What Is Filtration? Definition and Processes

What Is Filtration?
A filter separates solid particles from a liquid or gas in filtration.

Filtration is a process that separates solid particles from liquids or gases using a medium called a filter.

How Filtration Works

The requirements for filtration are:

  • Feed – The “feed” is a suspension of particles in a fluid, which may be either a liquid or a gas.
  • Filter – A filter is a lattice that won’t allow large particles to pass. The pore size and thickness of the filter determine the size of particles that are allowed. A surface filter traps particles on top, while a depth filter traps particles within the filter. Filter paper is an example of a surface filter, while sand is an example of a depth filter. The solid collected on a surface filter is called the “cake.” Trapped particles may clog a filter. This is called blinding. Sometimes filter aids are mixed with the fluid or placed on the filter to minimize blinding. Examples of filter aids include silica, cellulose, perlite, and diatomaceous earth.
  • Filtrate – The filtrate is the fluid that passes through the filter. Filtration is often imperfect, so it’s common for a few particles to escape into the filtrate.
  • Force – Filtration relies on some type of force to move the feed through the filter. This can be gravity drawing a fluid down, pressure pushing the mixture through the filter, or vacuum sucking the fluid through the filter.
Filter Diagram
Filter Diagram

Everyday Examples of Filtration

Filtration is important in the laboratory to separate and purify materials, but it’s also common in daily life. Here are some examples of filtration:

  • Brewing coffee – During the brewing process, hot water passes over ground coffee. A coffee filter separates the grounds from the coffee, which is the filtrate. Steeping tea works much the same way, where the filter is a tea bag (a type of paper filter) or a tea ball (a metal filter).
  • Face masks – Masks filter particles, minimizing the escape of pathogens from an infected person and the inhalation of outside particles. Pore size and thickness are significant factors in mask effectiveness.
  • Kidneys – The kidneys are a biological filter. The glomerulus filters particles out of blood. Water and essential molecules are reabsorbed, while undesirable particles are broken down or encapsulated and removed.
  • Air filters – Air conditioners often use HEPA filters to remove particles from air, while furnace use filters to prevent them from reaching heating elements.
  • Water filters – Water filters filter out large particles, which may include pathogens. Usually these filters also use ion exchange to remove contaminants. An aquifer is a natural water filter that cleans water as it passes through sand and permeable rock.
  • Oil filters – Oil filters prevent debris from harming engines.
  • Belt filters – Belt filters recover precious metals in mining.
  • Aquarium filters – Aquariums use fibrous filters to capture particulates.

Types of Filtration

There are several types of filtration. The best method for a task depends on whether the solid is suspended in the fluid or dissolved in it.

  • General Filtration: General filtration is a basic type of filtration that uses gravity to draw the feed through the filter. Coffee makers rely on general filtration.
  • Vacuum or Pressure Filtration: In one type of vacuum filtration, a B├╝chner flask and hose generate a vacuum to suck the feed through the filter. Usually, this vacuum filtration set-up gets help from gravity. Another type of vacuum filtration uses a pump to generate a pressure difference between the sides of the filter. Pump filters don’t need to be vertical.
  • Cold Filtration: Cold filtration allows collection of dissolved particles. In this method, rapid cooling crystallizes the solid so it can be collected on the filter. In the laboratory, chilling the solution in an ice bath before filtration often does the trick.
  • Hot Filtration: In contrast, hot filtration seeks to minimize crystal formation. Heating the feed, filter, and funnel help prevent the growth of crystals that could clog the filter or undesirable crystal formation in the filtrate. Stemless funnels find use in hot filtration because a long stem offers more surface area for crystal growth.

Difference Between a Filter and a Sieve

A sieve is a device that separates materials based on size using a screen or mesh. Sieves are also called sifts or strainers. Filtration and sieving work on the same principle, but a sieve only has a single layer of “holes.” Good examples of sieves are kitchen strainers and flour sifters. Some people consider a sieve to be a type of surface filter, while others consider them distinct separation methods.

Alternatives to Filtration

Filtration isn’t the only method used to separate mixtures. Two alternatives are decantation and centrifugation.

In decantation, a mixture separates based on differences in density and miscibility. The fluid is poured or siphoned off, leaving the solid. An advantage of decantation is that no material is lost on the filter medium. But, decantation doesn’t work for all mixtures.

Centrifugation uses centrifugal force to separate mixture components based on size, density, and viscosity. After centrifugation, solids form a firm cake. Decanting the liquid results in less loss of either liquid or solid after centrifugation.