The 'chemical oxygen demand' (COD) test measures:
Choose the correct answer
Only the dissolved oxygen available
Oxygen equivalent of ALL oxidisable organics (by K2Cr2O7) — faster than BOD; includes non-biodegradable fraction
Biochemical oxygen demand over 5 days
Total dissolved solids only
Correct Answer
B. Oxygen equivalent of ALL oxidisable organics (by K2Cr2O7) — faster than BOD; includes non-biodegradable fraction
Why is this the answer?
COD: all organics (biodegradable + non-biodegradable + toxic) oxidised by chemical oxidant (K2Cr2O7 in acidic condition with Ag2SO4 catalyst and heat for 2 hours). Results in g O2/L equivalent. COD > BOD always (COD includes non-biodegradable). Ratio BOD/COD: > 0.5 = biodegradable; 0.3–0.5 = moderate; < 0.3 = non-biodegradable/toxic waste (needs special treatment). COD result in hours vs BOD5 in 5 days.
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