New York State Wastewater Practice Exam – Complete Prep Guide 2025

The correct approach to calculate Biochemical Oxygen Demand (BOD) involves understanding how dissolved oxygen (DO) levels change over time due to the activity of microorganisms consuming organic matter in the water sample. BOD is represented by the decrease in DO from the initial reading to the final reading after a specified incubation period, typically 5 days.

By subtracting the final DO reading from the initial DO reading, you are effectively calculating the amount of dissolved oxygen that was consumed by the biological processes during that time period. This difference gives you the BOD value, which reflects the organic pollution level in the sample. A larger difference implies higher BOD, indicating more organic material present, while a smaller difference suggests less organic material.

Evaluating the other options shows that they don't accurately represent the calculation of BOD. Dividing the final DO by the initial DO does not provide a meaningful measure related to BOD. Adding the two readings together would yield a value that also does not pertain to BOD. The option involving multiplication with dilution may seem intuitive, but it lacks clarity without specifying what dilution factor is being accounted for and does not directly relate to the primary calculation of BOD itself.