How does a slow sand filter work?
When a slow sand filter is first put into operation or after it is “cleaned”, a living “community” of aquatic aerobic, predatory microscopic organisms grow and form what is called a “Schmutzdecke” or “biolayer” in the top 5 to 10 cm of wet sand which must always be under oxygen rich water. This biolayer is very effective at mechanically filtering very small particles out of the water flowing through it. Also, the living organisms in the biolayer literally “eat” pathogens in the water that get caught in the biolayer. Some filtering also occurs because of the physical action of the sand below the biolayer. Water must not flow through the filter faster than the biological action occurs in the Schmutzdecke. In small versions of slow sand filters the drainpipes at the bottom connect to a (usually pvc) pipe that runs out and up to an outlet several inches above the top of the sand. This way water drains slowly and never leaves the surface of the sand exposed to open air (this will kill the b
