Texas Wastewater Class B Practice Exam

The correct percentage for the removal of incoming Biochemical Oxygen Demand (BOD) in a properly designed and operated primary sedimentation tank typically falls within the range of 30-40%. Primary sedimentation is primarily focused on the physical separation of solids from wastewater. While there is some biological activity occurring, the main purpose of this process is the reduction of suspended solids and associated organic material, which contributes to the BOD.

In a well-functioning primary sedimentation tank, the removal efficiency can be impacted by several factors, including the hydraulic retention time, the characteristics of the wastewater, and the tank design. Although 10-20% removal may occur, it is not representative of the effectiveness expected from a primary sedimentation tank. Similarly, while higher percentages (50-60% and 70-80%) may indicate more advanced secondary treatment processes, they exceed the typical performance metrics of primary sedimentation.

Therefore, the range of 30-40% is aspirational for primary sedimentation systems, reflecting a standard expectation for capturing BOD from the incoming wastewater.