A Moving Bed Biofilm Reactor (MBBR) is one of the most common biological filtration technologies used in modern aquaculture. The process consists of thousands of small plastic carrier elements suspended in a continuously mixed and aerated tank. These carriers provide a large protected surface area where beneficial bacteria form a biofilm.
The primary role of an MBBR is nitrification: ammonia (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). By continuously removing toxic ammonia and nitrite, the biofilter helps maintain a stable environment for fish and enables much higher stocking densities than would otherwise be possible. See Ammonia / TAN and nitrite.
See also
How it works
Water containing dissolved waste enters the reactor and flows around freely moving plastic carriers. The biofilm growing on the carriers contains nitrifying bacteria that convert ammonia into nitrite and nitrite into nitrate.
Aeration, mechanical mixing or the water flow itself (pumped circulation through the reactor) keeps the carriers in motion, ensuring:
- Uniform oxygen distribution
- Good contact between water and biofilm
- Reduced risk of clogging
- Continuous self-cleaning of excess biomass
Unlike fixed-media biofilters, MBBRs do not rely on stationary packing material and are generally less prone to channeling and fouling.
Why MBBRs are popular in aquaculture
MBBRs have become a standard component in many RAS and intensive flow-through systems because they combine simplicity with high treatment capacity. Advantages include:
- Robust nitrification
- High treatment capacity per unit volume
- Scalability from small hatcheries to large commercial farms
- Tolerance to fluctuating feed loads
- Relatively low maintenance requirements
- Good operational reliability
- Simple expansion by adding more carrier media
For these reasons, MBBRs are commonly used in facilities producing salmon, trout, char, tilapia, shrimp and many other cultured species.
Oxygen and alkalinity demand
MBBRs do not remove waste for free. Nitrification consumes oxygen and alkalinity. For every amount of ammonia converted, significant quantities of oxygen are required and alkalinity is gradually consumed. As a result, biofilter performance depends on adequate dissolved oxygen, stable pH, sufficient alkalinity, appropriate temperature and healthy bacterial populations. Poor oxygenation or low alkalinity can quickly reduce nitrification efficiency. See dissolved oxygen, alkalinity and pH.
Simple sizing
The calculator below goes from peak feed to TAN load, biofilm area needed, media volume, reactor size, oxygen and mixing air. Pick a scenario (salmon/trout, sturgeon, tilapia, clarias) to set TAN yield and conversion per m² available area — you can change the value. Set mixing air to 0 if pumped circulation moves the carriers. Not a design calculation — media, temperature, salinity and load govern real conversion.
Simple sizing (orientation)
Typical culture temperature about 10–16 °C. Conservative conversion per m².
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MBBR start-up
A newly installed MBBR contains little or no nitrifying biomass. The biofilm must develop over time as bacteria colonise the carriers. During start-up, ammonia removal is initially limited, nitrite spikes are common, feeding may need to be restricted and water quality requires close monitoring. Biofilter maturation can take weeks to months depending on temperature, water chemistry, bacterial seeding and loading rates. See nitrite.
MBBR in RAS
In Recirculating Aquaculture Systems (RAS), the MBBR is often regarded as the biological heart of the facility. A typical RAS process train may include:
Fish tanks
↓
Solids removal
↓
Degassing
↓
MBBR
↓
Oxygenation
↓
Back to fish tanks
The order can vary (biofilter and degassing sometimes swap places). The biofilter works continuously and directly determines how much feed the system can process and therefore how much fish biomass the farm can support. Operators often monitor TAN, nitrite, nitrate, pH, alkalinity and dissolved oxygen as indicators of biofilter performance. See RAS.
MBBR vs other biofilters
| Technology | Typical strength |
|---|---|
| MBBR | Robust, simple, scalable |
| Fixed-bed biofilter | High efficiency, compact footprint |
| Trickling filter | Excellent CO₂ stripping and oxygen transfer |
| Bead filter | Solids removal and biofiltration combined |
| Fluidised sand filter | Very high surface area and treatment capacity |
Each technology has advantages, and many modern RAS facilities combine several treatment processes. Orientation, not a ranking.
Limitations
Although MBBRs are highly effective, they are not a complete water-treatment solution. They do not directly remove solids, carbon dioxide, phosphorus, colour or most dissolved organic compounds. These parameters require additional processes such as drum filters, settling systems, protein skimmers, degassing towers and ozone or UV treatment.
Practical rule of thumb
The MBBR is the engine room of biological filtration. As long as oxygen, alkalinity and carrier surface area are sufficient, an MBBR can provide highly reliable nitrification and support large fish biomasses. Most biofilter problems are caused not by the carriers themselves, but by insufficient oxygen, inadequate alkalinity, poor solids management, or attempting to feed the system faster than the bacteria can grow.
Sources/overview (selection): Timmons & Ebeling, Recirculating Aquaculture; established RAS practice on moving-bed biofilters. Orientation, not a design calculation. See also RAS, Ammonia / TAN, nitrite, alkalinity and dissolved oxygen.