The 'surface overflow rate' (SOR) in a sedimentation tank is defined as:
Choose the correct answer
Volume of tank divided by depth
Flow rate per unit plan area (m/day) — governs settling efficiency; particles settle if vs ≥ SOR
Number of settling tanks required
Sludge removal rate
Correct Answer
B. Flow rate per unit plan area (m/day) — governs settling efficiency; particles settle if vs ≥ SOR
Why is this the answer?
SOR = Q/A (m³/m²/day or m/day) where Q = flow rate, A = plan area of tank. A particle settles completely if its terminal settling velocity vs ≥ SOR. If vs < SOR, partial removal. Stoke''s law: vs = (ρs−ρ)gd²/(18μ). Typical SOR for primary clarifier: 30–50 m³/m²/day. SOR governs plan area design (not depth).
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