A backwashable water filter or media filter, uses a tank that’s filled with some type of filtering media to treat water. It can be used to remove particles from the water; neutralize the pH of acid water; oxidize iron and sulphur. They can also be used to absorb certain chemicals such as chlorine, chloramine, and fluoride from your families water. It’s function is totally determined by what media is used inside the tank.
Untreated water flows into the tank through the valve and passes through the media, which either removes the contaminants or changes them in some way. Cleaner, better water then can flow into your home for your family.
After a pre programmed period of time, usually about 5 days, the automatic back wash filter cycle begins. Controlled by the valve, water flows through the filter media in the reverse direction, that is back washing the media and removing the contaminants that have collected in the media and flushes them away, leaving the filter clean and ready to treat more water. In those systems where the media doesn’t pull out the contaminants, the backwash cycle serves to help prevent the media from being packed down and keeps it in good condition. 
A back wash cycle lasts for about 6 to 15 minutes. Water is forced down through a tube in the middle of the tank and then upwards through the filter bed, causing the media to expand, and as it does, the water carries the particles we’ve been filtering out to the drain. At the end of the back wash cycle, the valve changes the direction of water flow through the filter and the water flows quickly downward to re-pack the media bed so that it is ready for the next service cycle.
No chemicals to add or replenish and you get very long filter life, often 15 to 20 years!.
If we compare that to a cartridge type filter where water flows through trapping the particles inside the filter until it clogs. Since there is no way of backwashing or cleaning these filters, they are just thrown away either after 1 year of use or whenever they become clogged, whichever comes first.
Your initial investment is always lower with a filter cannister and disposable filter but the extra cost and maintenance (check out this video of mine to learn how to replace one of these filters) makes the automatic back washing filter a wiser investment for your family.