MBBR works by suspending thousands of plastic biofilm carriers inside an aerated reactor. Microorganisms colonize the protected inner surfaces of the carriers, forming a thin, highly active biofilm. Wastewater flows through the reactor, where dissolved pollutants diffuse into the biofilm and are biologically degraded. Aeration provides both oxygen for aerobic metabolism and the hydrodynamic energy to keep carriers moving.
Step-by-Step Process
Influent Entry
Pre-screened wastewater enters the MBBR reactor. Upstream screening (typically 3–6 mm) is essential to prevent clogging of carriers and retention screens.
Contact with Biofilm Carriers
The wastewater mixes with thousands of free-floating carriers, each providing 200–800 m²/m³ of protected internal surface area. The carriers are made of high-density polyethylene (HDPE) with a specific gravity of 0.95–0.97 g/cm³, ensuring they remain neutrally buoyant.
Biofilm Metabolism
Heterotrophic bacteria in the biofilm metabolize dissolved organic compounds (BOD/COD). Under aerobic conditions with sufficient dissolved oxygen (≥2 mg/L), autotrophic nitrifying bacteria oxidize ammonia (NH₄-N) to nitrite and nitrate. In anoxic zones, denitrifiers convert nitrate to nitrogen gas.
Aeration and Mixing
Fine-bubble or coarse-bubble diffusers provide oxygen and create a circulation pattern that keeps carriers tumbling. In anoxic stages, submersible mixers maintain carrier movement without introducing oxygen. The air flow rate is typically 6–12 Nm³/m²·h of reactor cross-section.
Biofilm Self-Regulation
Hydrodynamic shear forces and carrier-to-carrier collisions naturally limit biofilm thickness to 50–300 µm. Excess biomass sloughs off and becomes part of the mixed liquor, preventing diffusion limitation and maintaining high activity.
Effluent Separation
Treated water exits through a retention screen (5–10 mm openings) that retains carriers while passing effluent with detached biomass. This stream proceeds to secondary clarification (settling tank), DAF, or direct filtration (disc/drum filter) for solids removal.
Typical MBBR Operating Conditions
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Dissolved Oxygen (aerobic) | 2 – 6 | mg/L | |
| pH Range | 6.5 – 8.5 | - | |
| Temperature | 10 – 30 | °C | Optimal for nitrification: 15–25°C |
| Air Flow Rate | 6 – 12 | Nm³/m²·h | Reactor cross-section basis |
| Carrier Specific Gravity | 0.95 – 0.97 | g/cm³ | Near-neutral buoyancy |
| Retention Screen Opening | 5 – 10 | mm | Wedge-wire or perforated plate |
Aerobic vs. Anoxic MBBR Operation
In aerobic MBBR, diffused aeration supplies oxygen for BOD removal and nitrification while simultaneously circulating the carriers. The biofilm is dominated by heterotrophic bacteria (outer layers) and autotrophic nitrifiers (inner layers where competition for oxygen is lower in lightly loaded systems).
In anoxic MBBR, mechanical mixers replace aeration to keep carriers in motion without introducing dissolved oxygen. Denitrifying bacteria in the biofilm use nitrate (from recirculated aerobic effluent) as an electron acceptor and influent organic carbon as an electron donor, converting nitrate to nitrogen gas. Anoxic MBBR is commonly placed upstream of the aerobic stage for pre-denitrification.
Downstream Separation
MBBR effluent contains detached biofilm particles (typically 20–200 µm) that must be separated before discharge. Common options include conventional secondary clarifiers (for large plants), dissolved air flotation (DAF, especially for industrial applications with high-lipid waste), and direct tertiary filtration using disc filters or drum filters (compact footprint, suitable for retrofit). The choice depends on effluent quality requirements, space constraints, and sludge handling considerations.
Frequently Asked Questions
How does MBBR differ from a trickling filter?
Does MBBR need continuous aeration?
How long does it take for biofilm to establish on carriers?
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