The biggest operational obstacle to the application
of low-pressure membrane filtration is fouling, i.e.,
the reduction of flux or the increase in transmembrane
pressure (TMP) during operation because of
the accumulation of materials within the membrane
pores or on the surface of the membrane. Fouling
increases rapidly once a "critical permeate flux" has
been exceeded. However, no standard protocols exist
for measuring critical flux or TMP as a function of
coagulation or changing water quality. Such a protocol,
which could provide information analogous to
that provided by jar testing in conventional water
treatment, is necessary. This article describes benchscale
techniques that can be used to measure critical
flux and determine the effects of changing water
quality on membrane performance. The bench-scale
test is also used to demonstrate the effectiveness of
coagulation on the performance of low-pressure
membrane systems.
The concept of critical flux describes the maximum
permeate flux that can be applied without rapid fouling
of the membrane. Critical flux has been described
in a theoretical fashion, but the testing procedure proposed
in this article relies on an operational definition
of critical flux: the highest flux for which there was
only a small linear increase in TMP with time of filtration.
In the experiments presented in this article, fluxes
greater than the critical flux resulted in exponentially
increasing TMP with filtered volume and were labeled
"super-critical." The measured value for critical flux
decreased with increasing time of filtration (for each
step in the critical flux test). An empirical equation
was developed to allow prediction of critical flux for
longer filtration times.
The authors propose that similar bench-scale critical
flux measurements should be used by utilities and consultants
to assist in the design of low-pressure membrane
systems, to modify operation during periods of
changing water quality, and to determine the effects of
coagulation or other treatments on performance of
low-pressure membrane systems. The authors also
propose that water utilities and consultants consider
adopting bench-scale testing procedures for low-pressure
filters, and it is recommended that results are evaluated
using plots of average TMP versus flux, with
different time steps and with extrapolation of results to
longer time steps. This would provide a relatively conservative
estimate of critical flux. Includes 43 references, table, figures.
| Edition : | Vol. 97 - No. 7 |
| File Size : | 1
file
, 320 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 10 |
| Published : | 07/01/2005 |
