A hot tub filter is the most important spa component for keeping both the water and the tub clean, and the unit running properly. If the filter is clogged or broken, it can allow hair, dirt, oils, and other pollutants to build up in the water. These materials can clog the jets and get into the moving parts of the system. Hot tub water filters are generally available in three types: diatomaceous earth (DE), sand, and cartridge.
A DE hot tub filter pushes the water through this organic substance, which looks and feels like a fine, white sand. DE is actually fossilized remains of a certain type of algae. Though it is soft to the touch, DE is very jagged when viewed microscopically, and is capable of filtering out extremely small particles. It is a natural substance, but one that is also highly toxic if swallowed because of the heating process used to make it suitable for a hot tub filter.
It should be noted that there is a vast difference between food-grade DE and the type used in a pool or hot tub filter. The type used with edible items is safe to ingest and is often used in commercial food packaging to keep food from clumping or drawing insects. The type of DE used in hot tub filters has been superheated and is not safe to ingest. In fact, some areas have special rules governing the disposal of this substance, which can make cleaning a hot tub filter complicated. If local regulations specify procedures for DE use, a sand or cartridge filter is probably a better choice.
Some spa filters use sand, which is coarser than DE and therefore a little less effective at filtering. Sand won't pull some of the microscopic pollutants out of hot tub water the way DE can. The special type of sand used in these filters can usually be found anywhere hot tub supplies are sold. It is not the type of sand easily found at a home and garden store or plant nursery, but a specific variety sold only at specialty retailers.
The type of hot tub filter most commonly used is a cartridge filter. This is typically made of a mesh material that is folded into pleats to catch as much debris as possible. Sand and DE filters require back-washing to clean them, a process that pushes the water through the material and out of the filter casing. Cartridge filters must instead be rinsed off to be kept clean. A filter of this type should usually be replaced every year or two. The cartridge should be replaced sooner than that, however, if it looks dirty or damaged in any way.