It is found that a combined chlorine residual of 1.2 mg/L results in a reduction of an indicator microorganism by 99.0% for a contact time of 30 min in stabilization pond effluent.

a. Assuming the Chick-Watson model applies with n = 1, calculate the change in (i) combined chlorine residual or (ii) contact time required to increase removals to 99.9%. Assume that the contact chamber is PF.

b. Perform the same exercise as in (a) but use a value of n = 0.8.