgoing belts,
  • Leaf or plate filters are generally closed horizontal or vertical casing with leafs in the vertical or horizontal position. Leafs have one or two faces covered with a wire cloth. Cake is discharged by counter flow, vibration or rotation,
  • Rotating disc filters are of various technologies: similar to rotating drum filters or leaf filters or disc stacks. They generally allow a continuous process
  • Plate filters have vertical or horizontal plates which hold leafs tight or which create filtration chambers (filter presses). They have to be regularly open to allow cake discharge or leaf replacement,
  • Cartridge filters are closed vertical housings in which are removable filtering cartridges of various technologies: wounded pleated, agglomerated...
  • Membrane filters are closed housings of various geometry in which are enclosed thin film membranes themselves of various types : tubes, hollow fibers, plates or spiral wound, ... They work under pressure and the cross flow allows a quasi continuous process,
  • Centrifuge filters have a horizontal or vertical perforated rotating bowl in which is installed a filter cloth. The discharge is manual or automated. As many other solid recovery filters, they allow successive phases of filtration, drying, washing and discharge of the cake.
Guidance of Filter Selection



The choice of the best filter for a given application should be based on a methodic approach of the process in which the filtration step is located, the objectives to be achieved, the characteristics of the products to separate, waste disposal possibilities, safety, health and environmental considerations, all technical data being weight by economical data.

A good base for helping engineers defining a filtration process is the standard NF X 45-600 : "Solid-Liquid Separation – Specifications for defining solid-liquid separation equipment" (3).

A first step is to quantify the process characteristics: liquid minimum, maximum and mean flowrates, operating cycle duration, filtrate quality, continuity of downstream and upstream operations, available pressure and allowed head loss…. Then one has to know a minimum of physical chemical characteristics of the suspension:

  • Liquid characteristics: such as chemical nature, viscosity, pH, specific gravity, temperature and others (oxydability, flammability, corosivity, toxicity, ...)
  • Solid characteristics: concentra-tion, nature, particle size and shape factor distribution, specific gravity, solubility and same others.

At last, operating characteristics of the process itself have to be considered: possible pretreatment, temperature, upstream and downstream pressures, inertage requirement, degree of automatisation, allowed and/or forbidden materials, environment requirements (noise, vibration, odor, CIP ...) and at last size and weight requirements.


Filtering Media Selection

The ideal method for selecting a filtering media is to use it in actual conditions during the required separation time on the product itself. In many cases, if not all in industry, time, process and budget reasons impose to make a preselection based on technical data available in the supplier literature.

Then the need of having available written test methods which describe the operating protocol to measure the relevant parameters, characteristics or ability for use of the media appears. It is evidence that any such physical parameters take values which fully depend on the conditions applied to measure them.

Then the need of standard test methods appears. Standardization is the agreement by several parties involved in the filtration industry (media and filter manufacturers, expert end users and testing laboratories) on a compromise fixing the test operating conditions, telling how to express results and defining each technical word necessary for the test. If only standard test methods where referred to in filter media manufacturer brochures, the work of end users having to select the media the most suited to their needs would be greatly eased.

About thirty criteria for choosing a filter media could be listed. It is not intended to review them all in detail but only to recall the main ones:

  • The intrinsical and structural characteristics describe the porous and solid structures independent of their use,
  • The hydraulic properties quantify the impact of the media on a liquid flow. Their knowledge is necessary to size the filter media and to calculate the consumption of energy by the filtration process,
  • The performance characteristics are the key ones since they relate to the ability to retain particles and to the way the clogging, i.e. overall hydraulic properties, occurs throughout time,
  • The compatibility is the general term to define all interactions (other than hydraulic and performances) between the process or machine, the suspension and the porous media which may impact, generally negatively, their initial properties.

The best media will be the one giving the best technical and economical compromise between the sometimes contradictory requirements on the filtrate, the recovered solid, the cycle length and on all the selection criteria mentioned above.

Structural and Intrinsical Characteristics

When the media is made of individual particles, whatever loose or bound, its optimization requests to know particle size and shape factor distribution.

The size distribution directly affects the porosity, the bed or cake formation and final homogeneity, the clogging and the backwash processes, the filtration efficiency, the cycle duration and many other process conditions.

The porosity, defined by the ratio of the void volume to the apparent volume and expressed as a percentage (and not in micrometer) and the equivalent pore size distribution (expressed in micron) are main characteristics of the media internal structure geometry. They impact at the first degree the filtration efficiency, the differential pressure, the retention capacity and the ability to "backwash".

The physical chemical properties of the media material surface, e.g. its surface potential, wettability, roughness and others, will have a direct effect on the way the particles are captured and thus on the efficiency, the capacity and the duration of the process.

The intrinsical characteristics manage the filtration process. But their knowledge never allows to predict the filtration results but with the simplest ones. As an example, the pore size distribution of a wire mesh, because of its narrowness, allows to predict with a low margin of errors the initial absolute filtration rating with a simple glass sphere suspension. But as soon as clogging occurs, this rating will change.

Hydraulic Properties

The impact of the filter media on the way the liquid flows through is an important parameter to know for sizing/optimizing a filter. Permeability (Bo) is one value used to quantify it. It is calculated from the differential pressure (ΔP) measured across a thickness (e) of an area (A) of the media when flowed through by a liquid of viscosity (µ) at a flow rate (Q). Darcy law writes:

Bo = µ * Q * e / A * ΔP


Permeability is the preferred value to compare and size filtering media.

