Swimming Pool Filtration

The surface area on the top of the sand bed is the determining factor in sizing the proper filter. Consider a 36-inch (3-foot) diameter sand filter. The surface area is determined by calculating the area of a circle (pi x radius x radius), 3.14 x 1.5 feet x 1.5 feet, or 7 square feet of filter surface area. High rate sand filters can be horizontal or vertical, as shown on Exhibits 1 and 2.

High rate sand filters are considered the easiest to maintain and operate, but will only filter down to approximately 25 microns (1-millionth of a meter).

Cartridge Filtration

Cartridge filtration is used primarily in spas and pools where water is scarce and backwashing is prohibited. Cartridge filters are designed to filter pool/spa water down to 15 microns, which leads to better filtration than a high rate sand filter. Cartridge filtration utilizes an element made of spun-bonded, pleated fiber, which is placed in a cylinder vessel. The water passes through the cartridge, and the fiber traps dirt, oil and hair. The advantage of cartridge–besides filtering down to smaller microns–is that it takes less space in the filter room.

The filter surface area is the square footage of fiber material. Imagine taking the pleated cartridge apart, laying it flat and measuring the length x width. This will give you the filter surface area (2 feet wide by 25 feet long will yield a 50-square-foot cartridge filter). The downside of a cartridge is the dirty cartridge filter must be removed from the cartridge vessel and cleaned. Most pool operators use a pressure-washing technique, or soak the cartridge in a filter cleaner to remove the dirt and oils. Warning: never clean a cartridge filter with muriatic acid, as it sets oils. It is better to clean with trisodium phosphate (or filter cleaner) to remove oils; if scale persists, then rinse with a mild solution of muriatic acid.

Newer technology for cartridges—those made out of anitmicrobial fibers–is now available. Pool and spa cartridges are now impregnated with minerals, such as silver. Traditional fiber has been replaced with a new technology that weaves silver zeolite into the fabric. Silver is effective in killing a broad range of microorganisms, including mold, mildew and fungi. Since cartridge filters do not require traditional backwashing, there is little loss of water and chemicals

Diatomaceous Earth Filtration

Diatomaceous earth is ground-up fossilized skeletons of small sea plankton called diatoms. This powder is held against a grid device made of a cloth-like material when the filter is operating under pressure. The diatomaceous earth forms a coating on the cloth. As the water passes through the diatomaceous earth, it traps the suspended particulate, such as dirt. The more dirt and grime that are trapped, the higher the pressure. As in the other types, it is best to clean the grids and remove the dirty diatomaceous earth powder when the pressure gauge increases 8 to 10 pounds per square inch.

The dirty powder is removed and fresh powder added as the method of cleaning the filter. Some areas of the country require a separation tank, which will collect the diatomaceous earth powder for disposal. It is not allowed to go into a sanitary sewer. Due to some health concerns with diatomaceous earth, there are now man-made powders available for use by pool professionals.

The method of determining filter surface area is different from sand and cartridge. The grids, which can be round or rectangular, are measured on both sides. They are held in a manifold, and coated on each side. As an example, if you have 10 grids and each grid is 3 feet x 5 feet, the total filter surface area is 300 square feet ( 3 feet x 5 feet x 2 (both sides) x 10 grids = 300 square feet). The standard coating formula for diatomaceous earth filters is 1.25 pounds per 10 square feet of filter surface area, so in our example, you would need to add 37.5 pounds of diatomaceous earth powder. Pool managers should make an inspection of their grids and take measurements to be sure operators are adding the proper amount of powder. To clean the diatomaceous earth fabric, it is advisable to clean the same as a cartridge, either pressure-washing or soaking in filter cleaners, and avoid using muriatic acid.

At the recent National Swimming Pool Foundation-sponsored World Aquatic Health Conference, there was a presentation on Regenerative Media Filtration, which uses diatomaceous earth or one of the alternative powders.

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