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Wastewater Calculator

Compute wastewater using validated scientific equations. See step-by-step derivations, unit analysis, and reference values.

Reviewed by Daniel Agrici, Founder & Lead Developer

Reviewed by Daniel Agrici, Founder & Lead Developer

Formula

Loading (lbs/day) = Flow (MGD) x Concentration (mg/L) x 8.34

The 8.34 factor converts million gallons per day and mg/L to lbs/day (since 1 gallon of water weighs 8.34 lbs). Removal efficiency = (Influent - Effluent) / Influent x 100%. Population Equivalent = Total BOD load / 0.17 lbs/person/day. Sludge production is approximately 0.7-1.0 lbs dry solids per lb BOD removed.

Worked Examples

Example 1: Municipal Treatment Plant Performance

Problem:A 2.0 MGD plant has influent BOD of 180 mg/L and effluent BOD of 12 mg/L. Influent TSS is 200 mg/L, effluent TSS is 10 mg/L. Does it meet secondary standards?

Solution:BOD removal = (180-12)/180 x 100 = 93.3% (meets 85% minimum)\nBOD effluent: 12 mg/L (meets 30 mg/L limit)\nTSS removal = (200-10)/200 x 100 = 95.0% (meets 85% minimum)\nTSS effluent: 10 mg/L (meets 30 mg/L limit)\nBOD load removed: 2.0 x (180-12) x 8.34 = 2,802 lbs/day

Result:Meets all secondary standards. BOD: 93.3% removal, TSS: 95.0% removal.

Example 2: Industrial Discharge Loading

Problem:A food processing plant discharges 0.5 MGD at 800 mg/L BOD. What is the population equivalent and oxygen requirement?

Solution:BOD loading = 0.5 x 800 x 8.34 = 3,336 lbs BOD/day\nPopulation Equivalent = 3,336 / 0.17 = 19,624 PE\nOxygen requirement: 3,336 x 1.2 to 1.5 = 4,003 to 5,004 lbs O2/day\nThis small factory has the organic load of a city of ~20,000 people.

Result:PE: 19,624 | O2 needed: 4,003-5,004 lbs/day | BOD load: 3,336 lbs/day

Frequently Asked Questions

What is BOD and why is it important in wastewater treatment?

BOD (Biochemical Oxygen Demand) measures the amount of dissolved oxygen consumed by biological organisms to break down organic matter in water over 5 days at 20 degrees C (BOD5). It is the most important parameter for measuring organic pollution in wastewater. Typical domestic wastewater has a BOD of 150-300 mg/L. Secondary treatment must reduce BOD to below 30 mg/L (US EPA standards). High BOD in discharged water depletes oxygen in receiving waters, killing fish and other aquatic life. The BOD test mimics what happens in nature when organic waste enters a waterway, making it a practical measure of treatment effectiveness.

What is the 8.34 conversion factor used in wastewater calculations?

The factor 8.34 comes from the weight of one gallon of water: 8.34 pounds. It is used to convert between concentration (mg/L) and mass loading (lbs/day) in wastewater calculations. The formula is: lbs/day = Flow (MGD) x Concentration (mg/L) x 8.34. This works because 1 mg/L equals 1 part per million, and 1 million gallons weighs 8.34 million pounds, so multiplying MGD x mg/L x 8.34 gives pounds per day. This conversion factor is fundamental to virtually every wastewater calculation in the US system. In metric, the equivalent calculation is: kg/day = Flow (m3/day) x Concentration (mg/L) / 1000.

References

Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy