Read the following passage and answer the questions that follow :
Sewage is generated in large quantities in the urban areas. It is treated in sewage treatment-plants (STPs) to make it less polluted before discharging into any natural water body. Observe the flow chart given below and answer the questions that follow :Primary effluent is passed into large aeration tank
↓
Effluent is passed into settling tank to form the sediment (activated sludge)
(b) What happens to the activated sludge when it is passed into an anaerobic sludge digester ? Explain.
Sewage is generated in large quantities in the urban areas. It is treated in sewage treatment-plants (STPs) to make it less polluted before discharging into any natural water body. Observe the flow chart given below and answer the questions that follow :
Primary effluent is passed into large aeration tank
↓
Effluent is passed into settling tank to form the sediment (activated sludge)
(b) What happens to the activated sludge when it is passed into an anaerobic sludge digester ? Explain.
Show Hint
Solution and Explanation
In the aeration tank, bacteria grow as flocs and use up the organic matter.
The flocs settle in the settling tank and form the activated sludge.
A small part of it is sent back to the aeration tank as inoculum. The rest goes to the digester.
Step 2: What happens in the anaerobic digester:
The digester has no oxygen. Anaerobic bacteria grow in it.
They digest the bacteria and fungi present in the sludge.
During this digestion they produce a mixture of gases: methane, hydrogen sulphide and carbon dioxide.
This mixture is called biogas and it can be used as a source of energy.
Final Answer:
In the anaerobic digester, anaerobic bacteria digest the bacteria and fungi of the sludge and release biogas (methane, hydrogen sulphide, carbon dioxide).
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Assertion (A) : Flocs are masses of bacteria associated with fungal filaments in secondary treatment of sewage.
Reason (R) : Flocs help in digestion of solid waste by anaerobic respiration.
A laboratory assistant collected three water samples, namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant. But the laboratory attendant did not note which was which. The samples were labelled A, B and C and subjected to a BOD test. The BOD values of the three samples A, B and C were recorded as 30 mg/L, 8 mg/L and 400 mg/L respectively.
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