Given the following information, determine the required DO concentration in the treatment plant effluent discharge in order to ensure that at no point downstream the DO in the river drops below 4 mg/L:
QR = 16 m3/s
QW = 4 m3/s
T = 12°C
kd = 0.2 d-1
kr = 0.6 d-1
DOR = 8 mg/L
BODW = 50 mg/L
BODR = 5 mg/L