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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.

Pro Tip: The "one out of service" scenario is what keeps experienced operators up at night. Two 60-foot clarifiers at 1 MGD run at 354 gpd/ft² - comfortable for secondary treatment. Take one offline for sludge removal and the remaining clarifier jumps to 708 gpd/ft² - right at the edge of design limits. If that happens during a rain event, you are over the limit.

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Clarifier Loading Rate Calculator
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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

Frequently Asked Questions

Surface overflow rate (SOR) is the flow divided by the clarifier surface area, expressed in gallons per day per square foot (gpd/ft²). For a circular clarifier: SOR = Flow (gpd) / (π × r²). For secondary clarifiers, typical design SOR is 400-800 gpd/ft² at average flow. Exceeding design SOR means solids don't have time to settle and get carried over in the effluent.
Primary clarifiers can handle higher loading rates (800-1200 gpd/ft²) because they settle heavy, discrete particles. Secondary clarifiers (final clarifiers) must handle lighter biological floc, so their design rates are lower (400-800 gpd/ft²). Solids loading rate is also critical for secondary clarifiers because they must handle both incoming mixed liquor and return activated sludge.
High weir overflow rates create velocity currents near the effluent weir that can carry settled solids over. Design limits are typically 10,000-20,000 gpd per linear foot. Long weirs (like V-notch or double-sided launders) distribute flow more evenly and reduce localized velocity.
Disclaimer: This calculator uses standard design criteria from Ten States Standards and typical engineering practice. Actual performance depends on sludge characteristics, temperature, influent quality, and clarifier condition. Always verify loading rates against your permit and state design standards.

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