Chemists have developed a powerful household water purification system that puts the cleansing power of an industrial water treatment plant into a container the size of a ketchup packet. The tiny packets cost only a few cents each and can purify enough drinking water to last an average household for up to 3 days. The packets can kill water-born pathogens that cause cholera, typhoid and dysentery; remove a variety of toxic metals, including lead, arsenic and mercury; and also remove dangerous pesticides like DDT and PCB.