Surface overflow rate (SOR) for sedimentation with coagulants: Normal design values for overflow rate in sedimentation tanks using coagulants are typically within which range, expressed in L/hour/m²?

Introduction / Context:
Surface overflow rate (SOR), also called surface loading rate, is a fundamental parameter for sizing sedimentation (clarification) basins. When coagulation–flocculation precedes sedimentation, flocs settle faster than discrete particles, allowing higher SORs compared to plain sedimentation. Designers must choose an SOR that balances removal efficiency, footprint, and cost.

Given Data / Assumptions:

• Conventional treatment train with coagulation–flocculation.
• Clarifier is designed for steady average flow, with allowance for peak factors.
• SOR is expressed as L/hour/m² for clarity and quick checks.

Concept / Approach:
In the classical Hazen framework, particle removal depends on the ratio of settling velocity to overflow rate. With coagulant-aided flocculation, typical practice adopts SORs in the order of 1000–1250 L/hour/m² (equivalent to roughly 24,000–30,000 L/m²/day). Values much lower waste area; much higher values risk floc carryover and turbidity breakthrough.

Step-by-Step Solution:
Convert common daily units: 24,000–30,000 L/m²/day ≈ 1000–1250 L/m²/hour.Compare with offered ranges and select the matching interval.Choose 1000 to 1250 L/hour/m² as the normal design range for coagulated waters.

Verification / Alternative check:
Plant experience and standard design manuals list conventional clarifier SORs in this band for coagulant-aided settling; plate settlers and lamella units may use higher surface loadings due to increased projected area.

Why Other Options Are Wrong:

• 250–750 L/hour/m²: overly conservative for coagulated waters; excessive tank area.
• 750–1000 L/hour/m²: feasible but on the lower edge; many designs target higher values.
• 1250–2000 L/hour/m²: appropriate for high-rate lamella units, not conventional basins.

Common Pitfalls:

• Ignoring peak flow conditions; SOR should be checked at peak and average.
• Overlooking inlet/outlet hydraulics which can cause short-circuiting even at acceptable SOR.

Final Answer:
1000 to 1250