Where Does the Wastewater Go?
Basics on the topic Where Does the Wastewater Go?
Structure and Function of a Sewage Treatment Plant
Before explaining how a sewage treatment plant works, the sources of wastewater are usually discussed. Wastewater is generated nearly everywhere – mostly at home in the bathroom (toilet, shower, sink), laundry room (washing machine), and in the kitchen (sink, dishwasher). Huge amounts of waste water are also produced in schools, office buildings, and especially in many factories. And this water cannot simply flow into rivers or into the ground. That's why we have sewage treatment plants!
Structure of a Sewage Treatment Plant – Simplified Explanation
Many households are connected to the sewage system. This means that wastewater from toilets, showers, and sinks flows through pipes to the sewage treatment plant. The sewage treatment plant can be summarized as having three main treatment stages:
- The mechanical treatment stage
- The biological treatment stage
- The chemical treatment stage
In the diagram below, you can see how wastewater is treated in the sewage treatment plant, simplified for elementary school students:
Function of a Sewage Treatment Plant
What exactly happens in a sewage treatment plant? The three treatment stages of the sewage treatment plant are connected in sequence, meaning that the wastewater passes through various stations one after another.
1. Mechanical Treatment
In the first treatment stage – the mechanical treatment – the water is freed from large debris such as wood or paper using a screen. In the next tank, called the grit chamber, the small, heavy particles settle to the bottom and are pumped out.
In the third tank, called the sedimentation tank, or pre-clarification tank, the wastewater comes to rest for a while. Fine suspended matter settles as sludge at the bottom. Lightweight substances like fats and oils rise to the water surface and can be skimmed off or suctioned away.
2. Biological Treatment
In the second treatment stage, the wastewater is supplied with bacteria and a large amount of oxygen in a tank called the aeration tank, also known as the activated sludge tank. With the help of oxygen, the bacteria can multiply and consume the harmful substances dissolved in the water. This stage is called biological treatment because it utilizes natural biology in the process. By now, the water is already quite clean.
3. Chemical Treatment
In the secondary clarifier, or secondary settling tank, chemical solvents are added and the remaining impurities lump together. The dirt separated at various points in the cleaning process is directed to the digestion tower. There, the waste substances decompose; the resulting gasses can be used to generate energy. The remaining residues are then incinerated in a waste incineration plant.
Let’s review the three stages of wastewater purification with the table below:
| Treatment Stage | Tank | Cleaning Function |
|---|---|---|
| 1. Mechanical treatment |
Screen | Removal of coarse impurities |
| Grit chamber | Settling of heavier substances | |
| Pre-clarification/sedimentation tank | Removal of suspended matter and fats/oils | |
| 2. Biological treatment | Activated sludge/aeration tank | Addition of bacteria and oxygen |
| 3. Chemical treatment |
Secondary settling/clarifying tank | Addition of chemical solvents |
What Happens After the Sewage Treatment Plant?
After the last tank, the secondary settling tank, the water can be discharged into rivers or other bodies of water. The water returns to the natural water cycle. It will eventually evaporate, condense, fall to the earth as rain and trickle off into the ground. The water is further purified as it passes through various soil layers. In the end, it is collected as groundwater and pumped into our water pipes as drinking water.
Note: When the water leaves the sewage treatment plant, it cannot be pumped directly into our homes. It must go through the natural water cycle's purification processes once again.
In elementary school, many classes visit sewage treatment plants and get to know the treatment stages or stations directly on-site.
Transcript Where Does the Wastewater Go?
Before her trip, Uma had to quickly use the bathroom. Now, it's time to flush. The water comes out of the tank and flows into the toilet bowl, but "Where does the wastewater go?" First, it flows through a pipe into the sewer system. The sewer system carries wastewater from toilets, sinks, and bathtubs to the wastewater treatment plant. This is where the process really starts. In the wastewater treatment plant, the dirty water is purified in several stages. The cleaning process happens in different tanks, flowing from one basin to another. First, the water flows through a screen, which removes large floating items, such as dental floss, cotton swabs, or plastic bags. This ensures that nothing clogs or breaks the equipment. The wastewater then flows into the grit chamber. Here, the water flows much slower than before, allowing small stones, gravel, and sand to sink to the bottom. Oil and grease are also removed. Clear from sand, oil, and grease, the wastewater then enters the sedimentation tank. Here, small dirt particles sink to the bottom. Material that is lighter than water floats to the top. A scraper removes the material on the water's surface and bottom of the tank. The wastewater is now ready for the aeration tank. Here it is churned to add in air, this helps special bacteria break down organic matter in the wastewater. These bacteria eat organic matter, like the business you left in the toilet, or leftover food from washing dishes. The bacteria, together with other tiny organisms, form small flakes. Once the bacteria are finished, the wastewater flows into the secondary clarifier. Here, the flakes sink to the bottom and form a sludge. Finally, the water flows into the filtration tank. Then, a sand filter is used to remove chemical impurities. Now the water can be returned to the environment, which still cleans the water in its own natural ways. But, where does the sludge go? In some treatment plants, the sludge from the sand trap, primary sedimentation tank, aeration tank, and secondary sedimentation tank goes to the digestion tower. The sludge begins to rot, which means that bacteria breaks it down and produces a gas in the process. This gas can be used to generate electricity. Now let's briefly summarize: After flowing through the sewer system, wastewater reaches the treatment plant. Here the water is purified, or cleaned. The screen removes large pieces of garbage, then the waster water moves through the grit chamber, then, the sedimentation tank, and, after that, it flows to the aeration tank. Bacteria and debris settle in the secondary clarifier. Then, the remaining impurities can be filtered out in the filtration tank. Finally, the leftover sludge is broken down in the digestion tower, which produces a gas. Now, the purified water flows back into nature. Treating wastewater costs money, and adults pay for it through taxes and water bills. However, it is very important to clean wastewater. The environment would not be able to do the cleaning on its own, and all living things need clean rivers, lakes, and oceans to survive. Uma enjoys the clean water too! Thank goodness for wastewater treatment plants!
Where Does the Wastewater Go? exercise
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What is the purpose of the screen in the wastewater treatment process?
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After leaving the grit chamber, where does the wastewater go next in the treatment process?
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What is the role of the aeration tank in the wastewater treatment process?
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What happens to the small flakes formed by bacteria and other tiny organisms during the treatment process?
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Why is it necessary to remove large floating items from the wastewater early in the treatment process?
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Arrange the steps in the order that wastewater goes through at the wastewater treatment plant.
