Filtration

This article is about the process of solid-fluid separation. For the mathematical concept, seeFiltration (mathematics).For the stochastic concept, seeFiltration (probability theory).For filtration used in winemaking, seeClarification and stabilization of wine.For other uses, seeFilter (disambiguation).

Filtrationis a physicalseparation processthat separatessolidmatter andfluidfrom a mixture using afilter mediumthat has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter medium are described asoversizeand the fluid that passes through is called thefiltrate.Oversize particles may form afilter cakeon top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known asblinding.The size of the largest particles that can successfully pass through a filter is called the effectivepore sizeof that filter. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size, filter thickness and biological activity). Filtration occurs both in nature and inengineeredsystems; there arebiological,geological,andindustrialforms.In everyday usage the verb "strain" is more often used; for example, using acolanderto drain cooking water from cookedpasta.

Diagram of simple filtration: oversize particles in thefeedcannot pass through the lattice structure of the filter, while fluid and small particles pass through, becomingfiltrate.

Filtration is also used to describe biological and physical systems that not only separate solids from a fluid stream but also remove chemical species and biological organisms byentrainment,phagocytosis,adsorptionandabsorption.Examples includeslow sand filtersandtrickling filters.It is also used as a general term for macrophage in which organisms use a variety of means to filter small food particles from their environment. Examples range from the microscopicVorticellaup to thebasking shark,one of the largest fishes, and thebaleen whales,all of which are described asfilter feeders.

Physical processes

• Filtration is used to separate particles and fluid in a suspension, where the fluid can be a liquid, a gas or asupercritical fluid.Depending on the application, either one or both of the components may be isolated.
• Filtration, as a physical operation enables materials of different chemical compositions to be separated. Asolventis chosen which dissolves one component, while not dissolving the other. By dissolving the mixture in the chosen solvent, one component will go into thesolutionand pass through the filter, while the other will be retained.
• Filtration is widely used inchemical engineering.It may be combined with other unit operations to process the feed stream, as in thebiofilter,which is a combined filter and biological digestion device.
• Filtration differs from sieving, where separation occurs at a single perforated layer (asieve). In sieving, particles that are too big to pass through the holes of the sieve are retained (seeparticle size distribution). In filtration, a multilayer lattice retains those particles that are unable to follow the tortuous channels of the filter. Oversize particles may form acakelayer on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter (blinding). Commercially, the term filter is applied tomembraneswhere the separation lattice is so thin that the surface becomes the main zone of particle separation, even though these products might be described as sieves.
• Filtration differs fromadsorption,where separation relies onsurface charge.Some adsorption devices containingactivated charcoalandion-exchange resinare commercially called filters, although filtration is not their principal mechanical function.
• Filtration differs from removal ofmagneticcontaminants from fluids withmagnets(typicallylubricationoil, coolants andfuel oils) because there is no filter medium. Commercial devices called "magnetic filters" are sold, but the name reflects their use, not their mode of operation.
• In biological filters, oversize particulates are trapped and ingested and the resulting metabolites may be released. For example, inanimals(includinghumans),renal filtrationremoveswastefrom theblood,and inwater treatmentandsewage treatment,undesirable constituents are removed by adsorption into a biological film grown on or in the filter medium, as inslow sand filtration.

Methods

Filters may be used for the purpose of removing unwanted liquid from a solid residue, cleaning unwanted solids from a liquid, or simply to separate the solid from the liquid.

There are many different methods of filtration; all aim to attain theseparationof substances. Separation is achieved by some form of interaction between the substance or objects to be removed and the filter. The substance that is to pass through the filter must be afluid,i.e. aliquidorgas.Methods of filtration vary depending on the location of the targeted material, i.e. whether it is dissolved in the fluid phase or suspended as a solid.

Hot filtration, solution contained in the Erlenmeyer flask is heated on a hot plate to prevent re-crystallization of solids in the flask itself

There are several laboratory filtration techniques depending on the desired outcome namely, hot, cold andvacuum filtration.Some of the major purposes of obtaining the desired outcome are, for the removal of impurities from a mixture or, for the isolation of solids from a mixture.

