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

DESALINATION
ELSEVIER Desalination 15 l(2002) 229-238
www.elsevieccom/locate/desal
<strong>An</strong> <strong>assessment</strong> <strong>of</strong> <strong>the</strong> <strong>Silt</strong> <strong>Density</strong> <strong>Index</strong> <strong>based</strong> on RO membrane
colloidal fouling experiments with iron oxide particles
Stergios G. Yiantsios, <strong>An</strong>astasios J. Karabela.s*
Chemical Process Engineering Research Institute and Department <strong>of</strong> Chemical Engineering,
Aristotle University <strong>of</strong> Thessaloniki, PO Box 361, 570 01, Thermi, Thessaloniki, Greece
Tel. +30 (310) 996201; Fax +30 (310) 996209; email: karabaj@cperi.certh.gr
Received 9 July 2002; accepted 3 1 July 2002
Abstract
RO membrane colloidal fouling experiments were performed in <strong>the</strong> laboratory under well controlled and realistic
conditions. Iron oxide was selected as atypical inorganic colloidal foulant, due to its importance, as evidenced from well
known manufkcturer recommendations on iron concentrations in feed waters and from frequently encountered problems
in membrane installations. A range <strong>of</strong> iron concentrations was identified where a linearrelationship existed between flux
reduction rate and concentration. The performance <strong>of</strong> <strong>the</strong> <strong>Silt</strong> <strong>Density</strong> <strong>Index</strong> (SDI) was tested on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> RO
fouling data obtained. The range <strong>of</strong> iron concentrations where measurable and meaningful SD1 values could be obtained
was remarkably close to membrane manufacturer recommendations. A notable sensitivity <strong>of</strong> <strong>the</strong> SD1 was also observed
with particles for which retention is negligible. However, on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> RO fouling data obtained, it appears that
<strong>the</strong> SD1 is not conservative enough. Fur<strong>the</strong>rmore, since <strong>the</strong> SD1 cannot predict fouling rates, it cannot discriminate
between different types <strong>of</strong> membranes.
Kqwor&:
RO membrane colloidal fouling; Iron oxide colloidal particles; <strong>Silt</strong> <strong>Density</strong> <strong>Index</strong>
1. Introduction
One <strong>of</strong> <strong>the</strong> most serious concerns regarding
desalination membrane performance is <strong>the</strong>
presence <strong>of</strong> colloidal particles in <strong>the</strong> feed waters
and <strong>the</strong> associated fouling problems. The most
commonly occurring inorganic colloids in natural
waters are aluminum silicate clays, ranging in
*Corresponding author.
size between 0.3 to 1 pm, and colloids <strong>of</strong> iron,
aluminum and silica. Organic substances dissolved
in natural waters, such as humic and fulvic
acids, may be also characterized to a certain
degree as colloidal foulants since many features
<strong>of</strong> <strong>the</strong>ir behavior fall in <strong>the</strong> domain <strong>of</strong> colloid
science and are common with inorganic particles.
Organic deposits, toge<strong>the</strong>r with iron oxide, silica
and aluminum oxide, account for more than 70%
00 1 l-91 64/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved
PII:SOOll-9164(02)01015-9

230 S. G. Yiantsios, A.J. Karabelas /Desalination 151 (2002) 229-238
<strong>of</strong> <strong>the</strong> deposits detected in membrane autopsies
throughout <strong>the</strong> world [l]. The issues, <strong>the</strong>refore,
<strong>of</strong> prediction and mitigation <strong>of</strong> colloidal and
organic fouling remain critical after many years
<strong>of</strong> successful application <strong>of</strong> membrane technologies
to a variety <strong>of</strong> water sources including
well and surface, brackish and seawater, as well
as wastewater <strong>of</strong> various origins.
