Massachusetts Wastewater Practice Exam

If a wastewater sample is suspected to have a BOD of 150 mg/L, what volume of samples should be used for effective analysis?

1. 3, 4, 5 mL

2. 6, 8, 10 mL

3. 10, 15, 20 mL

4. 10, 20, 30 mL

The correct answer is: 6, 8, 10 mL

Selecting an appropriate sample volume for biochemical oxygen demand (BOD) analysis is crucial for obtaining reliable results. When a wastewater sample is suspected to have a BOD of 150 mg/L, the goal is to choose a volume that allows for accurate measurement while preventing dilution or contribution from background levels. In BOD testing, the typical procedure requires diluting the sample to ensure that the resulting BOD readings fall within a measurable range, usually between 1 mg/L and 500 mg/L after a 5-day incubation period. The higher the BOD level, the smaller the sample volume needed to achieve an accurate reading when the dilution factor is taken into account. Choosing a volume in the range of 6 mL, 8 mL, or 10 mL allows the analyst to prepare dilutions that are manageable and accurate given the expected BOD value. This range ensures that when accounting for the dilution factor, the final result will fall within optimal measuring limits. Volumes of 10 mL, 15 mL, or 20 mL, while potentially usable, may not provide the necessary concentration of oxygen depletion measurements against a suspected high BOD sample without causing excessive dilution, leading to inaccurate cycle results. In summary, utilizing sample