The minimum surface area required for clarification in the secondary clarifier Ac can be calculated as follows:
where vz = the zone settling velocity, which can be determined from the following procedure. The mixed liquor with a MLVSS concentration of X is placed in a settling column. Activated-sludge begins to settle under quiescent conditions, and an interface forms between the surface of the blanket of settling sludge and the clarified liquid above. Plotting the height of this interface as a function of settling time generates a settling curve corresponding to the MLVSS concentration X. The slope of a tangent drawn to the initial portion of the settling curve yields vz.
FIG. 7.29.1 Schematic representation of a continuous-flow, secondary clarifier under steady-state conditions.
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