Swimming pools can have different sizes, shapes, volumes and types, but they all work the same way more or less.
A combination of filtration and chemical treatment is used, whereby the main idea is to pump water from the pool or the compensation tank (depending on the type of pool), through the filtering and chemical treatment systems, and then redistribute the water through inlets (either floor or wall inlets).
The two most common types of swimming pools are: skimmer and overflow.
1.
Types of pools
SKIMMER TYPE
In this type of pools, water flows to the filtration system through skimmers located at the top, and the main drains at the bottom. All the debris that floats on the surface (leaves, hair, etc.) is drawn from the skimmers, whereas most of the dirt that sinks exits the pool through the main drains. Once the water is filtered, it returns to the pool through the wall inlets. The number of skimmers is determined by the area of the pool. In general, there are several main drains and skimmers; therefore if something blocks one of the drains, the pumping system pulls water from one of the other drains.
Filtration system of a residential skimmer pool
OVERFLOW TYPE
In this type of pools, which is the most common, the pool has an overflow channel, which is a trench that surrounds it, and is covered by a removable grille. Surface water flows over the edge of the pool, and runs by gravity to the compensation tank, from which the water is pumped, filtered and then returns to the bottom of the pool through the floor inlets.
Filtration system of a residential overflow pool
2.
Basic pool equipment
PRE-FILTER OR BASKET STRAINER
The pre-filter/ basket strainer is used to catch all floating leaves, hair, debris, etc. and can either be incorporated in the pump, or installed separately.
FILTER PUMP
The selection of the filter pump is determined by the turnover (flow rate), and the distance between the pump and the pool (head).
FILTER
Swimming pool filters come in various shapes and sizes, and are sized according to the volume of the pool. The two most common types are sand filters and diatomaceous earth filters.
SAND FILTER
Sand filters consist of large tanks, made of fiberglass, stainless steel or metal. During the filtering operation, dirty water from the pool comes in through the inlet pipe of the filter. Sand filters remove particles as small as 20 to 25 microns. Over time, the collected dirt and debris in the sand slow down the water flow. When pressure gauge difference increases by≈ 0.5 bar above the clean pressure drop difference, a cleaning cycle should be carried out. During backwashing, the flow of clean water is reversed back into the filter.
DIATOMACEOUS EARTH FILTER
In a diatomaceous earth filter, water passes through filter grids coated with diatomaceous earth, a fine powder made from the chemically inert, fossilized remains of sea organisms called diatoms. This type of pool filters is the most effective, as it removes particles as small as 5 microns.

The turnover of the pool determines the dimensioning of the filtration system; it is the theoretical time taken for the total pool volume to pass through the filtration system once.
RECOMMENDED POOL TURNOVER PERIODS
Residential pools: 4-5 hours
Children pools: 0.5 hours
Public pools: 2-3 hours
HEATING SYSTEMS
Different types of pool heaters are available, to heat up the pool water to the desired temperature. You may find here below the most common types of heating systems:
Heat exchanger
A boiler heat exchanger has two sets of pipes connected to it and running through it. The first set of pipes takes hot water from the boiler to the heat exchanger, and the second set of pipes circulates water to and from the pool. The system works as the cold pool water draws heat from the boiler’s pre-heated water.
Electric
A boiler heat exchanger has two sets of pipes connected to it and running through it. The first set of pipes takes hot water from the boiler to the heat exchanger, and the second set of pipes circulates water to and from the pool. The system works as the cold pool water draws heat from the boiler’s pre-heated water.
Solar
A solar heat exchanger, like the boiler heat exchanger, has two sets of pipes connected to it and running through it. However, the solar heaters utilize the sun to heat the water in the panels, usually located on the roof. A pump circulates the water from the panels to the pool.
Gas
Gas heaters use gas or propane, depending on the type of heater. The ones that burn natural gas have to be hooked up to a gas line, and the ones that burn propane require propane tanks. The burning gas warms a combustion chamber and the heat that is produced is then transferred to the pool.
Heat pump
Heat pumps use electricity to capture heat and move it from one place to another. They are actually a form of solar heaters, as the sun-warmed air contributes to the efficiency of these units. This heat is extracted from the air, upgraded with a compressor, and then transferred to the water.
3.
Disinfection techniques
Chlorination
Three forms of chlorine can be used in a pool. Chlorine gas (Cl2), sodium hypochlorite solution (NaOCl) or dry calcium hypochlorite, Ca(OCl)2. The addition of any of these to the water produces hypochlorous acid (HOCl). This disassociates into hypochlorite ions (OCL-) to some degree. The ratio of hypochlorous acid and hypochlorite ions depends on pH level. It is estimated that (HOCl) is 100 times more effective for disinfection than (OCL-). For this reason, it is important to control the level of pH in the water.
Parameter Chlorine gas (Cl2) Sodium hypochlorite (NaOCl) Calcium hypochlorite Ca(OCl)2
Physical form Gas Liquid Granular, Tablets, Pellets
pH 1.1 13.0 10.0-11.0
Available chlorine 100% 10-15% 65%
Ozonation
Ozone is generated when a molecule of oxygen is illuminated by high energy of ultraviolet light; it is formed when three atoms of oxygen are bound together instead of the normal two. The extra oxygen atom makes ozone a highly energetic oxidizer. Ozone destroys algae, viruses and bacteria. It also breaks down harmful chemicals and acts as a flocculent, causing dissolved solids to be suspended and easily removed by the filtration system.
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Ionization
Ionization is the process that causes an element to gain or lose electrons. Copper-silver ionization is the electronic release of copper and silver ions. A low voltage is passed through a set of metallic electrodes, which are placed in line with the circulation system. The voltage causes some of the atoms of the electrodes to lose an electron, thus becoming positive ions, which attempt to flow across the space between the electrodes, but instead are carried away by the flow of water. Copper ions will inhibit algae growth, while silver ions kill bacteria and viruses. The recommended level of copper ion in the water is between 0.15 and 0.2 ppm.
4.
Pool chemistry
Water should always be balanced and kept crystal clear, to protect the pool and the circulation system from scale and corrosion. Therefore, several parameters should be regulated:
Parameter Recommended value
Total available chlorine 1 - 3 ppm
pH 7.2 - 7.6
Alkalinity (1) 80 - 120 ppm
Calcium 200 - 400 ppm
Cyanuric acid (2) Below 70 ppm
Langelier saturation index (3) Between + 0.3 and -0.3
Temperature 25-28 °C
(1) Alkalinity is the quantity of alkaline material in the water. The level of bicarbonate alkalinity should be between 80 ppm and 120 ppm.

(2) Cyanuric acid is a chemical that protects chlorine from being destroyed by the sun. As chlorine is so susceptible to the ultraviolet rays of the sun, cyanuric acid is a necessary ingredient.

(3) The Langelier index is a calculation used to determine the adequacy of the pool water chemistry, as it relates to corrosion and scale formation. The adjustment of the pH, total alkalinity or calcium hardness can be used to adjust the Langelier index, so that it falls within the ideal range.