The most common and widely accepted tool
for prediction <strong>of</strong> colloidal fouling is <strong>the</strong> <strong>Silt</strong>
<strong>Density</strong> <strong>Index</strong> (SDI). O<strong>the</strong>r proposed indices are
<strong>the</strong> Modified Fouling <strong>Index</strong> (MFI), which was
developed in <strong>the</strong> Ne<strong>the</strong>rlands [2] and gained
acceptance mainly <strong>the</strong>re, and <strong>the</strong> Mini Plugging
Factor <strong>Index</strong> (MPFI ). All three are <strong>based</strong> on
batch filtration <strong>of</strong> feed waters through a 0.45 pm
Millipore micr<strong>of</strong>ilter. More recently, <strong>the</strong> UF-
MFI was introduced which uses ultrafiltration
membranes instead <strong>of</strong> <strong>the</strong> Millipore micr<strong>of</strong>ilter
[3,4]. O<strong>the</strong>r measurements such as turbidity,
particle counts, and particle electrophoretic
mobility are also used but are not considered
reliable criteria for <strong>assessment</strong> <strong>of</strong> fouling trends.
Membrane manufacturers recommend that SD1
should not exceed 4 or 5 and set limits <strong>of</strong> membrane
productivity depending on <strong>the</strong> SD1 [5] (i.e.,
8-14 gfd for SD1 2-4, 14-18 gfd for SD1 ~2 and
20-30 gfd for SD1 cl). It should be emphasized
that <strong>the</strong> SD1 test is, strictly speaking, an empirical
one and a very poor simulation <strong>of</strong> actual RO
conditions. The absence <strong>of</strong> actual membranefoulant
interactions, <strong>the</strong> absence <strong>of</strong> shear (as
opposed to <strong>the</strong> cross-flow operation <strong>of</strong> membrane
modules), <strong>the</strong> significantly higher permeation
rate (>l cm/s in <strong>the</strong> filter vs. c.001 cm/s in RO
membranes), and <strong>the</strong> doubtful rejection <strong>of</strong>
particles smaller than 0.45 pm (nominal pore size
<strong>of</strong> <strong>the</strong> filter) are <strong>the</strong> most important concerns.
These poorly rejected particles are more likely to
cause severe fouling to a membrane since <strong>the</strong>
resistance <strong>of</strong> a cake layer is inversely proportional
to <strong>the</strong> square <strong>of</strong> particle size. Schippers et
al. [6] state that particles smaller than 0.05 pm
are responsible for flux decline in RO mem-
branes. <strong>An</strong>o<strong>the</strong>r criticism is that <strong>the</strong> index is not
linearly related to colloid particle concentration
and does not distinguish between fouling
mechanisms during <strong>the</strong> test (pore blocking, cake
filtration, cake consolidation).
Despite <strong>the</strong> significance <strong>of</strong> colloidal fouling
problems in desalination, <strong>the</strong>re are relatively few
experimental studies at laboratory scale that
represent typical water sources, realistic foulants
and well controlled conditions at <strong>the</strong> same time.
Most studies use syn<strong>the</strong>tic waters <strong>of</strong> simple
composition (e.g., distilled water to which NaCl
is added) and model colloidal particles, which
cannot be considered typical <strong>of</strong> <strong>the</strong> foulants
encountered in practice [7-91. A second related
problem is <strong>the</strong> duration <strong>of</strong> <strong>the</strong> laboratory
experimental studies. First <strong>of</strong> all, long times are
required in order to obtain a stable membrane
flux under non-fouling conditions due to phenomena
referred to as “membrane setting”. In
addition, due to <strong>the</strong>ir high resistance to water
flow, RO membranes are not very sensitive to
colloidal fouling, in <strong>the</strong> sense that relatively thick
deposits <strong>of</strong> <strong>the</strong> typically employed particles
(which usually exceed 100 nm in diameter) need
to be developed in order to observe a detectable
flux reduction. This necessitates long experimental
times and accelerated fouling conditions
with unrealistically high particle concentrations
(200 ppm or more). It may also be added that in
a typically closed-loop laboratory system it is
difficult to maintain constant conditions due to
<strong>the</strong> limited stock <strong>of</strong> fouling particles and to
possible aggregation phenomena.
