An Overview of a Swimming Pool Circulation System



Sumps and overflow channel.


Most of the pollution in a swimming pool will sit in the top 150mm of pool depth.  Therefore, there needs to be an effective system for removing as much of this pollution as possible. There are three different types of surface water removal system:

  • Deck-level

  • Skimmer

  • Scum trough


As well as leaving the pool via the surface water draw off system, water is also leaving via the outlets (drains, or sumps). In a traditional swimming pool, these outlets are usually located on the floor of the deep end and are covered with a square grill.




A circulation pump and strainer basket.


The circulation pumps are the ‘heart’ of the circulation system. They are designed to continuously pump water around the system at a pre-determined rate called the flow rate. In larger installations there are usually several pumps working at the same time, with additional pump(s) on standby. In smaller installations, there may only be one single pump.

They work by having an impeller (which is similar to a propeller), which is housed within the pump casing and is connected to an electric motor, which rotates it at high speed. This causes water to be sucked into the pump on the suction side and forced out of the pump on the delivery side.

The pre-pump strainer is designed to trap the larger items of physical pollution before they can get into the pump itself, where it would cause damage. The strainer basket sits inside a vessel and can be removed for cleaning. They will need to be cleaned out regularly in order to prevent them becoming completely blocked with debris.




Swimming pool filters. The heat exchanger can be seen in between the filters.


Filtration is a fairly simple process; water leaves the pool via the deep end outlets and the surface water draw-off system and gets pumped into the top of the filter (or several filters in large pools), passes through the filter media (usually sand) where all the contaminants and pollution are trapped and the pool water comes out of the bottom and continues through the remaining components of the pool plant system.


Chemical Dosing


An automatic dosing unit between two disinfection solution day tanks. The yellow components on top of the days tanks are chemical pumps.


Automatic monitoring and dosing of water treatment chemicals should be the norm in a commercial swimming pool/ spa. Hand-dosing chemicals should only be undertaken in exceptional circumstances, after a thorough and robust risk assessment has been conducted, and only by people trained and competent to do so.

Chemical dosing should be continuous, 24 hours a day.  The automatic dosing system should be backed up by regular monitoring and verification.

Usual practice is to dose disinfectant prior to filtration. This is achieved by injecting a solution into the circulation pipework by means of a chemical dosing pump. A common type of pump used for this purpose is a diaphragm pump, which uses a rubber disc (the diaphragm) to create suction in the chemical feed line when it moves backwards, and then creates a pressure in the feed line on the discharge side when it moves forwards.

Dosing disinfectant before the filter(s) prevents inadvertent mixing of disinfectants and acids (which should be added post-filter). With ozone and ultraviolet systems (which removes residual disinfectant), dosing is always after the ozone or UV treatment.

The pH correctant (in the UK, usually acid-based), is usually dosed after filtration, in a similar manner to that described above for disinfectant dosing.


Ultra Violet


An ultra violet disinfection chamber.

Ultra violet disinfection is a process whereby the swimming pool water flows through a UV chamber and is exposed to UV light.

UV disinfection is a physical, not a chemical process and nothing is added to the swimming pool water when it passes through the UV chamber. This means that UV treatment is a non-residual form of disinfection and a secondary disinfectant such as chlorine will need to be added before the water recirculates back to the swimming pool.

Some advantages of UV disinfection systems are that there are no chemicals to handle or store and once installed, the system does not take up too much space or require high levels of expertise to operate (unlike ozone disinfection systems). Also, it does not produce chemical by-products like chlorine does.




A heat exchanger. The lagged pipes are the flow and return pipes from and back to the hot water system. The non-lagged pipes with the arrows on them are swimming pool water, into and out of the heat exchanger.


The heat exchanger is usually the final component that the pool water is circulated through before it is returned to the swimming pool.

There are two types of heat exchanger used in pool plant. They are the coil heat exchanger and the plate heat exchanger.

The plate heat exchanger is the newer type and is more efficient at heat exchange than the older coil heat exchanger, although the principles on which they work are very similar.