Question

The clean water should have BOD value
A ) 100 ppm
B ) less than 5 ppm
C ) 20 ppm
D ) 200 ppm

Answer 1

Hint: BOD is the Biochemical Oxygen Demand. It is the amount of dissolved oxygen required by aerobic bacteria to decompose the organic waste present in the water sample at a given temperature and for a particular time period.

Complete answer:
Drinking water has a BOD level of 1 - 2 ppm. When the BOD value of water is in the range 3 - 5 ppm, the water is moderately clean. Polluted water has a BOD value in the range of 6 - 9 ppm. In polluted water, some organic waste is present. This organic matter is decomposed by bacteria. A BOD value of more than 100 ppm suggests that water is highly polluted.
The clean water should have BOD value less than 5 ppm.

Hence, the correct option is the option B ).

Additional Information If Phytoplankton is present in water in significant amounts, it will be consumed by the bacteria that need dissolved oxygen. Due to this, the dissolved oxygen levels in water decrease and this will lead to suffocation of fish. A large number of fish can be found suddenly dead as they cannot find dissolved oxygen.

Note: When BOD value is high, the level of dissolved oxygen (DO) is less and the organisms that need high DO cannot survive. Such organisms include caddisfly larvae, mayfly nymphs. But organisms such as leeches, sludge worms require less DO and can survive.