Question

BOD test is commonly carried out at:

• A. 20$^\circ$C for 3 days
• B. 25$^\circ$C for 3 days
• C. 20$^\circ$C for 5 days
• D. 25$^\circ$C for 5 days

Hint:

Always remember: {BOD$_5$ = 5 days at 20$^\circ$C}. This is a very high-frequency exam fact.

Answer 1

The Correct Option is C

Step 1: Meaning of BOD.
Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required by aerobic microorganisms to biologically decompose organic matter present in wastewater. It is an important parameter for measuring the strength and pollution level of sewage.
Step 2: Purpose of standard test conditions.
Microbial activity depends strongly on temperature and time. To obtain consistent, comparable, and reproducible results worldwide, the BOD test is conducted under standardized laboratory conditions.
Step 3: Standard temperature and duration.
The internationally accepted standard for BOD testing is incubation at 20$^\circ$C for a period of 5 days. This duration allows sufficient time for microorganisms to stabilize their oxygen consumption without complete oxidation of all organic matter.
Step 4: Evaluation of options.
(A) 20$^\circ$C for 3 days: Duration is too short and does not represent standard BOD measurement.
(B) 25$^\circ$C for 3 days: Both temperature and time are non-standard.
(C) 20$^\circ$C for 5 days: Correct — This is the globally accepted standard condition for the BOD$_5$ test.
(D) 25$^\circ$C for 5 days: Higher temperature increases microbial activity and leads to inaccurate BOD values.
Step 5: Final conclusion.
Since the BOD test must be conducted at 20$^\circ$C for 5 days to ensure uniform and reliable results, the correct answer is (C) 20$^\circ$C for 5 days.