Pool Water Filtration

How Does It Work?

Unless pool water is circulated through a filter and disinfected it will become unsafe to bathe in within a few hours of the pool being filled from the water mains. This is because the mains water has chlorine in it that is removed by the action of sunlight within a few hours. The higher the water temperature the faster the water becomes unhealthy.

The Circulation System shown in the image below has been developed over the last 150 years and all pools (domestic or commercial) use this basic layout.

Diagram of how a pool circulation system works

The underlying principle of pool water filtration is that the floor outlets and the appropriate number of skimmers are positioned in the pool floor and walls. These must be in an arrangement which ensures 100% of the pool water is collected and pumped through a filter that traps the predominatly organic residue left in the water by bathers.

The water is then pumped through the water heating system and treated with disinfection system(s) before being discharged back into the pool by the appropriate number of inlets (normally 2 x number of skimmers). Again, these must be arranged correctly to help ensure that all the pool water is circulated and there are no "dead spots" where pathogens and algae can flourish.


Skimmers incorporate floating or rotating weirs that maintain a steady discharge of water even as the water level in the pool varies. As the name suggests, they are designed to skim off the surface water layer where most of the pathogens and organic residue float near the water surface. They can have wide throats (about 400mm) or the standard 225mm.

Skimmer dimensions

Picture of a skimmer in operation

Floor Outlets

Floor outlets must be installed in pairs to reduce the risk of a swimmer being trapped against them by the suction. The pairs need to be at least 2m apart but can be connected by a T to a single pipe cast in the floor of the pool.

floor outlets picture

Vacuum Point

Vaccum points are similar to inlets but are normally placed about 300mm below water level to allow being reached to take off the cover plate and insert the vacuum tube that is connected to the vacuum hed on the end of a telescopic rod.


Water inlets are provided on the basis of two inlets per skimmer with each pair of inlets being fed by a single supply pipe. They can incorporate a non-directional grid or a directional "ball - jet".

Inlets picure


There are normally two manifolds in a swimming pool. One is on the suction side of the pump where the pipework from the skimmers, the floor outlets and the vacuum point are connected together before all the flows are sucked into the pump inlet chamber. The other manifold is remote from the pump where the flow is distributed between the pipework that takes the water back to the pool inlets.

The inlet manifold will look like this.

Inlet manifold


The pool pump is of course the heart of a swimming pool filtration system because it provides the power that drives the water through the whole system.

The most important thing to realise about pumps is that the amount of water they move is directly related to the flow resistance of the pipework circuit. And so if the pump is a long way from the pool, the friction resistance of flow through the pipes can be so high that a bigger pump is required. In the same way if the sand in the filter is in need of a backwash the flow through the system can be so impeded that insufficient water is circulated for the disinfection system to work.

Typical pump sizes and output curves are shown as follows:

pump details


Filters are always the most prominent part in any pool plantroom because of their size and shape. They are also often provided in very bright colours. It is an element of the pool equipment that is often undersized to keep costs down. This is catastrophic from the point of view of the maintenance of pool water quality because the faster the water flows through the sand in the filter the less effective the filtration is.

Many domestic pools have filtration rates that are far too high (50m/hour is the norm) because the filters are too small.To achieve "Clear Clean Water" Bluepools limits the filtration speed in its filters to 30m/hour.

typical filter details