Just downstream of the outfall from a pollution point-source, the DO of a river is 6 mg/L and the mixture of the river and waste has a BOD of 20 mg/L. The saturation value of DO is 9 mg/L. The deoxygenation constant is 0.2/day, and reaeration constant is 0.375/day:

a) Find the critical time downstream at which the minimum DO occurs.

b) Find the minimum DO downstream.

(c) If the outfall is the only source of BOD, what percent removal of BOD would be needed to assure a minimum DO of 5 mg/L?