The present experimental efforts include two
steps: first, to generate reliable RO fouling data
under realistic, yet, well controlled conditions;
second, on <strong>the</strong> basis <strong>of</strong> such results, to assess <strong>the</strong>
performance <strong>of</strong> existing techniques for colloidal
fouling prediction (i.e., SDI, MFI). Iron oxide
was selected as a typical inorganic colloidal
foulant due to its importance as evidenced by <strong>the</strong>
frequent appearance in membrane autopsies [ 11,
and <strong>the</strong> specific reference in manufacturers’

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
A
FeC$ tank Acid tank
nrl
Sand
Filter
t
Cartridge
Filter
Fig. 1. Schematic diagram <strong>of</strong> <strong>the</strong> experimental set-up used in <strong>the</strong> RO fouling experiments.
Drain
Reject

232 S.G. Yiantsios. A.J. Karabelas / Desalination 151 (2002) 229-238
previously formed, a FeCl, solution <strong>of</strong> appropriate
concentration was injected into <strong>the</strong> feed
stream at a rate <strong>of</strong> 10 mL/min and pH was
adjusted to 7. The feed stream flow rate was
1.8 Wmin. At a pH close to neutral, iron oxide
particles are spontaneously formed and constant
fouling conditions are established regardless <strong>of</strong>
colloid aggregation phenomena. <strong>An</strong> important
consideration is to maintain iron in <strong>the</strong> stock
solution in dissolved form. According to Stumm
and Morgan [ 131, <strong>the</strong> following reactions take
place:
Fe3’ + H 2
0 f) FeOH’+ + H’ (14
Fe3’+ 2H,O c) Fe(OH): + 2H’
2Fe3’ + 2H,O f) Fe,(OH),4’ + 2H’
Fe3++ 3H,O f) Fe(OH),(aq) + 3H’
Fe3’ + 4H,O f) Fe(OH)i + 4H’
3Fe3’+ 2H20 t, Fe,(OH),4’ + 2H’
3Fe3’+ 4H,O f) Fe,(OH),S’ + 4H’
Fe(OH),(s)+ 3H’ f) Fe3++ 3H,O
(lb)
(lc)
(ld)
(Ie)
(lf)
(lg)
(lh)
Using <strong>the</strong> equilibrium constants listed in [ 121,
<strong>the</strong> pH necessary to avoid Fe(OH), precipitation
in <strong>the</strong> FeCl, stock solution can be calculated. In
<strong>the</strong> present experiments <strong>the</strong> pH <strong>of</strong> <strong>the</strong> stock
solution was maintained close to 2 by adding
appropriate amounts <strong>of</strong> HCl.
It is frequently mentioned in <strong>the</strong> literature that
a certain period <strong>of</strong> time is required for membrane
setting, which refers to membrane adjustment to
<strong>the</strong> pressure applied and to <strong>the</strong> ions present in <strong>the</strong>
feed water. During this period, flux drops and
rejection increases. <strong>An</strong> additional factor is <strong>the</strong>
influence <strong>of</strong> preservatives to which <strong>the</strong> membrane
is soaked for storage. These may cause flux
changes as <strong>the</strong>y leach, and <strong>the</strong> problem is more
Table 1
<strong>An</strong>alysis <strong>of</strong> <strong>the</strong> feed water used in <strong>the</strong> RO fouling
experiments
Physicochemical
PH
Conductivity, pS/cm
P-alkalinity
Total hardness, “F
Carbonate, “F
M-alkalinity
parameters
Chemical parameters, mg/L
Cations:
Na’
K’
Ca*+
MgZ
Fe*+
Zn’+
Mn2+
S13
<strong>An</strong>ions:
Cl-
HCO;
so:-
NO;
NO;
PO:-
F-
Neutral:
co2
SiO,
NaOCl
7.6
680
0
22.8
21.5
4.3
50.3
1.2
47.5
26.5
0.03
0.19
0.01
0.47
55.1
262.3
39.8
7.5
0
0.01
0.17
15
21.1
0.1
likely to occur in <strong>the</strong> typically closed laboratory
systems. In <strong>the</strong> present experiments, all membranes
were soaked in distilled water before use
and permeate and retentate were discarded.
During <strong>the</strong> initial operating periods <strong>of</strong> several
hours, flux changes were very severe and much
larger than any anticipated effect <strong>of</strong> colloidal
fouling. This behavior was found to be largely
independent <strong>of</strong> <strong>the</strong> type <strong>of</strong> feed water used (tap
water, sand filtered tap water, de-ionized water,
Nanopure water, RO water). Typically, 48 h or

S.G. Yiantsios, A.J. Karabelas / Desalination 151 (2002) 229-238 233
more were dedicated to membrane setting.
