- from Water
All of the nearly 8 billion human beings on Earth need clean water to drink. However, not every human being gets clean water.
Chemicals, both natural and artificial, can pollute lakes, rivers, and streams. Bacteria and other organisms can make a water supply deadly to drink. Heavy metals and other minerals can also make water unsafe to use. Keeping the world’s water supply clean is one big challenge.
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 naturally settle to the bottom. Other pollutants are lighter than water, so they float to the top. When they do, they are skimmed off. Settling and skimming remove about half of the pollutants from the stored water.
Wastewater contains very little oxygen. Special tanks aerate (provide oxygen to) the wastewater. The oxygen helps useful bacteria and microbes grow. These organisms eat the waste material. They remove about 90 percent of the 50 percent of pollutants left after the settling and skimming process.
Chemicals, such as chlorine, are used to kill some harmful organisms. Samples are taken to determine the presence of bacteria.
Filtering and Distilling
At this point in the waste treatment plant, the water may still contain metals, pesticides (chemicals used to fight crop pests), and other pollutants. Getting those things out of water is expensive. So, not all treatment plants can afford to do it. Filtering water through charcoal gets rid of carbon-based pollutants, such as pesticides. To remove salts and other minerals, a few special treatment plants distill water (boil it into steam). The pollutants stay behind as the steam rises and condenses. The liquid water is then sent to another chamber.
Sometimes, treatment plants add chemicals to remove other, harmful chemicals. For example, alum causes chemicals and waste particles to clump together. The clumps settle 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 that is rich in nutrients is used as fertilizer for crops.
◀ In many countries, children collect their family’s daily water supply from a community well. The groundwater from a well has filtered down through many layers of soil and rock. The layers help clean out some contaminants, but not all of them.
In the mid-1990s, children in Minnesota started seeing frogs with extra or missing legs. Since then, people around the United States have told scientists about frog deformities. Experts tested water in some frog habitats. They found it contained pesticides. That may be one of many causes of the deformed frogs. ▶