Sand filters are used for water purification. There are three main types;

• rapid (gravity) sand filters
• upflow sand filters
• slow sand filters

All three methods are used extensively in the water industry throughout the world. The first two require the use of flocculant chemicals to work effectively whilst slow sand filters can produce very high quality water free from pathogens, taste and odour without the need for chemical aids.

http://en.wikipedia.org/wiki/Sand_filter