The MBBR process uses attached-growth biofilm on free-moving carriers to perform biological treatment. Different configurations — single-stage aerobic, multi-stage aerobic, and staged anoxic/aerobic — target specific treatment objectives from simple BOD removal to complete nitrogen elimination.
Biological Mechanisms in MBBR
Three primary biological mechanisms operate in MBBR systems: heterotrophic carbon oxidation (BOD/COD removal), autotrophic nitrification (ammonia oxidation), and heterotrophic denitrification (nitrate reduction to nitrogen gas). These processes occur within the biofilm matrix, where substrate gradients and oxygen availability determine which organisms dominate.
Carbon Removal: Heterotrophic bacteria in the outer biofilm layers rapidly metabolize dissolved organic matter using oxygen as the terminal electron acceptor. Surface area loading rates (SALR) for carbon removal range from 5–25 g BOD₅/m²·d depending on temperature and wastewater characteristics.
Nitrification: Autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter/Nitrospira) oxidize ammonia to nitrite and then nitrate. These slow-growing organisms colonize the deeper biofilm layers where they face less competition from fast-growing heterotrophs. Nitrification requires low organic loading (≤5 g BOD₅/m²·d) and adequate dissolved oxygen (≥3 mg/L) to prevent heterotrophic overgrowth.
Denitrification: In anoxic conditions (no dissolved oxygen, nitrate present), denitrifying bacteria use nitrate as the electron acceptor and organic carbon as the electron donor to produce nitrogen gas. Anoxic MBBR stages achieve denitrification rates of 0.5–3.0 g NO₃-N/m²·d depending on carbon availability and temperature.
Common MBBR Process Configurations
Single-Stage Aerobic MBBR
One aerated reactor for combined BOD removal and partial nitrification. Suitable for high-strength industrial wastewater pretreatment or roughing applications where complete nitrification is not required.
Two-Stage Aerobic MBBR
First stage (high-rate) removes bulk BOD; second stage (low-rate, high DO) provides nitrification. This is the most common configuration for achieving both carbon removal and ammonia oxidation.
Pre-Denitrification MBBR
Anoxic MBBR (with mixers) before aerobic MBBR. Nitrate-rich effluent from the aerobic stage is recirculated to the anoxic stage, where denitrifiers use influent carbon to reduce nitrate. Total nitrogen removal of 70–85% is achievable.
Post-Denitrification MBBR
Anoxic MBBR after the aerobic stage, with external carbon dosing (methanol or acetate). Achieves very low effluent TN (<5 mg/L) but increases operating costs due to chemical carbon.
MBBR + IFAS Hybrid
Carriers added to an existing activated sludge reactor. Both suspended and attached biomass contribute to treatment. Ideal for upgrading existing WWTPs without new tanks.
MBBR Process Design Parameters by Application
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Carbon removal SALR | 5 – 25 | g BOD₅/m²·d | High-rate roughing to polishing |
| Nitrification SALR | 0.5 – 2.0 | g NH₄-N/m²·d | At 15°C, DO ≥3 mg/L |
| Denitrification rate | 0.5 – 3.0 | g NO₃-N/m²·d | Depends on C/N ratio |
| DO for nitrification | ≥ 3 | mg/L | Higher DO improves rate |
| C/N ratio for denitrification | 4 – 6 | g COD/g N | External carbon if insufficient |
| Pre-denitrification TN removal | 70 – 85 | % | With recirculation |
Frequently Asked Questions
Can MBBR achieve complete nitrogen removal?
What is the typical HRT for MBBR?
Does MBBR produce less sludge than activated sludge?
Related Pages
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