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Wastewater Treatment

Trickling Filter / Biofilter Design Simulator

Size a trickling filter — the classical fixed-film bioprocess that treats wastewater on a biofilm that grows over rock or plastic media. Move the influent BOD, flow, recirculation ratio and depth sliders and the required filter volume, area, hydraulic loading and organic loading update in real time using the NRC and Velz equations.

Parameters
Influent BOD
mg/L
Target effluent BOD
mg/L
Typical municipal discharge limit is ~20-30 mg/L
Flow rate Q
m³/day
Filter media
Rock is traditional and cheap; plastic supports deep, compact towers
Filter depth
m
Rock filters are usually <= 2 m; plastic towers up to 12 m
Recirculation ratio R
Fraction of effluent recycled back to the distributor
Operating temperature
°C
Velz correction k_T = k_20 x 1.035^(T-20)
Results
—
BOD load (kg/day)
—
Filter volume (m³)
—
Filter area (m²)
—
Hydraulic load (m³/m²/day)
—
Organic load (kg/m³/day)
—
BOD removal (%)
—
Trickling filter cross-section (influent → distributor → media → under-drain)

The rotating spray arm distributes sewage over the media surface, where the biofilm oxidises BOD. Colour shows the local BOD concentration (blue: high → yellow: low).

Efficiency vs BOD load
Recirculation ratio R vs removal efficiency
Theory & Key Formulas

$$E = \frac{100}{1 + 0.4423\sqrt{\dfrac{W}{V\,F}}},\qquad F = \frac{1+R}{(1+R/10)^{2}}$$

Single-stage NRC equation (US Army 1946). E: removal (%), W: applied BOD load (kg/day), V: filter volume (m³), F: recirculation factor, R: recirculation ratio.

$$W = \frac{S_0\,Q}{1000},\qquad \eta_T = 1.035^{(T-20)}$$

BOD load W (kg/day) and Velz temperature factor η_T. S₀: influent BOD (mg/L), Q: flow (m³/day), T: water temperature (°C).

$$\text{HL} = \frac{Q}{A},\qquad \text{OL} = \frac{W}{V}$$

Hydraulic loading HL (m³/m²/day) and organic loading OL (kg/m³/day). HL > 40 is "high-rate"; OL > 1.6 is "high-load" operation.

Trickling-filter BOD removal design

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A "trickling filter" is the thing that sprays sewage over a pile of rocks, right? Does just trickling water over stones really clean the wastewater?
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Exactly — that round tank with a rotating arm spraying water you sometimes see behind a small treatment plant or a food factory. The "pile of rocks" is the filter media, and on its surface a few millimetres of biofilm — a layer of aerobic microbes — has grown. As the wastewater trickles down in a thin film, the bacteria pull oxygen out of the air and oxidise the BOD into CO₂ and water. The first trickling filter started up at the Salford plant in England in 1893, and the process has been in continuous use for over 130 years.
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130 years is a long time! So the design must be totally settled by now — anyone gets the same answer?
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Not quite — that's actually the fun part: there are still rival design schools. The most famous is the NRC equation, E = 100/(1+0.4423√(W/(V·F))), which was fit empirically to data the US Army gathered from rock-media filters built in great numbers at military bases during WWII. The recirculation correction F = (1+R)/(1+R/10)² grows with R, so the higher R is, the higher the efficiency. Try sliding R from 0 to 4 on the left — you should see the removal climb and the required volume shrink.
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Yes — R = 2 makes the volume about half! So big R is always a good deal?
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Only half right. Recirculation pumps the effluent back up to the distributor, so doubling R doubles the pumping power. With R = 4 you are circulating five times the raw-sewage flow. The trade-off between pump electricity and filter capital cost lands real plants at R = 0.5 to 2.0. The media matters too: rock filters cap out around 2 m deep, but plastic media like Surfpac or Flocor let you stack 6 to 12 m tall and take only one-third of the footprint at the same throughput. Try changing the media and depth sliders.
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It sounds almost too good. So why do big cities mostly use activated sludge instead?
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Good question. Trickling filters are "low energy, simple to operate and cheap to build", but the downsides are real: (1) they need 3-5 times the land for the same effluent quality, (2) wintertime efficiency drops sharply (at 5 °C the Velz factor is 1.035^(5-20) ≈ 0.59 — almost half), (3) they smell, and tend to grow swarms of filter flies, and (4) they cannot do nitrogen or phosphorus removal on their own. So dense cities go with activated sludge, while small municipalities, low-density areas and food-industry pre-treatment still use trickling filters today.

