In a wastewater treatment plant, how much BOD is removed during primary treatment?

In a wastewater treatment plant, how much BOD is removed during primary treatment?

• A. 20%
• B. 30%
• C. 40%
• D. 60%

Answer: (B)

The key idea is that primary treatment relies on physical settling, so it can remove only the portion of BOD that is tied to settleable solids. Because much of the BOD in wastewater is dissolved or not easily removed by gravity, primary clarifiers typically reduce the BOD load by about a third. In practice, this is roughly 25–35% of the influent BOD, so 30% is the best fit among the options. The remaining BOD stays in the water to be treated biologically in secondary treatment.

The key idea is that primary treatment relies on physical settling, so it can remove only the portion of BOD that is tied to settleable solids. Because much of the BOD in wastewater is dissolved or not easily removed by gravity, primary clarifiers typically reduce the BOD load by about a third. In practice, this is roughly 25–35% of the influent BOD, so 30% is the best fit among the options. The remaining BOD stays in the water to be treated biologically in secondary treatment.