Approximately what percentage of BOD is removed during primary treatment in wastewater plants?

During primary treatment in wastewater treatment plants, the primary goal is to remove settleable solids and floating materials through physical processes such as sedimentation and flotation. Biochemical oxygen demand (BOD) is a measure of the amount of oxygen consumed by microorganisms in the decomposition of organic matter.

In this context, primary treatment typically achieves about 30% removal of BOD. This removal occurs through the sedimentation of solids and the separation of floating materials, which includes grease and oil. Essentially, the process is designed more to reduce the volume of solids and organic materials that then undergo further treatment rather than to drastically reduce BOD.

Subsequent treatment processes, such as secondary treatment, are responsible for much higher percentages of BOD removal, typically in the range of 85% to 95%. Thus, while primary treatment significantly reduces the load of organic materials, the expectation is that approximately one-third of the BOD is accomplished through this stage.