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Sludge Age of an Activated Sludge Tank

Hardactivated sludgesrt
An activated sludge aeration tank has a volume of 2,000 m³ and an MLSS of 3,000 mg/L. Sludge is wasted at 40 m³/day with a concentration of 10,000 mg/L. Neglecting solids lost in the effluent, the solids retention time is most nearly:
  1. A5 days
  2. B10 days
  3. C15 days
  4. D20 days
Show solution

Answer: C. 15 days

Solids retention time is the mass of solids held in the tank divided by the mass wasted each day: SRT = VX/(Qw Xw) = (2,000)(3,000)/[(40)(10,000)] = 6,000,000/400,000 = 15 days. Using the influent flow rather than the waste flow in the denominator is the usual error and gives a far shorter, meaningless value.
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