Water : From the River to the Bottle

How does water become clean enough to drink ?

Once water is received from the source it needs to be treated before we can use it.  In the previous sections of this module, you learned about microorganisms that need to be filtered out of drinking water.  Now well take a look  at how a typical water treatment plant purifies our drinking water.  The four processes commonly used to treat water are screeningcoagulation and settlingfiltration, and disinfection.

When water is taken from the source (Like a lake or river), wood, fish, and plants may still be present.  These things arescreened out as the water is drawn into the plant.  If the source happens to be groundwater, the screening process isnt as vital since the layers of the earth that the water travels through act as a natural screen, removing large contaminants.

When the water reaches the plant it is coagulated and settled.  In this process, water is mixed with aluminum sulfate (alum) and chlorine.  The alum forms sticky globs, which attract bacteria and other impurities.  The chlorine kills germs and improves taste and odor.  The water and the globs then flow into a sedimentation basin where the globs “settle” to the bottom and are removed.

Next, the water flows through special filters made of layers of sand and gravel.  The gravel layer of the filters is about 1 foot deep and the sand layer is about 2 ½ feet deep!  This filtering removes any remaining particles left in the water.

During disinfection, disinfecting chemicals are added and chlorine is used again.  This  process kills any surviving germs and keeps the water clean over time.  In some water treatment systems that use ground water, this is the only method needed to treat the water!

Source: People.Chem.Duke