Trickling filter design fixes the media volume, depth and hydraulic/organic loading so an attached-growth biofilm oxidises the applied BOD to the target effluent. The core levers are the media specific surface area, the hydraulic and organic loading rates, the recirculation ratio, and a first-order removal model such as the NRC or modified Velz equation.

What is a trickling filter and how does it work?

A trickling filter is an attached-growth (fixed-film) reactor. Settled wastewater is distributed over a bed of high-void media, and a biofilm of heterotrophic bacteria, fungi and protozoa grows on the media surface. Organics and oxygen diffuse from the trickling liquid film into the biofilm, where they are oxidised; metabolic products and CO2 diffuse back out. Unlike activated sludge, the biomass is immobilised rather than suspended, so the process is robust to hydraulic shocks and needs no return-sludge pumping.

Because the reaction happens inside a film fed by diffusion, trickling filters are governed by substrate mass transfer, not just bulk kinetics. That single fact shapes the media choice, the depth and the recirculation strategy that follow.

What controls the biofilm reaction rate?

Within the biofilm, substrate utilisation follows Monod kinetics, but the reactant must first diffuse inward. Combining Fickian diffusion with reaction gives a reaction–diffusion balance whose solution is captured by an effectiveness factor η — the ratio of the actual reaction rate to the rate if the whole film sat at the bulk concentration.

Df (d2S/dx2) = k S / (Ks + S)
Df = effective diffusivity in the biofilm (m2/s), S = substrate concentration (g/m3), x = depth into the film (m), k = maximum areal rate, Ks = half-saturation constant. A thick or dense film becomes diffusion-limited (η < 1): only the outer 50–150 µm is fully active, so adding biomass beyond that returns nothing.

The practical consequence: what matters is not filter mass but wetted specific surface area and how evenly liquid is distributed over it. This is why modern designs favour plastic media with a large, well-drained area, and why the same biofilm-diffusion physics underpins moving-bed and MBBR reactors, which push attached growth to much higher surface-area densities.

How are trickling filters classified by loading?

Filters are grouped by hydraulic loading rate (HLR, m3/m2·d, the flow per plan area) and organic (volumetric BOD) loading rate (OLR, kg BOD/m3·d). Together with recirculation these set the removal efficiency and the biofilm regime.

HLR = (Q + QR) / A   OLR = Q·S0 / V
Q = raw flow, QR = recirculated flow (m3/d), A = plan area (m2), V = media volume (m3), S0 = applied BOD (g/m3). Recirculation ratio R = QR/Q, typically 0.5–4.
TypeHydraulic load (m3/m2·d)Organic load (kg BOD/m3·d)MediaBOD removal
Low-rate (standard)1–40.08–0.4Rock80–90%
Intermediate4–100.24–0.5Rock50–70%
High-rate10–400.4–2.4Rock / plastic50–80%
Super-rate (plastic)15–900.6–3.2Plastic structured60–85%
Roughing40–2001.5–6Plastic40–65%

Low-rate rock filters give the best single-pass BOD removal and can nitrify; roughing filters are pre-treatment ahead of another stage. Recirculation keeps the media wet, dilutes strong influent and improves distribution, but beyond R ≈ 2–4 the marginal gain fades.

Which model sizes the media volume?

Several first-order models relate effluent BOD to depth and loading. The modified Velz / Germain form is the most transparent for plastic media:

Se / S0 = exp( −k · D / qn )
Se, S0 = effluent and applied BOD (g/m3), D = media depth (m), q = hydraulic application rate (m3/m2·min or L/m2·s), k = treatability rate constant (units to match), n = hydraulic exponent ≈ 0.5 for structured plastic media. k scales with media specific surface area and rises with temperature via kT = k20·1.035(T−20).

The alternative NRC equation (US National Research Council, developed for rock filters) predicts single-stage efficiency directly:

E = 100 / ( 1 + 0.4432 √(BOD load / (V·F)) )
E = % BOD removal, BOD load in kg/d, V = media volume (m3), F = recirculation factor = (1+R)/(1+0.1R)2. Both models are calibrated fits — always temper them with pilot data or a supplier guarantee.

Worked example: sizing a high-rate plastic filter

Design a single-stage plastic-media trickling filter to take settled municipal effluent from S0 = 200 g/m3 BOD to Se = 30 g/m3 at a flow of Q = 4,000 m3/d. Use the modified Velz form with k = 0.21 (m3/m2·min)0.5/m at 20 °C, n = 0.5, and a chosen media depth D = 6 m.

  • Required fraction remaining: Se/S0 = 30/200 = 0.15, so −ln(0.15) = 1.897 = k·D/q0.5.
  • Solve for q: q0.5 = k·D/1.897 = (0.21 × 6)/1.897 = 0.664, so q = 0.6642 = 0.441 m3/m2·min (≈ 635 m3/m2·d).
  • Plan area: convert Q = 4,000 m3/d = 2.78 m3/min. A = Q/q = 2.78/0.441 = 6.3 m2. That is a very high application rate, so add recirculation R = 3 to wet the media: applied flow = 4×2.78 = 11.1 m3/min, giving A = 11.1/0.441 = 25.2 m2 (≈ 5.7 m diameter).
  • Media volume: V = A × D = 25.2 × 6 = 151 m3 of structured plastic media.
  • Loading check: OLR = Q·S0/V = (4,000 × 0.200)/151 = 5.3 kg BOD/m3·d — at the top of the high-rate band, confirming this duty as a strong-rate plastic filter.

