Topic: Worked Problems — BOD Calculations and Sewage Treatment

Problem 1: BOD Reduction Percentage A municipal sewage plant receives influent with BOD = 280 mg/L. After secondary treatment, the effluent BOD = 14 mg/L. (a) Calculate the percentage BOD reduction. (b) Which treatment stage was most responsible for this reduction? (c) Is this effluent safe for river discharge (standard: <30 mg/L BOD)?

Solution: (a) BOD reduction = [(280 - 14) / 280] × 100 = $\frac{266}{280}$ × 100 = 95% (b) Secondary (biological) treatment — aerobic microbial digestion in aeration tanks is responsible for 85-95% BOD reduction. (c) Yes, 14 mg/L < 30 mg/L — the effluent meets standard discharge criteria.