High BOD in wastewater indicates:

Question

High BOD in wastewater indicates:

1. A. High inorganic salt content
2. B. High concentration of biodegradable organic matter (Correct answer)
3. C. High dissolved oxygen concentration
4. D. High alkalinity

Correct Answer

Option B — High concentration of biodegradable organic matter

Detailed Solution & Explanation

The correct answer is High concentration of biodegradable organic matter.

Key Points

• Biochemical Oxygen Demand measures the oxygen consumed by micro-organisms while they decompose organic matter in a sample. A high BOD therefore means there is a large quantity of biodegradable organic matter for them to consume.
• The other options misread what the test measures. BOD does not respond to inorganic salts or alkalinity, and a high BOD implies the opposite of high dissolved oxygen — the organic load depletes DO rather than raising it.

Additional Information

• The environmental significance is that a high-BOD discharge strips oxygen from the receiving water, and once DO falls below roughly 4-5 mg/L fish and other aerobic life cannot survive. Complete depletion turns the reach anaerobic and foul-smelling.
• Typical values worth carrying: raw domestic sewage 200-300 mg/L, and the Indian discharge standard into inland surface water 30 mg/L (10 mg/L for the stricter categories).
• BOD is an indirect, inferential measure — it quantifies oxygen demand, not the organic matter itself — which is why it takes five days and why COD, taking about three hours, is preferred for process control.
• The BOD/COD ratio indicates biodegradability: above about 0.5 the waste responds well to biological treatment; well below that it needs chemical or physical methods.