So roughly a 5.7 m diameter, 6 m deep plastic-media tower with 3:1 recirculation delivers the target 85% BOD removal on paper — to be verified against a supplier k value for the specific media and temperature.

What media, ventilation and depth should you specify?

Media sets the available biofilm area. Rock (crushed stone, 40–100 mm) offers only ~40–70 m2/m3 of specific surface area and ~50% voids, limiting depth to ~1.8–2.4 m before it clogs. Plastic media — random dumped packing or cross-flow/vertical structured sheets — provides 90–240 m2/m3 at >90% voids, so towers run 4–12 m deep with far less risk of ponding.

  • Structured (cross-flow) sheets: highest area and best liquid spreading, best for BOD roughing and high-rate duty.
  • Random plastic: cheaper, tolerant of uneven distribution, good for retrofits.
  • Rock: durable and free-draining but heavy and low-area; suited to small low-rate works.

Ventilation is not optional: the biofilm is aerobic and oxygen is supplied by natural draft driven by the temperature difference between the air in the voids and ambient. Underdrains, a peripheral vent area of ~1–2% of plan area, and adequate void space keep air moving; forced-draft fans are used on deep or enclosed towers. A well-designed distributor and depth ensure the whole media height stays wet and aerated.

How are sloughing and nitrification handled?

Biofilm grows until shear, substrate starvation at the base, or predation causes it to slough off in sheets. This is normal and self-regulating, but the detached biomass leaves with the effluent — so every trickling filter needs a downstream humus (secondary) clarifier sized for the sloughed solids, typically at 16–24 m3/m2·d surface overflow rate. Without it, effluent TSS and BOD both fail.

Nitrifying trickling filters exploit the fact that slow-growing autotrophic nitrifiers can only compete for the biofilm once BOD is low (typically <20 g/m3). A tertiary filter operated at low organic load will nitrify ammonia; because nitrification is oxygen- and area-limited, these are run at modest hydraulic loading on high-area plastic media. Where consent demands tight ammonia and total nitrogen, a filter is often paired with, or replaced by, a suspended-growth stage — compare the trade-offs in our MBR vs MBBR guide. For a duty-specific arrangement, MCBA can help with fixed-film process design and sizing.

How to size a trickling filter

  1. Characterise the load. Establish design flow Q and applied BOD S0 after primary settling, plus temperature and the effluent target Se.
  2. Classify the duty. Pick low-rate, high-rate, super-rate or roughing from the required removal and available footprint; this fixes media type.
  3. Choose media and depth. Select rock or plastic (random/structured) by specific surface area, then set depth D (4-12 m for plastic).
  4. Apply a removal model. Use the modified Velz Se/S0 = exp(-kD/q^n) or the NRC equation with the temperature-corrected k to solve for application rate q.
  5. Compute area and volume. Area A = flow/q (add recirculation R to keep media wet); media volume V = A x D. Check the organic loading rate is in band.
  6. Size the humus clarifier and ventilation. Add a secondary clarifier for sloughed solids and confirm natural or forced-draft ventilation supplies enough oxygen.

Frequently asked questions

What is the difference between a trickling filter and activated sludge?

A trickling filter is an attached-growth process: biomass grows as a fixed biofilm on media while wastewater trickles past. Activated sludge is a suspended-growth process holding biomass as a floc in aeration tanks with sludge recycle. Trickling filters use less energy and resist shock loads but generally achieve lower, less controllable effluent quality than well-run activated sludge.

How deep should a trickling filter be?

Depth depends on media. Rock filters are limited to about 1.8–2.4 m because deeper beds clog and starve of air. High-void plastic media allows towers of 4–12 m, commonly 6–7 m, which gives more biofilm contact per unit plan area. Depth is chosen with the removal model so the full height stays wetted and aerated.

Why is recirculation used in trickling filter design?

Recirculation returns filter or clarifier effluent to the inlet. It keeps the media continuously wet, improves liquid distribution, dilutes strong or toxic influent, and can seed active biomass. The recirculation ratio R (usually 0.5–4) raises the hydraulic application rate; beyond R of about 2–4 the extra BOD-removal benefit becomes marginal.

What is sloughing and why does it need a clarifier?

Sloughing is the periodic detachment of biofilm as it thickens and loses attachment at the base. The released solids leave with the effluent, so every trickling filter is followed by a humus (secondary) clarifier, typically at 16–24 m³/m²·d overflow rate, to settle them out. Without it the effluent TSS and BOD would both exceed consent.

Can a trickling filter achieve nitrification?

Yes. Slow-growing nitrifying bacteria only establish once BOD is low, so nitrification occurs in the lower reaches of a lightly loaded filter or in a dedicated tertiary nitrifying filter with high-area plastic media at modest hydraulic loading. Because it is oxygen- and area-limited, nitrification needs generous surface area and good ventilation to be reliable.

Which removal model should I use to size the media?

For rock media the NRC equation is the traditional choice; for plastic media the modified Velz/Germain form Se/S0 = exp(-kD/q^n) is more representative. Both are empirical fits, so calibrate the rate constant k to the actual media and temperature, and confirm with pilot data or a supplier performance guarantee before committing volumes.

Sources & further reading