Although flux drop was not completely eliminated,
it was reduced to about 0.2% per hour or
less, which is acceptable provided that higher
fouling rates can be manifested after <strong>the</strong>
introduction <strong>of</strong> <strong>the</strong> fouling species.
The type <strong>of</strong> membrane employed was a flatsheet
brackish-water RO membrane (Osmonics
AG). In all <strong>the</strong> experimental runs described in <strong>the</strong>
next section, a constant cross-flow rate <strong>of</strong>
0.5 L/min was maintained in <strong>the</strong> channel, which
corresponds to an average velocity <strong>of</strong> 17 cm/s.
This is in <strong>the</strong> range <strong>of</strong> typical cross-flow
velocities encountered in spiral-wound RO membrane
elements. The feed water flow rate in <strong>the</strong>
feed vessel was 1.8 L/min, and thus <strong>the</strong> mean
residence time <strong>of</strong> water and fouling species in <strong>the</strong>
system was about 15 min. Feed water passed
through a sand filter and a cartridge filter to reach
an SDI close to 2 and a turbidity typically less
than 0.1 NTU, whereas raw water has an SD1
close to 4 and a turbidity close to 1 NTU. <strong>An</strong>
analysis <strong>of</strong> <strong>the</strong> feed water is shown in Table 1.
2
E
3
5
a
..j
FeCl,addlthm : )
0 5 10 15 20 25 30 35
t Pf
10 I I , I
j
3. Results and discussion
3.1. RO fouling measurements
Typical raw data obtained in an experimental
sequence, which included pH adjustment, membrane
setting and fouling species introduction are
shown in Fig. 2.
Membrane flux prior to fouling species
introduction was kept close to 8~10~~ cm/s (or
28 LMH or 17 gfd), which is representative <strong>of</strong>
membrane fluxes typically encountered in RO
applications. As can be observed, a clearly
detectable decline <strong>of</strong> permeation rate, linear in
time, is obtained in <strong>the</strong> iron concentration range
<strong>of</strong> a few ppm. On <strong>the</strong> basis <strong>of</strong> such raw data,
normalized flux decline data can be obtained
prior to and after fouling species introduction, as
shown in Fig. 3. The difference in flux decline is
attributed to <strong>the</strong> fouling effects <strong>of</strong> <strong>the</strong> iron oxide
particles.
4
I I I
0 10 20 30 40 50 60
Fig. 2. Raw experimental data <strong>of</strong> permeate flow rate as a
function <strong>of</strong> time before and after fouling species introduction.
(a) Iron concentration: 5 ppm. (b) Iron concentration:
3.5 ppm.
Normalized flux reduction data (in percentage
reduction per hour) vs. iron concentration in <strong>the</strong>
feed water are shown in Fig. 4. These data are
considered representative <strong>of</strong> fouling rate variation
with iron concentration. As can be observed,
a linear relationship between iron concentration
and fouling rate is obtained with a satisfactory
degree <strong>of</strong> confidence. Experiments performed at
higher iron concentrations (10 and 20 ppm)
showed fouling rates similar to those obtained at
t W

234 S. G. Yiantsios. A.J. Karabelas /Desalination 151 (2002) 229-238
-y = lOO- 0.56x R= 0,79
y-100-3,84x R=O,99
+ 0,61x R= 0.98
1
I
,_,’
_,’
_,_’
,.
_,.’
_,”
,/
,I
70
0 2 4 6 8 10 12 14
105
t(h)
-y=lOO-0,22x
R=O.89
y= 100-2.02x R=0,99
Fig. 3. Normalized membrane flux data as a function <strong>of</strong>
time before and after fouling species introduction.
(a) Iron concentration: 5 ppm. (b) Iron concentration:
3.5 ppm.
5 ppm, and thus <strong>the</strong> concentration <strong>of</strong> 5 ppm is
considered an upper limit for linearity to hold
under <strong>the</strong> present experimental conditions (i.e.,
water pH, ionic strength and residence time in <strong>the</strong>
system).
