120.  Pick up the correct statement from the following;
A. Hydrogen sulphide gas in excess, may cause corrosion of concrete sewers
B. 4 ppm of Dissolved Oxygen (D.0.) is ensured before discharging the treated sewage in river
C. Solubility of oxygen in sewage is 95% of that of distilled water
D. All the above.

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121.  To test chemical oxygen demand (C.O.D.) of sewage, organic matter is oxidised by pottassium dichromate in the presence of
A. Hydrochloric acid
B. Sulphuric acid
C. Nitric acid
D. Citric acid.

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122.  The amount of oxygen consumed by the aerobic bacterias which cause the aerobic biological decomposition of sewage, is known
A. Bio-Chemical Oxygen Demand (B.O.D.)
B. Dissolved Oxygen (D.O.)
C. Chemical Oxygen Demand (C.O.D.)
D. None of these.

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123.  The standard B.O.D. of water is taken for
A. 1 day
B. 2 days
C. 3 days
D. 5 days

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124.  If the depletion of oxygen is found to be 5 ppm after incubating a 2.5% solution of sewage sample for 5 days at 21?C, B.O.D. of the sewage is
A. 50 ppm
B. 100 ppm
C. 150 ppm
D. 200 ppm

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125.  If the depletion of oxygen is found to be 2.5 mg/litre after incubating 2.5 ml of sewage diluted to 250 ml for 5 days at 20?C, B.O.D. of the sewage is
A. 50 mg/1
B. 100 mg/1
C. 150 mg/1
D. 250 mg/1,

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126.  Dilution method of disposing off sewage, is not preferred to
A. when sewage is fresh
B. when diluting water has high dissolved oxygen content
C. when diluting water is used for water supply near the point of sewage disposed
D. when the diluting water is having flow currents

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