Book 30, Question 104
What happens inside a big bed of sand and gravel?
A big filter bed has layers of sand and gravel, and sometimes charcoal on top.
Water trickles down through the tiny spaces between the grains. Bits of dirt and some germs get caught or stick to the sand, and the water comes out clearer. The filter is cleaned by sending water backward through it. Slow sand filters also grow a thin living layer that eats germs.