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

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