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Home Courses Treatment of Urban Sewage Course materials Lectures 4. Elaboration Question 3

4. Elaboration Question 3

Course subject(s) 4. Activated sludge process

Question 3: Activated sludge process in dry and wet weather conditions

A small wastewater treatment plant is designed for an average flow of 30,000 m3/day, with an influent concentration of 200 mgBOD/l.


  1. a. Calculate the volume of the aeration tank for a treatment plant with nitrification. The activated sludge concentration in the aeration tank is 4 g/l.

  2. b. Calculate the diameter of the settling tank.
    • Use a usual peak factor for Dutch treatment plants (2.5)
    • The sludge volume loading rate (\(SVLR = v_0 \cdot X_{AT} \cdot \frac{SVI}{1000}\)) is 0.4 m3/m2/h

    In which v0 is the surfae loading rate

    • SVI = 150 ml/g
    • Sludge recycle flow ratio (fr) = 0.6 (-)

  3. c. Heavy rainfall during 1 hour increases the maximum flow to the maximum hydraulic capacity of the influent works, 6,250 m3/hr. Calculate the effect on the sludge retention if the recycle flow remains unchanged.

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