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Percent removal equals the influent concentration minus the effluent concentration, divided by the influent concentration, times 100. For a process train, either use the first influent and last effluent directly or multiply the fractions that survive each stage; never add the stage percentages.
Removal (%) = (In − Out) ÷ In × 100
| Term | Meaning | Units |
|---|---|---|
| In | concentration entering the process | milligrams per liter (mg/L) (or NTU, or any consistent unit) |
| Out | concentration leaving the process | same unit as In |
All three sheets list removal percentage; the drinking water sheet also relates percent removal to log removal. 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).
Percent removal · Original practice item, not a real exam question.
Raw wastewater biochemical oxygen demand (BOD) is 265 mg/L, primary effluent is 178 mg/L and final effluent is 12 mg/L. Find the removal across the primary clarifiers, across secondary treatment, and overall.
(265 − 178) ÷ 265 × 100 = 32.8 %(178 − 12) ÷ 178 × 100 = 93.3 %(265 − 12) ÷ 265 × 100 = 95.5 %Answer: 32.8 percent primary, 93.3 percent secondary, 95.5 percent overall
Multiply what each stage lets through. Here 0.672 of the BOD survives the primaries and 0.067 of that survives secondary, so 0.672 × 0.067 = 0.045 remains, which is 95.5 percent removal.
Multiply the influent by the fraction allowed to remain. To show 85 percent removal on an influent of 265 mg/L, effluent must be no more than 265 × 0.15 = 39.8 mg/L.
Each log is a factor of ten: 90 percent removal is 1 log, 99 percent is 2 log, 99.9 percent is 3 log.
The free study app has a step-by-step lesson on this calculation, with fraction-tile unit cancellation and graded practice problems.
Open the percent removal 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.