It is clear that <strong>the</strong> present experimental results
represent accelerated fouling conditions since
iron concentrations are far above <strong>the</strong> recommended
values for membrane feed waters. Such
[Fe”1(ppm)
3 4
Fig. 4. Normalized membrane flux reduction rate as a
function <strong>of</strong> iron concentration in <strong>the</strong> feed water.
accelerated conditions are a necessity in laboratory
investigations due to time and resource
limitations. However, it is proposed that <strong>the</strong>
linear relationship found between fouling rate
and iron concentration can be extrapolated to
lower and more realistic concentrations, and here
lies <strong>the</strong> usefulness <strong>of</strong> <strong>the</strong> data obtained. For this
to be true two conditions need to be met. First,
iron precipitation at <strong>the</strong> chosen conditions (i.e.,
pH) must be complete down to lower concentrations,
and precipitation kinetics must be much
faster than <strong>the</strong> macroscopic characteristic times
in <strong>the</strong> system, i.e., iron residence time in <strong>the</strong> test
loop. Second, aggregation phenomena <strong>of</strong> <strong>the</strong>
precipitated crystallites, due to Brownian motion
or shear, must have a small effect on fouling,
namely on deposition rates <strong>of</strong> <strong>the</strong> particles on <strong>the</strong>
membrane and on deposit structure.
Regarding precipitation <strong>the</strong>rmodynamics,
according to <strong>the</strong> reactions given in <strong>the</strong> previous
section and <strong>the</strong> respective equilibrium constants
[13], <strong>the</strong> solubility <strong>of</strong> ferric ions at pH 7 is
estimated to be 5.9~10-‘~ moles/L. Therefore,
even at a total concentration <strong>of</strong> 1 ppb, almost all
iron will be in precipitated form. Regarding
kinetics, it may be recalled that <strong>the</strong> limiting step
for iron oxide particle formation in membrane
5

S.G. Yiantsios, A.J. Karabelas / Desalination 151 (2002) 229-238 235
feed waters is considered to be <strong>the</strong> oxidation <strong>of</strong>
<strong>the</strong> soluble ferrous ion to ferric. Thus, precipitation
kinetics is assumed here to be fast relative
to <strong>the</strong> iron residence times in <strong>the</strong> present
experiments.
Regarding <strong>the</strong> effects <strong>of</strong> aggregation on
deposition rates and deposit structure, an indirect
piece <strong>of</strong> evidence is presented below. Considering
a cake filtration mechanism, an estimate <strong>of</strong>
<strong>the</strong> characteristic particle size, a, may be
obtained as follows. Membrane flux as a function
<strong>of</strong> time is given by
J- -- A’
where AP is <strong>the</strong> applied pressure, R, <strong>the</strong> membrane
resistance and R, <strong>the</strong> fouling cake resistance.
The latter is given by
R, = R’, H (W
where
R’ = 2 (pap
c
and
2 a2(l-cp,Y
t
H = s
J(p,lcp,dt
0
(3b)
(3c)
Here, R, is <strong>the</strong> specific cake resistance, H <strong>the</strong>
cake thickness, qc <strong>the</strong> cake volume fraction, (Pi
<strong>the</strong> foulant bulk volume fraction, and p <strong>the</strong> water
viscosity. Combining <strong>the</strong> above equations and
differentiating with respect to time, we find that
<strong>the</strong> initial rate <strong>of</strong> flux decline is given by
q/Jo)
= _ R’c Jo % = _ %AJc~Jo
dt t=O %% 2~2%(1-%~ ,.\
(4)
where Jo is <strong>the</strong> initial membrane flux. The
parameters Jo and R,,, in <strong>the</strong> above equation can
be determined from <strong>the</strong> experimental measurements.