Hot filtration for the separation of solids from a hot solution

Hot filtrationmethod is mainly used to separate solids from a hot solution. This is done to prevent crystal formation in the filter funnel and other apparatus that come in contact with the solution. As a result, the apparatus and the solution used are heated to prevent the rapid decrease in temperature which in turn, would lead to the crystallisation of the solids in the funnel and hinder the filtration process. One of the most important measures to prevent the formation of crystals in the funnel and to undergo effective hot filtration is the use stemless filter funnel. Due to the absence of a stem in the filter funnel, there is a decrease in the surface area of contact between the solution and the stem of the filter funnel, hence preventing re-crystallization of solid in the funnel, and adversely affecting the filtration process.

Cold filtration, the ice bath is used to cool down the temperature of the solution before undergoing the filtration process

Cold filtrationmethod is the use of an ice bath to rapidly cool the solution to be crystallized rather than leaving it to cool slowly in the room atmosphere. This technique results in the formation of very small crystals as opposed to getting large crystals by cooling the solution at room temperature.

Vacuum filtrationtechnique is mostly preferred for small batches of solution to dry small crystals quickly. This method requires aBüchner funnel,filter paper of a smaller diameter than the funnel,Büchner flask,and rubber tubing to connect to a vacuum source.

Centrifugal filtrationis carried out by rapidly rotating the substance to be filtered. The more dense material is separated from the less dense matter by the horizontal rotation.

Gravity filtrationis the process of pouring the mixture from a higher location to a lower one. It is frequently accomplished via simple filtration, which involves placing filter paper in a glass funnel with the liquid passing through by gravity while the insoluble solid particles are caught by the filter paper. Filter cones, fluted filters, or filtering pipets can all be employed, depending on the amount of the substance at hand.Gravity filtration is in widespread everyday use, for example for straining cooking water from food, or removing contaminants from a liquid.

Filtering force

Only when a driving force is supplied will the fluid to be filtered be able to flow through the filter media.Gravity,centrifugation, applying pressure to the fluid above the filter, applying a vacuum below the filter, or a combination of these factors may all contribute to this force. In both straightforward laboratory filtrations and massive sand-bed filters, gravitational force alone may be utilized. Centrifuges with a bowl holding a porous filter media can be thought of as filters in which a centrifugal force several times stronger than gravity replaces gravitational force. A partial vacuum is typically provided to the container below the filter media when laboratory filtration is challenging to speed up the filtering process. Depending on the type of filter being used, the majority of industrial filtration operations employ pressure orvacuumto speed up filtering and reduce the amount of equipment needed.

Filter media

Filter media are the materials used to do the separation of materials.

Two main types of filter media are employed in laboratories:

• Surface filtersare solid sieves with ameshto trap solid particles, sometimes with the aid offilter paper(e.g.Büchner funnel,belt filter,rotary vacuum-drum filter,cross-flow filters,screen filter).
• Depth filters,beds of granular material which retain the solid particles as they pass (e.g.sand filter).

Surface filters allow the solid particles, i.e. the residue, to be collected intact; depth filters do not. However, the depth filter is less prone to clogging due to the greater surface area where the particles can be trapped. Also, when the solid particles are very fine, it is often cheaper and easier to discard the contaminated granules than to clean the solid sieve. Filter media can be cleaned by rinsing with solvents or detergents or backwashing. Alternatively, in engineering applications, such asswimming poolwater treatment plants, they may be cleaned bybackwashing.Self-cleaningscreen filtersutilize point-of-suction backwashing to clean the screen without interrupting system flow.

