Question

A wastewater treatment operator has been fined because the DO at a point 6 km downstream is below the permit limit of 5 mg/L. Determine the highest BODu concentration possible in the effluent which can still achieve the permitted requirement of a DO greater than 5 mg/L year round.

Q_R = 13.8 m³/s

Q_W = 4.4 m³/s

T_{W Summer} = 30°C

T_{R Summer} = 22°C

T_{W Winter} = 13°C

T_{R Winter} = 7°C

k_BOD@20° = 0.4 d^-1 (assume K_BOD is the same for the river and effluent)

H =4 m (assume no change due to wastewater flow)

V = 0.03 m/s (assume no change due to wastewater flow)

DO_R = 7 mg/L

DO_W = 1 mg/L

BOD_R = 5 mg/L

n = 0.4