NAMS 2002 Workshop - ICOM 2008

membranes were then challenged with different protein formulations (pH, ionic
strength, protein mass) for 5 to 25h depending on the fouling response. During all
tests, the in-situ detection of proteins associating with the membranes used
ultrasonic frequency- domain reflectometry (UFDR) integrated with a fast Fourier
transform protocol to process the signals. Time-domain signals of acoustic scans
from ultrasonic transducers mounted on cross-flow cells were transformed into
amplitude versus frequency distributions. From these distributions, reflected
power was obtained, and standard statistical indices were used to report and
characterize the distributions. Following cross-flow cell tests, membrane samples
were analyzed using ESEM, and the proteins associated with the membranes
were determined using a bicinchoninic protein assay.
Depending on the fouling challenge conditions, permeate flow-rate decreased in
a range between 40-90%. Permeate flow-rate responses, and patterns of
corresponding acoustic reflection power changes, indicated that both internal
membrane and surface deposition occurred, and could be identified as separate
fouling mechanisms during some challenge conditions. The permeate-flow rate
decline data can be described using the combined pore blockage, pore
constriction, and cake filtration model. The best fit model parameters can be
independently obtained based on the property of the feed and membrane
characteristics. In some instances however, transducers were unable to detect
reflected power changes even after significant permeate-flow rate decline
occurred. This phenomenon could be explained by the fact that permeate flowrate
observations are derived from overall membrane behavior, whereas UFDR
is applied in a sentinel format, which reports acoustic responses of small area
(point) observations on a very short timescales. In addition, membrane-
associated protein deposits are visco-elastic, and can (and likely do) reposition
on or through a membrane during the course of a fouling challenge; this can
manifest in a wide variability of reflected power changes as protein density
changes on a local scale during a test. Biochemical assays of protein
concentrations associated with membranes and ESEM micrographs confirmed
that a significant heterogeneity of protein deposition was in part responsible for
the fouling behavior and local density changes observed by UFDR. Where
protein concentration on the membrane varied between 5 to 100µg/cm 2 , ESEM
observations showed non- uniformity of protein deposition in a capricious patchy
array, with a significant amount of clean membrane area exposed; beyond this
range however (100µg/cm 2 ), continuous protein layers were observed on
challenged membrane surfaces.
The use of fouling models in combination with non-invasive, real-time monitoring
provides a unique capability to improve the fundamental understanding and
control of MF membrane fouling by commercially significant proteinaceous
biopolymers.