Does MBBR Require Secondary Clarification?

Short answer: Yes. MBBR produces detached biofilm flocs that must be separated from the treated water. A secondary clarifier is the most common option, but dissolved-air flotation (DAF), disc/drum filters or membranes can also be used, depending on the application and effluent requirements.

Why separation is needed

As biofilm grows on the carriers, excess biomass detaches and leaves the reactor as suspended flocs. Without downstream separation, these solids would appear in the effluent as elevated TSS. The separation step collects the detached biomass and produces a clear effluent that meets permit requirements.

Separation options

Solids separation after MBBR (indicative — confirm per project)

Parameter	Typical Value	Unit	Notes
Secondary clarifier	Most common	—	Gravity settling; proven, simple operation. No sludge return needed.
Dissolved air flotation (DAF)	Compact alternative	—	Smaller footprint; handles light biofilm flocs well.
Disc/drum filter	Tertiary polishing	—	Very compact; used after clarifier or DAF for low TSS targets.
Membrane (MBR)	Highest quality	—	Eliminates the need for a separate clarifier; highest CAPEX.

Key design consideration

Unlike activated sludge, the secondary clarifier after an MBBR does not need to return sludge. This means:

No RAS pumps or return-sludge line.
The clarifier only needs to settle and thicken excess sludge — not maintain a sludge blanket for recycling.
This can allow a somewhat smaller clarifier compared to an activated-sludge plant of the same capacity.

However, MBBR biofilm flocs can be lighter and more dispersed than activated-sludge flocs, which may require coagulant/flocculant addition or a DAF instead of gravity settling in some cases.

Frequently Asked Questions

Can MBBR operate without any clarifier?

Only if the downstream step provides separation (e.g. a membrane or a receiving biological stage that handles the solids). A standalone MBBR without any form of solids separation will not meet typical TSS effluent limits.

Is DAF better than a clarifier for MBBR?

DAF handles light biofilm flocs very well and has a smaller footprint. It is an excellent choice for industrial applications and space-constrained sites. For large municipal plants, gravity clarifiers are often more cost-effective. The choice depends on the specific project.

Does chemical addition help settling of MBBR flocs?

Yes. Adding a coagulant (iron or aluminium salt) or polymer can improve floc aggregation and settling, especially when the biofilm produces dispersed, light flocs. This is more common in high-rate applications and with certain industrial wastewaters.

MBBR + Secondary Clarification

Detailed clarifier design guide.

MBBR + DAF

Flotation after MBBR.

MBBR + Disc Filter

Compact filtration option.

MBBR + Membrane

Membrane bioreactor integration.

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