PRB Articles

Swimming Pool Filtration

When beginning any course on filtration, my first question to the class is, “Have you ever walked up to a pool and it looked as if diamonds were jumping off the water?” This imagery of sparkling water is the result of a conscious effort to maintain water clarity through filtration.

Water clarity is dependent upon proper filtration, circulation and chemical treatment. These articles have discussed circulation (hydraulics) and pool chemical treatment, but to ensure optimum clarity, the filtration system must be addressed. The pool water is cleansed, and particulate matter is removed when it passes through the filter media.

The opposite of clarity is turbidity--the lower the turbidity, the better clarity. Water clarity can be determined by several mechanical methods. A nephelometer, or a turbidimeter, is a device that measures the turbidity of water and industry standards set the Nephelometer Turbidity Units, not to exceed .5 NTUs. Most pool managers do not have one of these meters, and a visual inspection is mandated. The pool manager must be able to see the pool drain clearly from the pool deck. It is standard practice to close a pool if turbidity is too high, which can be determined by viewing the main drain. Some pool managers have a small 2-inch disk with black and red quadrants they place on the floor of the pool. The ability to differentiate the colors is necessary for water clarity.

Understanding Filter Media

The filter media traps particulate matter as it flows through the filter vessel. The type of media used will determine the desired size of the suspended, trapped particle. The particle size is measured in microns1-millionth of a meter. Silica sand, zeolite, crushed glass, diatomaceous earth, cellulose powder, perlite, spun-bonded polyester material and antimicrobial cartridge fabrics are just a few types of filter media used today. Each type will filter down to a different micron, and is placed in a different type of vessel, called a filter system.

Past, Present And Future Of Filtration

In the earliest known filter, boiled water was poured through sand or charcoal. During the exploration period, seashells were used as media for clarifying water on ships. From 1740 to 1800, the Scottish Enlightenment created an outpouring of scientific thought. One of the founders of swimming pool filtration was Robert Thom, a Scottish scientist and engineer. He built the first municipal water treatment plant in Paisley, using slow sand filtration. Subsequently, the rapid sand filter evolved. This filter is still being used in various swimming pool applications where the vessel contains gradients of gravel, and then a layer of sand.

High Rate Sand 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

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.

The basic mechanics of Regenerative Media Filtration are similar to those of a “bump filter.” There is a series of long, tubular elements made of fiber, which “dangle” from a manifold. The filter elements are flexible tubes that provide the support structure for the media. The outer wall of each element is fabricated of multi-filament, high-strength polyester braid. The diatomaceous earth , or man-made powder, adheres to the fiber and traps the particulate matter, and filters down to four to five microns. In order to dislodge the dirt and debris from the media, a bump mechanism is installed on the filter tank head. The mechanism pressurizes and depressurizes, causing the connected filter element to move up and down. This movement removes the accumulated solids. The final phase of operation is vacuuming the soiled media and regenerating with new filter powder. The growing popularity of this type of process is reduction in water (no backwashing as in sand) and reduction in chemical costs, requiring less space and maintaining a higher degree of water quality.

Aquatic managers should determine which type of filtration will suit their facility based on factors such as water availability and space requirements. In addition, managers should develop preventative maintenance programs for each type of filter in use at their facility. In addition, preventative maintenance should be in place to be sure the pump is providing the proper flow through the filters. High flow rates, greater than the system is able to tolerate, will lead to damage to the cartridge fibers, rip diatomaceous earth grids, and cause channeling in sand filter operation that will lead to poor water clarity. Water clarity is the goal and responsibility of any pool operation.

Connie Gibson Centrella is Program Director for the online Aquatic Engineering Program at Keiser College eCampus. Ms. Centrella is an industry veteran with over 40 years experience in the pool and spa industry. She is a former pool builder with extensive knowledge in pool construction and equipment installation as well as manufacturing.

About the Keiser College Aquatic Engineering Degree The Keiser College Associate of Science Aquatic Engineering Degree is a two-year degree consisting of 60 semester hours. Each student is required to complete 36 credit hours of major courses and 24 credit hours of general education courses. The degree program encourages students to broaden their knowledge in all aspects of swimming pool and spa management and operation. It is offered exclusively online, making the degree program available to the national and international community of pool and spa professionals, and those seeing employment in the field. The online format makes it possible to offer a global study program that enables schedule flexibility and increased access to those currently employed.

For more information about the Aquatic Engineering degree at Keiser College, visit and click on “Online Education”, or call 866-535-7371 or e-mail ccentrella@keisercollege.eduThis e-mail address is being protected from spambots. You need JavaScript enabled to view it and request a brochure.

Roll Out The Mats

The Power Of Sports 101