Wastewater Removal Efficiency Calculator

Enter influent and effluent concentrations and plant flow to get percent removal efficiency and the daily pounds of constituent loaded and removed.

Quick Facts

Formula
Efficiency = (Influent − Effluent) / Influent × 100
Mass load (lb/day) = concentration (mg/L) × flow (MGD) × 8.34.

Your Results

Calculated
Removal efficiency
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Percent of constituent removed
Influent load
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Pounds per day entering
Effluent load
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Pounds per day leaving
Mass removed
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Pounds per day removed

Ready

Enter influent, effluent, and flow, then calculate.

About wastewater removal efficiency

Removal efficiency is the single most common performance number in wastewater treatment. It answers a simple question: of the pollutant that came into the plant, what fraction did the process take out? Operators track it every day for constituents such as BOD (biochemical oxygen demand), TSS (total suspended solids), ammonia-nitrogen, and total phosphorus, because discharge permits are written in terms of both concentration limits and, often, minimum percent removal.

The formula

Removal efficiency compares influent and effluent concentrations of the same constituent:

Efficiency (%) = (Influent − Effluent) ÷ Influent × 100

Both concentrations must be in the same units — almost always milligrams per liter (mg/L), which for dilute aqueous samples is numerically equal to parts per million (ppm). The result is dimensionless because the units cancel. Effluent can never exceed influent for a real removal process, so a valid efficiency always falls between 0% and 100%.

Turning concentration into a daily mass load

Concentration alone does not tell you how much material a plant handles; that depends on flow. The "pounds formula" converts a concentration and a flow rate into a mass load per day:

Mass load (lb/day) = Concentration (mg/L) × Flow (MGD) × 8.34

Here MGD is million gallons per day and 8.34 is the weight of one U.S. gallon of water in pounds. The factor works because mg/L equals ppm: one million gallons carrying 1 mg/L of a substance holds 8.34 pounds of it. The mass actually removed each day is simply the influent load minus the effluent load, which also equals (Influent − Effluent) × Flow × 8.34.

Common reference points

  • Raw domestic (medium-strength) wastewater is often cited near 220 mg/L BOD and 220 mg/L TSS.
  • Well-run secondary treatment (activated sludge or trickling filter) typically achieves 85–95% removal of BOD and TSS.
  • The U.S. Secondary Treatment Standard (40 CFR Part 133) sets 30 mg/L for both BOD₅ and TSS as a 30-day average, and requires at least 85% removal.
  • Primary clarification alone removes roughly 25–40% of BOD and 50–65% of TSS.

Frequently Asked Questions

What counts as good removal efficiency?
For BOD and TSS, conventional secondary treatment is expected to remove at least 85%, and well-operated plants routinely reach 90–95%. Under 60% usually signals a process upset, sampling error, or that only primary treatment is in place. Nutrient removal (nitrogen, phosphorus) targets vary widely by permit and by whether the plant is designed for advanced treatment.
Do influent and effluent have to be in the same units?
Yes. The efficiency formula only cancels units correctly if both concentrations use the same basis, normally mg/L. If one sample is reported in µg/L or ppb, convert first (1 mg/L = 1,000 µg/L = 1,000 ppb) before dividing, or the percentage will be off by orders of magnitude.
Why is the mass load different from the concentration?
Concentration is how strong the water is; mass load is how much pollutant moves per day, and it scales with flow. A plant treating 5 MGD at 220 mg/L BOD carries five times the daily pounds of a 1 MGD plant at the same strength. Permits and design capacities are frequently written in pounds per day, so the 8.34 conversion is essential.

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