2. Elaboration Question 5

Course subject(s) 2. Primary treatment

QUESTION 5 Evaluating loading rates and retention time for primary sedimentation

A wastewater treatment plant has an average flow of 40,000 m3/day and a peak flow of 5,000 m3/hr. Following bar screens and grit removal the wastewater is treated in two rectangular primary sedimentation tanks (W = 13 m and L = 50 m). For primary sedimentation the following constants can be used:

  • Hydraulic surface loading rate: v0 = 2 – 4 m3/(m2.h).
  • Minimum retention time: tmin ≈ 1 h.
  • Critical scouring velocity:
    • vs = 0.30 m/s (sand),
    • vs = 0.03 m/s (primary sludge),
    • vs = 0.02 m/s (activated sludge).
  • Are the primary sedimentation tanks overloaded or not?
  • Calculate the average depth of the primary sedimentation tanks.
  • Check hydraulic retention time at average flow rate.

Elaboration Question 5

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