The initial decline rate found in Fig. 4 is
used. A typical value <strong>of</strong> 0.64 is used for cp=,
whereas (Pi is estimated from <strong>the</strong> iron concentration
assuming a density <strong>of</strong> 4 g/cm3 for <strong>the</strong> iron
oxide particles. Then, Eq. (4) can be solved for a
to obtain an estimate <strong>of</strong> 3 nm for <strong>the</strong> characteristic
particle radius. Such a size may be
expected to be characteristic <strong>of</strong> primary precipitated
crystallites ra<strong>the</strong>r than aggregates since
values <strong>of</strong> <strong>the</strong> same order <strong>of</strong> magnitude have been
obtained by low-angle x-ray scattering measurements
[ 141. It must be emphasized that <strong>the</strong> above
exercise is performed to provide only an order <strong>of</strong>
magnitude size estimate since several assumptions
have been made. In addition, iron oxide
crystallites resemble elongated ellipsoids ra<strong>the</strong>r
than spheres considered here. This size estimate
for <strong>the</strong> particles forming <strong>the</strong> fouling deposit does
not necessarily mean that aggregation effects are
absent. Indeed, it was observed by <strong>the</strong> naked eye
that settleable floes were formed at an iron
concentration <strong>of</strong> 10 ppm within a period <strong>of</strong> 1 h.
Ra<strong>the</strong>r, it suggests two things. First, that porosity
<strong>of</strong> <strong>the</strong> deposits does not dramatically change by
aggregation effects or that it is controlled by
pressure and cake consolidation. Second, that any
effects <strong>of</strong> reduced deposition efficiency <strong>of</strong> large
size aggregates or any particle detachment effects
are not significant.
Efforts to obtain more precise information on
aggregation kinetics by a dynamic light scattering
technique were not successful due to <strong>the</strong> small
scattering intensity <strong>of</strong> <strong>the</strong> colloidal dispersions in
<strong>the</strong> range <strong>of</strong> iron concentrations used in <strong>the</strong>
fouling experiments. In general, such measurements
are difficult with very small particles due
to <strong>the</strong> conflicting requirements <strong>of</strong> <strong>the</strong> technique
such as sufficient scattering and sufficiently slow
aggregation kinetics compared to <strong>the</strong> time
required for a particle size distribution measurement,
which is roughly 1 min.

236 S.G. Yiantsios, A. J. Karabelas /Desalination 151 (2002) 229-238
3.2. SDI measurements
It is interesting to examine <strong>the</strong> predictions or
recommendations that would be obtained by SD1
measurements if <strong>the</strong> same water were to be used
as RO feed water. For this task water was
prepared in <strong>the</strong> same way as for <strong>the</strong> fouling
experiments and ferric ions were added at various
concentrations. SD1 measurements were performed
according to <strong>the</strong> standard conditions, that
is, by use <strong>of</strong> 0.45 pm pore size Millipore filter
disks at a pressure <strong>of</strong> 30 psi. At a concentration
<strong>of</strong> 1 ppm almost complete plugging <strong>of</strong> <strong>the</strong> filter
occurred within <strong>the</strong> first 2 min <strong>of</strong> <strong>the</strong> test. The
upper limit <strong>of</strong> concentrations where measurable
and meaningful SD1 values could be obtained is
approximately 20 ppb, as can be seen from Fig. 5.
Several interesting observations may be made
by examination <strong>of</strong> <strong>the</strong> results in Figs. 4 and 5.
First <strong>of</strong> all, it may be recalled that manufacturers’
recommendations for membrane feed waters
under conditions favoring oxidation are that
dissolved iron concentration should be less than
50 ppb. It may also be mentioned that fouling
problems due to iron have been reported with NF
membranes at even lower concentrations, and,
<strong>the</strong>refore, this limit could be considered tentative
[ 151. The upper limit <strong>of</strong> roughly 20 ppb for
$3
2
1
.;
+
b
f .
/
c
0 ~~.~_~.~~_..~~~~ _--t..“_-.-__._-.. _-.._I
lFe3’~kW
15 20
Fig. 5. SD1 as a function <strong>of</strong> iron concentration in <strong>the</strong> feed
water.
precipitated iron obtained by <strong>the</strong> present SD1
measurements is not only remarkably close to
manufacturers recommendations, but also on <strong>the</strong>
correct side. A second observation is that <strong>the</strong>re is
a notable sensitivity to particles much smaller
that <strong>the</strong> pore size <strong>of</strong> <strong>the</strong> filter employed in <strong>the</strong>
SD1 test. In this case, <strong>the</strong> observed filter fouling
is more likely due to pore restriction and
plugging ra<strong>the</strong>r than to cake formation. One
might recall that a small change in pore diameter
results in significant changes in pressure drop,
<strong>the</strong> latter being proportional to pore diameter
raised to <strong>the</strong> fourth power. Thus, despite <strong>the</strong>
negligible particle retention, significant effects
are observed due to pore restriction and plugging.
