An effluent discharge with a flow of 1.5 m3/s and an ammonium-N concentration of 2 mg/L is released to a river with a flow of 12 m3/s and an ammonium-N concentration of 0.015 mg/L. What is the concentration of this mixture after they have become perfectly mixed downstream?

the final concentration is C=0.235 mg/L

Explanation:

doing a mass balance

ammonium mass outflow = ammonium inflow from river + effluent discharge of ammonium = 12000 L/s * 0.015 mg/L + 1500 L / s * 2 mg/L = 3180 mg/s

outflow rate = 12000 L/s + 1500 L / s = 13500 L/s

then the final concentration is

C = ammonium mass outflow/outflow rate = 3180 mg/s/13500 L/s = 0.235 mg/L