While biofilm is essential for MBBR function, too much biofilm is counterproductive. Excessively thick biofilm develops anaerobic inner layers that are biologically inactive, clogs carrier channels, increases carrier weight (impairing mixing), and can cause screening and downstream solids problems. The goal is an active, thin biofilm — not maximum biofilm.
Why excessive biofilm develops
High organic loading: A heavily loaded first-stage reactor promotes rapid heterotrophic growth that can outpace natural detachment. Low shear: Insufficient aeration or mixing energy means the erosion force is not strong enough to keep biofilm in check. Protected carrier geometry: Carriers with deep internal channels protect biofilm from shear, which is an advantage up to a point but can lead to channel clogging under high load.
Filamentous bacteria: In some cases, filamentous organisms colonise the biofilm and create a thick, slimy or hairy layer that traps solids and is resistant to normal erosion. This is analogous to filamentous bulking in activated sludge and may be triggered by low DO, nutrient deficiency, or specific substrate conditions.
Consequences and indicators
Excessive biofilm: signs and effects
| Parameter | Typical Value | Unit | Notes |
|---|---|---|---|
| Carrier channels clogged | Reduced protected surface area | — | Pull carriers and inspect. |
| Carriers sinking / rafting | Increased density or gas entrapment | — | Mixing energy insufficient. |
| Effluent TSS spike | Sloughing of thick biofilm | — | Downstream clarifier overloaded. |
| Reduced nitrification | O₂ cannot reach nitrifiers | — | Thick heterotrophic layer blocks O₂. |
| Screen clogging | Detached biofilm blocks screens | — | Increased backwash frequency. |
Control measures
Increase aeration / mixing energy: Higher shear promotes erosion and thins the biofilm. This is the primary control measure. Redistribute load: If the first stage is overloaded, divert some flow to later stages or activate standby volume to reduce the surface-area loading. Temporary air scouring: A short burst of high-intensity aeration can strip excess biofilm; this should be done carefully to avoid losing too much biomass.
In persistent cases, physical cleaning of carriers (removing and washing) has been used, but this is labour-intensive and disruptive. Preventing excessive growth through proper load management and aeration is always preferable to reactive cleaning.
Frequently Asked Questions
How thick is "too thick"?
Will increasing aeration strip all the biofilm?
Can filamentous biofilm be controlled?
Related Pages
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