Wastewater Treatment Operator Certification Practice Test

What is the typical BOD removal efficiency in a primary clarifier?

1. 10 to 15%

2. 40 to 60%

3. 20 to 50%

4. 95 to 99%

The correct answer is: 20 to 50%

The typical biochemical oxygen demand (BOD) removal efficiency in a primary clarifier falls in the range of 20 to 50%. Primary clarifiers are designed to remove settleable solids and to a lesser extent some organic material, which contributes to BOD. In the primary clarifier, the process of sedimentation allows heavier solids to settle to the bottom, while liquid layers above carry lighter solids and dissolved substances. The BOD reduction achieved in this stage is primarily due to the removal of solids that contain organic matter. While 20 to 50% is a common estimate for BOD reduction, it is important to recognize that this process does not provide complete removal due to the presence of finer particles and colloidal material that can remain suspended. Higher BOD removal efficiencies would typically be associated with additional treatment stages, such as secondary biological treatment processes, which are designed to significantly reduce organic material beyond what primary clarifiers accomplish. Thus, while primary clarifiers do remove some organic material, the efficiency remains moderately low compared to subsequent treatment steps.