The MBR Book: Principles and Applications of Membrane

Fundamentals 35
2.1.4.6 Critical flux
The critical flux concept was originally presented by Field et al. (1995). These authors
stated that: “The critical flux hypothesis for microfiltration/ultrafiltration processes is
that on start-up there exists a flux below which a decline of flux with time does not
occur; above it, fouling is observed”. Two distinct forms of the concept have been
defined. In the strong form, the flux obtained during sub-critical flux is equated to the
clean water flux measured under the same conditions. However, clean water fluxes are
rarely attained for most real feedwaters due to irreversible adsorption of some solutes.
In the alternative weak form, the sub-critical flux is the flux rapidly established and
maintained during start-up of filtration, but does not necessarily equate to the clean
water flux. Alternatively, stable filtration operation, that is, constant permeability for
an extended time period, has been defined as sub-critical operation even when preceded
by an initial decline in flux (Howell, 1995). Such conditions would be expected
to lead to lower critical flux values than those obtained for constant permeability operation,
however, since an initial permeability decline implies foulant deposition.
A number of slightly different definitions of sub-critical flux operation have been
proposed, largely depending on the method employed. The most microscopically precise
definition equates the critical flux to that flux below which no deposition of colloidal
matter takes place. Kwon and Vigneswaran (1998) equated critical flux to the lift
velocity as defined by the lateral migration theory of Green and Belfort (1980). This
rigorous definition is difficult to apply because of the relative complexity of the determination
of the lift velocity, particularly for heterogeneous matrices. On the other
hand, experimental determination of critical flux by direct observation of material
deposition onto the membrane has been conducted using model homodispersed suspensions
of polystyrene latex particles (Kwon and Vigneswaran, 1998), and some
authors have also used mass balance determinations (Kwon et al., 2000).
Given the limitations of applying particle hydrodynamics to the identification of
the critical flux in real systems, recourse generally has to be made to experimental
determination. By plotting flux against the TMP it is possible to observe the transition
between the linearly pressure-dependent flux and the onset of fouling, where
deviation from linearity commences. The flux at this transition has been termed “secondary
critical flux” (Bouhabila et al., 1998) and, more recently, the concept of “sustainable
flux” has been introduced, defined as the flux for which the TMP increases
gradually at an acceptable rate, such that chemical cleaning is not necessary (Ng
et al., 2005).
Whilst potentially useful in providing a guide value for the appropriate operating
flux, the absolute value of the critical flux obtained is dependent on the exact
method employed for its determination and, specifically, the rate at which the flux is
varied with time. A common practice is to incrementally increase the flux for a fixed
duration for each increment, giving a stable TMP at low flux but an ever-increasing
rate of TMP increase at higher fluxes (Fig. 2.13). This flux-step method defines the
highest flux for which TMP remains stable as the critical flux. This method is preferred
over the corresponding TMP-step method since the former provides a better
control of the flow of material deposition on the membrane surface, as the convective
flow of solute towards the membrane is constant during the run (Defrance and Jaffrin,
1999). No single protocol has been agreed for critical flux measurement, making