A municipal wastewater treatment plant receives an influent with 240 mg/L BOD and discharges an effluent with 18 mg/L BOD. What is the plant's BOD removal efficiency percentage?

Formula Used: Removal Efficiency (%) = [(Influent Concentration - Effluent Concentration) ÷ Influent Concentration] × 100

Step-by-Step Mathematical Derivation:

1. Calculate concentration removed: 240 mg/L - 18 mg/L = 222 mg/L.
2. Divide by influent concentration: (222 mg/L ÷ 240 mg/L) × 100 = 92.5%.

⚠️ Common Exam Trap: Always divide by the INFLUENT concentration in the denominator, never the effluent.