Harvest Rainwater with Sand Filters. “Essentially a sand filter is a large drum filled with sand. Water enters the top and slowly percolates through. A thin, biologically active layer (called the hypogeal layer) quickly forms on top, feeding on the bits of organic residue and other impurities in the water. By the time the water has made it through the several feet of sand, it is potable and remarkably clean. Eventually, the hypogeal layer becomes too thick and needs to be either scraped off or destroyed by drying and backflushing (the water from the flush being disposed of into a nearby thirsty plant). A new one quickly forms and water filtration can continue.” Also found on bottletop’s pages.