Filtration Filtration is a process used to separate solids from liquids or gases using a filter medium that allows the fluid to pass, but not the solid. The term "filtration" applies whether the filter is mechanical, biological, or physical. The fluid that passes through the filter is called the filtrate. The filter medium may be a surface filter, which is a solid that traps solid particles, or a depth filter, which is a bed of material that traps the solid. Filtration is typically an imperfect process. Some fluid remains on the feed side of the filter or embedded in the filter media and some small solid particulates find their way through the filter. As a chemistry and engineering technique, there is always some lost product, whether it's the liquid or solid being collected. Examples of Filtration While filtration is an important separation technique in a laboratory, it's also common in everyday life. Brewing coffee involves passing hot water through ground coffee and a filter. The liquid coffee is the filtrate. Steeping tea is much the same, whether you use a tea bag (paper filter) or tea ball (usually a metal filter). The kidneys are an example of a biological filter. Blood is filtered by the glomerulus. Essential molecules are reabsorbed back into the blood. Air conditioners and many vacuum cleaners use HEPA filters to remove dust and pollen from the air. Many aquariums use filters that contain fibers that capture particulates. Belt filters recover precious metals during mining. Water in an aquifer is relatively pure because it has been filtered through sand and permeable rock in the ground. Basic priniciple of filtration: Filtration is a simple technique used to separate solid particles from suspension in a liquid solution. There are many filtration methods available, but all are based on the same general principle: A heterogenous mixture is poured over a filter membrane. The filter membrane has pores of a particular size. Particles larger than the pores will be unable to pass through the membrane, so liquid will easily pass through it. The final result of a filtration process is a collection of residue on the filtration membrane. This residue is therefore effectively separated from the rest of the mixture that passed through the membrane. Filtration Methods 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. Which method is used depends largely on whether the solid is a particulate (suspended) or dissolved in the fluid. General Filtration: The most basic form of filtration is using gravity to filter a mixture. The mixture is poured from above onto a filter medium (e.g., filter paper) and gravity pulls the liquid down. The solid is left on the filter, while the liquid flows below it. Vacuum Filtration: A Büchner flask and hose are used to pull a vacuum to suck the fluid through the filter (usually with the aid of gravity). This greatly speeds the separation and can be used to dry the solid. A related technique uses a pump to form a pressure difference on both sides of the filter. Pump filters do not need to be vertical because gravity is not the source of the pressure difference on the sides of the filter. Cold Filtration: Cold filtration is used to quickly cool a solution, prompting the formation of small crystals. This is a method used when the solid is initially dissolved. A common method is to place the container with the solution in an ice bath prior to filtration. Hot Filtration: In hot filtration, the solution, filter, and funnel are heated to minimize crystal formation during filtration. Stemless funnels are useful because there is less surface area for crystal growth. This method is used when crystals would clog the funnel or to prevent crystallization of a second component in a mixture. Sometimes filter aids are used to improve flow through a filter. Examples of filter aids are silica, diatomaceous earth, perlite, and cellulose. Filter aids may be placed on the filter prior to filtration or mixed with the liquid. The aids can help prevent clogging of the filter and can increase porosity of the "cake" or feed into the filter. Filtration Versus Sieving A related separation technique is sieving. Sieving refers to use of a single mesh or perforated layer to retain large particles, while allowing the passage of smaller ones. In filtration, in contrast, the filter is a lattice or has multiple layers. Fluids follow channels in the medium to pass through a filter. Alternatives to Filtration In some situations, there are better separation methods than filtration. For example, for very small samples where it's important to collect the filtrate, the filter medium may soak up too much of the fluid. Figure : Vacuum filtration by Büchner funnel and flask Filtration occurs both in nature and in engineered systems; there are biological, geological, and industrial forms. For example, in animals (including humans), renal filtration removes wastes from the blood, and in water treatment and sewage treatment, undesirable constituents are removed by absorption into a biological film grown on or in the filter medium, as in slow sand filtration. Filter media Two main types of filter media are employed in laboratories: a surface filter, a solid sieve which traps 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 a depth filter, a bed of granular material which retains the solid particles as it passes (e.g. sand filter). The first type allows the solid particles, i.e. the residue, to be collected intact; the second type does not permit this. However, the second type 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. 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.
