Colorado Wastewater (WW) Operator D Practice Exam

The key idea is applying a percentage reduction to the influent BOD to find the post-treatment BOD. If the influent BOD is 150 mg/L and primary treatment reduces it by 30%, the remaining BOD is 150 × (1 − 0.30) = 150 × 0.70 = 105 mg/L. So the post-treatment BOD is 105 mg/L. The other values would correspond to no reduction (150 mg/L), a smaller reduction (135 mg/L), or an increase (180 mg/L), which don’t match a 30% reduction.