Swimming Pool Filtration

The advent of High Rate Sand Filtration came into existence during the Space Age. Newer materials were developed to withstand higher pressures within smaller vessels. The filter media is usually silica sand, but materials such as zeolite products have been introduced as a sand replacement. The mechanics of high rate sand filtration is as follows: the water from the pool is forced down into the filter sand. The sand media traps the particulate matter, and the cleansed water is returned to the pool through small laterals in the bottom of the tank. As the sand traps dirt and debris, the pressure within the tank rises. Since the pool operator cannot tell how dirty the sand is, the only way to determine when to clean the sand filter is to look at the pressure gauge. The industry standard is to backwash the filter when the pressure gauge rises 8 to 10 pounds per square inch. The filter is not completely full of sand; there is an area above the sand bed called freeboard. During the backwashing phase, the water is redirected from the bottom of the sand bed, and churns the water upwards into the freeboard area. The dirty water flows out of the top of the sand filter into the sanitary sewer. Depending on the size of the filter and the amount of dirt accumulated, the backwashing process will usually take 5 to 10 minutes. Pool operators can tell when the filter sand is clean by looking into the backwash site glass, located on the backwash line. When the water runs clear, the process can be reversed and filter valves can be changed back to filter mode. Larger pool facilities now incorporate automation to change the valves. 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

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