First, the dark beer-colored sewage is pulled through a series of tubes stuffed with thousands of fibers pierced with holes 1/300th the size of a human hair. Anything larger than 0.2 millionth of a meter - which includes suspended solids and bacteria - is left behind. The cleansed water is then forced at high pressure through hundreds of tubes that are filled with tightly wound plastic membranes. Reverse osmosis, as the process is called, stops nonwater molecules - including viruses and pharmaceuticals.