An assessment of the Silt Density Index based on RO membrane ...

S. G. Yiantsios, A.J. Karabelas / Desalination I51 (2002) 229-238 231
recommendations [ 10,111. Fouling data are obtained
by continuous introduction <strong>of</strong> ferric ions
into <strong>the</strong> feed water and spontaneous precipitation
<strong>of</strong> colloidal iron hydrous oxide. In <strong>the</strong> following
section <strong>the</strong> experimental materials and methods
are described. Then <strong>the</strong> RO fouling data obtained
are presented and discussed. Finally, SD1
measurements with <strong>the</strong> same feed water and
various iron concentrations are presented and an
<strong>assessment</strong> is made <strong>of</strong> <strong>the</strong> performance <strong>of</strong> SD1 in
light <strong>of</strong> <strong>the</strong> fouling data obtained.
2. Experimental materials and methods
RO fouling experiments are performed in a
laboratory unit with a narrow-channel type <strong>of</strong> test
section using flat-sheet membrane pieces with an
active filtration area <strong>of</strong> 130 cm*. A schematic
diagram <strong>of</strong> <strong>the</strong> experimental set-up is shown in
Fig. 1. A 30-L vessel contains <strong>the</strong> feed solution
which is pressurized via a Grundfos CRNE 2
pump, capable <strong>of</strong> reaching pressures up to
300 psi. Pressure and cross-flow rate are
monitored through digital sensors and controlled
through two needle valves at <strong>the</strong> entrance and
exit <strong>of</strong> <strong>the</strong> test section. Permeate flow is
monitored through a Humonics 1000 digital
flowmeter connected to a PC for automatic data
acquisition. Temperature is controlled in <strong>the</strong>
system through a cooling coil connected to a
water cooler. A controller and a flow control
valve maintain constant temperature in <strong>the</strong> feed
vessel <strong>of</strong> 25rtO. l°C. The system can run virtually
unattended for long periods <strong>of</strong> time. Experimental
runs last typically for about 1 week.
Initially, efforts were made to obtain fouling
data similar to those presented by Cohen and
Probstein [9] by employing monodisperse iron
oxide colloidal particles, 150 nm in diameter,
prepared according to <strong>the</strong> procedures described in
Scheiner and Matijevic [12]. However, several
problems related to <strong>the</strong> large particle size and
aggregation effects as well as difficulties to
maintain constant particle concentration in <strong>the</strong>
closed loop system were encountered. Thus, as in
Cohen and Probstein, results could be obtained
only with almost desalted water and insignificant
fouling rates were observed. It is noted
paren<strong>the</strong>tically that, in view <strong>of</strong> <strong>the</strong> above, <strong>the</strong>ir
“critical flux” observations must be viewed with
skepticism. In <strong>the</strong> experiments presented herein
a different route was followed. <strong>An</strong> open loop
operation was established by continuous
introduction and withdrawal <strong>of</strong> water and foulant
in <strong>the</strong> system. Instead <strong>of</strong> introducing particles
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Fig. 1. Schematic diagram <strong>of</strong> <strong>the</strong> experimental set-up used in <strong>the</strong> RO fouling experiments.
Drain
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