Frequently Asked Questions

The NRC equation (National Research Council, US Army 1946) is an empirical correlation that predicts the BOD removal efficiency of a single-stage trickling filter: E = 100 / (1 + 0.4423 * sqrt(W/(V*F))), where E is the percent removal, W is the applied BOD load (kg/day), V is the filter volume (m³), and F is the recirculation factor (1+R)/(1+R/10)². It was fit to data from rock-media trickling filters that the US Army built in large numbers during WWII, and is still recommended as a design equation by standard references such as AWWA and Metcalf & Eddy.
Rock media (50-100 mm) provides a specific surface area of about 45 m²/m³ and ~50% voidage. It has been used for over a century but its weight limits filter depth to about 2 m. Plastic cross-flow media (Surfpac, Flocor and similar) give 90-150 m²/m³ and >95% voidage, so towers can be 6-12 m deep and need only one third of the footprint of a rock filter at the same flow. Because the NRC equation was calibrated on rock filters, it is conservative for plastic media; Eckenfelder-style or Velz-style models are normally used together with the NRC result.
The recirculation factor F = (1+R)/(1+R/10)² is monotonically increasing: F=1 at R=0, F=1.65 at R=1, F=2.08 at R=2 and F=2.86 at R=4. Returning part of the effluent to the distributor (1) lets the biofilm treat the same molecules more than once, (2) evens out the hydraulic wetting and prevents short-circuiting, and (3) buffers wintertime temperature drops by mixing warmer effluent with the cold influent. The trade-off is the pumping power, so practical values are usually R = 0.5 to 2.0.
Pros: (1) much lower energy use because no aeration blowers are needed, (2) simple operation that does not need skilled operators, (3) very robust against influent-load shocks, (4) less and denser waste sludge that dewaters easily, (5) lower capital cost. Cons: (1) requires 3 to 5 times the footprint of an activated-sludge plant for the same effluent quality, (2) efficiency drops sharply in cold weather, (3) odour and filter-fly nuisance, (4) not well suited to nutrient (N/P) removal. Trickling filters are still common in small municipalities, developing countries and as roughing filters ahead of food-industry treatment.

Real-World Applications

Small municipal treatment plants: For populations of about 5,000-30,000 in suburban or rural areas, trickling filters are often preferred because they avoid the aeration blowers that activated sludge depends on. In Europe they are particularly common in Ireland, Scotland and the Nordics, where two-stage configurations (roughing + polishing) over rock or Flocor plastic media routinely deliver more than 90% BOD removal. Designers usually add 30-50% spare volume to absorb the wintertime efficiency drop.

Pre-treatment for food, dairy and brewery wastewater: High-strength industrial wastewater (BOD 1,000-5,000 mg/L) is often roughed out by a trickling filter that removes 70-80% of the BOD before the effluent is sent to a municipal sewer or a downstream activated-sludge polishing stage. Plastic-media high-rate towers (8-10 m deep, organic loading 2-4 kg/m³/day) keep the footprint small and the operation simple. Carlsberg in Denmark and Anheuser-Busch in the US are well-known examples.

Nitrifying towers after activated sludge: Once activated sludge has removed the carbon BOD, a polishing trickling filter dedicated to the nitrifiers (Nitrosomonas, Nitrobacter) can oxidise the remaining ammonia to nitrate. Because the BOD is already low, the slow nitrifiers are not out-competed and they can establish on the media. Plastic media at 150 m²/m³ is preferred for the surface area; in cold climates the tower is covered and the recirculation is heated to keep the water above 12 °C.

