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.