The MBR Book: Principles and Applications of Membrane

32 The MBR Book
Physical Chemical
Base
Caustic soda
Citric/oxalic
Backflushing
with air
without air
Relaxation
Chemically
enhanced
backwash
Figure 2.10 Membrane cleaning methods
Acids
Hydrochloric/sulphuric
Citric/oxalic
Oxidant
Hypochlorite
Hydrogen peroxide
have been proposed for particular matrices and/or applications. Classical dead-end filtration
models can be adapted for crossflow operation if the proportion of undeposited
solute material can be calculated.
2.1.4.3 Physical and chemical cleaning
Since the flux and driving force are interrelated, either one can be fixed for design
purposes. For conventional pressure-driven water filtration, it is usual to fix the value
of the flux and then determine the appropriate value for the TMP. The main impact
of the operating flux is on the period between cleaning, which may be by either physical
or chemical means (Fig. 2.10). In MBRs physical cleaning is normally achieved
either by backflushing, that is, reversing the flow, or relaxation, which is simply ceasing
permeation whilst continuing to scour the membrane with air bubbles. These two
techniques may be used in combination, and backflushing may be enhanced by combination
with air. Chemical cleaning is carried out with mineral or organic acids, caustic
soda or, more usually in MBRs, sodium hypochlorite, and can be performed either
in situ (“cleaning in place” or CIP) or ex situ (Section 2.3.9.2). Alternatively, a low
concentration of chemical cleaning agent can be added to the backflush water to produce
a “chemically enhanced backflush” (CEB).
Physical cleaning is less onerous than chemical cleaning on a number of bases. It
is generally a more rapid process than chemical cleaning, lasting no more than
2 min. It demands no chemicals and produces no chemical waste, and also is less
likely to incur membrane degradation. On the other hand, it is also less effective than
chemical cleaning. 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, which is obviously something of a misnomer. 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”. It is this fouling which builds up over a number of years and ultimately
determines membrane life.