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 mg_BOD/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.