What Is a DE Pool Filter?

Swimming pool cleaning and repairs

MATJAZ SLANIC/Getty Images

DE stands for diatomaceous earth, which is a soft, crumbly sedimentary rock composed of chemically inert, fossilized (or skeletal) remains of billions of microscopic algae-like organisms, called diatoms. Chemically, diatomaceous earth consists mostly of silica (also known as silicon dioxide), one of the most abundant compounds on earth. Silica makes up about 26 percent of the earth's crust by weight and is found in many natural substances, including sand, emerald, quartz, feldspar, mica, clay, asbestos, and glass. The silica in DE is in fairly raw form, however, not yet transformed in any way by heat or pressure.

Diatomaceous earth is mined from various places in the world where sediments from former seas are found. Deposits have been found on dry land, under stream beds and lakes, or beneath the muck of wetland areas. Scattered deposits have been found in many U.S. states, including California, Nevada, Oregon, Washington, Maryland, and Virginia. DE deposits sometimes form chalky layers that crumble upon digging, but it sometimes is found in a compressed rocky form known as diatomite. The composition of diatomaceous earth varies, depending on the species of diatoms that comprise it and the location where it is mined.

When the soft diatomite stone is ground into a white or grayish powder, the resulting diatomaceous earth is a fine abrasive powder that has many practical uses. It is the principal ingredient in one form of filter used to purify water for swimming pools and spas, but also has many other uses.

DE in Pool Filters

There are three types of filters used for pools: sand, cartridge, and diatomaceous earth (DE). Cartridge filters trap contaminants by straining the water through paper-like materials; sand filters pump the water through sand, which captures many contaminant particles; DE filters pump the water through a filter canister filled with DE, which has particles considerably finer than sand and with a structure better suited to capturing contaminants. Many pool owners and pool maintenance workers, therefore, consider DE filters to be the best type for residential swimming pools.

When examined under a microscope, diatomaceous earth (DE) reveals tiny sponge-like organisms that have a remarkable ability to absorb or filter water's impurities. Since the DE powder is so fine, it can filter microscopic contaminants from the water that passes through the filter. The DE used in pool filters has been heat-treated (calcinated) to give the silica a crystalline structure that improves its filtering ability. So-called food-grade DE is not the type used in pool filters.

DIY Tip

When adding fresh DE to a filter, some pool pros recommend using 1/2 cup less than the instructions of whatever brand you use. This is because there is typically some leftover DE powder in the filter, no matter how well you clean it. Too much DE can end up clogging the skimmer, creating additional problems.

Food Industry Uses

Some types of DE are rated as food-grade, but this does not mean it is a food additive. Fresh-water food-grade diatomaceous earth is the type used in agriculture as an anti-caking agent in grain storage, and as a feed supplement for livestock and farm animals. It is sometimes used as a "marker" in livestock experiments to measure the digestive quality of livestock feeds.

Agricultural Uses

Freshwater DE can be used as a growing medium for growing plants in hydroponic gardening.  It is often a component in specially formulated potting mixes, such as bonsai mix.

In uncalcinated form, DE has a variety of uses as a pesticide, especially against arthropods and gastropods such as snails and slugs. On these pests, the DE powder causes the outer layer of the creature's exoskeleton to dissolve, thereby killing the pest through water loss. DE can be found in snail and slug repellents and is used to control cockroaches and fleas. In the garden, DE can be a lethal dust to aphids, ants, thrips, mites, and earwigs, but it is not harmful to beneficial worms.

And yes, the infamous bedbug can be eradicated by DE, although it can take several weeks to do the job.

Other Industrial Uses

Diatomaceous earth has a surprising number of commercial uses:

  • DE is a useful stabilizing agent for nitroglycerine and is used in many commercial and military explosives.
  • Its excellent thermal properties make DE useful for insulating fireproof safes and furnaces.
  • Various forms of commercial chemistry and food processing use DE in a variety of filtering applications.
  • DE is a component in many potable water filtration systems.
  • DE is used in cat litter, and as a filler in cement, paints, and many dry chemical products.
  • Because of its abrasive properties, DE has been used in toothpaste, metal polishes, and facial scrubs.

Purchasing and Disposal Considerations

DE can be purchased in various quantities at hardware stores, pool supply stores, and grain and feed supply stores. It is also available from many online retailers. Make sure to buy a product that is specified for your use. So-called "food-grade" DE, for example, is not the proper product to use in pool filters.

Because DE can potentially clog pipes, some cities and counties do not allow it to be dumped into sewer systems. To accommodate proper disposal, most pool DE filters include a separation tank next to the filter container. When the filter is drained or backwashed, the dirty water passes through a canvas strainer bag inside a small tank before it reaches the sewers or storm drains. Most of the DE is retained in this strainer bag, which can then be discarded with household trash.