Spa Pools (Hot Tubs)

Design and Capacity
Filtration
Chemical Treatment
Operational Criteria

A spa pool consists of a relatively small volume of warm water (35°C – 39°C) in which bathers sit as opposed to swim. The water is circulated inwards through nozzles set at frequent intervals around the perimeter and out through, main drains or skimmers. Alternatively the design may be of the overflow type in which the water flows into channels constructed around the outside of the shell. Even with this design, there will usually be a low suction point or sump. It is essential that the recirculating water is cleaned by an adequate filtration system and disinfected by an appropriate chemical treatment because of the high water temperature. Most spas have some form of air injection system to create turbulence in the water, and this, with the increased temperature, can affect the efficiency of some of these treatments.

IMPORTANT: If there is water in a Spa, it must be treated and tested, even if no-one is using it, and records of this must be kept.

Capacity and Bathing Load of Spa Pools

Each type of spa pool will have been designed to contain a maximum number of bathers at any one time. The recommendations are that a minimum surface area of 0.37 m2 (4 sq ft) and a minimum of 0.25 m3 (55 gallons) of water be provided for each bather.
It is also important to ensure that the maximum bathing load (number of bathers using the spa per hour) is not exceeded.

Filtration of Spas

The most efficient type of filter for a spa is the high rate sand filter. This can handle between 2 and 20 gallons of water per minute per square foot of filter area. A particle size for the sand of 0.40 to 0.55 mm and effective distribution is critical for proper operation of this type of filter.
Alternatively a Diatomaceous Earth (DE) filter system may be used. In both cases above, body oils and cosmetics removed from the bathers by the warm water accompanied by aeration, collect on the filter media. With sand filters, backwashing does not remove all of the oil coating and gradually all of the sand particles become coated, until oil passes straight through and a slight milkiness is imparted in the water. Regular, annual sand changes will prevent this from happening.

In domestic spas, cartridge filters are popular. In these cases the fabric filaments become coated with oil until they are saturated, at which point the oils pass through, again imparting a slight milkiness in the water. The filter element is removed and cleaned with a proprietory cleaner after which it is suitable for re use.