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Biochemical Oxygen Demand (BOD) in a river water
A
Increases when sewage gets mixed with river water
B
Remains unchanged when algal bloom occurs
C
Has no relationship with concentration of oxygen in the water
D
Gives a measure of $Salmonella$ in the water
Detailed Solution
BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria.
It is therefore an indirect measure of the organic matter present in the water: the greater the BOD, the greater the polluting potential.
Sewage contains a large amount of biodegradable organic matter, so when it mixes with river water the BOD increases, and the dissolved oxygen falls sharply.
An algal bloom also adds organic matter when the algae die, so BOD does not remain unchanged.
BOD is not a count of any particular bacterium such as $Salmonella$.
Hence BOD increases when sewage gets mixed with river water.
It is therefore an indirect measure of the organic matter present in the water: the greater the BOD, the greater the polluting potential.
Sewage contains a large amount of biodegradable organic matter, so when it mixes with river water the BOD increases, and the dissolved oxygen falls sharply.
An algal bloom also adds organic matter when the algae die, so BOD does not remain unchanged.
BOD is not a count of any particular bacterium such as $Salmonella$.
Hence BOD increases when sewage gets mixed with river water.