What is the typical percentage of BOD reduction achieved without chemical addition in primary clarification?

• A. 15% to 20%
• B. 25% to 35%
• C. 50% to 60%
• D. 70% to 80%

Answer: (B)

In primary clarification, the process primarily relies on physical separation methods such as sedimentation to remove suspended solids and some influent biodegradable organic matter, measured by biochemical oxygen demand (BOD). Without the addition of chemicals to enhance separation, primary clarification typically results in a BOD reduction of about 25% to 35%. This reduction comes from the settling of solids and the removal of some organic materials, but it does not achieve the higher percentages observed with additional treatment processes or chemical aid.

Understanding this percentage is essential because it helps wastewater treatment operators set realistic expectations for pollutant removal in the primary phase and plan further treatment stages accordingly. The range of reduction highlights the effectiveness of primary treatment and the necessity of subsequent biological treatment to achieve more significant reductions in BOD, which is critical for meeting regulatory standards and protecting the environment.