1. Provide at least 4 methods you could use to determine the sludge pumping rate out of a secondary clarifier and the sludge return rate based on the mixed liquors suspended solids (MLSS) characteristics.
2. Two secondary clarifiers are operating with an overflow rate of 0.8 m/h and the activated-sludge tank MLSS concentration is 3800 mg/L. Based on sludge thickening tests, the interfacial settling velocity can be described by the following relationship:

V_i = V_o×e^-kX

where V_i =interfacial settling velocity, m/d

X = MLSS concentration, g/L

V_o = 180 m/d

k = 0.401 L/g

Determine the MLSS concentration possible with only one clarifier in operation using the above underflow concentration. Show operational conditions if two clarifiers are used. Indicate the new overflow rate operating flux line as a dashed line and show the corresponding operating state point.