Answer to problem 3

3. In a oxidation ditch the level of oxygen is controlled by changing the level of the rotating brush aerators in the mixed liquor. Why do you suppose that the rotation velocity is not just increased instead?
  1. It would require more power
  2. The increased torque would prematurely damage the rotors.
  3. It doesn't really matter, either method would be effective.
  4. The detention times in the anaerobic zones would be insufficient.

    The correct answer is D. If the mixed liquor travelled through the anaerobic zones with too high a velocity, there simply would not be enough time for the microbes to do the job. Also, a high velocity could create surface aeration, inhibiting denitrification. The changes in the operation to the rotors for a small increase in velocity are relatively insignificant, so A and B are not relevant. C is precluded by the explanation for D.

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