In an MBBR, the biofilm on the carrier surface is the treatment engine. An ideal biofilm is thin enough for substrates and oxygen to penetrate to the inner layers, yet thick enough to harbour a diverse microbial community. Thick, overgrown biofilm suffers from substrate limitation at its base, reduces effective treatment and can increase carrier weight to the point where mixing is impaired. Biofilm control is therefore a balancing act between promoting growth and preventing excess.
What governs biofilm thickness?
Biofilm thickness is the result of growth versus detachment. Growth is driven by substrate (COD, ammonia) and oxygen availability; detachment is driven by hydrodynamic shear from aeration, carrier collisions and endogenous decay. In a well-operated MBBR, these forces balance at a steady-state thickness — typically in the range of 50–500 µm depending on load and aeration intensity — but disturbances (load spikes, aeration failure, temperature drops) can upset this balance.
Carriers with internal protected surfaces (the area inside grooves or channels) tend to develop thicker biofilm than those with exposed flat surfaces, because the shear inside the channels is lower. This is an advantage for nitrifiers (slower growing, benefiting from protection) but can become a disadvantage if the channels clog with excess biomass.
Biofilm thickness indicators
Indicative biofilm data (confirm per project)
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Typical thin biofilm (nitrification) | 50–200 | µm | Thin for O₂ penetration to nitrifiers. |
| Typical thick biofilm (carbon removal) | 200–500 | µm | Higher COD load supports thicker growth. |
| Overgrown biofilm | >500–1 000 | µm | Inner layers likely inactive/anaerobic. |
| Carrier-collision shear | Design-dependent | — | Mixing energy controls detachment rate. |
Operational control measures
Aeration intensity: Increasing airflow rate raises shear, promoting biofilm detachment and keeping thickness in check. This is the primary control knob but must be balanced against DO requirements and energy cost. Carrier fill ratio: A higher fill ratio means more carrier-to-carrier collisions and more abrasion, which helps control biofilm but also increases mixing energy demand. Load management: Reducing organic load to a reactor (e.g. by spreading load across stages) slows biofilm growth and can prevent excessive accumulation.
Visual inspection of carriers is a practical diagnostic: healthy carriers in an aerobic reactor should have a thin, even brownish biofilm. Black, thick or slimy biofilm indicates anaerobic conditions within the film; bare or patchy carriers indicate too much shear or insufficient substrate.
Frequently Asked Questions
How can I tell if biofilm is too thick?
Is it possible to have too little biofilm?
Does biofilm thickness affect energy consumption?
Related Pages
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