In the premium US Spa models, 24 hour circulation pumps are used to continuously filter the water.  Water is circulated through the skimmer door.  Floating debree gets trapped in a basket in the skimmer.  Water is circulated through dual cartridge filters that total 100 sq ft of filtering surface area.  The filters should be cleaned weekly with a brus, some water and a filter cleaning solution available at spa stores

Combining this continuous filtration process with ozone gas and you get very effective water sanitization.  This minimizes the need to use harsh chemical sterilizers such as chlorine or bromine.