Swimming Pool Filtration Systems

Various Filtration units

There are a variety of filtration systems utilized in swimming pool sanitation.  Each could be a blog in itself.  However, below are the most notable systems used and a brief description of each.

Sand
A pressure-fed sand filter is typically placed in line immediately after the water pump. The filter typically contains a medium such as graded sand (called ’14/24 Filter Media’ in the UK system of grading the size of sand by sifting through a fine brass-wire mesh of 14 to the inch (5.5 per centimeter) to 24 to the inch (9.5 per cm)). A pressure fed sand filter is termed a ‘High Rate’ sand filter, and will generally filter turbid water of particulates no less than 10 micrometers in size.[20] The rapid sand filter type are periodically ‘back washed’ as contaminants reduce water flow and increase back pressure. Indicated by a pressure gauge on the pressure side of the filter reaching into the ‘red line’ area, the pool owner is alerted to the need to ‘backwash’ the unit. The sand in the filter will typically last five to seven years before all the “rough edges” are worn off and the more tightly packed sand no longer works as intended[citation needed]. Recommended filtration for public/commercial pools is 1 ton sand per 100,000 liters water (10 ounces avdp. per cubic foot of water) [7.48 US or 6.23 UK gallons].

Introduced in the early 1900s was another type of sand filter – the ‘Rapid Sand’ filter, whereby water was pumped into the top of a large volume tank (3′ 0″ or more cube) (1 cubic yard/200US gal/170UK gal/770 liters) containing filter grade sand and returning to the pool through a pipe at the bottom of the tank. As there is no pressure inside this tank, they were also known as “gravity filters”. These type of filters are not greatly effective, and are no longer common in home swimming pools, being replaced by the pressure-fed type filter.

Diatomaceous earth
Some filters use diatomaceous earth to help filter out contaminants. Commonly referred to as ‘D.E.’ filters, they exhibit superior filtration capabilities.[21] Often a D.E. filter will trap waterborne contaminants as small as 1 micrometer in size. D.E. filters are banned in some states, as they must be emptied out periodically and the contaminated media flushed down the sewer, causing a problem in some districts’ sewage systems.

As of 2020, several companies now produce regenerative media filters, sometimes called precoat media filters, which use perlite as the filtration media rather than diatomaceous earth. As of 2021, perlite can safely be flushed down the sewer and is approved and NSF listed for use in the United States.

Cartridge filters
Other filter media that have been introduced to the residential swimming pool market since 1970 include sand particles and paper type cartridge filters of 50 to 150 square feet (4.6 to 13.9 m2) filter area arranged in a tightly packed 12″ diameter x 24″ long (300 mm x 600 mm) accordion-like circular cartridge. These units can be ‘daisy-chained’ together to collectively filter almost any size home pool. The cartridges are typically cleaned by removal from the filter body and hosing-off down a sewer connection. They are popular where backwashed water from a sand filter is not allowed to be discharged or goes into the aquifer.

Automated pool cleaners

Automated pool cleaners more commonly known as “Automatic pool cleaners” and in particular electric, robotic pool cleaners provide an extra measure of filtration, and in fact like the handheld vacuums can microfilter a pool, which a sand filter without flocculation or coagulants is unable to accomplish[22]

These cleaners are independent from the pool’s main filter and pump system and are powered by a separate electricity source, usually in the form of a set-down transformer that is kept at least 10 feet (3.0 m) from the water in the pool, often on the pool deck. They have two internal motors: one to suck in water through a self-contained filter bag and then return the filtered water at a high rate of speed back into the pool water. The second is a drive motor that is connected to tractor-like rubber or synthetic tracks and “brushes” connected by rubber or plastic bands via a metal shaft. The brushes, resembling paint rollers, are located on the front and back of the machine and help remove contaminating particles from the pool’s floor, walls (and in some designs even the pool steps) depending on size and configuration. They also direct the particles into the internal filter bag.[23][24]

Other systems
See also: Salt water chlorination
Saline chlorination units, electronic oxidation systems, ionization systems, microbe disinfection with ultra-violet lamp systems, and “Tri-Chlor Feeders” are other independent or auxiliary systems for swimming pool sanitation.

Consecutive dilution
A consecutive dilution system is arranged to remove organic waste in stages after it passes through the skimmer. Waste matter is trapped inside one or more sequential skimmer basket sieves, each having a finer mesh to further dilute contaminant size. Dilution here is defined as the action of making something weaker in force, content, or value.

The first basket is placed closely after the skimmer mouth. The second is attached to the circulation pump. Here the 25% of water drawn from the main drain at the bottom of the swimming pool meets the 75% drawn from the surface. The circulation pump sieve basket is easily accessible for service and is to be emptied daily. The third sieve is the sand unit. Here smaller organic waste that has slipped through the previous sieves is trapped by sand.

If not removed regularly, organic waste will continue to rot down and affect water quality. The dilution process allows organic waste to be easily removed. Ultimately the sand sieve can be backwashed to remove smaller trapped organic waste which otherwise leaches ammonia and other compounds into the recirculated water. These additional solutes eventually lead to the formation of disinfection by-products (DBP’s). The sieve baskets are easily removed daily for cleaning as is the sand unit, which should be back-washed at least once a week. A perfectly maintained consecutive dilution system drastically reduces the build-up of chloramines and other DBP’s. The water returned to the pool should have been cleared of all organic waste above 10 microns in size.

Mineral sanitizers
Mineral sanitizers for the swimming pool and spa use minerals, metals, or elements derived from the natural environment to produce water quality benefits that would otherwise be produced by harsh or synthetic chemicals.

Companies are not allowed to sell a mineral sanitizer in the United States unless it has been registered with the United States Environmental Protection Agency (EPA). Currently, two mineral sanitizers are registered with the EPA: one is a silver salt with a controlled release mechanism which is applied to calcium carbonate granules that help neutralize pH; the other uses a colloidal form of silver released into water from ceramic beads.[25]

Mineral technology takes advantage of the cleansing and filtering qualities of commonly occurring substances. Silver and copper are well-known oligodynamic substances that are effective in destroying pathogens. Silver has been shown to be effective against harmful bacteria, viruses, protozoa and fungi. Copper is widely used as an algicide. Alumina, derived from aluminates, filters detrimental materials at the molecular level and can be used to control the delivery rate of desirable metals such as copper. Working through the pool or spa filtration system, mineral sanitizers use combinations of these minerals to inhibit algae growth and eliminate contaminants.

Unlike chlorine or bromine, metals and minerals do not evaporate and do not degrade. Minerals can make the water noticeably softer, and by replacing harsh chemicals in the water they lower the potential for red-eye, dry skin and foul odors.

This article published: https://en.wikipedia.org/wiki/Swimming_pool_sanitation#Filtration_units