Sewage Treatment and Biogas: Quick Revision

A brief note on Sewage Treatment and Biogas. Read it before a test. Try each question at the end before you open its answer.

Sewage treatment

• Sewage: the waste water of a town. It holds much organic matter and many disease causing microbes. It is treated before it goes into a river.
• Primary treatment (physical): filtration removes floating debris, sedimentation removes grit, the settled solids are the primary sludge. The liquid above is the effluent.
• Secondary treatment (biological): the effluent goes into aeration tanks. Air is pumped in and the liquid is stirred. Aerobic microbes grow as flocs and lower the BOD.
• Floc: bacteria held together with fungal filaments.
• BOD: the oxygen that bacteria would use to oxidise the organic matter of 1 L of water. A greater BOD means a greater power to pollute.
• Activated sludge: the sediment of the flocs in the settling tank. A small part is the inoculum. The rest goes to the anaerobic sludge digesters.
• Digesters: anaerobic bacteria make methane, hydrogen sulphide and carbon dioxide, the biogas.
• The effluent of secondary treatment is generally released into rivers and streams.

Biogas

• Biogas: a mixture of gases, mostly methane, made by methanogens without oxygen. A methanogen is Methanobacterium.
• Methanogens live in anaerobic sludge and in the rumen of cattle, so dung is rich in them.
• Gobar gas plant: a tank 10 to 15 feet deep with a slurry of dung, a floating cover, an outlet for the gas and an outlet for the spent slurry. The spent slurry is a fertiliser.
• The technology in India was developed by IARI and KVIC.

A worked example

A plant receives 5000 m³ of sewage in a day. The BOD is 200 mg per litre. How much oxygen would the microbes consume?

1. $200$ mg per litre is $200$ g in each m³.
2. $5000 \times 200 = 1{,}000{,}000$ g, which is $1000$ kg of oxygen in a day.

Practice with new numbers

A program to try

The program works out the oxygen demand of a flow of sewage.

Mistakes to avoid

• Saying that microbes work in primary treatment. It is physical.
• Saying that BOD is the oxygen made by microbes. It is the oxygen that they consume.
• Saying that all the activated sludge goes back to the aeration tank. Only a small part does.
• Saying that methanogens are aerobic. They are anaerobic.

Check yourself

1. What is a floc?

2. What happens to the sediment of the flocs?

3. Which microbes make methane?

4. What is the use of the spent slurry?

Sources

• 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, McGraw Hill.