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Advanced Membrane Fouling Characterization 129
For similar reason, the use of average permeate flux is not appropriate when assessing the
efficiency of membrane cleaning in the full-scale RO processes. Because of the existence of
the surplus membrane area in the long channel, partial removal of the foulants from the
channel can fully restore the average permeate flux. The failure of the average flux in
assessing cleaning efficiency is often experienced by many plant operators, who observe
that this fully restored average flux cannot be maintained for as long as when new RO
membranes are initially used (23). The duration of fully restored flux becomes shorter in
subsequent cleanings until the initial average flux cannot be fully restored. These observations
suggest that some foulants still remain on the membrane surface after cleaning even
when average flux is fully restored. The use of flux restoration as a measurement of the
effectiveness of membrane cleaning will lead to inaccurate and most likely overestimated
conclusions.
5.2. Filtration Coefficient of a Long Membrane Channel
By definition, membrane fouling always increases the total resistance of the RO membrane
regardless of decline in the average permeate flux. Therefore, the basic requirement of an
effective membrane fouling measurement technique is to reflect the increase in membrane
resistance because it is the most fundamental feature of membrane fouling. For this purpose, a
lumped parameter, the filtration coefficient F (m/Pa s), is introduced to reflect the overall
membrane resistance as
F ¼ 1
ðL 1
dx: (20Þ
L 0 Rm
It can be seen from Eq. (20) that the increase of membrane resistance on any point of the
channel length will cause corresponding decrease in the filtration coefficient F. When there is
membrane fouling in a membrane channel, the value of the filtration coefficient of the
membrane channel will decrease with time. Therefore, the filtration coefficient is an intrinsic
indicator of fouling in a long membrane channel that is independent of average flux. Only in
cases when the membrane channel is controlled by mass transfer, the average permeate flux
can be related to the filtration coefficient by the following equation:
v ¼ FðDp DpÞ: (21Þ
Although the filtration coefficient is virtually a different way to represent the membrane
resistance, its advantages are obvious in fouling characterization of a long membrane channel
over the direct use of the membrane resistance. First, filtration coefficient is a collective
parameter that describes the whole system with a single value while the membrane resistance
is a distributive parameter that varies along the membrane channel. Secondly, it is practically
impossible to measure or determine membrane resistance distribution in the membrane
channel. On contrary, the filtration coefficient of a membrane channel can be easily determined
with a simple filtration test. Conducting filtration experiment at a pressure under the
characteristic pressure of the clean membrane channel, the filtration coefficient can be readily
calculated from the measured average flux and the net driving pressure with Eq. (21).