Trickling Filter NRC Efficiency Calculator

The National Research Council formula predicts trickling filter BOD removal from organic loading and recirculation. Enter your loading to see the expected single-stage efficiency.

BOD₅ load to filter (lb/day)Settled influent BOD applied to the filter.

Media volume (1,000 ft³)Enter 40 for 40,000 ft³ of media.

Recirculation ratio RRecirculated flow / influent flow. 0 = no recirculation. Default 0.

Influent BOD₅ (mg/L, optional)Adds an estimated effluent BOD. Leave blank to skip.

NRC Predicted Efficiency

Item	Value

How the calculation works

NRC formulas (single stage, 20°C, US units)
Recirculation factor F = (1 + R) / (1 + 0.1 x R)²
Efficiency E = 1 / (1 + 0.4432 x sqrt(W / (V x F)))
Organic loading = W / V (lb BOD₅/day per 1,000 ft³)
where W = BOD₅ load (lb/day), V = media volume (1,000 ft³), R = recirculation ratio
Estimated effluent BOD = influent BOD x (1 – E)

Trickling filter classification	Organic loading (lb/day/1,000 ft³)
Low rate	5–25
Intermediate rate	25–100
High rate	100–300

Notes & limitations

The NRC equations are empirical, developed from operating records of stone-media trickling filters at U.S. military installations (Metcalf & Eddy). The 0.4432 constant applies to US customary units (lb/day, 1,000 ft³); the metric form uses 0.532 with kg/day and m³.

The formula predicts performance at 20°C. Cold weather lowers biological activity and real efficiency; allow a margin in winter.
For plastic-media high-rate filters, the Schulze or Germain equations often fit better than NRC — use NRC as a screening estimate for those.
The recirculation factor F counts effective passes through the filter; it improves wetting and efficiency but does not change the total BOD load.
Two-stage systems need the second-stage NRC equation, which discounts the first stage’s efficiency — this tool covers single-stage only.

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