A Guide to Swimming Pool Filters


The purpose of a filtration system is to pump the water from the pool into a filter to remove dirt and debris, and then return the filtered water back to the pool.

The importance of proper swimming pool filtration cannot be overstated as it removes insoluble matter such as dirt and algae, assists keeping the pool safe and sanitary for swimmers and helps produce clear, sparkling water.

Filtering removes much of the material suspended in the water such as plant and animal sources which provides food for bacteria and algae. The filter also removes the remaining dirt, debris, oils, skin, hair and organisms not trapped by the skimmer box.

How long you need to run a filter depends on the size of your swimming pool and the horsepower of your pool pump. If you are unsure check your instruction manual or consult with your local pool professional.

The three most popular types of filtration currently in use in domestic pools are high-rate sand or zeolite, cartridge and diatomaceous earth (DE).

Sand or Zeolite Filters

Modern high-rate sand or zeolite is a natural low maintenance filter system which works on a very simple principle. Sand or zeolite is packed into a filter tank and the water is drawn from the surface by the pool skimmer box and pumped through the filter. As the water passes through the sand or zeolite bed, the dirt particles and impurities attach themselves to the grains. The larger particles are trapped at the surface of the sand or zeolite bed and the smaller particles are collected deep inside the bed where the finer grains are more tightly packed.

Over time, dirt and debris trapped by the sand or zeolite will start to restrict the water flow and cause pressure in the system. This is quite normal and is easily fixed by “backwashing”.

Backwashing – most sand or zeolite filters have a pressure gauge which lets you know at a glance when backwashing is required. To backwash a sand filter you simply switch off the pump and rotate the valve handle to the backwash setting and then switch the pump back on. This takes only two or three minutes and is followed by a rinse cycle of about half a minute. You can tell when the water is clean by watching it as it comes out of the waste backwash line.

Modern filters now have a sight glass next to the filter, which allows you to watch without leaving the filter.

Sand or zeolite filters will filter pool water to a satisfactory 10-12 microns. Their ease of use, and the fact the sand or zeolite only needs replacing only every three to five years, makes these type of filters one of the most popular

Diatomaceous Earth (DE) Filters

The DE medium is actually fossilised diatoms, which are similar to coral. When crushed they create a very fine powder and each grain has microscopic holes running through it to trap even the finest particles.

The modern DE filter is a pressure type, similar to a sand filter that it is placed on the pressure side of the pool pump. The filter housing contains a number of grids, or pads, which are covered with fine material such as dacron or polypropylene. While this material has some filtering capacity its purpose is really to hold the DE.

The DE is mixed into a bucket of clean water to form slurry which is then fed into the skimmer box with the pump turned on. Once inside the filter, pads, septums or tubes trap the DE to form layers or cakes of DE, and these layers or cakes then begin to trap the particles in the water flowing through.

When the filter has trapped a certain amount of debris, the water flow becomes restricted and registers on a gauge. This indicates the need for backwashing and although it is the same process used for sand filters (see above) the difference is that some of the DE is also flushed away and new slurry needs to be added after the backwashing is complete.

For the system to operate efficiently, DE filters need to be taken out at least once a year and soaked in a light acid solution, which is available from pool shops.

Despite the slight increases in cost and maintenance effort, DE is amazingly efficient because of its capacity of filtering to levels as low as 5.0 or 6.0 microns.

Cartridge Filters

There are two types of cartridge filter – vacuum and pressure.

In a pressure type cartridge filter, the cartridge is contained in a sealed tank, located on the pressure side of the pump. In a vacuum type, the cartridge is usually housed in a skimmer box, drawing the water directly from the pool. The cartridge consists of one or a number of concertina-shaped filters to trap the dirt or debris.

Cartridge filters do not need backwashing which makes them ideal for use in areas where space is limited and backwashing is difficult. They are also suitable in limited water supply areas where the loss of water due to backwashing is a problem.

As with other filters, clogging eventually takes place. This is registered on a pressure gauge as pressure builds up. The filter element is then simply removed and hosed off.

Due to the large filtering service provided by the concertina design, a cartridge filter enjoys a long filter life and provides quality filtration. The larger the cartridge filters the less often it will require cleaning.


Whatever method of filtration is selected, it’s important to remember that it alone does not keep water looking its best. Proper water chemistry must also be maintained for safe, clean water. Also see A Guide to a Clean Sanitised Swimming Pool and A Guide to Maintaining Swimming Pool Water Quality.

Many pool problems are directly related to water chemistry or improper sizing of filters and pumps, rather than failure of the equipment to do its job.

Ultimately, your choice will depend on your particular requirements, environment and preference so seek the advice of your local pool shop professional.


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