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Where Does the Wastewater Go?

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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:

  1. The mechanical treatment stage
  2. The biological treatment stage
  3. 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:

SEO_27333_Where_Does_the_Wastewater_Go-01.svg

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!

2 comments
2 comments
  1. very cool!

    From Jonah Miles kerley , 6 days ago
  2. good video!

    From Jace, about 2 months ago

Where Does the Wastewater Go? exercise

Would you like to apply the knowledge you’ve learned? You can review and practice it with the tasks for the video Where Does the Wastewater Go?.
  • What is the purpose of the screen in the wastewater treatment process?

    Hints

    Consider why it's necessary to physically separate certain items from wastewater in the early stages of treatment.

    Could these items cause blockages or damage to equipment if not removed early in the treatment process?

    Solution

    The screen removes large floating items such as dental floss, cotton swabs, or plastic bags to prevent clogs and equipment damage.

  • After leaving the grit chamber, where does the wastewater go next in the treatment process?

    Hints

    Think about the various types of particles and materials present in wastewater.

    How might the density or weight of these particles play a role in their separation within the treatment process?

    Solution

    The wastewater flows into the sedimentation tank after leaving the grit chamber.

  • What is the role of the aeration tank in the wastewater treatment process?

    Hints

    Imagine a process where the water is vigorously mixed and stirred, creating movement and introducing air bubbles.

    This action supports the growth and activity of certain microorganisms.

    There are two correct answers.

    Solution

    The aeration tank churns the wastewater and adds air to aid special bacteria in breaking down organic matter.

  • What happens to the small flakes formed by bacteria and other tiny organisms during the treatment process?

    Hints

    After the wastewater has undergone several treatment steps, what is the final outcome in terms of the particles that have settled at the bottom?

    Think about the characteristics of these settled particles and how they might be described in terms of their physical state.

    Remember that tiny pieces gather together and sink to the bottom. This forms something called sludge.

    But, unlike before when special tiny creatures helped, this time it's not because of them.

    Think about what happens to these tiny pieces in the end.

    Solution

    The small flakes settle to the bottom of the secondary clarifier and form a sludge.

  • Why is it necessary to remove large floating items from the wastewater early in the treatment process?

    Hints

    Think about why it's important to take out big things from the water at the start.

    What might happen if these large items were not removed early in the cleaning process?

    How could the presence of large items hinder the smooth progression of water and the effectiveness of subsequent cleaning stages?

    Solution

    Removing large items early helps keep the pipes and machines working well.

    If big things stay, they can block and break the equipment, making it hard to clean the water.

    So, taking them out early makes everything work better and keeps the water cleaning process going smoothly.

  • Arrange the steps in the order that wastewater goes through at the wastewater treatment plant.

    Hints

    First, the water flows through a screen, which removes large floating items, such as dental floss, cotton swabs, or plastic bags.

    Next, the wastewater flows into the grit chamber, where it moves much more slowly than before, enabling small stones, gravel, and sand to settle at the bottom.

    Oil and grease are also removed.

    After the wastewater is thoroughly cleaned and purified, what is left behind that needs further treatment?

    Think about the different stages the sludge might go through in a wastewater treatment plant.

    Solution

    The key stages of the treatment process, from the moment the wastewater enters the plant, are:

    • screening
    • grit removal
    • sedimentation
    • aeration
    • secondary clarification
    • filtration
    • sludge digestion