Understanding how to read the pressure gauge will help tell you what’s going on inside the filter.


Keeping the pool clean is a dirty job, which is why to clear dirt and debris effectively, pool filters need periodic cleanings themselves.

The three most common types of swimming pool filters are diatomaceous earth (DE), sand and cartridge filters.

Understanding how these different pool filters work and how to maintain them can make your job easier, keep your pools looking good, and keep your customers happy.

Micron ratings

The filter’s job is to trap all of the fine dust, dirt and sand particles that pass through the skimmer basket and the hair-and-lint pot.

Filters can make the pool water look beautifully blue and crystal clear, but the water is not necessarily pure. The misconception that a filter can purify water leads too many homeowners and pool technicians to blame cloudy water on filters instead of proper water balance and sanitation.

Filters remove only solid particles from swimming pool water. They generally cannot remove dissolved contaminants, such as oils, bacteria or disease-causing pathogens.

A swimming pool filter’s efficiency is measured by its micron rating, meaning how many microns can pass through the filter. A micron is a unit of length equal to one millionth of a meter (1/1,000,000 m), or 0.0000394 of an inch.

A grain of ordinary table salt is about 100 microns, and the human eye can see down to about 35 microns without the aid of magnification. The lower the micron rating on a pool filter, the smaller the particles it can remove from the water.

Diatomaceous Earth filters

The DE filter is a favorite of pool builders and service technicians because it has the best micron rating. A DE filter can strain out particles as small as 1 to 3 microns.

Diatomaceous earth is a white powder that is made of the tiny skeletal remains of sea creatures, called diatoms, which died prehistorically. Inside the DE filter tank are eight semi-circular grids. The DE coats the grids and forms a filter cake, which acts as a micro-screen to strain out tiny pieces of debris.

Pure DE will not compact, so the pool water can easily pass through it. The dirt and debris that is trapped in the filter cake will cause the filter cake to pack and restrict water flow.

The DE filter may be partially cleaned by backwashing. All DE filters have a valve that allows the pool water to run backwards through the filter. Pool water enters the filter and rinses most of the filter cake, dirt and debris away to the sewer drain.

The filter grids must be re-coated with a new filter cake after each backwashing. Because backwashing does not remove all of the dirt and debris from the filter, it is necessary to periodically open the filter and clean each grid individually.

Separation tanks

The separation tank, found on many systems, is used to backwash a DE filter. The job of the separation tank is to separate the DE and filter dirt from the water while it is being backwashed.

Many cities have banned DE and filter dirt from being backwashed down the street and into storm drains. Some cities also don’t want DE and filter dirt entering the sewer system, lest it clog the pipes. Therefore, during backwashing, the discharged water must pass through a separation tank.

The separation tank contains a strainer bag, which traps the DE dirt and muck. Depending on the city ordinance, the clean, DE-free pool water is either returned to the pool or sent down the sewer drains.

The separation tank should be cleaned every time the filter is cleaned, and its contents must be disposed of in a proper trash receptacle. It is important to clean the separation tank at regular intervals — i.e., every time you clean the filter — because the more impacted it gets, the less effective it becomes; the heavier the strainer bag gets, the harder it is to remove; and the more oils it collects, the worse its odor will be.

Adding DE
Filter Size

Adding DE

After backwashing the filter, you will need to add more DE to it through the skimmer. Some pool techs will put the DE into a bucket, add water, then pour the whole mixture down the skimmer; others will scoop the DE into the skimmer directly.

DE is added at a rate of 1 pound per 10 square feet of filter (always round up). For years, a 1-pound coffee can was used as a DE measure; a 1-pound coffee can holds 1/2 pound of DE, so you would use two 16-ounce coffee cans per pound of DE. But coffee cans now hold only between 10 and 13 ounces — not 16 — so they are not recommended anymore. A 1-pound scoop is available at any supply house. A 44-ounce drink cup also holds a pound of DE.

Sand filters

A sand filter is the oldest and simplest filter in use: The water used in the famous Roman baths was filtered by running it through sand. The sand inside the filter is called the sand bed. As the pool water passes through tiny openings in the sand bed, dirt particles and other debris are trapped.

Sand filters are virtually maintenance-free. A good sand filter can go years and years on backwashing alone, if the backwashing is done right. There are no grids to clean and no DE or sand to add — just backwash and go.

The edges of the sand particles create tiny crevices to filter the water and catch the debris. As dirt passes through the filter, it becomes trapped in these crevices while the clean water passes through.

Just as you would with a DE filter, when backwashing a sand filter, you run the water backwards through the system so that all the dirt in the crevices and gaps rises to the top of the filter and goes down the backwash line.

Sand filters have the highest micron rating. They start out at about 40 microns and go down to 20 microns over time. “A sand filter starts to clean better the dirtier it gets,” says John Ott, Western Regional Technical Training Manager at Hayward Pool Products, Elizabeth, N.J.

Because of this high micron rating, the water in a sand-filtered pool could start to look cloudy. When this happens, simply add a good clarifier. A clarifier will gather all of the small pieces of debris that pass through the filter and clump them together into a larger piece of debris that will become trapped in the sand filter’s crevices.

Backwashing a sand filter

The only way to clean a sand filter is by backwashing it. And it’s important to remember that this is a two-step process.

First, backwash the filter for at least two minutes, or until the water runs clear. In normal run position, the sand gets packed down. During backwashing, the sand rises and separates, thus releasing the debris trapped inside and allowing it to flow out of the filter.

Second, allow the filter to sit for 15 to 30 seconds. This lets the sand inside settle down again. Then, set the backwash valve to the rinse setting and rinse the filter for 30 seconds, or until the water runs clear.

A common complaint about sand filters is that a small amount of dirt will shoot back into the pool after backwashing. This is often because the second backwashing step — allowing the filter to sit for 15 to 30 seconds so the sand can settle and re-trap the dirt, followed by rinsing the filter — was not performed.

Bypassing this step may cause some dirt to escape the filter and re-enter the pool. With proper backwashing and rinsing, and the addition of a clarifier, a sand-filtered pool should stay clean and blue.

Cartridge filters

Developed in the 1950s, cartridge types are the newest of the swimming pool filters. They are also the simplest to maintain, which is why they are so popular today. 

The cartridge of the filter closely resembles a car’s air filter, but is much taller. The cartridge material is made from a pleated polyester cloth. As water passes through the pleated material, dirt particles and debris are trapped within the pleats. A new cartridge filter can strain out particles at about 20 microns, but will go down to as low as 5 microns. The micron rating actually gets lower as the filter gets dirtier.

The cartridge must be removed for cleaning, as the filter cannot be backwashed. It’s a good idea to clean a filter every six months to a year, depending on the bather load and the size of the cartridges. (See “The Pressure’s On” for more information on when to clean a filter.)

To clean the filter, remove the cartridge and simply hose it off. 

After hosing the filter clean, take a minute to inspect it. The pleats should be straight, not buckled or crooked. Also, inspect the molding on the top and bottom of the cartridge. Is it in good shape, or is it starting to tear away from the pleats? Buckling and tearing are signs that the cartridge is getting old and needs to be replaced.