What happens to water after it goes down the drain?

1. Every time you wash your hands, take a bath or flush, the used water, called wastewater, goes down the drain into pipes under your home.
2. Sewer pipes under the street slope gently downhill, so gravity carries the wastewater along. Where the land rises, pumps lift it up.
3. At the plant, screens catch rags, wipes and plastic. Then the water slows down in a grit tank, where sand and gravel sink to the bottom.
4. In big, calm settling tanks, heavier solids sink to the bottom as sludge, and grease floats to the top. Scrapers and skimmers remove both.
5. In the aeration tank, billions of tiny microbes eat the dissolved dirt. Air bubbles give them the oxygen they need to do their job.
6. In a final settling tank the microbes clump together and sink. Ultraviolet light stops germs, and the clean water flows back to the river.
7. The leftover sludge goes into big digester tanks. Microbes there break it down without air and make biogas, which can help power the plant.

Follow the water from the drain to the river. Tap each part to see what it does.

• Sewer pipes. Sloping pipes under the street carry wastewater from every building to the plant, pulled along by gravity.
• Bar screen. Metal bars catch rags, wipes and plastic. A rake lifts the trash out into a bin.
• Grit chamber. The water slows down so sand and gravel sink. Grit would wear out the pumps.
• Settling tank. In this calm tank, solids sink to the bottom as sludge and grease floats to the top. Both are scraped away.
• Aeration tank. Blowers push air through holes in the bottom. The bubbles mix the tank and give the microbes oxygen.
• Helpful microbes. Bacteria and other tiny living things eat the dissolved dirt and turn it into carbon dioxide, water and more microbes.
• Final settling tank. The microbes clump together and sink. Some are sent back to the aeration tank to keep working.
• UV disinfection. Ultraviolet lamps shine through the water and stop germs from multiplying.
• Back to the river. The cleaned water, called effluent, flows back into a river or the sea. Some cities reuse it to water parks.
• Digester. Sludge goes into big closed tanks. Microbes that live without air break it down and make biogas.

Change the air and the time the water stays in the tank. Watch the microbes and the 'out' jar. What do the microbes need to clean the water?

Experiment: What do the microbes need to clean the water?

1. Turn the air all the way off and watch the microbes.
2. Now turn the air up.
3. Compare the water in the 'out' jar.

What we found: With no air, the microbes can't breathe, so they hardly eat and the water stays dirty. With air bubbles, they eat the dissolved dirt quickly and the water comes out clean. Microbes need oxygen, just like you!

Turn on JavaScript to try it yourself.

Put the six stages in the right order, choose how much air the microbes get, and run the plant. Make the river fish happy!

When water goes down a drain or a toilet is flushed, it doesn't disappear. Pipes carry it to sewers under the street, which slope downhill so gravity can move it. In the end it reaches a wastewater treatment plant.

The plant cleans the water in steps, from big things to tiny things. Screens catch trash such as wipes and plastic. A grit tank lets sand settle. Big, calm settling tanks let heavier solids sink and grease float to the top.

Next comes the most surprising step: microbes do the cleaning! In the aeration tank, air bubbles give oxygen to billions of tiny bacteria, which eat the dissolved dirt. Later they clump together and sink, and some go back to keep working.

Finally, ultraviolet light stops germs, and the clean water flows back to a river. The leftover sludge goes to digesters, where microbes make biogas that can help power the plant. You can help too: only flush toilet paper, and never pour grease down the sink.

Key words

Wastewater

Used water from sinks, showers, toilets and washing machines.

Sewer

A pipe under the street that carries wastewater to the treatment plant.

Sludge

The solid material that settles out of wastewater in the tanks.

Activated sludge

A mix of water and helpful microbes that eat the dissolved dirt.

BOD

Biochemical oxygen demand: how much oxygen microbes would use to break down the organic matter in water.

Effluent

The treated water that leaves the plant.

Check your thinking

1. Why shouldn't you flush wipes?

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   They don't break apart and can clog pipes and screens. Toilet paper falls apart in water, but wipes stay strong and tangle into clogs.

2. What do microbes do in the aeration tank?

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   Eat the dissolved dirt. They use the dissolved dirt as food and turn it into harmless gas, water and more microbes.

3. What happens to the leftover sludge?

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   Digesters turn it into biogas. Microbes in digesters break it down without air and make biogas for energy.

4. Raising dissolved oxygen from 2 to 4 mg/L barely speeds up BOD removal. Why?

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   The microbes are already close to oxygen-saturated, so oxygen is no longer the limit. The rate follows DO ÷ (K + DO). With K around 0.5 mg/L, going from 2 to 4 mg/L only raises the rate from about 80 to 89 percent of its maximum.

5. Why must wastewater be cleaned before it reaches a river?

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   Otherwise microbes in the river use up the oxygen fish need. Dirty water feeds river microbes, and they use up the oxygen that fish breathe.