Any physical, mechanical or biological operations which are used to separate suspended solid particles from fluids (Liquids or Gases) by passing them through a medium through which only a fluid can pass, so that the solid particles can be separated is filtration. Either the solid particles are separated in the process or the liquid free of solid particles may be the desired product in this process. The process of filtration was employed in ancient times, where people obtained clear water from muddy rivers by digging a hole in the sand on the bank of the river.
Clearwater filtered by the sand would trickle into the hole and be available for use. Similarly, water found in wells is relatively clean and not muddy even though it is sometimes very deep because it undergoes natural filtration due to the soil particles. The fluid that is separated from the process of filtration is called the filtrate. In physical filters, the solid particles are trapped and retained by allowing the filtrate to pass through small openings which do not permit oversized solids from doing the same. In the case of biological filters, some particulates are trapped and ingested by certain enzymes and certain metabolites are retained and removed. An example of a biological filter is the renal system which filters and removes waste from the blood in the form of urine. An alternative to filtration is centrifugation followed by decantation.
Applications of Filtration:
1) When fluid and particles are in suspension, filtration is one of the easiest ways to separate it, regardless of whether the fluid is a liquid, gas, or supercritical fluid. Depending upon the specific need, either one or both of the components in suspension can be isolated and recovered.
2) Filtration is an important physical operation in chemistry for the separation of mixtures of materials with different chemical composition. If in a mixture of two materials, a solvent can dissolve one of them, that solvent can act as a filter, and the filtrate can be obtained by passing the mixture through the solvent so that one of the components can dissolve in the solvent and the needed component can be retained. This is one of the most important techniques used in the laboratory to purify compounds.
There are multiple methods which can be applied for the process of filtration. The method used depends upon whether the solid is particulate or dissolved in the fluid and the nature of the fluid itself. Some classifications are
1) General Filtration: This is the most basic form of filtration and uses gravity and a physical filter. The mixture is poured from above onto the physical barrier like filter paper and gravity pulls the liquid down. The liquid flows below the barrier while the solid is left on the barrier used.
2) Vacuum filtration: A hose and Buchner flask are used to pull a vacuum to suck the liquid through the filter with the aid of gravity. This is used when the solid is needed for something. This can also greatly speed up the separation and also helps to dry the solid.
3) Cold Filtration: This is used when the solid is dissolved in the liquid. Cold filtration is done by quickly cooling a solution, prompting the dissolved solid to form small crystals. This method is usually performed by placing the container with the solution in an ice bath before the filtration process.
4) Hot Filtration: In the process of hot filtration, the solution, filter, and funnel are heated to minimize the formation of crystals during filtration. This method is used when the crystals that form tend to clog the physical filter and prevent the passage of the filtrate. Stemless funnels are used as they have less surface area for the growth of crystals.
5) Solvent Filtration: A solvent is used as a filter medium and it separates two components by dissolving a single component and allowing the other to pass through. It is extensively used in laboratories to separate components.
The requirements for performing the process of filtration are:
1) A filter medium – Filter medium can roughly be divided into two classes
(a) Thin barriers exemplified by things like filter screen, filter paper or filter cloth. These are also called surface filters since the filtration process takes place in a surface. This type allows the solid particles, that is the residue to be collected intact, but it is more prone to clogging, as the solid particles can block the fluid flow after a certain accumulation.
(b) Thick and en masse barriers are used, like sand beds, coke beds, porous metal, and filter aid. This type does not allow the solid particle to be separated, but it is much less prone to clogging due to the greater surface area. It is cheaper than surface filtrating since it is easier to discard the contaminated granules than clean the sieve.
2) A fluid with suspended particles – Weather it is a homogenous mixture or heterogeneous one, or the solid particles are dissolved or not, a fluid with solid particles in it can go through filtration to get the particles separated.
3) A driving force – The driving force is the force which performs filtration. The most common force is the force of gravity, observable when a liquid is poured down onto a filter from above. The other driving forces include vacuum suction during vacuum filtration and also applying pressure onto the mixture to facilitate smooth separation. In centrifuges fitted with a physical filter, the centrifugal force is the driving force.
4) A mechanical device to function as the filter – Mechanical devices like the filter, basic sieve and funnel is needed to perform filtration. It has different specifications depending upon the usage and the mixtures, mainly because the accumulation of the residue is quite common, and it clogs these mechanical devices.
5) A filter aid (optional) – Certain methods can be employed to increase the effectiveness of filtration. And one of these methods is by using filter aids. The aids commonly used are diatomaceous earth, kieselguhr, wood cellulose, and perlite.
These aids are either used as a precoat before the filtration process, by which it can prevent gelatinous solids from plugging the filter medium while simultaneously ensure a clearer filtrate. Or they can also be added to the mixture just before filtration, by which it can increase the porosity of the filter cake while also reducing the resistance of the cake during the process of filtration. The use of filter aids is only when the soli filter residue can be discarded or when the precipitate can be separated from the filter aids after the process of filtration.
Alternatives to filtration:
Filtration can be said to be a very efficient method for separating mixtures, but it has its own drawbacks. It is time-consuming, especially when the solid particles that are supposed to be separated are very fine, and block the pores on the filter medium, and thus the filtration process becomes very slow. Also, if very small amounts of solutions are involved, most of the solution is likely to end up being soaked by the filter medium and the mechanical components.
The best alternative which can be used in the place of filtration is centrifugation followed by decantation. Centrifugation is especially useful for separating gelatinous or fine particles, along with solids that do not separate well.
Examples of an everyday process applying filtration:
i) The coffee filter is used to separate coffee from the beans.
ii) Air conditioners use filters to remove particles from the air.
iii) In the mining process, belt filters are used to extract precious metals.
iv) The filtration process is applied in furnaces to prevent fouling of particulates.
v) Combustion engines, ventilation systems, and industrial goods employ air filters.
vi) Oil filters are used in automobile canisters and cartridges.
vii) Filters that purify water can be found in modern aquariums.
viii) Our kidneys perform filtration to absorb waste from the blood.