Wastewater Calculator — BOD Removal & Organic Loading
Enter influent BOD, effluent BOD, flow rate, and reactor volume to calculate BOD removal efficiency, organic loading rate (kg BOD/day), hydraulic retention time, and volumetric BOD loading for a wastewater treatment system.
mg/L
mg/L
m³/day
m³
Fraction of incoming BOD eliminated by treatment
- 1
BOD removed
BOD_in − BOD_out = 250 − 20 = 230 - 2
Removal fraction
230 ÷ 250 = 0.92 - 3
BOD removal efficiency
0.92 × 100 = 92
How does this calculator work?
BOD removal (%) = (BOD_in − BOD_out) / BOD_in × 100. Organic load (kg/day) = BOD × flow / 1 000. Hydraulic retention time (hours) = tank volume / flow × 24. A well-designed secondary treatment plant removes 85–95% of incoming BOD. Volumetric loading should stay below ~2 kg BOD/m³/day to avoid overloading.
Formula
How this is calculated
Wastewater treatment performance is measured by how effectively the plant reduces Biochemical Oxygen Demand (BOD) — the amount of oxygen microorganisms consume when breaking down organic matter. BOD removal efficiency is the fraction of incoming BOD that is eliminated, expressed as a percentage. A well-designed secondary treatment system (activated sludge, trickling filter) typically achieves 85–95% BOD removal.
Organic loading converts concentrations to mass flows: multiplying influent BOD in mg/L by the daily flow in m³/day and dividing by 1 000 gives kilograms of BOD entering the plant per day. Volumetric loading then divides this mass by the reactor volume to give kg BOD per m³ per day — a key design parameter for biological treatment sizing. Typical activated-sludge systems operate at 0.5–2.0 kg BOD/m³/day.
Hydraulic Retention Time (HRT) is the average time wastewater spends inside the reactor, calculated as volume divided by flow rate. Longer HRT gives microorganisms more contact time to remove pollutants but requires larger — and more expensive — tanks. Typical secondary treatment HRTs range from 4 to 24 hours. All values in this calculator use average daily conditions; peak-flow design requires applying a peak factor (often 2–4× average flow) to the results.
Frequently asked questions
Secondary biological treatment (activated sludge, trickling filter, constructed wetland) typically achieves 85–95% BOD removal. Regulatory effluent limits vary by jurisdiction; the EU Urban Wastewater Directive requires ≥ 70–90% removal depending on plant size and receiving water sensitivity. Tertiary treatment can push removal above 98%.
Volumetric loading (kg BOD/m³/day) measures how hard the biological reactor is being worked per unit volume. Values above ~2 kg/m³/day usually indicate the reactor is overloaded and effluent quality will deteriorate. Conversely, values below ~0.3 kg/m³/day may indicate the reactor is oversized. This parameter guides capacity expansion and operational decisions.
HRT is the average time water spends inside the treatment reactor. Longer HRT (more tank volume or lower flow) gives bacteria more time to break down organic matter, improving BOD removal but increasing capital and land costs. Most activated-sludge systems target an HRT of 4–24 hours; higher-rate processes can operate as low as 2 hours.
Also known as
TG we-Calculate Editorial Team. (2026). Wastewater Calculator — BOD Removal & Organic Loading [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/wastewater-calculator
TG we-Calculate Editorial Team. "Wastewater Calculator — BOD Removal & Organic Loading." TG we-Calculate. 2026. https://we-calculate.com/calculator/wastewater-calculator.
TG we-Calculate Editorial Team, "Wastewater Calculator — BOD Removal & Organic Loading," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/wastewater-calculator
@misc{wecalculate_wastewater_calculator, title = {Wastewater Calculator — BOD Removal & Organic Loading}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/wastewater-calculator}}, year = {2026}, note = {TG we-Calculate} }
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