How is Water Purified?
 

Have you ever stopped to think about where that water comes from and just how it makes its way into your home?  In this section we will take a look at where we get our water and how it is purified for our use.  Let’s take a trip… 

Where do we get our water?  

Cities tap their water from lakes and rivers or from the ground. (Maybe more on ground water and water cycle)  Here in San Diego, we receive an average of only 10 inches of rain each year.  Incredibly, our local water resources provide only 10 percent of the total need!  Since San Diego has few streams or natural lakes, this 10 percent that makes up our local water supply comes primarily from ground water.  So where does San Diego get the other 90 percent of its water? 

Well, the city purchases the rest through the Metropolitan Water District of Southern California.  The water comes from Lake Oroville in northern California and/or the Colorado River by way of large pipes called aqueducts. 

Miami, Honolulu, San Antonio and Mexico City depend mostly on groundwater sources.  Chicago, Montreal, Quebec, Toronto, St. Louis, and Pittsburgh are on the shores of rivers or lakes, so they use surface water.  Cities such as New York, Los Angeles, San Diego, and Denver have to bring surface water through pipes for many miles. 
 

You Tell Us  Where does your city or town get its water?

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 we’ll take a look  at how a typical water treatment plant purifies our drinking water.  The four processes commonly used to treat water are screening, coagulation and settling, filtration, and disinfection. 

When water is taken from the source (Like a lake or river), wood, fish, and plants may still be present.  These things are screened out as the water is drawn into the plant.  If the source happens to be groundwater, the screening process isn’t 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! 
 
 
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