Another approach is to measure the overall differential pressure (ΔP) across the media at a given flowrate (Q) and to draw the curve ΔP = f(Q). Q may be replaced by v, the fluid velocity (v = Q / A).


 Performance Characteristics

The filter media performance is quantified by one or more characteristics of the filtrate or by comparing the upstream and downstream value of these characteristics.

The Filtration Efficiency (E%), e.g. the percentage of what has been retained to what was in the upstream flow, may be measured by global parameters of the suspension (turbidity, concentration) or at different particle sizes by using particle counters. Then, the curve of filtration efficiency vs particle size can be drawn (figure 1).

Figure 1: Typical filtration efficiency curve


All characteristics of the suspension, suspended solids and the liquid directly influence the result of the efficiency measurement what makes it highly recommendable to standardize efficiency test methods. As well, only efficiency values measured in identical conditions should be compared.

Standardization committees gather experts of a given sector (fluid power, automotive, aerospace, drinking water, membrane manufacturers, swimming pool, pharmaceutical, ...) who look for laboratory conditions simulating as close filter actual working conditions as possible : that explain that test fluids may be mineral oils, fuels or waters and test contaminant be carbon black, metallic oxides, bacteria or silica. The efficiency may be measured on a clean media sample (initial efficiency) or all along its clogging cycle (average or minimum efficiency). From the curve of figure 1 one efficiency value can be used to rate the filter in micrometer.



Some recent standards (EN 13343-2, ISO 19438) have clarified the definition of filter rating by fixing the relevant efficiency value and they adopted the word "Reference" to differentiate it from existing but commercial and undefined "nominal" and "absolute" ones. 

Absolute rating can only be referred to when reporting the diameter of the largest glass bead able to flow through the clean media in standard conditions.



The filtration efficiency curve and the "Reference" rating are key characteristics of filtering media used in liquid clarification whilst absolute rating is more relevant to cake filtration or to so call "last chance" filters.

Because of the continuous clogging of an efficient filter media, it is necessary to know the mass of contaminant, the volume of prefilt or the filtration time elapsed before the media needs to be backwashed or changed. This phenomenon is measured by the Retention Capacity and illustrated by the clogging curve as defined in several standards (see figure 2). The shape of this curve, as well as the efficiency one, is a function of the test conditions, fluid and contaminant.


Figure 2: Typical clogging curve

Other Characteristics: The "Compatibilities"

We gather together in this section all the characteristics which quantify the negative effects of the filter media on to the filtrate and those of both the manufacturing process and the liquid on the media "integrity".

The filtering media may degrade the filtrate quality if its materials dissolve or migrate or leak within the filtrate: these are measured though media migration, extractable and leakage tests.

On the contrary the intrinsical characteristics of the media may be altered by "extreme" fluid conditions, high temperature, oxidation, high/low pH, light ray degradation.... Several standard test methods consist in comparing a characteristic of the filtering media new and after having been submitted to the "aggression" in fixed and accelerated conditions.

At last some mechanical actions such as pleating, high or cyclic differential pressure, traction or compression, may change media performance or shorten its life. Other standards explain how to measure these resistances.

Media Choice Optimization

Once all these technical data are available through standard tests, what is the only way to guarantee the comparability of values claimed, the filtration engineer has to apply a matrix of choice including several steps.

First is the compatibility (chemical, thermal, …) which ensures that over time of use, sometimes several years, the media structure and characteristics will not change and that the filtered liquid has no chance of being altered by some compounds of the media. 

Second are conditions of applications. Considerations on available space, acceptable weight, safety requirements, continuity of the process and several other “non filtration” parameters open or close access to the type of media.

Then the degree of residual contamination acceptable in the filtered liquid, what defines the rating of the filter, orientates toward a family of media. E.g. microfiltration at reference rating lover than 1 micron is not achievable with sintered fibres whereas allowed by nearly all synthetic polymeric membranes.

At last, the filtration cycle duration determines the technology (backwashable or consumable) and the size of the filtering media, its surface area. This surface area (in m2) is leaked to the specific retention capacity (in g/m2) of the media by the level of contamination of the liquid (in g/m3) to the liquid flow rate (m3/h) and to the filtration cycle duration (h)

The final choice is always the result of a compromise as show figures 3 and 4.

Fig 3 : Typical particle counts downstream filter during an efficiency test (single pass)

Fig 4 : Typical clogging curves of a retention capacity test (single pass)

Based on these results, a druggist obliged to change filters for any new batch he produces will choose filter A ensuring the lowest filtrate contamination level even with the lowest retention capacity.

Adversely, a continuous chemical process would require filter C with the highest capacity i.e. requiring the less changes even if the quality of the filtrate may not be rigorously guaranteed at the start of the filtration process.


Liquid filters represent a very large family of industrial products in which the filtering media plays the key role. 

The choice of the media the most appropriate for a given application must be based on several considerations among which the technical characteristics have to be measured and claimed in standard conditions so as to make them clear unbiased and comparable.

The performance and hydraulic characteristics are less important in concentration applications of filtration, i.e. when the actual efficiency is ensured by a cake. They are essential in clarification. As far as they are concerned, the compatibilities have to be evaluated in all cases since they impact the continuity of the good process with time. 


T.C. Dickenson (1997) “Filters and Filtration Handbook”, 4th edition – Elsevier Advanced Technology, Oxford (UK).

D.B. Purchas and K. Sutherland (2002), “Handbook of Filter Media”,  2nd edition, Elsevier Advanced Technology, Oxford (UK).

NF X 45-600: “Solid-liquid separation – Specifications for defining solid-liquid separation equipment”, AFNOR, Saint-Denis, (F).