Porous media are substances with spaces through which liquids or gases can pass, including sand, earth, stone, sponges, and foams. Porosity affects permeability and is important in filtering and oil and gas exploration. Porous media are used in filtration for purification and water treatment, as well as in aquaculture and aquarium systems.
A porous medium is a substance that contains pores or spaces between solid material through which liquids or gases can pass. Examples of naturally occurring porous media include sand, earth, and some types of stone, such as pumice and sandstone. Sponges, ceramics, and crosslinked foams are also produced for use as porous media. The possible applications of these materials in science, industry and everyday life are vast, although they are perhaps most commonly used as filters.
Physically, a porous medium can be distinguished from other materials, including other porous media, by its porosity or by the size of its pores. Materials with low porosity are less permeable and typically have smaller pores, making it more difficult for gases or liquids to pass through, while materials with high porosity have large pores and are easily permeable. Porosity is an important consideration in filtering, because if particles are to be removed from a porous medium, the pores must be small enough to trap them effectively. Geologists also consider the porosity of the surrounding stone and soil when conducting observations of oil and natural gas reservoirs. Natural gas trapped in low-porosity stone is known as “tight gas” and is more difficult to access than other reserves.
Porosity varies from a low percentage in dense shale and sandstone to about 50% in sand and up to 70% in clay. Man-made materials can be even more porous. For example, crosslinked foam, a porous medium used in air conditioner filters and cosmetic applicators, has a porosity of up to 98%.
One of the most common applications of porous media in science and industry is filtration. In manufacturing plants, these materials can be used to filter gases or liquids either mechanically, by trapping particles, or chemically, by selectively removing certain compounds. The substance to be filtered is filtered through the porous medium and is purified as it passes through the pores of the material. Examples of industrial uses of this method include water treatment and petroleum refining.
Porous media are also often used in aquaculture and home aquarium systems. Carbon filters, made up of porous activated carbon, are used in aquariums to absorb unwanted organic compounds and metals from the water. Activated carbon is an example of chemical filtration, but mechanical filtration is also an option. In the latter system, a porous sponge mechanically filters the particulates, creating a cleaner environment for the organisms in the tank.
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