Clarifier Design & Loading Calculations
Clarifier design and calculation MUHAMMAD BILAL
Objective: To provide an in-depth and advanced engineering reference for designing and
evaluating clarifiers used in water and wastewater treatment plants. This document includes
basic formulas, process-based design values, and advanced modeling techniques.
1. Surface Area Calculations
a. Rectangular Clarifiers
• Formula:
A= L × W (in ft2)
• Used for primary, secondary, and tertiary clarifiers in space-constrained systems.
b. Circular Clarifiers
• Formula:
A= πr2 = 0.785×D2
• Preferred for large-scale municipal or industrial applications.
Design Insight: Always verify space availability, influent hydraulic profile, and sludge collection
mechanism to choose geometry.
2. Volume Calculations
a. Rectangular Tank
• Formula:
V=L×W×H (ft3)
b. Cylindrical Tank
• Formula:
V=πr2h
Clarifier design and calculation MUHAMMAD BILAL
c. Conversion
• 1 ft3=7.48 gallons
d. Cone Bottom Tank (Advanced)
• Vtotal=πr2h1+1/3πr2h2
h1 = height of cylinder, h2 = height of cone
3. Detention Time (DT)
Formula:
DT (hours)= Tank Volume (MG)×24 / Flow (MGD)
• Typical Design Range: 2 to 3 hours
Advanced Note: CFD (Computational Fluid Dynamics) modeling can be used to evaluate flow
patterns, short-circuiting, and dead zones that are not captured by theoretical DT.
4. Surface Overflow Rate (SOR) / Surface Loading Rate (SLR)
• Formula:
SOR (gpd/ft2)= Flow (gpd/ Surface Area (ft2)
• Design Range: 400 to 800 gpd/ft², max 1200 for tertiary polishing
Advanced Technique: Use dynamic process simulators (e.g., GPS-X, BioWin) to evaluate loading
under varying influent conditions and peak flows.
5. Weir Overflow Rate (WOR)
• Formula:
WOR (gpd/ft)= Flow (gpd)/ Weir Length (ft)
• Typical Design Limit: ≤ 10,000 gpd/ft
Clarifier design and calculation MUHAMMAD BILAL
a. Rectangular:
• Weir length = 2 x width (1 trough) or 4 x width (2 troughs)
b. Circular:
• Lweir=π× Dweir
Design Optimization: Implement V-notch or multi-point weirs to maintain uniform overflow
velocity and prevent localized carryover.
6. Solids Loading Rate (SLR)
• Formula:
SLR (lbs/day/ft2) = lbs /day of Solids/ Surface Area (ft2)
• Pounds Calculation:
• lbs/day=FlowMGD×Concentrationmg/L×8.34
• Design Range: 25 to 30 lbs/day/ft²
Advanced Tool: Integrate online MLSS (mixed liquor suspended solids) sensors with PLC/SCADA
to continuously monitor and control SLR in secondary clarifiers.
7. Advanced Clarifier Design Practices
a. CFD Modeling
• Simulate inlet energy dissipation, density currents, and sludge blanket stability.
• Tools: Ansys Fluent, FLOW-3D, OpenFOAM
b. Process Simulation
• Model clarifier behavior over diurnal and storm events.
• Software: BioWin, GPS-X, WEST
c. Real-Time Monitoring & Control
• Combine ultrasonic sludge blanket monitors with turbidity and DO sensors.
• Connect to automation systems (PLC/SCADA) to maintain operational setpoints.
Clarifier design and calculation MUHAMMAD BILAL
d. Baffle & Trough Design
• Use energy-dissipating inlet baffles.
• Optimize effluent weir and scum baffle design to reduce short-circuiting and floating
sludge.
8. Summary Table
Parameter Formula Typical Range
DT (hrs) VMG×24 / QMGD 2–3 hrs
SOR Qgpd / Aft2 400–800 gpd/ft²
WOR Qgpd /Lwier ≤ 10,000 gpd/ft
SLR lbs/day / Aft2 25–30 lbs/day/ft²
Clarifier design and calculation MUHAMMAD BILAL
