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?

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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.

Drinking Water in Developing Countries

Developed countries like the U.S. can provide clean drinking water to most of their citizens because they have the technology and infrastructure to do so. However, in developing nations, this is not the case since there are far fewer water treatment facilities, if any.

In order to have safe drinking water, many people in developing countries have to boil their water to disinfect it. And while this may sound like an easy alternative, finding fuel for boiling water can be time-consuming and laborious. Water itself may not even be nearby, and in many countries, families will spend entire days walking to a water source and then carrying it home for disinfection.

In response to this crisis, some companies and manufacturers have developed affordable, easy-to-use disinfectant systems for those who lack access to clean water. These systems don’t require any fuel other than sunlight, and they can be used in remote locations.

Over 400 people in the world die every hour because of the unavailability of fuel and clean drinking water. These people die from preventable diseases they get from the pathogens in unclean water. In addition to pathogens, untreated water may contain toxic chemicals that get into the water supply. In Bangladesh, very deep wells are made to avoid contaminants found close to the surface. However, the wells are so deep that they collect water that is very highly contaminated with arsenic.

Lesson Summary

We all need clean water that is safe for drinking, otherwise known as potable water. In the U.S., we take for granted that this is provided for us. However, in many developing countries, this is not the case. A lack of clean drinking water kills hundreds of people each day and may force people to walk great distances for water as well as for fuel for disinfecting it.

We are also fortunate that our wastewater is treated for us, either through septic systems in rural areas, which separates waste from water in septic tanks, or through primary, secondary, and tertiary treatment in urban areas.

You may have even noticed that the municipal process for treating wastewater is quite similar to purifying drinking water. For each process, it’s important to first remove large particles, aerate the water and then disinfect it. Though the final destination may seem different, it’s important to treat wastewater as thoroughly as possible because we really are using the same water over and over again.

Treated wastewater ends up in streams, lakes and rivers, and this is where we get our drinking water from. So, you can see that since we are recycling the same water over and over again on Earth, thorough treatment is a necessary process.

Learning Outcomes

After watching this lesson, you should be able to:

  • Describe the process of water purification
  • Explain how wastewater is treated so that it’s reusable
  • Discuss water purification in developing countries