The MBR Book: Principles and Applications of Membrane

126 The MBR Book
Key design parameters relating to membrane cleaning (Sections 2.1.4.3 and
2.3.9.2) are:
● period between physical cleans (t p), where the physical clean may be either
backflushing or relaxation;
● duration of the physical clean (� p);
● period between chemical cleans (t c);
● duration of the chemical clean (� c);
● backflush flux (J b);
● cleaning reagent concentration (c c) and volume (v c) normalised to membrane
area.
If it can then be assumed that a complete chemical cleaning cycle, which will contain
a number of physical cleaning cycles (Fig. 2.11), restores membrane permeability
to a sustainable level then the net flux J net can be calculated:
J
net
where n is the number of physical cleaning cycles per chemical clean:
tc
n �
t � t
(3.3)
(3.4)
t c and t p may be determined by threshold parameter values, specifically the maximum
operating pressure or the minimum membrane permeability. Note that it is
normally the net flux which is most appropriate to use for energy demand calculations,
and that the pumping energy in Equation (3.1) can be modified for chemical
cleaning down time:
W W tc
h,net � h
t � t
(3.5)
Other costs of chemical cleaning relate to the cost of the chemical reagent itself. The
total mass of cleaning of chemical cleaning reagent is simply the product of volume
and concentration. For a periodic chemical clean in place (CIP), either maintenance or
recovery (Section 2.3.9.2), the specific mass per unit permeate product is simply:
M
c
nJt ( p � Jbtp)
�
t � t
p p
cv c c
�
J A ( t � t )
(3.6)
where A m is the membrane area. If the cleaning reagent is flushed through the membrane
in situ then the volume of cleaning reagent used can be found from:
vc � JcAmtc c c
c c
net m c c
(3.7)