RAS Biofilters — Moving-Bed, Fixed-Bed and Sand Filters Compared
The biofilter converts toxic ammonia from fish and shrimp waste into nitrate. It is the slowest part of a RAS to start and the hardest to fix later, so it deserves the most careful specification.
Short answer: A RAS biofilter grows nitrifying bacteria on media to turn ammonia into nitrate. Moving-bed filters keep plastic media moving with air; fixed beds also trap fine particles. Compare quotes on the stated surface-area basis and the design ammonia load, not media volume alone.
What ras biofilters do
- A reactor filled with media on which nitrifying bacteria grow; they oxidise ammonia (from feed) to nitrite and then nitrate.
- Moving-bed biofilm reactors (MBBR) keep buoyant plastic carriers circulating with aeration, which also scours old biofilm.
- Fixed-bed filters hold media still; besides nitrification they can retain fine particles smaller than the drum filter removes.
- Sand-based and trickling filters are older or niche options; sand moving-bed reactors were piloted again in seawater RAS in 2026.
Where it applies
- All RAS for fish and shrimp
- Fresh, brackish and seawater (design differs)
- Biofloc ponds use in-pond bacteria instead
Technology maturity: MBBR and fixed bed: established. Sand moving-bed in seawater RAS: pilot (2026).
Variants compared
| Variant | Best fit |
|---|---|
| Moving-bed (MBBR) | Aerated buoyant media, self-cleaning, widely used in grow-out. |
| Fixed bed | Static media; also retains fine particles. |
| Sand / fluidised | Compact; sand moving-bed piloted in seawater RAS in 2026. |
| Trickling filter | Simple and degasses CO₂; larger footprint. |
Typical specifications
| Parameter | Typical range | What it means for you |
|---|---|---|
| Media specific surface area | ≈500–5,500 m²/m³ (manufacturer-stated) | Suppliers quote total, protected or active area — these are not interchangeable. Ask which one. |
| Moving-bed fill ratio | ≈15–70% of reactor volume (manufacturer-stated) | Higher fill means more area but less movement; confirm the design fill. |
| Upstream solids removal | Drum filter ≈40–60 µm ahead of the biofilter | Solids reaching the biofilter consume oxygen and clog fixed beds. |
| Start-up time | Weeks; a 2026 seawater sand-MBBR pilot needed 53 days | Plan biofilter maturation before stocking — it cannot be rushed. |
| Nitrification rate | No independent like-for-like figure | Manufacturer loading claims depend on temperature, salinity, oxygen and organic load; ask for the design basis. |
Advantages
- Removes toxic ammonia
- Modular — add chambers to expand
- MBBR media are self-cleaning
Limitations
- Weeks to mature
- Consumes oxygen and needs degassing
- Performance depends on temperature and salinity
Maintenance
- Check media movement and screens
- Monitor ammonia and nitrite trends
- Inspect aeration grids and diffusers
Risks to manage
- Biofilter crash from chemicals or medication
- Clogging when solids removal fails
- Nitrite spikes during start-up
Compliance points to check
- Check medicines and disinfectants are compatible with the biofilter
- Nitrate and sludge discharge under local permits
Compliance points are prompts for your own review. Producers, suppliers and buyers remain responsible for the rules that apply to them; FishMatch does not certify equipment or guarantee compliance.
Sizing guidance
Size the biofilter from the daily feed load (ammonia production), water temperature, salinity and the ammonia limit for your species — not from tank volume.
| Project scale | Typical biofilter approach | Design notes |
|---|---|---|
| Hatchery / quarantine | Small MBBR or fixed bed per system | Separate systems per biosecurity zone; easy media cleaning matters. |
| Grow-out RAS | MBBR, often in parallel chambers | Parallel units allow cleaning or repair without stopping production. |
| Marine / fine-particle sensitive | Fixed bed or MBBR + fine filtration | Discuss degassing and fine solids control together with the biofilter. |
Ask suppliers to state the design daily feed, ammonia removal capacity at your temperature and salinity, oxygen demand of the biofilter, surface-area basis and the start-up plan.
How to choose
- Design basis in writing
Daily feed (kg/day), water temperature, salinity and target ammonia concentration — the same inputs for every supplier.
- Surface-area definition
Total, protected and active area differ; comparing different definitions gives misleading results.
- Oxygen and degassing
Nitrification consumes oxygen and produces CO₂; confirm aeration, oxygen supply and CO₂ stripping are sized with the filter.
- Redundancy
Split the biofilter into chambers so one can be maintained while others keep working.
What drives the price
- Media volume and type
High-area media cost more per m³ but may need less volume.
- Reactor construction
Tank material, screens to retain media, aeration grid and access hatches.
- Integration
Degassing, pumping, controls and connection to the rest of the loop.
No sourced price benchmark is published for this product type. Quotations depend on capacity, materials, controls, redundancy, installation scope and delivery terms — FishMatch requests comparable quotations against your written specification instead of quoting a list price.
Common specification mistakes
- Comparing media by volume or headline surface area without the definition.
- Stocking before the biofilter has matured.
- No upstream solids removal, so the biofilter clogs or loses capacity.
- One single biofilter with no way to isolate part of it for maintenance.
Components that work with it
- Drum filters
Solids removal upstream
- Oxygen generators
Oxygen for fish and nitrification
- Water quality sensors
pH, ammonia and nitrite checks
RAS biofilters — buyer questions
Get quotes for ras biofilters
Send your duty, site conditions and target capacity once. We turn it into a comparable specification, send a confidential RFQ to producers and suppliers that may fit the project, and coordinate their replies on the same scope. Free for buyers; qualifying projects start at USD 250,000. FishMatch does not manufacture equipment, and buyers do not contact suppliers directly.
Related pages
Sources and review
- Pilot-scale sand moving-bed biofilm reactor for seawater RAS — Bioresource Technology Reports, 2026-06
- Filter medium, temperature and ammonia load in nitrifying trickling filters — Universitat Politècnica de València, 2016
Ranges marked "manufacturer-stated" come from public manufacturer documentation reviewed by FishMatch; they are examples, not guarantees, and supplier names are not shown. Last reviewed 2026-10-24.