Filtration is a process used to separate solids from liquids or gases using a filter medium that allows the fluid to pass through 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 the 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 containing 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.
There are different types of filtration. 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 create 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 prevent crystallization of the 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 the filter from clogging and can increase the porosity of the “cake” or feed into the filter.
Filtration vs. 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 contrast, during filtration, the filter is a lattice or has multiple layers. Fluids follow channels in the medium to pass through a filter.
Alternatives to Filtration
There are more effective separation methods than filtration for some applications. For example, for very small samples in which it’s important to collect the filtrate, the filter medium may soak up too much of the fluid. In other cases, too much of the solid can become trapped in the filter medium.
Two other processes that can be used to separate solids from fluids are decantation and centrifugation. Centrifugation involves spinning a sample, which forces the heavier solid to the bottom of a container. In decantation, the fluid is siphoned or poured off of the solid after it has fallen out of solution. Decantation can be used following centrifugation or on its own.