In measurement of `BOD_x`, x is generally taken as

Explanation

To solve the question regarding the measurement of `BOD_x`, we need to identify what `x` stands for in the context of Biochemical Oxygen Demand (BOD).

### Step-by-Step Solution:

1. **Understanding BOD**: BOD stands for Biochemical Oxygen Demand. It measures the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in a water sample.

2. **Identifying the Time Frame**: The `x` in `BOD_x` typically refers to the duration over which the oxygen demand is measured. This is crucial because BOD is a time-dependent measurement.

3. **Common Time Frame**: The standard time frame used for measuring BOD is 5 days. Therefore, `BOD_5` is the most commonly used term, indicating the biochemical oxygen demand measured over a period of 5 days.

4. **Temperature Consideration**: The measurement is usually conducted at a temperature of 20 degrees Celsius, which is the standard condition for BOD testing.

5. **Conclusion**: Thus, in the measurement of `BOD_x`, `x` is generally taken as 5 days, leading to the notation `BOD_5`.

### Final Answer:
In measurement of `BOD_x`, x is generally taken as **5 days**.

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