Question

1. Decay of a pollutant is a 1^st order reaction with a half-life of 55 days. How many days are needed to remove 90% of its initial concentration?

2. A factory is discharging wastewater containing 200 mg/L BOD into a river. The river has a discharge of 10 m³/s and the wastewater discharge is 2 m³/s. If the upstream of this river has a BOD of 10 mg/L. Please compute BOD after wastewater is completely mixed with river (any BOD degradation is not considered).

Answer 1

drs. Decay of pollutant is 1st order reaction. half life = 55 days For 1st order reaction, tha = half life = 0.693 Soi 1 k = ka = 0.0126 days' 0.693 tlik 0.693 55 days . Now to 10 days to remove 90% of initial concentration=? t=?", k= 0.0126 days', 0o = initial concentration at= concentration at = alo-90 ao - time to = 0.190

Now for first order reaction t = 2.303 loga t = 2.303 00126 dayst log og 0.01 t = 182078 x logi t = 182.78 days (Days can't be infraction) To remove 90% pollutant 183 days are required.