Decentralised treatment in developing countries: In regions where large sewer networks are not affordable, trickling filters built as gravel-filled concrete tanks run entirely by gravity (no pumps) are deployed by NGOs in India, East Africa and Southeast Asia. The "required footprint and depth" produced by this kind of calculator is often used directly as the basis for the on-site contractor's cost estimate.

Common Misconceptions and Pitfalls

The biggest pitfall is taking the NRC equation literally for any media or wastewater. The NRC fit, made from data gathered by the US Army in 1946, assumes (1) rock media, (2) typical domestic sewage at 200-300 mg/L BOD with readily biodegradable carbon, and (3) a temperate climate. Plastic media or industrial wastewater (containing oils, surfactants, antimicrobials) is outside that calibration range, and efficiency predictions can be 20-40% off. Use Eckenfelder-style (k × D^x for surface-area correction) or Velz-style (k₂₀ × 1.035^(T-20) for temperature correction) models alongside NRC, and ultimately calibrate against a pilot plant. Treat this tool as a first-order sizing estimate only.

Next, "just raise the volume and the hydraulic loading takes care of itself" is wrong. The NRC equation makes hydraulic loading (HL) and organic loading (OL) look independent, but in reality hydraulic loading above 40 m³/m²/day causes continuous biofilm "sloughing" that drops the apparent efficiency sharply. On the other side, dropping below about 2 m³/m²/day dries out the media and creates channelling, which also lowers efficiency. The verdict in this tool warns at HL > 100, but practical design windows lie inside HL = 1-40 m³/m²/day.

Finally, do not assume a trickling filter "also removes nitrogen". A standard trickling filter is sized for aerobic oxidation of carbon BOD; ammonia either passes through untreated or is at most nitrified to nitrate. Denitrification (nitrate → N₂ gas) requires an anoxic stage, so a separate anoxic zone or anaerobic digester has to be added. In low-BOD polishing applications with BOD/NH₄ < 5, the filter must be re-designed as a dedicated nitrifying filter. A trickling filter is not a universal pollutant remover.

How to Use

  1. Enter influent BOD concentration (mg/L) — typical municipal wastewater ranges 150–300 mg/L; industrial sources 500–2000 mg/L
  2. Set target effluent BOD (mg/L) — secondary treatment targets 20–30 mg/L; advanced systems aim below 10 mg/L
  3. Input wastewater flow rate (m³/day) and filter media depth (m, typically 1.5–2.5 m for stone or 1–1.5 m for plastic media)
  4. Simulator calculates required filter volume, footprint area, hydraulic loading rate, and organic volumetric load
  5. Adjust depth or diameter iteratively to keep hydraulic load below 5–10 m³/m²/day and organic load below 0.5–1.2 kg BOD/m³/day for stable nitrification

Worked Example

Municipal treatment plant: influent BOD 220 mg/L, target effluent 25 mg/L, flow 500 m³/day, filter depth 2.0 m. BOD load = 500 × 0.220 = 110 kg/day. For 80% removal (176 kg removed), organic load ≈ 0.7 kg/m³/day requires filter volume ≈ 158 m³. This yields circular area ≈ 79 m² (diameter 10 m). Hydraulic load = 500 ÷ 79 = 6.3 m³/m²/day — acceptable for single-stage stone media with recirculation.

Practical Notes

  1. Plastic media filters tolerate higher loads (1.5–2.0 kg/m³/day) than stone media but require better distribution; stone media (0.4–0.8 kg/m³/day) favors nitrification
  2. Include recirculation ratio 0.5–2.0 to prevent ponding, improve surface wetting, and enhance BOD removal in low-flow scenarios
  3. Account for seasonal temperature effects — biofilm activity halves from 20 °C to 10 °C; increase surface area by 15–25% in cold climates
  4. Monitor clogging and channeling; add 0.3–0.5 m underdrain depth for air distribution and sludge collection