2-16. 158 60 A water tower containing 4,000 m³ of water has been taken out of service for installation of a chlorine monitor. The concentration of chlorine in the water tower

was 2.0 mg/L when the tower was taken out of service. If the chlorine decays by first-order kinetics with a rate constant k = 1.0 d-¹ (Grayman and Clark, 1993), what is the chlorine concentration when the tank is put back in service 8 hours later? What mass of chlorine (in kg) must be added to the tank to raise the chlorine level back to 2.0 mg/L? Although it is not completely mixed, you may assume the tank is a completely mixed batch reactor. Page 77