In most places in the US, like here in the Truckee Meadows, you can fill a glass from the tap and drink safe water.  How does this happen?  In short, water is purified by treatment processes before it reaches your tap.  The Environmental Protection Agency (EPA) sets treatment standards for the 151,000 public systems providing about 280 million people drinking water.  More detailed descriptions of public water distribution systems are available here

Several important treatment processes happen between the source of water and the tap: coagulation, flocculation/sedimentation, and disinfection.  Processes are depicted in the graphic below.  At intake, water is diverted or pumped from a source, such as a lake, river, or groundwater, towards a treatment plant.  Water passes through a large screen/rack to prevent debris from entering the system. 

During coagulation, water enters the treatment plant and the purification process begins.  Coagulant chemicals are used to help many fine particulates stick together to form larger particles known as floc.  Coagulants are mixed into the water.      

Next, the water and floc particles flow into a sedimentation basin.  The large flocs sink to the bottom of the tank and periodically removed.  Clean water at the top of the tank then passes into the next purification step – filtration.  Layers of sand, gravel, and charcoal naturally filter any remaining flocs and from the water.

After filtration, water is disinfected to protect against viruses, parasites, and bacteria.  Disinfection is done with chemicals such as chlorine or processes using ultraviolet light.  The water flows into the distribution system and storage tanks that you might see around town.  Read more about water treatment processes at this Centers for Disease Control (CDC) website.  

Looking for more local information?  Learn about water treatment at the Truckee Meadows Water Authority (TMWA) website

From Gaurab Karki, online biology notes.