- from Water
All of the nearly 8 billion human beings on Earth need clean water to drink. However, not everyone gets it.
Chemicals, both natural and artificial, can pollute lakes, rivers, and streams. Bacteria and other organisms can make water deadly to drink. Heavy metals and other minerals can also make it unsafe. Keeping the world’s water clean is a big job.
What Happens to Wastewater?
What happens to toilet water after you flush, to the water that flows out of the sink, shower, or tub? Until the last few decades, individuals as well as cities dumped this wastewater (or sewage) directly into rivers, lakes, and oceans. Today, most treatment plants have several ways to clean water so it can be reused or returned to the environment.
Settling and Skimming
Water may pass slowly through a sedimentation tank. Some pollutants are heavier than water, so they settle to the bottom. Other pollutants are lighter than water, so they float to the top. Those are skimmed off. Settling and skimming take out about half of the pollutants.
There is not much oxygen in wastewater. So, special tanks put oxygen into the water. Oxygen helps useful bacteria and microbes grow. These organisms eat waste material. They remove about 90 percent of the pollutants left after the settling and skimming process.
Chemicals can kill some harmful organisms in the water. One common chemical used for this is chlorine. Scientists take samples to see if bacteria are in the water.
Filtering and Distilling
At this point in the waste treatment plant, the water may still contain bad stuff. It may have metals, pesticides (bug-killers), and other poisons in it. Getting those things out of water is expensive. So, not all treatment plants can afford to do it. Some places filter water through charcoal. That gets rid of carbon-based pollutants, such as pesticides. To take out salts and other minerals, a few plants distill water. That means they boil the water into steam. The steam rises and turns into a liquid again, but without pollutants in it. The liquid water then flows into another chamber.
Sometimes, treatment plants add useful chemicals to remove bad ones. For example, they might use a chemical called alum. It makes chemicals and waste particles clump together. The clumps go to the bottom of the storage tank. They become a gooey liquid called sludge.
Removing or Reusing Sludge
Treatment plants usually bury or burn sludge. That keeps it from getting back into the water supply. Sometimes, sludge is rich in nutrients. Then it may be used to fertilize crops.
◀ In many countries, children collect the family’s daily water supply from a community well. The groundwater from a well is filtered by the Earth itself. Water flows down through many layers of soil and rock. Those layers help take out some contaminants but not all.
In the mid-1990s, kids in Minnesota found something weird. They reported seeing frogs with extra or missing legs. Since then, people around the United States have been finding deformed frogs. Experts tested water in some places where frogs live. They found lots of pesticides. They may be one reason that frogs are deformed. ▶