Step 1: Why a fixed BOD test is needed.
Wastewater contains organic matter that is broken down by aerobic bacteria.
The oxygen used during this biological breakdown gives an idea of how polluted the water is.
To compare pollution levels of different water samples, a fixed and uniform testing method is essential.
Step 2: Origin of the 5-day test.
The BOD test was originally developed in temperate regions where rivers typically took about 5 days to flow from source to sea.
Measuring oxygen demand over this period gave a realistic estimate of organic pollution affecting natural waters.
Step 3: Role of temperature control.
Microbial activity increases rapidly with temperature.
To avoid variations in oxygen consumption, the test temperature is fixed at a moderate value:
\[ 20^\circ\text{C} \] This temperature ensures steady microbial action without excessive acceleration or inhibition.
Step 4: Identifying the standard condition.
A duration shorter than 5 days underestimates pollution.
Higher temperatures exaggerate oxygen demand.
Thus, the internationally accepted condition is incubation at 20°C for 5 days, known as the BOD\(_5\) test.
Final Conclusion:
The standard conditions for measuring Biochemical Oxygen Demand are:
\[ \boxed{20^\circ\text{C for 5 days}} \]