Viable pathogens are bacteria harmful to the human body that can reproduce, creating more of themselves and contaminating a greater amount of water. The EPA has a zero-tolerence policy for pathogenic bacteria, and various water purification standards have been put in place for resiviors and water bottling plants to make sure that the water is as pure as possible. The most effective methods for removing pathogens is disinfection with chlorine, boiling, or ultraviolet light; no one method of disinfection treats every pathogen, so for a bottling plant, multiple methods -- including physical filtration -- must be used. Water flowing from outdoor reservoirs is filtered and disinfected before it enters common piping; various agencies take care of disinfecting tank trucks and standing water tanks, usually with chlorine. Since chlorine is poisonous in large quantities, the disinfected tank must be emptied and tested before drinking water is placed inside. In general, city water is safe to drink, but many people put it through additional filtering using an after-market product on the faucet or in a pitcher to remove trace elements and other bacteria that can grow in water pipes.