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by Steve Frey, National Sales Manager
Originally printed in Swimming Pool / Spa Age, July 1998

Since the first filters appeared in commercial pools in the early 1900s, hydraulic engineers and pool designers have worked to develop better methods of water circulation and filtration. It is these two operations combined with proper water chemistry that allow pool owners to enjoy the same volume of water for years without having to continuously empty and fill the swimming pool.

Let's take a closer look at this process. Filtration is simply the mechanical process by which suspended solids are removed from water. Water treatment, on the other hand, addresses sanitation (removal of pathogenic organisms), pH, total alkalinity, calcium hardness and total dissolved solids (TDS), which is everything in solution or dissolved in the water.

Prior to World War II, commercial pools relied on sand and gravel filters. These systems often had open tops and relied on gravity for water flow. This resulted in very large systems with flow rates of only 3 GPM per square feet. Furthermore, they had to be manually disassembled for cleaning.

At the end of World War II, this process dramatically improved with the rapid rate sand filter.This system had a closed top and could be cleaned by backwashing. Rapid sand filters, while smaller and more efficient than their predecessor, still were not practical for residential use.

Modern residential pool filtration had its birth in the 1950s when Diatomaceous Earth (DE) pressure filters became available, followed by the development of high-rate sand filters and later cartridge filters. Today, after years of refining, consumers have their choice of three types of filtration systems: high-rate sand, DE, and cartridge.

By nature, sand filters have a limited surface area and use depth or torturous path filtration to entrap particulate. Since the 1960s, the accepted flow rate for these filters is 15 GPM to 20 GPM. As an example, a typical 24-inch, high-rate sand filter has 3.14 square feet of surface area and should be sized to flow at a maximum of 63 GPM.

Higher flow rates can cause the laminar or smooth water flow to become turbulent, resulting in channeling. The water will then take the path of least resistance and filtration will be compromised.

Sand filters are considered the easiest type of filter to operate because of the multiport valve system. Backwashing is accomplished by simply rotating the valve to the desired function. Typically, backwashing should last approximately three minutes or until the effluent is clear. Once the backwash is complete, the valve is turned to the "rinse" position and then back to the "filter mode" to resume operation.

During the backwash process, a considerable amount of heated and chemically treated water will be lost. This factor should be taken into consideration in areas where water use is a concern.

Sand filters use a special pool-grade No. 20 silica. This sand is between .45mm and .55mm and has very irregular and sharp edges. This irregular shape is critical to the filter's efficiency and therefore should not be substituted. Under normal conditions, this sand should last between five and ten years before it needs to be replaced. Local conditions, such as water hardness, could reduce the life span if the sand edges become coated with minerals and round off.

Sand filters are considered the least efficient in micron retention and while they do a very good job of filtration, they do not give the water the sparkle of cartridge or DE filtration.

DE filters were developed after sand. Diatomaceous earth is not "earth" at all, but actually the skeletons of microscopic water plants called diatoms. As such, DE is not biodegradable and has been here for millions of years. This fact should be noted when installing a DE system. Many municipalities will not allow used DE powder to be disposed of via the city sewer system. DE filter manufacturers offer a backwash mode to divert the dirty water through a separation tank bag so the DE powder can be captured.The most efficient method of cleaning a DE filter, however, is to do a filter teardown and flush the DE powder off the grids, followed by a mild acid wash.

DE systems for residential pools commonly come in sizes from 24 square feet to 72 square feet of surface area. Figuring a flow rate of 2 GPM per square feet, residential filters can flow up to 144 gallons per minute at optimum efficiency. This is more than adequate to handle just about all residential applications.

DE filters use a series of support septums covered with a specialty woven cloth to hold the DE powder in place. The powder forms a cake, which does the actual filtration. DE filters are the most efficient of the three types and can filter out particles down to three microns in size. (Twenty microns is considered the smallest particle visible to the naked eye.)