Water clarity is a crucial aspect of pool maintenance, whether a job site involves a small family pool or a huge commercial vessel.

But experts agree that there’s no “magic bullet” when it comes to filter media — each type has its ups and downs, and is ideally suited for some situations while potentially disastrous in others.

Here, veteran service technicians and scientists share their perspectives on selecting filter media, and walk through the process of assessing a pool’s filtration needs.


As many service techs know, sand is the oldest type of filter media in the pool industry — but what’s less well known is that it’s the oldest filter media in human history, dating back to the baths of ancient Egypt.

Like many tried-and-true technologies, sand filtration works as well as ever in a limited range of circumstances — but as today’s pool circulation systems have grown more complex, and filtration expectations more stringent, it’s no longer the most efficient option available.

Still, sand is widely used in commercial pool systems — particularly those with relatively slow circulation rates. In the 1950s, when the pool industry was still in its infancy, many public pools used rapid-rate sand filters, which filtered water through a bed of sand with a gravel substrate. This system was soon made obsolete by high-rate sand filtration, which uses much smaller sand particles and no gravel. Thus, most of today’s “sand filters” are more precisely termed “high-rate sand filters.”

These filters typically use sand particles between 0.018 in. and 0.022 in. In size — often called “#20 standard silica sand” — though some operators substitute other media such as zeolite, or a mixture of crushed glass and gravel. Material at this level of fineness can usually entrap particles between 20 and 100 microns in size.

This might sound tiny, but sand actually misses many particles that would be caught by more modern filters. 

Operators who choose sand filtration are often motivated by the desire to keep costs down, or simply by the knowledge that the pool’s bather load isn’t particularly high. In some cases, this is a sensible decision.

However, a sand filter can bring along other costs that might not be as obvious. As the top bed becomes caked over with debris, its sand will sometimes start to form vertical channels, experts explain. These channels can reduce the area where optimal filtration is taking place because the water is mostly flowing through the channels that have formed, instead of diffusing evenly through the filtration media.

Another inconvenience of sand filters is that they must be backwashed frequently — the exact timing varies according to factors such as bather load and flow rate, but most systems require a backwash approximately once per month. In addition, the effectiveness of a sand filter can drop drastically if the sand isn’t replaced at least once every five years. This process can get fairly involved — it entails scooping large amounts of sand out of the filter, finding a place to dispose of the material, then refilling the filter with an even layer of clean new sand.

In short, sand filters are best suited for applications where low cost is a top consideration, bather loads are fairly limited, and yearly “refreshes” are acceptable to both the site operator and the service tech.

A traditional sand filter is composed of a large central chamber — filled with sand and/or gravel — through which water is circulated. A diatomaceous earth filter’s overall design is similar, but its central chamber is more cylindrical, and is filled with grids specially designed to be coated with DE.

Diatomaceous Earth

A significant step up from sand in several ways, diatomaceous earth (DE) is composed of the skeletons of microscopic prehistoric organisms. The complex structure of these skeletons results in much finer filtration — down in the range of 3 to 5 microns — than #20 sand can achieve. This fine texture, combined with high permissible flow rates, has made DE the filter media of choice for many of today’s service technicians.

This effectiveness comes with a price, though. First, DE is toxic to humans, which means it’s crucial to wear facial protection to prevent anyone from accidentally inhaling or swallowing the substance. Care must also be taken to ensure that the powder doesn’t contaminate nearby chemicals or equipment, where it can cause corrosion or other unwanted reactions.

Because of concerns like these, many cities and counties also have regulations against disposing of used DE in the street, or even down public wastewater lines.

These restrictions have led many techs to develop workarounds. Supporters of DE recommend disposing of used media in the customer’s yard (after securing permission, of course) where it acts as a fertilizer for plants. Another option is to use catch basins, which allow water to drain away from the DE, which can then be thrown into the trash.

Still, cleanup and disposal of DE can be nearly as messy as cleaning a sand filter — or, in a way, even more so, given the media’s toxic nature. And DE filter’s require more backwashing, experts say.

DE is ideally suited for pools with high bather loads where pristine water is a major priority. Maintenance won’t be a cinch by any means, but proper care will ensure effective filtration of most particles, even at high flow rates and temperatures.


A major leap forward in filtration technology came with the development of cartridge filters. Rather than using a chamber filled with loose filter media, these filters strain water through a compact array of pleats that catch particles down in the range of 5 to 10 microns in size — about a third the width of a human hair.

Though their don’t catch particles quite as tiny as those caught by DE filters, cartridge filters are more efficient, especially in pools with relatively low bather loads and flow rates. In these conditions, a cartridge filter with a footprint of a few square feet can filter a volume of water in the range of 500 square feet; far more than a sand filter of the same size.

Techs also report that these filters tend to be resistant to breakdowns and serious clogs, and are much easier to clean than sand filters.

And unlike sand and DE, cartridges don’t require backwashing or regular media replacement. Instead, all a tech needs to do is remove the cartridge from the filter tank and hose it off; or, in some cases, dip it in a mild muriatic acid solution to remove particularly stubborn particles.

The downside is that if this cleaning isn’t performed regularly, cartridge filters have a tendency to clog, especially in pools with high bather loads, techs report.

Thus, cartridge filters are best suited for applications with low to moderate bather loads and water volume, such as residential pools that see usage a few times a week. If simplicity of maintenance is a high priority, a cartridge system may be ideal.


Pool & Spa News wishes to thank the following people for providing their expertise for this article:

• Richard Medina, vice president of

operations and engineering at Pleatco, LLC in Glen Cove, N.Y.

• Steve Boykins, owner of AquaPoolCo in Los Angeles

• John Shonfield, owner of Shonfield Pools in Los Angeles

• Scott Gleason, national sales manager at Unicel in Chatsworth, Calif. 

• Bruce Stump, vice president of sales and marketing at Filbur Manufacturing in Buena Park, Calif.