Tennessee Wastewater Operator Practice Exam

Which substance can be measured with the COD test but not with the BOD test?

1. Biodegradable organics

2. Non-biodegradable organics

3. Soluble nutrients

4. Suspended solids

The correct answer is: Non-biodegradable organics

The Chemical Oxygen Demand (COD) test is designed to measure the total amount of oxygen required to oxidize both biodegradable and non-biodegradable organic materials in a sample, making it a more comprehensive measurement of organic pollution. This means that COD can account for substances that do not break down easily or take a long time to degrade biologically, which is a significant factor when evaluating wastewater treatment efficiency and potential environmental impact. In contrast, the Biological Oxygen Demand (BOD) test specifically measures only the amount of oxygen consumed by microorganisms while degrading biodegradable organic matter under aerobic conditions over a set period—typically five days. Consequently, BOD is unable to accurately quantify non-biodegradable compounds that do not contribute to oxygen demand in that biological context. Therefore, the capability of the COD test to include non-biodegradable organics while the BOD test does not illustrates why non-biodegradable organics specifically can be measured with COD but not with BOD. This distinction is important for wastewater operators and environmental regulators to assess the complete impact of organic pollutants in water bodies.