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A loading rate is a flow or a mass divided by the size of whatever receives it: surface overflow rate is gallons per day divided by surface area in square feet, and weir overflow rate is gallons per day divided by weir length in feet. Solids loading is pounds of solids per day, from the pounds formula, divided by the clarifier's surface area.
surface overflow rate (SOR) (gpd/ft²) = Flow (gpd) ÷ Area (ft²) · weir overflow rate (WOR) (gpd/ft) = Flow (gpd) ÷ Weir length (ft)
| Term | Meaning | Units |
|---|---|---|
| Area | 0.785 × diameter² for a circle | ft² |
| Weir length | 3.14 × diameter for a weir around the full rim | ft |
| Flow | flow over the clarifier | gpd |
| Solids loading | million gallons per day (MGD) × milligrams per liter (mg/L) × 8.34 ÷ area | lb/day/ft² |
The drinking water sheet lists surface loading and weir overflow rate; the wastewater sheets add hydraulic, solids and organic loading. Official sheets: Exam Conversion Sheet (revised 4/30/2026) (State Water Resources Control Board, Drinking Water Operator Certification Program; Water Treatment and Water Distribution exams); Equivalents and Formulas Sheet, Version 1 (State Water Resources Control Board, Wastewater Operator Certification Program; Wastewater Treatment Plant Operator exams); Equivalents and Formulas Sheet, Version 2 (State Water Resources Control Board, Wastewater Operator Certification Program; Wastewater Treatment Plant Operator exams).
Loading rates · Original practice item, not a real exam question.
A 75-ft diameter secondary clarifier receives 2.4 MGD of forward flow, with its effluent weir around the full rim. Mixed liquor enters at 3.2 MGD (forward flow plus return activated sludge (RAS)) carrying 2,600 mg/L mixed liquor suspended solids (MLSS). Find the surface overflow rate, the weir overflow rate and the solids loading.
0.785 × 75 ft × 75 ft = 4,415.6 ft²2,400,000 gal/day ÷ 4,415.6 ft² = 543.5 gpd/ft²3.14 × 75 ft = 235.5 ft; 2,400,000 gal/day ÷ 235.5 ft = 10,191 gpd/ft3.2 MGD × 2,600 mg/L × 8.34 = 69,389 lb/day; 69,389 ÷ 4,415.6 ft² = 15.7 lb/day/ft²Answer: SOR 543.5 gpd/ft², WOR 10,191 gpd/ft, solids loading 15.7 lb/day/ft²
No. Return activated sludge leaves through the bottom of the clarifier, not over the weirs, so overflow rates use forward flow. Solids loading includes it.
Pounds of biochemical oxygen demand (BOD) per day, from the pounds formula, divided by the area or volume of the process receiving it, such as pounds per 1,000 ft³ of trickling filter media per day.
The free study app has a step-by-step lesson on this calculation, with fraction-tile unit cancellation and graded practice problems.
Open the loading rates lessonFormulas follow the official sheets linked above, which are the reference at the exam. The worked example and the mistakes are original practice material written for this page; they are not real exam questions, which are confidential. Written by the site's author.
Questions, flashcards, and explanations are AI-written under a sourcing policy that builds on the exam content outlines and candidate materials each certifying body publishes without restriction and the regulations and technical references they cite, and that excludes actual exam questions, recollections of any exam sat by the author, other study products, and any handbook or document released under a confidentiality agreement. Any item found outside that policy is rewritten from primary sources or removed. Content is then put through repeated review — regulatory values checked against primary sources, every calculation recomputed, and errors corrected — plus spot checks by the author, who holds California water operator certifications and a California civil engineering license. Those spot checks are sampling, not a professional review: nothing here is stamped, sealed, or offered as engineering advice, and not every item has been individually verified, so mistakes can still slip through.
Not affiliated with or endorsed by the State Water Resources Control Board (SWRCB), its Division of Drinking Water (DDW) or Wastewater Operator Certification Program (WWOCP), the California-Nevada American Water Works Association (CA-NV AWWA), the California Water Environment Association (CWEA), the National Council of Examiners for Engineering and Surveying (NCEES), the California Board for Professional Engineers, Land Surveyors, and Geologists (BPELSG), the Contractors State License Board (CSLB), or the American Public Works Association (APWA). All questions, flashcards, and references are independent study aids; verify against the official exam materials.