Achieving flow through the filter

Fluids flow through a filter due to a pressure difference—fluid flows from the high-pressure side to the low-pressure side of the filter. The simplest method to achieve this is by gravity which can be seen in thecoffeemakerexample. In the laboratory, pressure in the form of compressed air on the feed side (or vacuum on the filtrate side) may be applied to make the filtration process faster, though this may lead to clogging or the passage of fine particles. Alternatively, the liquid may flow through the filter by the force exerted by apump,a method commonly used in industry when a reduced filtration time is important. In this case, the filter need not be mounted vertically.

Filter aid

Certain filter aids may be used to aid filtration. These are often incompressiblediatomaceous earth,or kieselguhr, which is composed primarily ofsilica.Also used are woodcelluloseand other inert porous solids such as the cheaper and saferperlite.Activated carbonis often used in industrial applications that require changes in the filtrate's properties, such as altering colour or odour.

These filter aids can be used in two different ways. They can be used as a precoat before theslurryis filtered. This will prevent gelatinous-type solids from plugging the filter medium and also give a clearer filtrate. They can also be added to the slurry before filtration. This increases theporosityof thecakeand reduces the resistance of the cake during filtration. In a rotary filter, the filter aid may be applied as a precoat; subsequently, thin slices of this layer are sliced off with the cake.

The use of filter aids is usually limited to cases where the cake is discarded or where theprecipitatecan be chemically separated from the filter.

Alternatives

Filtration is a more efficient method for theseparation of mixturesthandecantationbut is much more time-consuming. If very small amounts of solution are involved, most of the solution may be soaked up by the filter medium.

An alternative to filtration iscentrifugation.Instead of filtering the mixture of solid and liquid particles, the mixture is centrifuged to force the (usually) denser solid to the bottom, where it often forms a firmcake.The liquid above can then be decanted. This method is especially useful for separating solids that do not filter well, such as gelatinous or fine particles. These solids can clog or pass through the filter, respectively.

Biological filtration

Biological filtration may take place inside an organism, or the biological component may be grown on a medium in the material being filtered. Removal of solids, emulsified components, organic chemicals and ions may be achieved by ingestion and digestion, adsorption or absorption. Because of the complexity of biological interactions, especially in multi-organism communities, it is often not possible to determine which processes are achieving the filtration result. At the molecular level, it may often be by individual catalytic enzyme actions within an individual organism. The waste products of some organisms may subsequently broken down by other organisms to extract as much energy as possible and in so doing reduce complex organic molecules to very simple inorganic species such as water, carbon dioxide and nitrogen.

Excretion

Main article:Renal physiology § Filtration

In mammals, reptiles, and birds, thekidneysfunction by renal filtration whereby theglomerulusselectively removes undesirable constituents such asurea,followed by selective reabsorption of many substances essential for the body to maintain homeostasis. The complete process is termedexcretionbyurination.Similar but often less complex solutions are deployed in all animals, even theprotozoa,where thecontractile vacuoleprovides a similar function.

Biofilms

Main article:Biofilm

Biofilms are often complex communities of bacteria, phages, yeasts and often more complex organisms includingprotozoa,rotifersandannelidswhich form dynamic and complex, frequently gelatinous films on wet substrates. Such biofilms coat the rocks of most rivers and the sea and they provide the key filtration capability of theSchmutzdeckeon the surface ofslow sand filtersand the film on the filter media oftrickling filterswhich are used to create potable water and treat sewage respectively.

An example of a biofilm is a biological slime, which may be found in lakes, rivers, rocks, etc. The utilization of single- or dual-species biofilms is a novel technology since natural biofilms are sluggishly developing. The use of biofilms in the biofiltration process allows for the attachment of desirable biomass and critical nutrients to immobilized support. So that water may be reused for various processes, advances inbiofiltrationmethods assist in removing significant volumes of effluents fromwastewater.

Systems for biologically treating wastewater are crucial for enhancing both human health andwater quality.Biofilm technology, the formation of biofilms on various filter media, and other factors have an impact on the growth structure and function of these biofilms. To conduct a thorough investigation of the composition, diversity, and dynamics of biofilms, it also takes on a variety of traditional and contemporary molecular approaches.

