MBBR + Secondary Clarification

A pure MBBR has no return sludge, but the effluent still carries suspended solids — sloughed biofilm fragments and influent solids that were not intercepted upstream. A downstream solids-separation step is therefore almost always needed. The traditional option is a gravity secondary clarifier, though DAF and filtration are alternatives depending on the application.

Settling behaviour of MBBR solids

Biofilm-origin solids from an MBBR are different from activated-sludge flocs. They tend to be smaller, more irregular and sometimes lighter, which can make gravity settling less efficient than for conventional activated sludge. The solids characteristics depend on the loading, the carrier type and whether the MBBR is a high-load carbon stage (denser, faster-settling solids) or a low-load nitrification stage (lighter, slower-settling solids).

For this reason, the secondary clarifier for an MBBR may need a lower surface loading rate (i.e. larger area) than one designed for conventional activated sludge. Chemical addition (coagulant/polymer) can improve settling where gravity alone is insufficient. In some projects, DAF or disc/drum filters replace the clarifier entirely.

Aerial view of a wastewater treatment plant showing circular secondary clarifiers alongside rectangular aerated biological basins connected by stainless-steel pipework — Circular gravity clarifiers (left and right) separate biological solids downstream of the aerated basins. Where footprint is tight, compact lamella/plate settlers can achieve the same clarified-water quality in a fraction of the area.

Compact alternative: lamella / plate settlers

Where site footprint is limited, an inclined-plate (lamella) settler can replace a conventional circular clarifier. By stacking many inclined settling surfaces, a plate settler provides a large effective settling area in a compact tank — often a good match for the footprint-conscious philosophy behind MBBR retrofits. Our sister site plate-settler.com covers lamella/plate settler technology in depth, including how inclined-plate separation is applied to biological and industrial solids.

Design considerations

Secondary clarification after MBBR (indicative — confirm per project)

Parameter	Typical Value	Unit	Notes
Solids type	Biofilm fragments	—	Smaller and lighter than AS flocs.
Surface loading	May need to be lower	m³/m²·h	Compared to AS clarifiers.
Chemistry	Coagulant/polymer option	—	Improves settling if needed.
Alternatives	DAF, disc/drum filter	—	For compact or stringent applications.

Frequently Asked Questions

Does a pure MBBR need a secondary clarifier?

It needs a downstream solids-separation step — a secondary clarifier, DAF, or filter. Unlike activated sludge, no sludge is returned; the clarifier only settles and removes solids. Without it, the effluent carries suspended biofilm fragments above most discharge limits.

Why are MBBR solids harder to settle?

Biofilm fragments from an MBBR tend to be smaller and lighter than activated-sludge flocs, especially from low-loaded nitrification stages. The clarifier may therefore need to be larger, or chemical flocculation may be needed to aggregate the solids.

Is return sludge needed from the clarifier?

No — in a pure MBBR there is no sludge return. The clarifier collects and wastes the settled solids. If the process is IFAS (hybrid with suspended sludge), then sludge return is needed just as in conventional activated sludge.

MBBR + DAF

DAF as downstream separation.

MBBR + Disc Filtration

Direct filtration alternative.

MBBR + Tertiary Filtration

Filtration for stringent limits.

Plate / Lamella Settlers (plate-settler.com)

Compact inclined-plate clarification — R+F sister site.

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