How does water get filtered while passing through sand, charcoal, etc.?
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With sand filtration, there are two main mechanisms.
1. physical filtration: the spaces between the grains of sand are tiny, and that removes most large sediment particles that make the water cloudy. But the bacteria and viruses are too small to be stopped by this.
2. biological filtration: the upper layers of sand (about the top 5 inches) get colonized by a biofilm of bacteria and other micro-organisms. It’s basically a microscopic ecosystem made of micro-organisms surrounded by a gel that they secrete. This coats all the sand grains. Many of these micro-organisms are predators which feed on bacteria. So the slimy gel of the biofilm traps the viruses and bacteria, which are then consumed by the microbes living in the biofilm. This biofilm is often referred to as the “Schmutzdecke”, which is a german word which literally means “slime layer”.
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