Good filtration of the pool water is essential for clear water. The filter removes dirt and larger particles and allows the water and very small particles to pass through. The size of the particles that will pass through the filter is measured in microns. A micron is 1/1000 of a millimeter. The smallest size particle that can be seen without the aid of magnification is about 40 microns.

There are 3 types of filters used in pools, sand, DE, and cartridge. Each type has advantages and disadvantages and the choice of which filter to use may be determined by cost and the size of the area it is to be located in.

Sand filters are the oldest type of filtration and the simplest to use. A sand filter works by passing water through a bed of silica sand. As the water passes through the sand from top to bottom, solid particles lodge in the small crevices and voids between the individual grains of sand. Oils and mucous-like substances tend to cling to the sand and help to trap the smaller particles. A clean sand filter will allow particles 40 microns and smaller to pass through. As the filter becomes dirty, the voids between the sand are filled with the filtered particles and it can filter particles in the 20-25 micron range. Clarifiers which use polymeric substances are available, and they increase the efficiency of a sand filter.

Eventually, the filter becomes plugged and the water flow diminishes. At this point, pressure at the top of the filter rises and it must be cleaned by backwashing. Backwashing reverses the flow of water through the filter and the filtered particles are lifted from the sand and carried away to waste.

Advantages: Easy to use and clean, the media (sand) lasts for years,
Disadvantages: High initial cost, takes up more space, requires several hundred gallons of water for backwash.

D E Filters are capable of filtering the finest particles of the three types. Generally, they can filter down to 3 microns. Diatomaceous Earth is the tiny skeletal remains of old plants called diatoms. Each fossil particle contains passages and channels that the water can pass through. These channels trap the particulate matter. There have been some environmental and health safety concerns with the use and disposal of DE and cellulose fiber alternatives are avialable to replace DE.

D E works by coating fabric covered grids to prevent the dirt, oils, and other matter from plugging the fabric. Most residential systems are designed to be shut down daily. When the pump restarts, the water agitates the DE that has fallen off of the grids and recoats them.

Bump style filters can be bumped to drop the DE, and the tubular elements are recoated to maximize the capacity of the DE. This type of filter uses much less water to change the DE than backwashing a vertical grid filter.

Vertical grid filter elements are the most widely type used in the industry. Residential filters today are usually the pressure type shown in the diagram. When the pressure rises 7-10 psi above the starting pressure, the filter is backwashed in much the same way a sand filter by reversing the flow of water. However, not all of the dirt and DE are removed and the filter needs to be disassembled and cleaned annually. After backwashing, the filter is recoated with new DE.

Advantages: Filters the finest particles to produce maximum water clarity, longer cycles than a sand filter.
Disadvantages: More labor intensive for annual cleaning, most expensive type of filter to purchase.

Cartridge filters continue to grow in popularity because of the simplicity of the installation, compact size, and price. A drop in cartridge is made of a pleated polyester material and are usually rated to a nominal 20 microns. When the element gets dirty, it is removed and cleaned by washing off the dirt with a pressure nozzle on a garden hose. Since no water is used from the pool or spa for cleaning the filter, no makeup water is needed, thus saving on chemicals needed to keep the water in balance. When the cartridge media can no longer be cleaned or is worn, it is replaced with a new one.

Advantages: Lowest inital cost, easy to install, compact size, no water from pool or spa used to clean the filter.
Disadvantages: More time needed to clean the cartridge compared to backwashing, your feet may get wet cleaning the cartridge, cartridge and o-rings need periodic replacement.

The pump should be run long enough each day to pump the volume of the pool through the filter, typically about 6-8 hours. It does not all need to be on one cycle, however. Electric utilities recommend having the pump off during peak times, and they may give a monthly credit if a load management device is installed. Click here to calculate your pump's operating cost.

This chart from Service Industry News, Volume 19, Number 14, July 31, 2005, shows the percentages of filters used in various regions of the US.

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