Fixed bed biofilters are biological filters that use stationary media to provide a large protected surface area for nitrifying bacteria. The media may consist of plastic modules, structured packing, gravel, rock, lava media, synthetic materials or other high-surface-area substrates that remain fixed in place while water flows through them.
Like all biological filters, their primary purpose is nitrification: ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). By converting toxic ammonia and nitrite into less toxic nitrate, fixed bed biofilters help maintain water quality and support higher fish stocking densities. See Ammonia / TAN and nitrite.
See also
How it works
Water passes through stationary filter media where biofilms containing nitrifying bacteria grow on the media surfaces. As water flows through the biofilter, ammonia is converted to nitrite and nitrite to nitrate. Oxygen and alkalinity are consumed. Because the media remains fixed, biofilms can develop large and stable bacterial populations with excellent treatment capacity. Many fixed bed systems operate under low-energy conditions and require little or no mechanical mixing — flow through the bed is often enough.
Why fixed bed biofilters are used
Fixed bed biofilters have been used in aquaculture for decades and remain popular in hatcheries, small RAS, flow-through farms, recirculation systems and public aquaria. Advantages include:
- Very high protected surface area
- Efficient nitrification
- Low energy requirements
- Simple construction
- Stable bacterial communities
- Good performance under steady operating conditions
Because the biofilm is protected within the media structure, these filters can support large bacterial populations in a relatively compact footprint.
The importance of solids removal
The biggest challenge with fixed bed biofilters is fouling. If suspended solids are not removed before the biofilter, waste particles become trapped within the media and gradually block water flow. Potential consequences include reduced hydraulic performance, channeling and dead zones, oxygen depletion within the media, reduced nitrification rates, increased maintenance and formation of anaerobic zones.
For this reason, fixed bed biofilters are normally installed after effective solids removal such as drum filters, settling systems, microscreens or hydrocyclones. Good solids management is often the difference between a highly efficient fixed bed biofilter and a poorly performing one.
Oxygen and alkalinity demand
Like all nitrifying systems, fixed bed biofilters require dissolved oxygen, alkalinity, stable pH and suitable temperature. Nitrification continuously consumes both oxygen and buffering capacity. If oxygen or alkalinity becomes limiting, ammonia removal declines, nitrite may accumulate and efficiency falls. See dissolved oxygen, alkalinity and pH.
Simple sizing
The same model as for MBBR: peak feed → TAN load → area needed → media volume → bed size. Fixed-bed defaults are a lower specific area (structured plastic, orientation) and a higher fill, with air 0 because the bed is not normally mixed. Conversion per m² is the same variable as in the MBBR calculator — fouling reduces the real area. Not a design calculation.
Simple sizing (orientation)
Typical culture temperature about 10–16 °C. Conservative conversion per m².
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Fixed bed vs MBBR
| Feature | Fixed bed | MBBR |
|---|---|---|
| Media movement | Stationary | Moving |
| Surface area | Very high | High |
| Energy demand | Low | Moderate |
| Solids sensitivity | Higher | Lower |
| Risk of clogging | Higher | Lower |
| Expansion | Less flexible | Easily expanded |
| Maintenance | More cleaning | Generally lower |
Both perform the same biology. Fixed beds often achieve excellent nitrification; MBBRs are typically more forgiving when feed loads fluctuate or solids removal is imperfect.
Fixed beds in hatcheries
Fixed bed biofilters are particularly common in hatcheries where feed loads are moderate, water quality is closely controlled, stable operation is prioritised and tank biomass is relatively predictable. Many trout, salmon and char hatcheries have successfully used fixed bed biofilters for decades.
Common media types
- Structured plastic blocks and cross-flow plastic media
- Corrugated plastic sheets
- Lava rock and gravel beds
- Synthetic fibre media and modular biofilter elements
The goal is always the same: maximising surface area while maintaining effective water flow and oxygen supply. Specific area differs greatly between gravel and structured plastic — enter the right figure in the calculator.
Limitations
Fixed bed biofilters do not directly remove solids, carbon dioxide, phosphorus, colour or dissolved organic compounds. Additional treatment is typically required, including drum filtration, degassing, oxygenation, UV and sometimes ozone. Poor solids removal can eventually force shutdown and cleaning of the biofilter.
Practical rule of thumb
A fixed bed biofilter can provide extremely efficient nitrification, but only if solids are removed before the water reaches the media. Good solids removal protects the biofilter. Poor solids removal eventually turns the biofilter into a solids filter. The greatest strength is the large protected bacterial surface. The greatest weakness is fouling.
Sources/overview (selection): Timmons & Ebeling, Recirculating Aquaculture; established hatchery and RAS practice. Orientation, not a design calculation. See also MBBR, Ammonia / TAN, nitrite, alkalinity and dissolved oxygen.