Sand filters are the best option for all around performance. With typical pool sand, it should be changed every 3-5 years. Typical size filters for spas and pools are 300lb and 600lb filters. Sand will begin to "round out" the edges, and become eroded, which help trap debris making it less effective. If sand is left unchanged for longer periods, it will literally begin to for sedimentary rock, as the water compacts and erodes the sand into more solid chunks, with channels flowing though at the points of least resistance. This allows the water to push through the filter, without removing any debris and is referred to as "channeling." Sand filters with standard pool sand are generally rated to filter down to 20-40 microns in size. However, sand is not the only media available for filtering! Activate glass media is an alternative that looks like sand to the naked eye, but provides even finer filtration at 5-10 microns in size. That's even finer than cartridge filters are capable removing! It would still require replacement every 3-5 years. With the addition of the multiport valve, it provides a recirculation option for bypassing the filter, and still being able to circulate water, allowing for chlorination, heating and surface skimming, in the event there is an issue with the filter. It also provides a winterization option, closed position, backwash AND rinse (unlike "push/pull" valve). Sand filters offer a quality, reliable means of filtration with that is easily upgradeable with media at ANY sand change without any costly equipment changes
Cartridge Filters were really designed as a less maintenance version of a D.E. filter. They offer better filtration over standard pool sand in a sand filter. Generally cartridge filters are able to filter particles anywhere from about 5-20 microns in size. Cartridges are removable and cleanable for a finite period. The best way to clean cartridge filters are to pressure wash them, with a wide nozzle. There are chemical cleaners, available as well. Most recommended would be a "soak style" cleaning agent that would be added to water, and allow the cartridges to soak in the solution for the recommended period of time, remove and pressure wash thoroughly to remove any additional solvents or detergents, and then replace. Cartridge life is not as long as sand or glass. Generally we would recommend having 2 sets of cartridges for each cartridge filter. When the active set is dirty, they can be removed and cleaned, and the alternate set can go into the filter, so the system is never down. The alternating cartridges also extend the life of both sets. At the end of each season, evaluate both sets and if a set needs replacement.... whichever set is worse, should be replaced with a new set. Use the worse looking filters for any algae cleanups, openings, etc. The cost on cartridges (depending on the specific model, design and number of cartridges) range on average between $100-$800.
D.E. or Diatomaceous Earth filters have the highest level of filtration available, able to filter out particulate from 1-6 microns in size! However the increased filtration comes at a cost. D.E. filters work by flowing water through a set of concentric screens inside the filter. The screens themselves are not what filter the water. The screens are what "catch" and hold the D.E. Once the screens are coated in D.E. the water is filtered through that coating. D.E. (like sand) will need to be cleaned out periodically. This is done with either a multiport valve like the sand filter and backwashing by flushing water "backwards" through the screens and then out a waste line, or by "bumping" the filter and backwashing depending on the style of D.E. filter. After the backwash is done, new D.E. needs to be added again, in order for the filter to be functioning again. To do this....Fresh D.E. must be added with the system running, through the skimmer, 1 cup at a time. D.E. is made from the fossilized remains of tiny, aquatic organisms called diatoms. Their skeletons are made of a natural substance called silica. Over a long period of time, diatoms accumulated in the sediment of rivers, streams, lakes, and oceans. Today, silica deposits are mined from these areas. Silica is also a known carcinogen when inhaled, and caution should be taken when handling. Additionally the screens inside must be removed and cleaned thoroughly to remove deposits of old d.e as well as debris.