Filter feeders

Main article:Filter feeders

Filter feeders are organisms that obtain their food by filtering their, generally aquatic, environment. Many of the protozoa are filter feeders using a range of adaptations including rigid spikes ofprotoplasmheld in the water flow as in thesuctoriato various arrangements of beatingcilliato direct particles to the mouth including organisms such asVorticellawhich have a complex ring of cilia which create a vortex in the flow drafting particles into the oral cavity. Similar feeding techniques are used by theRotiferaand theEctoprocta.Many aquaticarthropodsare filter feeders. Some use rhythmical beating of abdominal limbs to create a water current to the mouth whilst the hairs on the legs trap any particle. Others such as somecaddis fliesspin fine webs in the water flow to trap particles.

Examples

Filter flask (suction flask, with sintered glass filter containing sample). Note the almost colourless filtrate in the receiver flask.

Many filtration processes include more than one filtration mechanism, and particulates are often removed from the fluid first to prevent clogging of downstream elements.

Particulate filtration includes:

• Thecoffee filterto separate the coffee infusion from the grounds.
• HEPAfilters inair conditioningto remove particles from air.
• Belt filtersto extractprecious metalsinmining.
• Vertical plate filter such as those used inMerrill–Crowe process.
• TheNutsche filteris typically used in pharmaceutical applications or batch processes that need to capture solids.
• Furnaces use filtration to prevent the furnace elements from fouling with particulates.
• A pneumatic conveying system such as anindustrial exhaust duct systemoften employs filtration to stop or slow the flow of unwanted material that is transported, through the use of abaghouse.
• In the laboratory, aBüchner funnelis often used, with afilter paperserving as the porous barrier.
• Air filtersare commonly used to remove airborne particulate matter in building ventilation systems, combustion engines, and industrial processes.
• Oil filterin automobiles, often as a canister or cartridge.
• Aquarium filter
• Straining water from food with acolander

Adsorption filtration removes contaminants byadsorptionof the contaminant by the filter medium. This requires intimate contact between the filter medium and the filtrate, and takes time for diffusion to bring the contaminant into direct contact with the medium while passing through it, referred to asdwell time.Slower flow also reduces pressure drop across the filter. Applications include:

• Carbon dioxideremoval frombreathing gasinrebreathersandlife-support systemsusingscrubber filters,
• Activated carbonfilters to remove volatile hydrocarbons, odours, and other contaminants from recirculated breathing gas in closed habitats.

Combined applications include:

Small stationary Bauer HP breathing air compressor installation showing water separator (centre), and two high-pressure product filter housings (gold anodised) to produce oxygen compatible breathing air for diving gas mixtures.

• Compressed breathing airproduction, where the air passes through a particulate filter before entering the compressor, which removes particles likely to damage the compressor, followed bydroplet separationafter post-compression cooling and final product adsorption filtration to remove gaseous hydrocarbons contaminants and excessive water vapour. In some cases prefilters using adsorption media are used to control carbon dioxide levels,pressure swing adsorptionmay be used to increaseoxygen fraction,and where the risk ofcarbon monoxidecontamination exists,hopcalitecatalytic convertersmay be included in the filtration media of the product. All these processes are broadly referred to as aspects of the filtration of the product.
• Potable watertreatment using biofilm filtration in slow sand filters.
• Wastewater treatmentusing biofilm filtration using trickling filters.

See also

• Separation process– Method that converts a mixture or solution into two or more distinct products
• Microfiltration– Physical process where a fluid is passed through a special pore-sized membrane
• Ultrafiltration– Filtration by force through a semipermeable membrane
• Nanofiltration– Filtration method that uses nanometer sized pores in biological membranes
• Reverse osmosis– Water purification process
• Cross-flow filtration– filtration techniquePages displaying wikidata descriptions as a fallback
• Sieve– Tool for separation of solid materials by particle size
• Sieve analysis– Procedure to assess particle size distribution

External links

• Filtration modeling(constant rate and pressure)