MBBR biofilm carriers are small, engineered plastic elements — typically made from HDPE or PP — designed to provide maximum protected internal surface area for biofilm growth while maintaining near-neutral buoyancy in water. The carrier design directly influences biofilm development, mass transfer, and treatment performance.
Carrier Types and Geometries
Over three decades of MBBR development, numerous carrier designs have emerged. The most common geometries include:
Chip / Cylinder
Small cylindrical elements (7–15 mm diameter, 7–10 mm height) with internal cross-walls. Protected surface area typically 500–800 m²/m³. The original Kaldnes K1/K3/K5 design belongs to this category.
Wheel / Wagon Wheel
Larger elements (22–25 mm diameter) with radial fins and central hub. PSA typically 350–500 m²/m³. Better suited for high-rate carbon removal due to larger internal passages.
Saddle / Helix
Saddle-shaped or helical elements that provide good mixing characteristics and biofilm distribution. PSA ranges from 300–600 m²/m³.
Cubic / Block
Cube or rectangular elements with internal grid structures. Higher PSA (up to 800+ m²/m³) but may have diffusion limitations in the innermost voids at high biofilm thickness.
Carrier Selection Criteria
- Protected Surface Area (PSA): Higher PSA provides more biofilm per unit volume of carrier, enabling higher volumetric loading rates. However, very high PSA carriers with small internal passages may experience diffusion limitations.
- Specific Gravity: Must be close to water (0.94–0.98 g/cm³) for uniform distribution and efficient circulation. Too light → carriers float and accumulate at the surface. Too heavy → carriers settle and require more energy to keep suspended.
- Size and Shape: Larger carriers have better internal mixing but lower PSA per volume. Smaller carriers provide more surface area but may clog retention screens more easily. Typical sizes range from 10–25 mm.
- Material Durability: Virgin HDPE or PP provides UV resistance, chemical stability, and 20+ year service life. Recycled material carriers are available at lower cost but may have variable density and shorter lifespan.
- Application Match: High-rate carbon removal favors larger carriers with open channels. Nitrification benefits from smaller, high-PSA carriers that support thin, active biofilm. Denitrification requires carriers that maintain biofilm under lower mixing intensities.
Common MBBR Carrier Specifications
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Material | HDPE / PP | Virgin or recycled | |
| Specific Gravity | 0.94 – 0.98 | g/cm³ | |
| Size Range | 10 – 25 | mm | Diameter |
| Protected Surface Area | 200 – 800 | m²/m³ carrier | |
| Void Ratio | 60 – 90 | % | |
| Bulk Density (dry) | 120 – 180 | kg/m³ | |
| Service Life | > 20 | years | Under normal conditions |
Frequently Asked Questions
How many carriers are needed for an MBBR system?
Can different carrier types be mixed in one reactor?
Do carriers need replacement over time?
Related Pages
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