Sample sizing inputs and outputs
| Daily feed (kg/day) | Protein (%) | TAN fraction | Nitrif. rate (g/m²·day) | Media SSA (m²/m³) | Safety factor | Flow (m³/h) | Design media (L) | Footprint (m²) |
|---|---|---|---|---|---|---|---|---|
| 0.50 | 35 | 0.70 | 0.60 | 500 | 1.30 | 5.00 | 85.0 | 1.25 |
Formula used in this calculator
How to use this biofilter sizing calculator
- Enter daily feed, protein percentage, and TAN excretion fraction.
- Choose media, then keep or override the effective surface area.
- Set nitrification rate based on temperature and oxygen levels.
- Pick a safety factor that matches your operation risk.
- Provide flow directly or estimate using system turnover.
- Review media volume, footprint, and loading rate outputs.
- Download CSV for records or PDF for sharing.
Design basis: nitrogen loading from feed
Biofilters are sized from expected ammonia production, not pond volume alone. This calculator estimates total ammonia nitrogen (TAN-N) from daily feed, protein percentage, nitrogen-in-protein, and the fraction excreted as TAN. For steady operation, use a 7–14 day average feed rate and apply a safety factor for spikes.
Media performance: effective surface area and fouling
Media rating matters when expressed as effective surface area, because biofilm thickness, solids, and channeling reduce active area. Moving media resists clogging, while static media may need larger margins. If you use “Custom”, enter an effective value that already includes fouling allowances.
Hydraulics: flow, loading rate, and contact time
Hydraulic loading rate (HLR) links flow to footprint. Higher HLR can promote short-circuiting and uneven distribution. The calculator reports contact time as design media volume divided by flow, helping you compare layouts. Example data: feed 0.50 kg/day, protein 35%, TAN fraction 0.70, nitrification 0.60 g/m²·day, SSA 500 m²/m³, safety 1.30, flow 5.0 m³/h.
Oxygen and temperature: maintaining nitrification capacity
Nitrifying bacteria consume oxygen and slow down in cold water. Use a lower nitrification rate when dissolved oxygen is limited, pH swings, or temperature drops. If winter performance matters, size for the colder month using a reduced areal rate, and verify aeration supports the load.
Operations: start-up, monitoring, and scaling
New biofilters need time to mature. Seed media when possible, increase feed gradually, and track TAN, nitrite, and nitrate trends. Maintain prefiltration to limit solids, rinse mechanical stages weekly, and keep water moving to avoid dead zones locally. Sudden cleaning or antibiotic treatments can reduce biofilm activity; your safety factor helps absorb those events. When scaling up, re-run the calculator using updated feed and flow.
Frequently asked questions
What does TAN-N mean in the results?
TAN-N is total ammonia nitrogen expressed as nitrogen mass. It combines ammonia and ammonium on an N basis, matching many test kits and nitrification capacity data.
How do I choose a nitrification rate value?
Start with conservative data for your water temperature, oxygen, and pH stability. If performance is uncertain, select a lower rate and use a higher safety factor, then refine using real monitoring results.
Why is a safety factor required?
Biofilters face swings in feeding, temperature, and solids loading. A safety factor adds extra surface area and media volume to reduce risk of TAN or nitrite spikes during upset conditions.
When should I use “Custom” media surface area?
Use Custom when you have a verified effective surface area for your exact media and configuration. Enter values that already account for clogging, biofilm thickness, and any manufacturer performance notes.
What is better: direct flow or turnover mode?
Use direct flow if you know pump flow through the filter. Use turnover when you only know system volume and desired turnovers per hour. Both produce the same design flow once converted.
Why does footprint use the maximum of two checks?
One check ensures the footprint can pass the design flow at your target hydraulic loading. The other ensures the footprint is large enough to physically hold the media volume at the chosen depth.
Can I use this for ponds and aquaponics?
Yes, it is suited for freshwater pond and aquaponics planning. For saltwater, very cold systems, or unusual wastewater, use conservative rates and validate with water testing and manufacturer guidance.