9.5.6 Contact for Bubble Aerators

Bubble aerators were discussed in a previous Chapter 6 (Mixing and Flocculation). In that chapter, the rise velocities of bubbles vb as derived by Peebles and Garber were presented. The formulas are:

PO a

Re is a Reynolds number; g is the acceleration due to gravity; ^ is the absolute viscosity of fluid; pl is the mass density of fluid; a is the surface tension of fluid; pg is the mass density of the gas phase (air); and r is the average radius of the bubbles. To give Gj a name, we called it the Peebles number. If the depth of submergence of the bubble diffuser and the rise velocity computed from one of the above equations are known, the time of contact between the gas phase in the bubbles and the surrounding water can be determined. This is illustrated in the next example.

Example 9.11 An aerator in an activated sludge reactor is located 3.5 m below the surface. The temperature inside the reactor is 30°C. What are the rise velocity of the bubbles, contact time for aeration, Re, Gj and G2? The approximate average diameter of the bubbles at mid-depth is 0.25 cm.


9V V


Re - 2P'Vt>r - 2(996)(0.43)(0.0025/2) - j338>>2 V 8 (10-4)

Thus, formula is not applicable.

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