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Feature mbr channel clogging fig
Nocardia foaming in the process tank of an activated sludge plantCredit: ©2012, COLE Publishing Inc.,

MBR challenges

The key challenges associated with MBRs relate largely to the membrane separation component. This makes the MBRs more complex to operate compared to the conventional activated sludge process (CAS).

Sustaining membrane permeability is essential to ensure that the flow capacity of the process is maintained. This becomes problematic if the installed pretreatment is insufficient or ineffective.

Image of ragging on an MBR
Credit: Jim Impero

What can go wrong with an MBR?

As with most other wastewater treatment processes, a design which is not conservative enough can impair the routine operation of the plant and lead to widely recognised technical challenges of membrane surface fouling and, in particular, membrane channel clogging.

What can go wrong in an MBR?
Graphic to illustrate 'Permeability decline in MBRs − fouling and clogging'. Fouling is split into Reversible and Irreversible fouling. Clogging is split into Sludging and Ragging/braiding.
Credit: Judd Water & Wastewater Consultants

Membrane fouling in MBRs

'Fouling' refers to the coating of the membrane surface or the plugging of the membrane pores with dissolved, colloidal or fine solids.

This is normally removed by the physical and chemical cleaning cycles and so does not usually demand the removal of the membranes for ex-situ manual cleaning.

Membrane fouling in MBRs

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Foaming sludge

Foaming in MBRs

Foam, sometimes called 'floating sludge', can form in both CAS and MBR plants and has a number of undesirable impacts.

There are two types of foam: chemical, or biochemical, foam that can usually be dispersed by antifoaming agents, and biological foams that are more stable and demand more extensive remedial measures.

Foaming in membrane bioreactors
Pictures of the ragged membranes. A, Top of the cassette. B, inside the cassette

Clogging: sludging and ragging (braiding)

'Clogging' describes problems associated with the agglomeration of the mixed liquor solids. There are two types of clogging: sludging and ragging (or braiding).

'Sludging' is when the solids block the membrane channels, impeding the passage of air and mixed liquor. 'Ragging' is the formation of long lengths of rag-like macroparticles.

Clogging: sludging and ragging (braiding)

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