Rapid Gravity Sand Filter with Backwash

The slow sand filter, already published here, cleans water with a living skin of microbes on its surface, and it does it at about 0.1 to 0.3 metres an hour. To supply a city that way you need acres of it. The rapid filter runs at five to fifteen metres an hour — fifty times faster — through coarser sand, and it has no living skin at all. It cannot: nothing survives being scoured daily. It works only because the water reaching it has already been coagulated, so the particles arrive as flocs that stick to the grains instead of passing between them. And because it clogs in hours rather than months, it needs the invention that actually made it practical — reversing the flow to blow the bed clean. Build one in a clear pipe so you can watch the dirt go in, and watch it come back out.