For a given industrial wastewater sample, BOD is usually less than COD because:

Question

For a given industrial wastewater sample, BOD is usually less than COD because:

1. A. COD measures only biodegradable organics
2. B. COD measures more organic compounds (including non-biodegradable) than BOD (Correct answer)
3. C. BOD is determined at higher temperature
4. D. COD excludes toxic organics

Correct Answer

Option B — COD measures more organic compounds (including non-biodegradable) than BOD

Detailed Solution & Explanation

The correct answer is COD measures more organic compounds (including non-biodegradable ones).

Key Points

• COD exceeds BOD because the two tests oxidise different fractions of the same sample. COD uses a powerful chemical oxidant that breaks down nearly all organic matter; BOD relies on micro-organisms, which consume only what they can biodegrade in five days.
• The remaining options are factually wrong: it is BOD, not COD, that is limited to biodegradable material; BOD is run at 20°C, a standard low temperature; and COD does not exclude toxic organics — indeed it oxidises them, whereas they may inhibit the bacteria in a BOD bottle.

Additional Information

• The difference COD − BOD estimates the refractory organic load, and tracking it tells a designer whether biological treatment alone can meet the discharge standard.
• Toxic industrial samples can produce a misleadingly low BOD because the toxicant kills the seed organisms. Sample dilution and seeding are used to counter this, and a very low BOD alongside a very high COD should always prompt suspicion of toxicity.
• For a given wastewater, once the ratio is established it becomes a useful shortcut: operators estimate BOD from a same-day COD rather than waiting five days.
• Domestic sewage typically shows a BOD/COD ratio of about 0.5-0.6, which is why it treats readily by biological means.