We occasionally get asked about swimming pool sand filters from people who have never owned a swimming pool before. Apart from the lessor used diatomáceos earth (D.E.) filters there are two basic choices for pool filtration systems which are sand filters and cartridge filters.
In Spain and Portugal it is far more common to find a sand filter system for domestic swimming pools. The video, by an Indianapolis based company, explains how a sand filter system works. Although the video is from the USA, the filter system is the same as most of those you find in Spain or Portugal.