Diagram of simple filtration: oversize particles in the feed cannot pass through the lattice structure of the filter, while fluid and small particles pass through, becoming filtrate.
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a filter medium that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter medium are described as oversize and the fluid that passes through is called the filtrate.[1] Oversize particles may form a filter cake on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known as blinding. The size of the largest particles that can successfully pass through a filter is called the effective pore size of 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 in engineered systems; there are biological, geological, and industrial forms.[2]
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 by entrainment, phagocytosis, adsorption and absorption. Examples include slow sand filters and trickling 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 microscopic Vorticella up to the basking shark, one of the largest fishes, and the baleen whales, all of which are described as filter feeders.
Physical processes
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Methods
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There are many different methods of filtration; all aim to attain the 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.
Hot filtration, solution contained in the Erlenmeyer flask is heated on a hot plate to prevent re-crystallization of solids in the flask itselfThere are several laboratory filtration techniques depending on the desired outcome namely, hot, cold and 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.
Hot filtration for the separation of solids from a hot solutionHot 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.[3] 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 processCold 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.
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.[4]
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.[4]
Filtering force
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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.[5]
Filter media
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Filter media are the materials used to do the separation of materials.
Two main types of filter media are employed in laboratories: surface filters, which are solid sieves that trap the solid particles, with or without the aid of filter paper (e.g. Büchner funnel, belt filter, rotary vacuum-drum filter, cross-flow filters, screen filter), and depth filters, a bed of granular material which retains the solid particles as they pass (e.g. sand filter). The surface filter type allows the solid particles, i.e. the residue, to be collected intact; the depth filter does not permit this. 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.[6]
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. Self-cleaning screen filters utilize point-of-suction backwashing to clean the screen without interrupting system flow.[clarification needed]
Achieving flow through the filter
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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 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
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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 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
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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 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.
Biological filtration
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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
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Inside mammals reptiles and birds, the kidneys function by renal filtration in which the glomerulus selectively removes undesirable constituents such as urea, followed by selective reabsorption of many substances essential for the body to maintain homeostasis. The complete process is termed excretion. Similar but often less complex solutions are deployed in all animals even the protozoa where the contractile vacuole provides a similar function.
Biofilms
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Biofilms are often complex communities of bacteria, phages, yeasts and often more complex organisms including protozoa, rotifers and annelids which 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 the Schmutzdecke on the surface of slow sand filters and the film on the filter media of trickling filters which 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 in biofiltration methods assist in removing significant volumes of effluents from the wastewater.[7]
Systems for biologically treating wastewater are crucial for enhancing both human health and water 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.[8]
Filter feeders
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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 of protoplasm held in the water flow as in the suctoria to various arrangements of beating cillia to direct particles to the mouth including organisms such as Vorticella which 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 the Rotifera and the Ectoprocta. Many aquatic arthropods are 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 some caddis flies spin fine webs in the water flow to trap particles.
Applications and examples
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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:
Adsorption filtration removes contaminants by adsorption of 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 as dwell time. Slower flow also reduces pressure drop across the filter. Applications include:
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.See also
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Pages displaying wikidata descriptions as a fallback
References
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The air surrounding us comprises 78% nitrogen, 21% oxygen and 1% various gases and solid particles. This last component listed comprises such elements and compounds as noble gases, carbon dioxide, fine particles, salts and gas emissions from motor traffic and industry. Although one percent sounds like very little, it does determine whether the air quality is considered healthy or unhealthy.
Although the operation of a filter may appear very simple in theory, filters are in fact highly complex products. The filter fibres have to allow sufficient air to pass through – without offering too much resistance – while also trapping harmful particles. This is the strength of good filters.
A human being inhales and exhales some twenty kilograms of air daily. Twenty kilos! This is quite an impressive figure, particularly when one considers that a human being also consumes around one and a half kilos of food and two and a half kilos of water. People are inclined to pay close attention to what they eat and drink, while government bodies also issue dietary recommendations. It therefore appears only logical that we should devote greater attention to the quality of the air we breathe. How might airborne substances affect our performance and health? And what can we do to ensure the optimum quality of the air that we breathe?
During the past few years, increasing attention has been drawn to the hazards of fine particles; air pollution in the form of particles which are smaller than 10 microns. Busy roads, industry, combustion engines and the bioindustry are major sources of fine particles. The human body is poorly equipped to deal with fine particles. The nose and windpipe act as natural filters for relatively large particles – larger than 5 microns. However, smaller particles can penetrate deep into our lungs, where they may cause substantial damage to health. Children, the aged and people with respiratory complaints are particularly susceptible. The concentration of fine particles in the air can vary greatly from region to region and from one country to another.
People in the western world spend around 70% of their time indoors. Countless health problems can consequently be attributed to ‘indoor conditions’. Air quality in the workplace is sometimes also far from perfect. This can cause sick building syndrome (SBS). Almost three quarters of cases of SBS can be attributed to the dust particles present within the premises. Common symptoms of SBS include listlessness, concentration and respiratory problems, headaches, drowsiness, skin and eye irritation and fatigue. Adequate air filtration is a relatively simple means of combating SBS and protecting people from its harmful effects. AFPRO Filters’ range of appropriate products enables us to vouch for the air quality. Our sales staff are equipped to provide a suitable solution for a healthy indoor or outdoor climate in any circumstances. These applications are widely used in business premises, hotels and conference centres.
Apart from protecting people, filters can also be used to guarantee the progress of operating processes. The applicable filter requirements naturally vary, depending on the type of operating process in question. AFPRO Filters can nevertheless provide a suitable filter, whatever the process. Many of our products are ultimately destined for the nuclear industry, in gas turbines, in the field of semiconductor manufacturing and the pharmaceuticals sector.
The nuclear filter industry plays an essential role in the global supply of energy and the military sector. Air filtration systems perform crucial roles in nuclear plants, such as power stations, fuel processing plants, research facilities and waste management. These nuclear air filters comply with the most stringent environmental standards, in terms of the requirements applicable for the minimisation of radioactive air pollution.
The primary function of an air filter inlet system is to protect the gas turbine and other rotating machinery from pollution present in the ambient air. Dust particles (> 5 µm) can cause erosion. Fine particles (submicron) contaminate the vanes, which has a detrimental effect on the performance of the gas turbine. A wellbalanced filter system is therefore crucial to optimum output.
Highly stringent standards are applicable in this industry. The products, which are often manufactured in cleanrooms, are highly susceptible to disruption. The slightest level of pollution in the air – comprising even the most minute particles – can significantly raise the percentage of rejects from the production process. Prefilters, fine filters and HEPA filters ensure that the air present in the cleanroom is of the highest quality.
Poor air quality during the execution of production processes in the pharmaceuticals sector can have far-reaching consequences. The contamination of drugs can affect their efficacy or render them altogether ineffective, which could naturally prove hazardous to health. The use of superior quality filters is therefore crucial if the production of medicines in a manufacturing plant is to proceed without complications.