Clarifier Loading Rate Calculator - Surface Overflow, Solids Loading & Weir Rates

How It Works

1. Select Clarifier Type and Shape

   Choose primary or secondary clarifier, and circular or rectangular shape. Design standards differ significantly between primary and secondary.

2. Enter Dimensions and Flow

   Input diameter (circular) or length × width (rectangular), sidewater depth, number of clarifiers in service, and plant flow rate.

3. Add Optional Parameters

   Enter influent TSS for solids loading rate and weir length for weir overflow rate. RAS rate applies to secondary clarifiers only.

4. 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

Frequently Asked Questions