Question

Biochemical oxygen demand (BOD) value can be a measure of water pollution caused by the organic matter. Which of the following statements is correct?
A. Polluted water has a BOD value higher than 10 ppm.
B. Aerobic bacteria decrease the BOD value.
C. Anaerobic bacteria increase the BOD value.
D. Clean water had BOD value higher than 10 ppm.

Answer 1

To answer this question, you must recall the concept of BOD. Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed by aerobic bacteria to digest the waste organic matter present in a given water sample.

Complete step by step answer:
The value of BOD is most commonly expressed as the amount of molecular oxygen in milligrams required to convert the organic waste contained in $1.0Litre$ of a water sample.
Thus, BOD values are reported in $mg/L$ values or $ppm$ values (parts per million).
Bacteria are responsible for decomposing organic waste. When organic matter like dead plants, manure, leaves, sewage or food waste is present in the water supply, the bacteria begin the process of breakdown of this waste. Due to this, much of the dissolved oxygen in the water supply is used up by the aerobic bacteria, thus reducing the amount of available dissolved oxygen for other aquatic organisms.
Thus, we can infer that, more the amount of organic waste in water, more is the BOD and hence, lesser is the dissolved oxygen making the water sample unfit to support life.
Clean, unpolluted water generally has a BOD less than $1ppm$. For moderately polluted water, the BOD value varies between $2 - 8mg/L$. Polluted water has a BOD higher than $10ppm$.
The BOD level of untreated sewage is between $200 - 600mg/L$.
Anaerobic bacteria decrease the BOD value as they remove the organic compounds by anaerobic digestion (in the absence of oxygen).

Thus, the correct option is A.

Note:
BOD level is a common metric system for measuring water pollution. The simplest method to determine the BOD level is by comparing the dissolved oxygen levels of the water sample before and after five days of incubation in the dark. The difference between the final and initial levels of DO represents the amount of oxygen required for decomposition of organic material in the sample which gives an approximation of the BOD level.