How is BOD typically measured in a water sample?

Biochemical Oxygen Demand (BOD) is a crucial metric used to assess the organic pollution levels in water. It measures the amount of oxygen that microorganisms will consume while decomposing organic matter under aerobic conditions.

The standard method for determining BOD involves incubating a water sample at a controlled temperature (20°C) for a specified duration (5 days) while keeping the sample in darkness. This incubation method is intentional because light can promote photosynthesis in aquatic plants, leading to increased oxygen levels that would skew the results.

Measuring BOD at 20°C for 5 days provides a consistent baseline under which the microbial activity is observed, allowing for reliable comparisons between different water samples. This standardization is critical in environmental science and water quality management for determining the extent of pollution and the efficiency of wastewater treatment processes.

Other temperature and light conditions, such as those mentioned in the other choices, deviate from this protocol and could yield inconsistent or misleading results. Therefore, option B is the correct representation of the standard practice for measuring BOD in water samples.