Sewage and its primary treatment · Biology Class 11–12 | StemPy Lab
Course 26.2·Lesson 1 of 3
Sewage and its primary treatment
The used water of a town is called sewage. It holds a great deal of organic matter and many germs, so it is treated before it goes back to a river. Primary treatment is physical: bars hold back the floating debris, the grit settles, and the solids that settle make the primary sludge.
about 25 min1 simulation1 practice question1 Python program5 questions to check yourself
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Where the water goes
Every day, a town uses water to wash, to cook and to flush the toilets. The used water runs down the drains. This waste water of a town or city is called sewage.
Sewage holds a large amount of organic matter, and it holds many microbes. Many of them cause disease. So sewage cannot be let into a river or a lake just as it is. It would pollute the water that people drink and that fish live in.
A sewage treatment plant, or STP, cleans the sewage first. The cleaning is done by microbes that feed on the organic matter and that are already present in the sewage. They are heterotrophic microbes, which means that they take their food from other living or dead matter.
The treatment has two stages: primary treatment, which is physical, and secondary treatment, which is biological.
A flow chart of a sewage treatment plant in two parts. Primary treatment, physical, runs along the top: raw sewage, screens, grit chamber and primary settling tank, joined by arrows. An arrow leaves the primary settling tank to the right, labelled primary sludge. A downward arrow labelled effluent leads from the primary settling tank to secondary treatment, biological, in the lower part. There, the effluent enters the aeration tank, where flocs form. An arrow leads from it to the secondary settling tank, and on to a box effluent to a river. From the secondary settling tank an arrow labelled activated sludge goes down to the anaerobic sludge digester, and a curved arrow labelled a small part goes back as inoculum returns to the aeration tank. An arrow from the digester leads to a box labelled biogas.
Primary treatment
In primary treatment, particles are removed from the sewage by filtration and sedimentation. No microbe has any part in it.
Screening. The sewage is filtered in turn, through screens with bars. The floating debris is removed.
Grit removal. The sewage flows slowly through a chamber, and the grit, which is soil and small pebbles, settles to the bottom.
Settling. In the primary settling tank, the solids that remain settle. They form the primary sludge.
The effluent. The liquid above the sludge is called the effluent. It goes on to secondary treatment.
Three panels. Panel 1, screening: water flows through a channel and floating debris, drawn as brown blobs, is held back by a row of vertical bars. Panel 2, grit chamber: water flows through a tank and grit, soil and small pebbles, settles to the bottom as grey dots. Panel 3, primary settling tank: a tank that narrows toward the bottom, with solids settling into a brown layer called primary sludge, and the liquid above it labelled the effluent, which goes to secondary treatment.
A word list helps. Sludge is a thick mass of settled solids. Effluent is the liquid that flows out of a stage of the plant. Sedimentation is settling under gravity.
Some numbers for a town
The numbers below are round teaching values. Real sewage differs from one town to another.
A person adds about 120 L of sewage each day.
Each litre holds about 220 mg of solids, which is 220 g in each cubic metre.
Sewage stays in the primary settling tank for a few hours, here 2 hours.
About 60 per cent of the solids settle as primary sludge.
For a town of 50,000 people, the flow is 50000×120=6,000,000 L in a day, which is 6000 m³. A tank that holds the sewage of 2 hours is 6000×2/24=500 m³. The solids in the sewage are 6000×220=1,320,000 g, which is 1320 kg in a day. Of these, 60 per cent settle, so the primary sludge is 792 kg in a day.
You should find that the sludge follows the flow, and that a longer time in the tank needs a bigger tank. In a real plant, the time in the tank is only one of the things that decide how much settles.
Try it in Python
The program works out the flow, the size of the tank and the sludge for a town.
Practise with new numbers
Common mistakes
Saying that microbes work in primary treatment. It is physical: filtration and sedimentation.
Mixing up sludge and effluent. The sludge is the solid that settles. The effluent is the liquid above it.
Saying that primary treatment removes the dissolved organic matter. It removes particles. Secondary treatment lowers the organic matter.
Forgetting to change litres to cubic metres. One cubic metre is 1000 litres.
Remember these
Sewage: waste water of a town, rich in organic matter and with many germs. It is treated in an STP before it is let into a river.
Primary treatment is physical. Filtration removes floating debris. Sedimentation removes grit. The solids that settle are the primary sludge. The liquid above is the effluent.
The effluent goes to secondary treatment, which is biological.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
What is sewage, and why is it treated before it is let into a river?
What are the two stages of sewage treatment, and which one is physical?
How is the floating debris removed, and how is the grit removed?
What is the primary sludge, and what is the effluent?
A town adds 8,000,000 L of sewage a day. How many cubic metres is this?
References
NCERT (2023). Biology, Textbook for Class XII, chapter 8, Microbes in Human Welfare.
Tchobanoglous G., Stensel H. D., Tsuchihashi R. and Burton F. (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th edition, Metcalf and Eddy, McGraw Hill, the chapters on the characteristics of wastewater and on physical unit processes.
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