Editing Filtration (section)

==Physical processes==
* Filtration is used to separate particles and fluid in a suspension, where the fluid can be a liquid, a gas or a [[supercritical 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. A [[solvent]] is chosen which dissolves one component, while not dissolving the other. By dissolving the mixture in the chosen solvent, one component will go into the [[Solution (chemistry)|solution]] and pass through the filter, while the other will be retained.
* Filtration is widely used in [[chemical engineering]]. It may be combined with other unit operations to process the feed stream, as in the [[biofilter]], which is a combined filter and biological digestion device.
* Filtration differs from sieving, where separation occurs at a single perforated layer (a [[sieve]]). In sieving, particles that are too big to pass through the holes of the sieve are retained (see [[particle 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 a [[Filter cake|cake]] layer 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 to [[membrane]]s where 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 from [[adsorption]], where separation relies on [[surface charge]]. Some adsorption devices containing [[Activated carbon|activated charcoal]] and [[ion-exchange resin]] are commercially called filters, although filtration is not their principal mechanical function.
* Filtration differs from removal of [[magnetic]] contaminants from fluids with [[Magnet#Common uses of magnets|magnets]] (typically [[lubrication]] oil, coolants and [[fuel 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, in [[animal]]s (including [[human]]s), [[renal physiology#Filtration|renal filtration]] removes [[metabolic waste|waste]] from the [[blood]], and in [[water treatment]] and [[sewage treatment]], undesirable constituents are removed by adsorption into a biological film grown on or in the filter medium, as in [[slow sand filter|slow 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 the [[Separation of mixtures|separation]] of 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 a [[fluid]], i.e. a [[liquid]] or [[gas]]. 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.
[[File: Hot Filtration set up.jpg|thumb|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 and [[Suction filtration|vacuum 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.
[[File: Hot FIltration.jpg|thumb|left|Hot filtration for the separation of solids from a hot solution]]
'''Hot filtration''' method 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.<ref>{{cite web|title=ORGANIC LABORATORY TECHNIQUES 3: Filtration Methods|url=http://www.chem.ucalgary.ca/courses/351/laboratory/filtration.pdf|publisher=[[University of Calgary]]|date=19 September 2013<!--from PDF source-->|archive-date=13 February 2015|archive-url=https://web.archive.org/web/20150213060129/http://www.chem.ucalgary.ca/courses/351/laboratory/filtration.pdf|url-status=live}}</ref>
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.

[[File: Cold Filtration.jpg|thumb|Cold filtration, the ice bath is used to cool down the temperature of the solution before undergoing the filtration process]]
'''Cold filtration''' method 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.

'''[[Suction filtration|Vacuum filtration]]''' technique is mostly preferred for small batches of solution to dry small crystals quickly. This method requires a [[Büchner funnel]], filter paper of a smaller diameter than the funnel, [[Büchner flask]], and rubber tubing to connect to a vacuum source.

'''Centrifugal filtration''' is 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.<ref name="Filtration">{{cite web |title=Filtration - Definition, Types, Functions & Quiz |url=https://biologydictionary.net/filtration/ |website=Biology Dictionary |date=3 March 2017}}</ref>

'''Gravity filtration''' is 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.<ref name="Filtration"/> 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 or [[vacuum]] to speed up filtering and reduce the amount of equipment needed.<ref>{{cite web |title=filtration {{!}} Definition, Examples, & Processes {{!}} Britannica |url=https://www.britannica.com/science/filtration-chemistry |website=www.britannica.com |language=en}}</ref>

===Filter media===
{{Unreferenced section|date=July 2020}}
Filter media are the materials used to do the separation of materials.

Two main types of filter media are employed in laboratories: 
*''Surface filters'' are solid sieves with a [[mesh]] to trap solid particles, sometimes with  the aid of [[filter paper]] (e.g. [[Büchner funnel]], [[belt filter]], [[rotary vacuum-drum filter]], [[cross-flow filtration|cross-flow filters]], [[screen filter]]).
* ''[[Depth filter]]s'',  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.<ref>{{Cite book|chapter=Chapter 10 - Liquid Filtration |date=2019-04-19 |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780081010983000111 |title=Coulson and Richardson's Chemical Engineering |pages=555–625 |edition=Sixth |editor-last=Chhabra |editor-first=Raj |publisher=Butterworth-Heinemann |doi=10.1016/B978-0-08-101098-3.00011-1 |language=en |isbn=978-0-08-101098-3 |s2cid=239117840|editor2-last=Basavaraj |editor2-first=Madivala G.}}</ref>
Filter media can be cleaned by rinsing with solvents or detergents or backwashing. Alternatively, in engineering applications, such as [[swimming pool]] water treatment plants, they may be cleaned by [[Backwashing (water treatment)|backwashing]]. Self-cleaning [[screen filter]]s utilize point-of-suction backwashing to clean the screen without interrupting system flow.{{clarify|point of suction backwashing|date=May 2021}}

====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 the [[coffee filter|coffeemaker]] example. 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 a [[pump]], 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 incompressible [[diatomaceous earth]], or kieselguhr, which is composed primarily of [[silica]]. Also used are wood [[cellulose]] and other inert porous solids such as the cheaper and safer [[perlite]]. [[Activated carbon]] is 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 the [[slurry]] is 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 the [[porosity]] of the [[Filter cake|cake]] and 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 the [[precipitate]] can be chemically separated from the filter.

===Alternatives===
Filtration is a more efficient method for the [[separation of mixtures]] than [[decantation]] but 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 is [[centrifugation]]. 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 firm [[Filter cake|cake]]. 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.