Finally, from <strong>the</strong> linear relationship obtained
in Fig. 4 on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> present RO fouling
data, it may be projected that a similar membrane
operating under similar conditions would exhibit
a 6% drop in productivity within less than 45 or
30 days, if <strong>the</strong> feed water contained 10 ppb or
15 ppb <strong>of</strong> iron; <strong>the</strong>se concentrations correspond
to SD1 values close to 3 or 4, respectively, which
are within <strong>the</strong> range <strong>of</strong> recommended values.
However, <strong>the</strong> above rates <strong>of</strong> flux decline are
ra<strong>the</strong>r severe, and hence, under <strong>the</strong> conditions <strong>of</strong>
<strong>the</strong> present tests, <strong>the</strong> SD1 does not appear to be
conservative enough. Moreover, <strong>the</strong> SD1 cannot
predict fouling rates, and, hence, it cannot
discriminate between different types <strong>of</strong> membranes.
For example, under <strong>the</strong> same conditions,
an NF membrane would be expected to foul more
rapidly and a seawater membrane to be less
affected since <strong>the</strong>ir resistances are different
compared to that <strong>of</strong> <strong>the</strong> fouling cake. It is,
<strong>the</strong>refore, suggested that an improvement <strong>of</strong> <strong>the</strong>
SDI, in <strong>the</strong> direction <strong>of</strong> predicting fouling rates
and discriminating between various membranes
is needed.
4. Conclusions
In <strong>the</strong> present study iron oxide was selected as
a typical inorganic colloidal foulant due to its

S.G. Yiantsios. A.J. Karabelas / Desalination 151 (2002) 229-238 237
importance, as evidenced from well known
manufacturers’ recommendations on iron concentration
in feed waters and from frequently
encountered problems in membrane installations.
Fouling data were obtained by continuous
introduction <strong>of</strong> ferric ions in <strong>the</strong> feed water and
spontaneous precipitation <strong>of</strong> colloidal iron
hydrous oxide. Thus, <strong>the</strong> experimental procedure
followed simulates phenomena taking place in
actual membrane installations and only skips <strong>the</strong>
oxidation step <strong>of</strong> dissolved ferrous ions. A range
<strong>of</strong> iron concentrations was identified where a
linear relationship existed between flux reduction
rate and concentration under <strong>the</strong> experimental
conditions employed. Although <strong>the</strong> results
obtained represent accelerated fouling conditions,
it is proposed that <strong>the</strong> linear relationship can be
extrapolated to lower and more realistic concentrations.
The upper limit <strong>of</strong> <strong>the</strong> concentration
range where measurable and meaningful SD1
values could be obtained was roughly 20 ppb,
which is remarkably close to membrane
manufacturers’ recommendations. A notable
sensitivity <strong>of</strong> <strong>the</strong> SD1 was also observed with
particles for which retention is negligible, due
mainly to filter pore restriction and plugging.
However, on <strong>the</strong> basis <strong>of</strong> <strong>the</strong> RO fouling data
obtained, it may be projected that <strong>the</strong> SD1 is not
conservative enough. Fur<strong>the</strong>rmore, since <strong>the</strong> SD1
cannot predict fouling rates, it cannot discriminate
between different types <strong>of</strong> membranes (i.e.,
NF vs. high pressure RO). It is, <strong>the</strong>refore,
suggested that an improvement <strong>of</strong> <strong>the</strong> SD1 is
needed in <strong>the</strong> direction <strong>of</strong> predicting fouling rates
and discriminating between various membrane
types.
Acknowledgments
Financial support <strong>of</strong> this work by <strong>the</strong> Middle
East Desalination Research Center (Project No
98-BS-034) is gratefully acknowledged.
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