The pores in a reverse osmosis membrane are between 0.0001-0.0005 µm (one-ten-thousandth to five-ten-thousandths) microns in diameter, which is only slightly larger than individual water molecules. Using your house water pressure, water molecules are forced through these microscopic pores, leaving behind many contaminants, too large to pass through. This purified water or "permeate", is stored in a holding tank, while the water that didn't pass through the membrane, containing the contaminants, is flushed down the drain.