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MBR operation & maintenance – fouling, clogging and cleaning

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Image of ragging on an MBR membrane module
Credit: Jim Impero

Fouling, clogging and cleaning

Permeability decline is caused by both membrane fouling and membrane channel clogging:

  • 'fouling' refers to the coating of the membrane surface or the plugging of the membrane pores with dissolved, colloidal or fine solids. It is normally removed by the physical and chemical cleaning cycles.
  • 'clogging' refers to the agglomeration of gross solids within or at the entrance to the membrane channels. Clogging within the channels is sometimes referred to as ‘sludging’.

In municipal wastewater treatment, membranes may sometimes also become clogged with ‘rags’ (or ‘braids’) formed from aggregated filamentous matter (specifically textile fibres such as cotton wool) in the feedwater. This is usually referred to as 'ragging' or 'braiding'. Sludging and ragging are the main causes of unscheduled manual intervention in municipal MBRs.

Figure 1. Permeability decline in MBRs − fouling and clogging Credit: Judd Water & Wastewater Consultants
Figure 1. Permeability decline in MBRs − fouling and clogging
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 cleaning

Membrane cleaning can be by physical or chemical means, and both are employed in MBR operation and maintenance. Physical cleaning is normally achieved either by ‘backflushing’, i.e. reversing the flow back through the membrane, or ‘relaxation’, which is simply ceasing permeation while continuing to scour the membrane with air bubbles (for an immersed process) or the crossflow (for a sidestream one). These two techniques may be used in combination, and backflushing may be enhanced by combination with air.

Chemical cleaning usually employs sodium hypochlorite, an oxidative chemical, in combination with mineral or organic acids (most often citric acid). Cleaning is normally conducted without removing the membrane from the tank or skid (‘cleaning in place’ or CIP). If chemical cleaning is combined with backflushing this is normally referred to as a ‘chemically-enhanced backflush’ (CEB). CEBs are routinely carried out on a weekly/monthly basis for iHF MBRs, and such protocols are normally referred to as ‘maintenance’ cleans.

Physical cleaning removes gross solids attached to the membrane surface, generally termed ‘reversible’ or ‘temporary’ fouling, whereas chemical cleaning removes more tenacious material often termed ‘irreversible’ or ‘permanent’ fouling.

Physical cleaning is less onerous than chemical cleaning in that it is generally more rapid, demands no chemicals, generates no chemical waste, and is less likely to degrade the membrane. However, its effectiveness is limited, meaning that chemical cleaning is always required at some point. Since the original virgin membrane permeability is never recovered once a membrane is fouled through normal operation, there remains a residual resistance which can be defined as ‘irrecoverable fouling’ which may build up over a number of years and so ultimately determine membrane life.

Chemical cleaning employed specifically to recover permeability requires higher reagent concentrations and longer contact times than maintenance cleans. Such ‘recovery cleaning’ is not always effective normally because the membrane has become clogged with sludge solids.

Neither the routine physical clean nor the chemical clean can be expected to counter clogging, which demands manual intervention (removal of the immersed membrane from the tanks) to remove the agglomerated material.


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Hi, recently our plant had hoist up and clean the MBR, but found out the membrane module is coated with greenish things (suspect is algae), is there any solution to solve it?
Airflow, DO, pH and MLSS is in normal range value.
Is top up trisodium phosphate excessive will cause this issue?
Fyi: This membrane is install on July 2015.

Gopi Natesan

I am working with kubato SMU, I am not getting sufficient flow, how to increase permeate flow, kindly advise me. Membrane cleaning purpose 3 options

1. Flushing without water

2. Air scouring

3. Relaxation

No backwash option. How to solve the issue.

Thank you.

Simon JuddGopi Natesan

It's probably clogged, which means chemical cleaning won't work. You'll need to remove the membrane and inspect it for clogging. This is what the above article is about.


We have two MBRs one of them does not run immediately after backwash. The amount of water processed by this MBR is less compared to the other. We changed solenoid valves but it did not help. There is a delay for some reason. Kindly advise.

Jason Deglow

What is most effective CIP process for crossflow MBR at a waste activated sludge plant. Currently we use a 2 wash cycle cleaning mixing chlorine and caustic 1st wash and citric and nitric acid 2nd wash?

Simon JuddJason Deglow

Is this a pumped (sidestream, external) system using a tubula membrane? If so, is it an industrial effluent?

Jason DeglowSimon Judd

Yes and yes

Simon JuddJason Deglow

In that case it will depend on the effluent. The hypo - citric combination would normally be expected to be effective, but if there is a strong hydrophobic element (free oil, for example) then i may require proprietary reagents. Sorry, but can't really offer much more than this.

Jason DeglowSimon Judd

In your opinion Would it be more effective to split the hypochlorite and caustic into there own cycles followed with the acid wash then soak. Or continue with current cycles?

Gopi NatesanJason Deglow

First clean with sodium hypochlorite and soaking in 3 or 4 hours and flushed it then soak with citric acid 3 or 4 pH.

About this page

'MBR operation & maintenance – fouling, clogging and cleaning' was written by Simon Judd

This page was last updated on 07 September 2021


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