As well as leaving the pool via the surface water draw off system, water is also leaving via the sumps (drains). In a traditional swimming pool, these outlets are usually located on the floor of the deep end and are covered with a square grill. The following picture shows three outlets.
The outlets of a swimming/spa pool can be extremely hazardous. The hazards are groups under the headings of entrapment hazards and entanglement hazards.
The outlets of a swimming/spa pool are connected to a powerful circulation pumping system.
If the outlets are blocked, whatever is causing the blockage will be exposed to the suction force of the circulation pumping system.
The circulation pumps will be sucking on to whatever is causing the blockage, causing a vacuum.
At this point, it will be very difficult to remove the blockage without turning off the circulation pump(s).
If the blockage is in the form of a person, then tragic consequences can occur, including drowning, disembowelment and transanal evisceration, which is where internal organs are forcefully drawn out through the anus.
There are various ways that suction entrapment can be avoided:
Emergency cut-off devices that automatically turn off the suction pumps when an increase in suction force is detected
Multiple outlets being fitted so that even if one of the outlets gets covered, the remaining outlets take the increased water flow and prevent a vacuum being created at the blocked outlet.
Outlets being designed so that it is impossible to cover them and form a seal. This can be achieved via having the grill surface area of sufficient size (450mm2 is the standard size usually used). It can also be achieved by the use of outlets that are designed to prevent a seal being formed around them when they are covered. These are called anti-vortex drain covers. An example below.
There are also fittings that can be placed over a drain cover that is not an anti-vortex type that will go some way to gaining a similar effect. Example below.
Drain covers must be maintained. The picture below is an example of a drain cover that has been allowed to fall into a state of disrepair, with obvious hazards. In order to prevent children getting fingers/toes trapped in the grill, the apertures should be no wider than 8mm.
Once the above design-based precautions have been considered, other precautions can be implemented such as providing training for all relevant staff regarding the dangers of suction entrapment.
Entanglement hazards are slightly different to entrapment hazards.
They involve hair being drawn into the outlet and then entwining on the other side of the outlet due to the circular motion of the water is it goes through the outlet.
It may be impossible to free the hair from the outlet, even if the circulation pumps are turned off.
People using spa pools are at an increased risk of suction entanglement due to the fact that they will be in close proximity to an outlet no matter where they are situated within the spa pool.
For this reason, people using spa pools should be advised to tie long hair back and refrain from submerging their head under the water.
Staff responsible for supervising the spa pool should be trained sufficiently so that they are aware of the hazards associated with outlets.