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)
(c) (ii) Define BOD. What happens to the BOD when organic matter present in the wastewater increases ?
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)
(c) (ii) Define BOD. What happens to the BOD when organic matter present in the wastewater increases ?
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Solution and Explanation
BOD means biochemical oxygen demand.
It is the amount of oxygen that bacteria would consume to break down all the organic matter in one litre of water.
Step 2: Effect of more organic matter:
More organic matter needs more oxygen for its breakdown.
So BOD increases when organic matter increases.
A high BOD means the water is more polluted.
Final Answer:
BOD is the oxygen needed by microbes to oxidise all the organic matter in one litre of water. BOD rises as organic matter increases.
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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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