What are blanks used for in the BOD test?

Blanks in the Biochemical Oxygen Demand (BOD) test are specifically used to calculate the dilution water dissolved oxygen (DO) uptake. This is an essential part of the BOD testing process because it accounts for the consumption of oxygen by the dilution water itself, rather than the sample being tested.

When performing the BOD test, some oxygen will be consumed by organic and inorganic matter present in the dilution water during the incubation period. The blank is a sample that consists solely of the dilution water, which is incubated under the same conditions as the actual samples. By measuring the DO concentration in the blank before and after the incubation period, the laboratory can ascertain how much oxygen was consumed by the dilution water alone. This value is then subtracted from the DO consumption measured in the water sample to accurately reflect the BOD of that sample.

This process ensures a more precise measurement of BOD by isolating the contribution of the dilution water, allowing for a clearer understanding of the organic load present in the wastewater being tested. The other options pertain to different testing processes or outcomes unrelated to the primary purpose of blanks in the BOD test.