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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.
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.
'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.
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.
'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.
