Water Treatment: Improving Water Quality

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  • 0:07 Water Purification
  • 1:52 Treating Wastewater
  • 4:16 Drinking Water in…
  • 5:49 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this lesson, you will learn how water is purified for drinking, and how wastewater is treated before being put back into the environment. You will also learn about water purification issues in developing countries.

Water Purification

When you're thirsty, you go to the sink and fill up a glass of water. In the United States, we often take this luxury for granted because most of us have access to clean drinking water. But where does that water come from and how is it purified so that you can drink it without getting sick?

Potable water is water that is safe for drinking. You may be surprised to learn that we actually use purified, potable water for everything - cooking, laundry and flushing our toilets. We get this water from natural sources, such as groundwater, rivers and lakes, and then take a number of steps to make it safe for use.

First, the water is mixed with minerals (like aluminum sulfate) that create large, gel-like clumps in the water. These clumps act like sheepdogs herding cattle as they round up the bacteria and dirt particles in the water, eventually settling to the bottom. After all the large particles have been separated, the water can then be filtered through sand and gravel, which separates it from any remaining clumps that may not have settled to the bottom.

Many water treatment facilities also aerate the water to improve its odor and flavor. Aeration is the process of circulating air through the water, which removes strong-smelling sulfur compounds (and sulfur smells like rotten egg!). The addition of dissolved air into the water also gives the water a nice taste - otherwise it tastes a bit flat.

After water has been filtered and aerated, it's then disinfected to kill any pathogens, such as bacteria and viruses, that remain after filtration. Chlorine gas and ozone are the two most commonly used disinfectants.

Treating Wastewater

Once we have used potable water, it becomes wastewater. Any water that has been used by people in some way ends up as wastewater, and this includes the water than runs down your sink after doing dishes, taking a shower, washing your car and storm water runoff. Natural systems aren't able to adequately process the large amount of wastewater we produce, so some wastewater treatments have been developed to clean the water before it goes back into the environment.

Depending on where you live in the U.S., your wastewater treatment may be different. For example, if you live in a rural area, you likely use a septic system, which directs wastewater from the house into an underground septic tank where the waste can separate from the water. The water that separates then travels downhill to a drain field where any remaining pollutants can be decomposed by microbes. The solid waste that is left behind in the tank is removed from the tank and then taken to the landfill.

If you live in the city, your wastewater is treated in a central location after it's carried away through the sewer system. There are three steps to treating municipal wastewater that involve physical, chemical and biological techniques.

First, is primary treatment, which is when the water flows through settling tanks or clarifiers and the contaminants are physically removed. Lighter material, like grease and oil, settle at the top and the heavier sludge settles at the bottom. This process removes about 60% of suspended solids from the wastewater that comes through.

After the primary treatment, the water goes through secondary treatment, which is when wastewater is aerated, allowing bacteria to break down organic pollutants. This process is highly effective, as it removes about 90% of suspended solids from the water.

In the final stage, tertiary treatment, wastewater is disinfected with chlorine or ultraviolet light before being released back into the environment. Once water has been disinfected, it's usually piped back into rivers, lakes and oceans, but sometimes it may be reclaimed, which is when it is reused for purposes that don't require purified water, such as watering golf courses, agricultural irrigation, and even some of those grand water fountain displays in Las Vegas.

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