Clarifier Loading Rate Calculator - Surface Overflow, Solids Loading & Weir Rates
Check primary and secondary clarifier design standards with one-out-of-service scenario
Calculate surface overflow rate, solids loading rate, and weir overflow rate for primary and secondary clarifiers. Supports circular and rectangular clarifiers with built-in design standard ranges. Includes one-out-of-service scenario to model peak loading when a clarifier is taken offline for maintenance.
Check detention time in your clarifier to confirm adequate settling at peak flow
Detention Time Calculator →Screen sludge volume prompts after clarifier solids assumptions are checked
Sludge Production & Disposal Cost Calculator →Calculate aeration energy costs for the secondary system downstream of your clarifier
Aeration Energy Calculator →Verify weir overflow rate matches your clarifier effluent discharge conditions
Weir Flow Calculator →How It Works
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Select Clarifier Type and Shape
Choose primary or secondary clarifier, and circular or rectangular shape. Design standards differ significantly between primary and secondary.
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Enter Dimensions and Flow
Input diameter (circular) or length × width (rectangular), sidewater depth, number of clarifiers in service, and plant flow rate.
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Add Optional Parameters
Enter influent TSS for solids loading rate and weir length for weir overflow rate. RAS rate applies to secondary clarifiers only.
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Review Loading Rates
See all loading rates with pass/warn/fail status against design standards. Toggle the N-1 scenario to see what happens when one clarifier goes offline.
Built For
- Operators verifying clarifier performance against design criteria
- Engineers checking plant capacity for flow increases
- Inspectors evaluating whether a plant can handle growth
- Operators planning clarifier maintenance around flow conditions
- Permit writers evaluating expansion needs
Assumptions
- Surface overflow rate (SOR) is calculated as flow divided by clarifier surface area, assuming uniform flow distribution across the entire surface
- Solids loading rate uses influent TSS concentration and assumes uniform solids distribution across the clarifier floor
- Weir overflow rate assumes flow is evenly distributed along the entire weir length (no localized channeling)
- Design criteria ranges follow Ten States Standards and ASCE/WEF Manual of Practice No. 8 for primary and secondary clarifiers
- The N-1 (one-out-of-service) scenario distributes total plant flow equally among remaining clarifiers
- RAS (return activated sludge) flow for secondary clarifier solids loading is added to the influent flow per standard practice
Limitations
- Does not account for density currents caused by temperature differences between influent and basin water
- Wind effects on large-diameter clarifiers can disrupt settling patterns - outdoor clarifiers over 100 feet diameter are particularly affected
- Does not model sludge blanket depth, which reduces effective settling volume and increases SOR for the active zone
- Inlet energy dissipation and feed well design significantly affect clarifier performance but are not evaluated here
- Does not calculate sludge removal rates or desludging frequency needed to maintain clarifier performance
- Biological floc characteristics (SVI, settling velocity) vary with process conditions and are not directly modeled - only loading rates are checked
References
- Ten States Standards (Great Lakes-Upper Mississippi River Board) - Recommended Standards for Sewage Works (clarifier design criteria)
- WEF/ASCE Manual of Practice No. 8 - Design of Municipal Wastewater Treatment Plants (Chapter 13: Clarification)
- AWWA/ASCE - Water Treatment Plant Design, 5th Edition (primary sedimentation basin design)
- Tchobanoglous, Burton & Stensel - Wastewater Engineering: Treatment and Reuse, 5th Edition (settling theory and clarifier design)
- WEF Manual of Practice FD-3 - Clarifier Design (secondary clarifier performance and design guidelines)
- EPA 625/1-75-003a - Process Design Manual for Suspended Solids Removal