The lowest-energy biological process there is: wastewater trickles over a fixed film and
the bugs do the rest. Size the media from the organic load, check the hydraulic load that keeps the film
wet and scoured, and read the NRC removal efficiency — including what it costs you to reach 20 mg/L.
1Duty & configuration
Pick the configuration first — it sets the acceptable organic load, the media depth and
the hydraulic window that the results are checked against.
2Media sizing
A trickling filter is sized on organic load (kg BOD per m³ of media per day), not
on detention time. Depth is then chosen from the media type, and the area falls out of V/D.
Results — media & hydraulics
Media volume V
—m³
Plan area A
—m²
Hydraulic load HLR
—m³/m²·d
Item
Value
Basis
3Removal efficiency — NRC
The NRC (National Research Council) empirical equation, converted to metric. Recirculation
enters through the factor F: it dilutes the applied load and passes it through the film twice.
Item
Value
Basis
4Distributor & ventilation
The distributor has to keep the film wet and thin. Too slow and the media dries and
ponds; too fast and you lose the contact time — the dose per pass is the lever.
Item
Value
Basis
Sludge, air & downstream
Item
Value
Basis
5Instrument schedule
Part of the appeal of a biofilter is how little it needs. This is the honest minimum —
and the two flowmeters are the ones people forget.
Point
Instrument
Why here
Quote the biofilter instruments at factory price
Influent and recirculation flow, effluent DO, pH and solids — all mappable to our catalogue and
supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).
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Sources & parameter basis
[1]
METCALF & EDDY / EPA — trickling filter loading: low-rate rock OLR 0.08–0.32 kg BOD/m³·d, HLR 1–4 m³/m²·d, depth 1.8–2.4 m; high-rate rock OLR 0.3–0.6, HLR 4–10; high-rate plastic OLR 0.5–1.5, HLR 10–40, depth 3–8 m; tower plastic OLR 1–3, HLR 40–150, depth 6–12 m.
[2]
NRC equation, metric form: E = 1/(1 + 0.4433·√(OLR/F)) — the published coefficient 0.0561 is tied to lb BOD/d per 1000 ft³ of media; 0.0561·√62.43 = 0.4433 converts it to kg/m³·d. Recirculation factor F = (1+R)/(1+0.1R)². Second stage: E₂ = 1/(1 + (0.4433/(1−E₁))·√(OLR₂/F)).
[3]
Rotary distributor: revolutions per day = HLR(mm/d) / (arms × dose per pass); typical dose 50–150 mm/pass and 0.05–0.5 rev/min.
[4]
Humus (secondary) tank after a biofilter: SOR 20–33 m³/m²·d — more conservative than an activated-sludge clarifier because biofilm sloughings are light and fluffy.
[5]
Sludge yield 0.4–0.8 kg TSS/kg BOD removed (biofilm processes produce less than activated sludge); oxygen ≈1.2–1.5 kg O₂/kg BOD removed; air carries ≈0.28 kg O₂/m³.
Full derivation and worked example: Trickling-Filter-Calculation-Sheet.md. The NRC
equation was developed on rock media — on plastic it is generally conservative, and real plants often
beat the prediction. Treat the result as a floor, not a forecast.
