An MBBR system consists of five core components: (1) the reactor tank, (2) biofilm carriers, (3) an aeration or mixing system, (4) carrier retention screens, and (5) downstream solids separation. Understanding the function and specification of each component is critical for successful design and operation.
Reactor Tank
The MBBR reactor is typically a reinforced concrete or welded steel tank. Rectangular or circular geometries are common. The reactor must be designed to ensure adequate carrier circulation without dead zones. For aerobic applications, the water depth is usually 4–6 m to provide sufficient oxygen transfer efficiency from the diffuser grid. Reactor hydraulics should achieve complete mixing, with aspect ratios (depth-to-width) of 0.8–1.5 for rectangular tanks. Internal baffles may be used to create distinct aerobic and anoxic zones within a single tank.
Biofilm Carriers
Biofilm carriers are small polyethylene (HDPE or PP) elements, typically 10–25 mm in diameter, with a specific gravity of 0.95–0.97 g/cm³. They are designed with internal fins, cross-walls, or grid structures that provide protected surface area (200–800 m²/m³ of carrier) for biofilm attachment. The most commonly used designs include cylindrical chips, saddle shapes, and wheel-type elements. The carrier fill ratio (volume of carriers / total reactor volume) typically ranges from 30% to 67%, with 50–60% being the most common design range.
Aeration System
The aeration system serves a dual purpose in aerobic MBBR: delivering oxygen for biological treatment and providing the energy for carrier mixing and circulation. Fine-bubble membrane diffusers or coarse-bubble diffusers are installed at the reactor floor in a grid pattern. The total air flow must satisfy both the process oxygen demand (for BOD oxidation and nitrification) and the minimum mixing requirement (typically 6–12 Nm³ air per m² of reactor floor area per hour). For anoxic stages, low-speed submersible mixers (banana-blade type) at 5–15 W/m³ are used instead of aeration.
Retention Screens
Retention screens are installed at the reactor outlet to prevent carriers from escaping with the effluent. Common types include wedge-wire screens with 5–8 mm slot openings, perforated stainless steel plates, and cylindrical drum screens. The screen area must be sized to handle peak hydraulic flows while maintaining low approach velocities (<1 m/s) to prevent carrier accumulation and blinding. Regular cleaning mechanisms (air scour, mechanical wipers, or backwash) are essential to prevent screen clogging by biomass and debris.
Downstream Solids Separation
MBBR effluent contains detached biofilm particles that require separation before discharge. Options include secondary clarifiers (conventional settling, surface loading 0.4–0.8 m/h), dissolved air flotation (DAF, effective for light biofilm particles from industrial applications), disc filters (compact footprint, 10–20 µm filtration), and drum filters. The choice depends on effluent quality targets, space availability, and sludge handling preferences. For retrofit applications, existing secondary clarifiers can often be reused.
MBBR Component Specifications
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Reactor Depth | 4 – 6 | m | Water depth for aerobic reactors |
| Carrier Size | 10 – 25 | mm | Diameter of typical elements |
| Carrier Specific Gravity | 0.95 – 0.97 | g/cm³ | |
| Protected Surface Area | 200 – 800 | m²/m³ carrier | |
| Fill Ratio | 30 – 67 | % | Most common: 50–60% |
| Air for Mixing | 6 – 12 | Nm³/m²·h | Minimum for carrier circulation |
| Screen Opening | 5 – 10 | mm | Wedge-wire or perforated |
| Mixer Power (anoxic) | 5 – 15 | W/m³ | Submersible slow-speed |
Frequently Asked Questions
What material are MBBR carriers made of?
How often do retention screens need cleaning?
Can existing tanks be used for MBBR?
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