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What Is a Porous Medium?

By E.A. Sanker
Updated May 17, 2024
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A porous medium is a substance that contains pores, or spaces between solid material through which liquid or gas can pass. Examples of naturally occurring porous media include sand, soil, and some types of stone, such as pumice and sandstone. Sponges, ceramics, and reticulated foam are also manufactured 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 the size of its pores. Materials with low porosity are less permeable and typically have smaller pores, making it more difficult for gas or liquid to pass through them, while materials with high porosity have large pores and are easily permeated. Porosity is an important consideration in filtering, since if particles must be removed by a porous medium, the pores must be small enough to effectively trap them. 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 ranges 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, reticulated 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 strained through the porous medium and becomes purified as it passes through the pores in 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, which consist of porous activated carbon, are used in aquariums to absorb undesirable organic compounds and metals from the water. Activated carbon is an example of chemical filtration, but mechanical filtration is also a possible option. In the latter system, a porous sponge mechanically filters out particulate matter, creating a cleaner environment for the organisms in the tank.

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Discussion Comments

By wavy58 — On Feb 12, 2012

@Perdido – Sponges come in handy for many things. I use sponge applicators to apply my makeup, and this works much better than using my finger.

A sponge can hold onto liquid foundation and disperse it across my face, whereas if I put it on the tip of my finger, the skin will absorb much of it, so it's wasted. Also, if I use a sponge, I don't have to worry about the natural oils from my fingers combining with the makeup and messing it up.

My eyeshadow applicator tip is also a sponge. It glides easily across my eyelid and holds an amazing amount of color. In fact, it is hard to get all the color out of the sponge, so I use different applicators for each shade of eyeshadow.

By Perdido — On Feb 11, 2012

I use a sponge in my kitchen to clean my countertops and dishes. It can hold quite a bit of water and liquid soap, so I can lather it up and use it for awhile before needing to re-wet it.

For the countertops, I saturate the sponge and add a small amount of soap to it. I then squeeze the soapy water onto the surface and wipe it down. The sponge reabsorbs the liquid as I wipe it across the counter, scrubbing spots of grease or dried food.

I prefer using a porous sponge instead of a dish cloth. Rags simply can't hold a candle to the long-lasting performance and moisture of a sponge. It is true that a sponge can also hold more germs and bacteria, but I put mine in the microwave for about two minutes to kill the germs.

By lighth0se33 — On Feb 10, 2012

I increase my soil porosity by adding sand to it. I do this for the plants that don't do well when trapped in moist environments.

Some plants love wet soil and thrive with their roots in constantly moist conditions. Many other plants need good drainage, because their roots will rot if the soil stays wet all the time.

This is why many people add gravel to the bottom of plant containers. If you are going to be keeping your plant in the container for good, then it helps to have something for the water to run through, and this gives the soil a chance to air out. The water can run through the rocks and out the holes in the bottom of the container.

By orangey03 — On Feb 10, 2012

Sand is the porous medium in my swimming pool filter that helps trap debris. Though the basket that sits on top of the hose leading to the filter catches the big stuff, like insects and frogs, the sand catches smaller things, like dirt and bacteria.

I have heard that I should replace my sand soon, because I have been using the same sand for five years. The guy who works at the pool store told me that the grains become too smooth after several years to capture as much junk, so they need to be replaced with new, rougher grains.

I think he's right. Last year, my pool and sand turned five years old, and I had a lot more problems with algae and bacteria than I usually have. I needed to vacuum the pool more than once a week!

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